SOURCES AND EFFECTS
OF IONIZING RADIATION
United Nations Scientific Committee on the Effectsof Atomic Radiation
UNSCEAR 2000 Report to the General Assembly,
with Scientific Annexes
VOLUME II: EFFECTS

UNITED NATIONSNew York, 2000

NOTE
The report of the Committee without its annexes appears as Official Records of theGeneral Assembly, Fifty-fifth Session, Supplement No. 46 (A/55/46).
The designation employed and the presentation of material in this publication do not
imply the expression of any opinion whatsoever on the part of the Secretariat of the United
Nations concerning the legal status of any country, territory, city or area, or of its authorities,
or concerning the delimitation of its frontiers or boundaries.
The country names used in this document are, in most cases, those that were in use
at the time the data were collected or the text prepared. In other cases, however, the names
have been updated, where this was possible and appropriate, to reflect political changes.

UNITED NATIONS PUBLICATION
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ISBN 92-1-142239-6

ANNEX G
Biological effects at low radiation doses

CONTENTSPage

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
I.

CELLULAR EFFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. DESIGNATION OF LOW DOSES AND LOW DOSE RATES . . . . . . . . .
1. Physical factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Biological approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. DAMAGE TO DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Dose response for low-LET radiation . . . . . . . . . . . . . . . . . . . . . . . . .
2. Dose response for high-LET radiation . . . . . . . . . . . . . . . . . . . . . . . .
3. Influence of track structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. CELLULAR DAMAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Chromosome aberrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Cell transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Mutagenesis in somatic cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

II.

ANIMAL EXPERIMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
A. CANCER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
1. Dose-response relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
2. Cancer risks at low doses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
B. HEREDITARY DISEASE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
C. SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

III. EPIDEMIOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. STATISTICAL CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. HIGH-DOSE AND HIGH-DOSE-RATE EXPOSURES . . . . . . . . . . . . . .
1. Dose-response relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Minimum doses for a detectable increase in cancer risk . . . . . . . . . .
3. Effect of dose and dose rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. LOW-DOSE-RATE EXPOSURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Occupational exposures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Environmental exposures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

77
77
77
79
80
80
81
83
84
85
86
89
91

108
109
109
110
114
116
117
117
119
120

IV. MECHANISMS AND UNCERTAINTIESIN MULTI-STAGE TUMORIGENESIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
A. MULTI-STAGE PROCESSES IN TUMORIGENESIS . . . . . . . . . . . . . . . 122

74

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES
Page

B.

C.

D.

E.

F.

G.
V.

1. Initiation of neoplasia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Promotion of neoplasia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Neoplastic conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Progression of neoplasia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MUTATIONAL EVENTS MEDIATING
THE TUMORIGENESIS PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Proto-oncogenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Tumour-suppressor genes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Genes involved in cell-cycle control and genomic stability . . . . . . . .
4. Early events in multi-stage tumorigenesis . . . . . . . . . . . . . . . . . . . .
5. Non-mutational stable changes in tumorigenesis . . . . . . . . . . . . . . .
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CELLULAR AND MOLECULAR TARGETS
FOR TUMOUR INITIATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Monoclonal origin of tumours . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Molecular targets for radiation tumorigenesis . . . . . . . . . . . . . . . . .
3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CELLULAR FACTORS THAT COUNTERONCOGENIC DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Control of cellular proliferation and genomic stability . . . . . . . . . . .
2. Programmed cell death and gene expression . . . . . . . . . . . . . . . . . .
3. Cellular differentiation and other cellular interactions . . . . . . . . . . .
4. Cellular surveillance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DNA REPAIR AND TUMORIGENESIS . . . . . . . . . . . . . . . . . . . . . . . . .
1. DNA repair as a determinant of oncogenic response . . . . . . . . . . . .
2. Implications and uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BIOLOGICAL MODELLING OF TUMORIGENIC RESPONSES . . . . . .
1. Implications of current data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Basic premises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Error-free DNA repair at low doses . . . . . . . . . . . . . . . . . . . . . . . . .
4. Epigenetic events in tumorigenesis . . . . . . . . . . . . . . . . . . . . . . . . .
SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

BIOLOGICALLY BASED MODELLINGOF RADIATION CARCINOGENESIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. GENERAL ASPECTS OF THE PROBLEM . . . . . . . . . . . . . . . . . . . . . . .
B. EMPIRICAL AND MECHANISTIC MODELS . . . . . . . . . . . . . . . . . . . .
1. Armitage-Doll multi-stage model . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Two-mutation models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Generalized Moolgavkar-Venzon-Knudson (MVK)
multi-stage models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Multiple pathway models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. DOSE-RESPONSE RELATIONSHIPS . . . . . . . . . . . . . . . . . . . . . . . . . .
D. SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

122
122
122
122
122
123
124
125
126
127
128
129
129
130
131
131
131
132
134
135
136
137
137
137
140
140
140
141
142
143
144

144
145
146
146
148
149
151
152
156

SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES75

INTRODUCTION
1. Biological effects of ionizing radiation in humans, due
to physical and chemical processes, occur immediately
following the passage of radiation through living matter.
These processes will involve successive changes at the molecular, cellular, tissue and whole organism levels. For acute
whole-body exposures above a few gray from radiation of low
linear energy transfer (LET), damage occurs principally as aresult of cell killing. This can give rise to organ and tissue
damage and, in extreme cases, death. These effects, termed
earlyor deterministic, occur principallyabove a threshold dosethat must be exceeded before they are manifested as clinicaldamage, although damage to individual cells will occur at
lower doses. Protracted delivery of such high doses over
several hours or days will usually result in effects of lower
severity. Information on the early effects of radiation inhumans was reviewed in the UNSCEAR 1993 and 1982
Reports [U3, U6].
2. A second type of damage can occur at late times after
exposure. This damage consists primarily of damage to the
nuclear material in the cell, causing radiation-induced cancer
to develop in a proportion of exposed persons or hereditarydisease in their descendants. Although the probability of bothcancer and hereditary disease increases with radiation dose, itis generally considered that their severity does not. They are
termed stochastic effects and were reviewed in the UNSCEAR
1977, 1988, and 1994 Reports [U2, U4, U7].
3. Direct information on radiation-induced cancer isavailable from epidemiological studies of a number of humanpopulations. These include the survivors of the atomic
bombings in Japan and groups that have been exposed toexternal radiation or to incorporated radionuclides, either for
medical reasons or occupationally. Such studies provide
quantitative information on the risk of cancer at intermediate
to high doses and are reviewed in Annex I, “Epidemiologicalevaluation of radiation-induced cancer”. At lower levels of
exposure, however, quantitative estimates of risk are not soreadily obtained, and inferences need to be made by downward extrapolation from the information available at higher
doses.

tarydisease following radiation exposure is presentlybased on
the hypothesis that the frequency of their induction increases
proportionally with radiation dose. A linear, no-thresholddose-response relationship has generally been adopted bynational and international bodies for assessing the risks
resulting from exposures to low doses of ionizing radiation(see, e.g. [I2, U4]). This hypothesis implies that the risk ofcancer increases (linearly) with increasing exposure and that
there is no threshold, i.e. no dose below which there isabsolutely no risk. As yet no definitive experimental data are
available on this issue (see Chapter IV).
6.
Experimental and epidemiological data on whichquantitative evaluations of the risk of cancer following
exposure to low-LET radiation are based come principallyfrom studies involving exposures at moderate to high doses
and dose rates. Most organizations have extrapolated linearly
and then applied a reduction factor to estimate risks at low
doses and low dose rates. This reduction factor has beenvariously termed a dose and dose rate effectiveness factor(DDREF) [I2], a dose-rate effectiveness factor (DREF) [N1],a linear extrapolation overestimation factor (LEOF), and a low
dose extrapolation factor (LDEF) [P1, P14]. The basis for theapplication of such a reduction factor was described in the
UNSCEAR 1993 Report [U3]. For high-LET radiations, suchas neutrons and alpha particles, no reduction factor has
generally been applied, because the dose response forradiation-induced cancer and hereditary disease is essentiallylinear between the lowest dose at which effects have been
observed and that at which cell killing becomes a factor in thedose response [I2, U3, U4].

4. In the case of radiation-induced hereditary disease,studies on human populations have not provided quantitative
information, so risk estimates have to be based on the resultsof animal studies. There is again the difficulty that quantitative data are available only following exposures to intermediate to high doses. Information on radiation-induced
hereditary disease has been reviewed previously by theCommittee [U3, U4].

7.
There has been extensive debate as to the shape of thedose-response relationship below the range at which effects
can be directly measured. It has been argued that
irradiating cells and tissues with small radiation doses canresult in an adaptive response that reduces the amount ofdamage caused by subsequent radiation exposure [U2, W6]
or even results in a beneficial effect, termed hormesis [A9,
T11, W13]. There have been suggestions that, at very low
doses, radiation may have no effect at all; these suggestions
are based on the proposition that there could be a thresholdfor a response, in the same way as there is for clinicallyobserved deterministic effects. This situation may arise, forexample, if damage to a number of cells is needed before
any adverse effect occurs or if interaction between cells isa prerequisite for an effect [K19, M34]. An apparent
threshold may also arise if the latent period between exposure and the appearance of a cancer exceeds the normal
lifespan of the individual [R1, R14].

5. For the majority of situations in which human beingsare exposed to ionizing radiation in the home, in the naturalenvironment, and in many places of work, the principal
concern is the consequence of exposure to low doses and low
dose rates. For the purposes of radiation protection, theestablishment of the expected incidence of cancer or heredi-

8.
Several mechanistic models have been proposed todescribe the effects of radiation at the different levels of
biological organization. There has been considerable effort indeveloping such models to quantitatively describe cellularsurvival, repair and transformation, based on the stochastic(probabilistic) process of energy deposition in radiosensitive

76

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

targets representing elements of cell structure, or employing
track structure concepts. Other models have concentrated on
representing the processes of repair and misrepair of damaged
cell structures. In general, a mechanistic model should, apartfrom quantitative description of available data, have a
predictive capability and offer crucial tests of its validity.
Some mechanistic models support the linear no-thresholdexpressions used to fit epidemiological data, while others point
to power law dose-effect relationships, implying a zero initialslope. There have been suggestions that the limits of dosebased quantities have been reached and that fluence and anaction cross section are more appropriate concepts forassessing damage to cells. No quantitative attempt has,
however, been made to apply these concepts in radiologicalprotection [S5].

11. Dose-response relationships for radiation effects incellular systems are reviewed in Chapter I. Considered first of
all is the definition of a low dose and a low dose rate, as they
may be described either physically or biologically. This will
depend upon the level of biological organization considered.
Also addressed are theoretical aspects of the interactions ofradiation with cells and tissues; the influence of track structureon radiation response; the concept of dose as it applies totissues, cells, or subcellular targets; and the possible implications for dose-response relationships. The results of cellularstudies are then reviewed. The range of endpoints of thesestudies include cell killing, cell transformation, chromosome
aberrations, and mutation, which occur principally as aconsequence of damage to the nuclear material in individual
cells.

9. It has been recognized by the Committee for sometime that information is needed on the extent to which both
total dose and dose rate influence the induction of cancer
and hereditary disease. A number of considerations areimportant in determining the risks of exposures toradiation at low doses and low dose rates. These include (a)careful analysis of epidemiological studies to determine thelowest doses at which effects are statistically evident, (b)examination of the shape of the dose-response relationships
in the low-dose region using available experimental andepidemiological data, and (c) assessment of the possibilities
for extrapolation to lower levels of dose based on anunderstanding of the mechanisms involved in the radiation
response of tissues. Extrapolation based on mechanistic
considerations can, in principle, be made using information on relevant biological factors such as cellular/
molecular targets for tumour initiation, the nature ofradiation-induced damage to deoxyribonucleic acid (DNA)and the fidelity of its repair, together with information on
adaptive responses and cellular surveillance. Many of thesefactors were discussed in the UNSCEAR 1993 and 1994
Reports [U2, U3].

12. The results of animal studies related to radiationinduced cancer and hereditary disease are considered in
Chapter II. For tumour induction, animal studies have demonstrated that dose-response relationships can be complex,
depending on the age, gender, and species or strain of theanimal, the sensitivity of individual tissues, the tumour type,
and the dose rate. The results obtained for dose-response
relationships for life-shortening and tumour induction withdifferent animal models following exposure to externalradiation or incorporated radionuclides are illustrated, andinformation is presented on the extent to which animal data
can provide information on the risks of exposure at low doses.

10. The objective of this Annex is to examine the sourcesof data that are available for assessing the risks ofradiation-induced cancer and hereditary disease at low
doses for both sparsely ionizing (low-LET) and densely
ionizing (high-LET) radiation and their associateduncertainties. This Annex brings together information
reviewed by the Committee in separate specialized
Annexes, material from previous UNSCEAR reports, andadditional data from dosimetric and cellular studies,
epidemiological investigations, recent advances inmolecular biology, and developments in mechanistic
models. The aim is to provide an overview of the data
available on the relationship between radiation exposureand the induction of cancer and hereditary disease, withemphasis on the extent to which radiation effects can beobserved at low doses. This information, coupled withknowledge on the mechanisms of damage to cells andtissues, provides a basis for informed judgements to bemade about the likely form of the dose response at
exposures below those at which direct information isavailable.

13. In the case of damage to germ cells, the mutational
events resulting from DNA damage generally arise as asimple function of dose and dose rate and depend principallyon the radiation sensitivity of the specific gene locus. Doseresponse relationships are reviewed in Chapter II. Radiationinduced hereditaryeffects were comprehensively examined by
the Committee in the UNSCEAR 1986, 1988, and 1994
Reports [U2, U4, U5].
14. Epidemiological studies give information on doseresponse relationships for tumour induction and provide thebasis for quantitative risk estimates for human populations.
The available data have been the subject of substantive
reviews by the Committee [U2, U4, U5], and a further review
is contained in Annex I, “Epidemiological evaluation ofradiation-induced cancer”. The information available on
dose-response relationships is described in Chapter III, withemphasis on the extent to which data are available at low
doses. These data relate to the consequences of exposure inutero as well as the exposure of infants, children, and adults.
15. The direct information on tumour induction, both fromexperimental and epidemiological studies, is insufficient, on
its own, to elucidate the shape of the dose-response
relationship at low doses. In Chapter IV, present knowledge
is examined on the mechanisms of radiation tumorigenesisthat can be used to gain further insight into effects at low
doses. Emphasis is placed on gaps in knowledge and theconsequent uncertainties. This topic was last reviewed by theCommittee in the UNSCEAR 1993 Report [U3], and otherissues relevant to those discussed here are considered inANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

Annex F, “DNA repair and mutagenesis” and Annex H,“Combined effects of radiation and other agents”.
16. As modern molecular methods are developed andapplied, the understanding of the mechanisms of tumorigenesis has, in recent years, increased substantially. At thesame time there has been an equivalent increase in knowledge of radiation action on cellular DNA; of control of the
reproductive cell cycle; of the mechanisms of DNA repair,genomic maintenance, and mutagenesis; and of non-mutational mechanisms of stable cellular changes. All thisinformation could be relevant to assessing the shape of thedose response for both radiation-induced cancer and heredi-

77

tary disease at low doses and dose rates and the effects ofradiation quality at exposures below those at which direct
information is available.
17. An important aim of Chapter IV is, accordingly, tohighlight the critical elements of the current understanding of
the mechanisms of tumorigenesis in order to relate them todata on dose-effect relationships and permit extrapolation todoses beneath those at which quantitative information isavailable. In Chapter V, the judgements developed in Chapter
IV are used to examine biologically based computationalmodels that may in turn be used to assess the risk of radiationinduced cancer at low doses and low dose rates.

I. CELLULAR EFFECTS
18. Damage to DNA, which carries the genetic informationin chromosomes in the cell nucleus, is considered to be themain initiating event by which radiation damage to cells
results in the development of cancer and hereditary disease[U3]. Either one or both strands of the DNA helix in cells
may be damaged or broken, resulting in cell death, damage tochromosomes, or mutational events. Radiation is thought tohave an effect on DNA either through the direct interaction ofionizing particles with DNA molecules or through the actionof free radicals or other chemical intermediates produced by
the interaction of radiation with neighbouring molecules.
Damage can also be caused to other cellular structures,
resulting in death or sublethal damage in individual cells;such damage does not in general result in radiation-induced
cancer or hereditary disease. An exception is damage to cells
that results in fibrosis, as this seems to be a precursor to thedevelopment of some tumour types (see Chapter II). It is also
possible that other more indirect mechanisms can influence
tumour development.

1993 UNSCEAR Report [U3], the physical and biologicalfactors that need to be considered in making theseevaluations were examined in the context of the doses anddose rates below which it would be appropriate to apply areduction factor when assessing risks (per unit dose) at low
doses and low dose rates from information on risks
obtained at high doses and dose rates.
21. The following Sections deal with physical and biologicalapproaches todesignating exposures that maybe considered tobe either low-dose or low-dose-rate and with experimental datathat can give information on the dose-response relationship forstochastic effects in cells either in vitro or in vivo.
1. Physical factors

A. DESIGNATION OF LOW DOSES
AND LOW DOSE RATES

22. Various models have been developed to account for thefeatures of dose-response relationships obtained in experimental studies. A common aspect of many of these models is thata single radiation track, for any radiation quality, is taken to
be capable of producing the initial damage and hence thecellular effect. The fundamental physical quantity used todefine the deposition of energy in organs and tissues fromionizing radiation is the absorbed dose. The tissue or organ
absorbed dose, DT, is generally taken to be the mean energy
absorbed in the target organ or tissue divided by the mass, T.
This definition of the absorbed dose does not, however,characterize the fluctuation of energy absorption resulting
from the stochastic nature of the energy deposition events(tracks) in individual cells. The fluctuation in the energy
deposition between cells in a tissue is generally disregarded
but can be significant when the possible effects of ionizing
radiation on cells at low doses are considered. The number of
independent tracks within each cell follows a Poisson
distribution, and thus the numbers of cells receiving zero or
few tracks will depend on the fluence of tracks through theorgan or tissue.

20. In interpreting the responses of cells and tissues toionizing radiation, judgements need to be made as to thebounds for low and high doses of low-LET radiation. In the

23. The physical factors that can influence the effect ofradiation on cells and tissues are generally well understood asa result of advances that have taken place in recent years in

19. This Annex is concerned with the examination of thebiological effects of radiation at low radiation doses. It isappropriate, therefore, to consider first how these should be
defined. The designation of low doses and low doses rates
has been considered in earlier reports by the Committee[U3, U5] and is summarized here briefly. The following
Sections then consider radiation damage to DNA, relative
biological effectiveness (RBE) of radiations of different
quality, and the influence of track structure on cellularresponse. Cellular studies related to the determination of
dose-response relationships for chromosome aberrations,cell transformation, and mutation induction in somatic
cells are then summarized.

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ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

microdosimetry at the cellular and subcellular levels [B31,
B32, G6, G12, P13, R18]. A microdosimetric argument for
defining low doses and low dose rates can be based on
statistical considerations of the occurrence of independent
radiation tracks within cells or nuclei. For 60Co gamma rays,
for example, and a spherical cell or nucleus (taken to be thesensitive target) assumed to be 8 µm in diameter, there willbe, on average, one track per nucleus when the averaged
absorbed dose is about 1 mGy [B31, B32]. If the induction ofdamage in the nucleus depends on energy deposition in single
nuclei, with no interaction between them, a departure fromlinearity is unlikely unless there have been at least two
independent tracks within the cell nucleus. The number oftracks within cells follows a Poisson distribution, as illustrated
in Table 1, with the mean number of tracks proportional to theaverage absorbed dose. For average tissue absorbed doses of
0.2 mGy from low-LET 60Co gamma rays, for example,
spherical nuclei of say 8 µm diameter would each receive, on
average, about 0.2 tracks. In this case, just 18% of cells would
receive any radiation track at all and less than 2% of cells
would receive more than one track. Halving the exposure
would simply halve the fraction of the total cells affected, andso, at such low doses, the dose-effect should be linear. This
microdosimetric argument for a low dose (taken here to be 0.2
tracks per cell) would apply to biological effects where theenergy deposited in a cell produces effects in that cell and noother cell. It might apply, for example, to cell killing, theinduction of chromosome aberrations, and mutations. Itsapplicabilitytocell transformation and cancer induction is lesscertain. It would need modification if, for example, the pro-

Table 1
Proportions of a cell population traversed by tracks for various mean doses a from gamma rays and alphaparticles

Mean tracks
per cell
0.1
0.2
0.5
1
2
5b
10 b

Percentage of cells in population suffering
0 track

1 track

2 tracks

3 tracks

4 tracks>5 tracks

Percentage of
hit cells with
only one track

90.5
81.9
60.7
36.8
13.5
0.7
0.005

9
16.4
30.3
36.8
27.1
3.4
0.05

0.5
1.6
7.6
18.4
27.1
8.4
0.2

0.015
0.1
1.3
6.1
18
14
0.8

0.2
1.5
9
17.5
1.9

0.4
5.3
56
97.1

95.1
90.3
77.1
58.2
31.3
3.4
0.05

a Approximately 0.1 mGy for gamma rays, 300 mGy for alpha particles.
b At these values appreciable proportions of the cell population will incur more than five tracks.

24. To develop the microdosimetric argument forassessing a low dose, knowledge is required of the sensitive
volume in the cell. A sphere of 8 µm diameter, as describedabove, is typical of the size of some cell nuclei, although
they may be larger or smaller. If only a part of the nucleus
responds autonomously to radiation damage and repair,then a smaller sensitive volume may be more appropriate,
and the estimate of a low dose would increase. Figure I
illustrates, for various volumes, the specific energy of lowLET radiation that would correspond to this microdosimetric criterion of a low dose when less than 2% of cells
receive more than 1 track. Thus for a nucleus of diameter
4 µm, a low dose (0.2 tracks per cell, on average) would be
about 0.8 mGy, and for 32 µm it would be about 0.01 mGy.
As described in the UNSCEAR 1993 Report [U3], thisdefinition of a low dose could also take into accountinformation on the time characteristics for DNA repair, which
would give a low dose rate of 103 mGy min1, or be based on
only a single track traversing a cell in a lifetime (say, 60
years), allowing essentially no scope for track interactions,which would give a low dose rate of 108 mGy min1.

100

5 MeV alpha particles
10
MEAN DOSE (mGy)bability of an effect was influenced by a subsequent track at alater time, as could be the case for multi-stage carcino-genesis,
or if there was interaction between cells in the development ofa specific radiation effect, as, for example, has been suggested
for so-called bystander effects. This is con-sidered further in
Chapter IV.

160

Co gamma rays

0.1

0.01
0

10

100

DIAMETER OF CELL NUCLEUS (µm)

Figure I. Mean dose from an average of 0.2 radiation
tracks per cell nucleus as a function of diameter (<2%of nuclei receive more than one track).

25. The situation is quite different for exposures to highLET radiation. When a tissue receives an average dose of
1 mGy from alpha particles, only about 0.3% of the nuclei
are struck by a track at all; the remaining 99.7% are totallyunirradiated. When a single track does strike, it delivers to
the nucleus a very large dose, of about 370 mGy on
average. In individual nuclei the dose may be any value up
to about 1,000 mGy [G22, G23].

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

2. Biological approaches
26. Biological approaches. The definition of a low doseand a low dose rate can also be based on direct observationof damage in experimental systems or in epidemiologicalstudies. One approach to assessing a low dose is based onparametric fits to observed dose-response data for cellulareffects at low to intermediate doses, below those at whichcell killing will become important. For the induction ofcellular damage, the incidence, I, of an effect can then be
related to the dose, D, by an expression of the form
(1)
I(D)  α D  β D 2in which α and β, the coefficients for the linear and quadratic
terms fitted to the radiation response, are constants and are
different for different endpoints. This equation has beenshown to fit data on the induction of chromosome aberrations
in human lymphocytes [U3]. It can also be extended to covercell killing as described in Section I.B.1. For some types of
unstable chromosome aberrations in human lymphocytes, theα/β quotient (which corresponds to the average dose at which
the linear and quadratic terms contribute equally to the
biological response) is about 200 mGy for 60Co gamma rays
[L34], and thus the response is essentially linear up to about
20 mGy, with the dose-squared term contributing only 9% of
the total response. Even at 40 mGy, the dose-squared term
still contributes only about 17% to the overall response. On
this basis a low dose might be judged to be 20!40 mGy.
27. Another approach to assessing the low dose range
can be based on animal studies. The results of studies
designed to examine the effect of dose and dose rate on
tumour induction were comprehensively reviewed in the
UNSCEAR 1993 Report [U3]. The results obtained withexperimental animals, predominantly mice, comparing theeffect of various dose rates of low-LET radiation on the
induction of leukaemia and solid tumours have suggested
that, on average, a dose rate of about 0.06 mGy min1 overa few days or weeks may be regarded as low. At lower dose
rates no further reduction in tumour incidence, per unit

79

exposure, was obtained. The choice by the Committee inthe UNSCEAR 1986 Report [U5] of a low dose rate toinclude values up to 0.05 mGy min1 appears to have been
based on dose rate studies in experimental animals.
28. The analysis of information from epidemiologicalstudies, in particular the data from the survivors of theatomic bombings in Japan, can also be used for estimating
a low dose. Analysis of the dose response for mortality
from solid cancers in the range 0!4 Gy (adjusted for
random errors) has suggested an α/β quotient from a minimum of about 1 Gy, with a central estimate of about 5 Gy
[P1, P14]. An α/β quotient of 1 Gy suggests that at a doseof 100 mGy the dose-squared term contributes less than
10% to the response and at 200 mGy still less than 20%. It
was suggested in the 1993 UNSCEAR report that for solid
tumour induction in humans, a low dose could be taken to
be less than 200 mGy [U3]. There is, in practice, little
evidence of a departure from linearity up to about 3 Gy. In
the case of leukaemia in the survivors of the atomic
bombings, where there is a significant departure fromlinearity at doses above about 1.5 Gy, a central estimate ofα/β has been calculated to be 1.7 Gy, with a minimum
value less than 1 Gy [P1, P14]. On the basis of this central
estimate, the dose-squared term would contribute about
10% to the response at a dose of 200 mGy and about 23%
at 500 mGy. A low dose might therefore be considered to
be any exposure up to about 200 mGy [U3].
29. On the basis of these various analyses of physical andbiological data, the Committee concluded in the
UNSCEAR 1993 Report [U3] that for the purpose of
assessing the risk of tumour induction in humans at low
doses and dose rates of low-LET radiation, a reduction
factor (dose and dose rate effectiveness factor, DDREF)
should be applied, either if the total dose is less than
200 mGy, whatever the dose rate, or if the dose rate is
below 0.1 mGy per min1 (when averaged over about anhour), whatever the total dose. The various approaches toassessing a low dose and low dose rate from low-LET
radiation are summarized in Table 2.

Table 2
Alternative criteria for upper limits of low dose and low dose rate for assessing risks of cancer induction inhumans (low-LET radiation)[U3]

Basis of estimationabc

Low dose(mGy)

Low dose rate(mGy min-1)

200

0.05
0.1 c

UNSCEAR 1986 Report [U5]
UNSCEAR 1993 Report [U3]Linear term dominant in parametric fits to single-cell dose responses
Microdosimetric evaluation of minimal multi-track coincidences in cell nucleus

20-40
0.2

Observed dose-rate effects in animal carcinogenesis
Epidemiological studies of survivors of the atomic bombings in Japan

10-8 (lifetime)
10-3 (DNA repair)
0.06

200



Approximately 0.1 mGy for gamma rays, 300 mGy for alpha particles.
At these values appreciable proportions of the cell population will incur more than five tracks.
Averaged over about an hour.

80

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

30. For high-LET radiation, the experimental datasuggested that little consistent effect of dose rate or dose
fractionation on the tumour response at low to intermediate
doses has been obtained. It was therefore concluded [U3]that there was no need to apply a reduction factor to risks
calculated at high doses and dose rates.

B. DAMAGE TO DNA
31. Cells are able to repair both single- and double-strand
breaks in DNA over a period of a few hours, but this repair
can be imperfect, resulting in long-term cellular damage andmutation. It has been assumed in previous reports by theCommittee that damage to DNA causing mutational events ingerm cells is the result of a single biological event but thatcarcinogenesis is a multi-stage process in which the radiation
can induce one or more of the stages involving damage to
DNA and interference with cellular homeostatic mechanisms[U3].
32. The vast majority of endogenous DNA lesions take theform of DNA base damage, base losses, and breaks to one of
the sugar-phosphate backbone strands of the double helix[A14, W7]. Such single-strand damage may be reconstituted
rapidly in an error-free fashion by cellular repair processes
since the enzyme systems involved will, for all these lesions,have the benefit of the DNA base sequence on the undamaged
strand acting as a template on which to recopy the damaged
or discontinuous code. Single ionizing tracks of radiation will
induce a preponderance of such single-strand damage as aresult of energy loss events occurring in close proximity to asingle DNA strand in the double helix. A cluster of such
events within the diameter of the DNA duplex (about 2 nm)
has, however, a finite probability of simultaneously inducing
coincident damage to both strands. In support of this, anapproximately linear dose response for double-strand breakinduction by low-LET radiation has been observed [J6],confirming that breakage of both strands of the duplex may beachieved by the traversal of a single ionizing track and does
not require multiple-track action. There is also evidence thata propor-tion of radiation-induced double-strand breaks arecomplex and involve locally, multiply damaged sites, LMDS
[J6]. On the basis of a body of experimental evidence it may
be judged that the ratio of low-LET radiation-induced singlestrand DNA breaks plus base damages to double-strand
breaks is around 50:1. The probability of a double-strand
break per cell has been judged to be about 4 per cell per
100 mGy [G10].
33. A fraction ofradiation-induced double-strand damage
will be repaired efficiently and correctly, but error-free
repair of all such damage, even at the low abundance
expected after low-dose exposure, is not anticipated. Unlike
damage to a single strand of the DNA duplex, a proportionof double-strand lesions, perhaps that component
represented by LMDS, will result in the loss of DNA
coding from both strands. Such losses are inherently
difficult to repair correctly, and it is believed that misrepairof such DNA double-strand lesions is the critical factor

underlying the principal hallmarks of stable mutations
induced by ionizing radiation of various qualities [T2,
T14]. Double-strand DNA losses may in principle berepaired correctly by DNA repair recombination, but suchdamage may be subject to error-prone repair, which can
result in the formation of gene and chromosomal mutations
that are known to characterize malignant development[C23]. This interpretation would, however, be flawed if
cellular repair processes were totally effective in repairingdamage in the case of small numbers of double-strand
breaks in the affected cells. In such a case, a threshold dose
before any response could occur would be possible The
most basic, although not necessarily sufficient, condition
for a true dose threshold would be that any single track of
the radiation should be unable to produce the effect. Thus,
no biological effect would be observed in the true low-dose
region, where cells are traversed only by single tracks. Thisis considered further in Chapter IV.
1. Dose response for low-LET radiation
34. The approach that has been frequently used to describe
both the absolute and the relative biological effectiveness of agiven radiation exposure from low-LET radiation is based on
the assumption that the induction of an effect can beapproximated by an expression of the following form:
I(D)  (α D  β D 2) e(p1 D  p2 D 2)

(2)

where α and β are coefficients of the linear and quadratic
terms for the induction of stochastic effects and p1 and p2
are linear and quadratic terms for cell killing. Thisequation has been shown to fit much of the published dataon the effects of radiation on cells and tissues resulting
from damage to DNA in the cells, including the inductionof chromosome aberrations, mutation in somatic and germ
cells, and cell transformation.
35. The nature of the initial damage to DNA wasconsidered in the UNSCEAR 1993 Report [U3]. The
theoretical considerations were described in terms of thegeneral features of target theory, because the insult of
ionizing radiation is in the form of finite numbers of
discrete tracks. On this basis, it was proposed that thenature of the overall dose response for low-LET radiation
could be subdivided into a number of regions:(a) Low-dose region. At the lowest doses, a negligible
proportion of cells (or nuclei) would be intersected bymore than one track, so the dose response for singlecell effects would be dominated by individual track
events acting alone and would therefore be expected
to increase linearly with dose and be largely independent of dose rate;(b) Intermediate-dose region. In this dose region, where
there may be several tracks per cell, it has beencommonly assumed that tracks act independently if alinear term (α) can be obtained by curve-fitting toequations such as (1). For most of the experimental

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES(c)dose-response data used for curve-fitting, the lowest
dose at which a significant effect is obtained is usually
towards the higher end of this dose region, when
individual cells may, in fact, have been traversed byconsiderable numbers of tracks. The assumption of onetrack action for this region considers that the relevant
metabolic processes of the cell are not influenced by theadditional tracks in any way that could alter the
expression of the ultimate biological damage of eachindividual track. On this assumption, it is conventional
to interpolate linearly from this region to zero dose to
deduce the effectiveness of low doses and low dose rates
of radiation. Such interpolation is based on the coefficient α in equation (1) and on the assumption that itremains unchanged down to very low doses and very
low dose rates. There are a number of radiobiologicalstudies, mostly with cells in vitro but also from animals
exposed at different dose rates, that suggest that thiscommon assumption is not universally valid.
High-dose region. In this region, where there are many
tracks per cell, multi-track effects are clearly seen as
non-linearity of dose response, with upward or
downward curvature of the dose response. The simpler
forms of the dose-response relationship that are
observed experimentally can commonly be fitted by ageneral polynomial with terms for dose and dose rate.
At high doses, a separate term is needed to account for
the effects of cell killing.

36. Equation (1), or some modification of it, has been
conventionally used to estimate the biological effectiveness ofradiation at minimal doses, assuming a constant value of α
from the intermediate-dose region down to zero dose, withindependence of dose rate. There are, however, instances inwhich this may not apply.
37. The assumption of such a dose response for single-cellstochastic effects may not hold if there are significant multitrack events in the intermediate-dose region. Such events
could include, for example, the induction of multiple
independent steps in radiation carcinogenesis, cellular
damage-fixation processes influencingrepair ofDNA damage;the induction of enhanced repair by small numbers of tracks;
multiple tracks or enhancement of misrepair; and variations
in cell sensitivity with time. The dose response may also be
modified if the biological effect of interest required damage tomore than one cell or if it was influenced by damage toadditional or surrounding cells.
2. Dose response for high-LET radiation
38. There are extensive radiobiological data indicating thathigh-LET radiations (neutrons and alpha particles) have agreater biological effect, per unit of average absorbed dose,than low-LET radiation. The influence of radiation quality on
a biological system is usually quantified in terms of its relative
biological effectiveness (RBE). The RBE of a specificradiation, R, can be defined as the absorbed dose of reference
radiation required to produce a specific level of response
divided by the absorbed dose of radiation, R, required to81

produce an equal response, with all physical and biologicalvariables, except radiation quality, being held constant as far
as possible. This definition does not depend on the dose
response for the two radiations being the same, it simply
depends on comparing the dose to give a specific level of
effect for a particular endpoint. Low-LET radiation (x rays or
gamma rays) is normally used as the reference radiation. A
particular form of RBE is RBEm, which is the maximum RBE
that would be obtained at low doses and low dose rates.
Various authors and committees (see for example [M18, S35,
U6]) have reviewed the relevant biological data. A
comprehensive review of the literature relevant to thedetermination of values for RBE may be found in NCRP
Report No. 104 [N6]. Maisin et al. [M53] reported information on tumour induction in 7- or 21-day old C57BL mice
exposed to 3.1 MeV neutrons (0.5, 1 or 3 Gy) or 250 kVp
x rays (0.125, 0.25, 0.5 or 1 Gy). When the incidence of all
malignant tumours and of hepatocellular cancer was fitted toa linear of linear-quadratic function, an RBE in the range 5 to
8 was obtained.
39. It is apparent from the studies summarized above that
the RBE for high-LET radiation is dependent on the
biological response being studied. For early effects in tissues
caused by cell killing (e.g. skin burns, cataracts, and sterility),an ICRP task group [I13] concluded that for a range of tissues
and for both neutrons and alpha particles, the RBE wasgenerally less than 10. For damage to the lung from inhaledalpha particles causing fibrosis and loss of fluid into the lung(pneumonitis), the RBE for rats and beagle dogs wasestimated to be between 7 and 10. Similarly, for the inductionof chromosome aberrations in human blood lymphocytes byalpha particles from 242Cm, RBE values of about 6 have beenobtained in comparison with x rays and 18 in comparison
with gamma rays [E12]. For the induction of micronuclei(caused by fragmentation of chromosomes) in lymphocytes byalpha particles from plutonium, an RBE of 3.6 has been foundat low doses (<1 Gy), and for DNA double-strand breaks inEhrlich ascites tumour cells, RBEs in the range 1.6!3.8 havebeen reported [B40].
40. In a few experimental studies a biological effect has
been obtained for alpha particle irradiation although a similareffect has not been found with low-LET radiation. Studies inwhich sister chromatid exchanges (SCE) have been measured
in human lymphocytes in the G0 stage of the cell cycle give ameasurable frequency of SCEs following exposure to alphaparticles from 241Am, but no effect of x-ray irradiation was
obtained. From the definition of RBE given above, this
implies an infinite RBE, although it is solely a consequence of
there having been no observable effect of x rays at low doses[A16]. Similar results have been reported for SCEs in Chinese
hamster ovary cells irradiated in the G1 phase of the cell cycle
by 238Pu alpha particles or x rays. High values of RBE, up to
about 245, have also been reported [N10] for sperm headabnormalities in mice when the effect of external exposure to
x rays was compared with the effects of tissue-incorporated
241
Am. This may partly be accounted for by the heterogeneous
distribution of 241Am incorporated in the testis; it is known
that actinides such as 241Am tend to concentrate in interstitial

82

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

tissue in the mouse testis, in close proximity to the developing
sperm cells.
41. For tumour induction, a number of studies have demonstrated that both high- and low-LET radiation may induce
cancer in a range of tissues. Data relevant to the choice ofRBEs for neutrons and alpha particles are summarized below.
Values of RBE obtained for long-term effects can be useful fortransferring information on risks calculated following exposure to high-LET to assess risks in populations exposed to
low-LET radiation, and vice versa.

(a) Neutrons
42. Values for RBEm obtained for various biological endpoints in mammals and in mammalian cells for fission
neutrons compared with gamma rays are summarized inTable 3. Similar reviews have been published by UNSCEAR[U5] and Sinclair [S35]. Information on the variation of RBEm
with neutron energy comes partly from data from cellularstudies, in particular using point mutations, chromosomal
aberrations, and cell transformation as endpoints.

Table 3
Estimated RBEm values for fission neutrons compared with gamma rays
[N6]
Endpoint

RBEm

Cytogenetic studies, human lymphocytes in culture
Cell transformation
Genetic endpoints in mammalian systems
Life shortening (mouse)
Tumour induction

3453
380
570
1046
1659

43. There is uncertainty in the value of RBEm for fission
neutrons. This uncertainty comes principally from how thedata for low-LET radiation, mainly for cancer induction
and life shortening in mammals, are extrapolated to low
doses and low dose rates. The derivation of values for
RBEm is illustrated in Figure II. The straight line A of
slope αH represents the dose-response relationship for highLET radiation. The data points shown in the Figure arerepresentative of data for low-LET radiation and can beextrapolated to low doses by the linear relationship B ofslope αLH or by curve C, based upon a linear-quadratic
dose-response relationship. Curve D represents theextrapolated linear portion with slope αLL of the low dose
response of curve C. The ratio of the slopes of curves A(slope αH) and D (slope αLL) represents RBEm.
Curve A
High-LET α

H

44. There are few data in whole animals that measure thevariation of RBEm for specific tumour induction or life
shortening with neutron energy. Knowledge of the variationof RBEm with neutron energy is confined to cellular studies.
Chromosomal aberrations in human lymphocytes [E12, E13]
indicate a monotonic decrease by a factor of about four from
1 MeV to 14 MeV. Mutations in a human hamster hybrid cellline (AL) indicated [H29] a monotonic decrease by a factor of
about seven from 0.3 to 14 MeV. The oncogenic transformation of C3H10T½ cells showed a more erratic variation
with neutron energy [M15], but an overall variation by afactor of three from 230 keV to 14 MeV. The cellular datasuggest a decrease in RBEm by a factor of about four from100 keV with an increase of neutron energy. There are very
few experimental data at lower neutron energies. Some
cellular data observing chromosome aberrations in human
lymphocytes [E14] suggest an RBEm close to that for fission
neutrons, whereas similar data from Sevan’kaev et al. [S36]suggest lower values.
(b) Alpha particles

EFFECT

Curve C
Curve B
Low-LET α

L HCurve D
Low-LET α

L L

DOSE

Figure II. Typical dose-effect relationship for low- andhigh-LET radiations [M18].

45. Alpha particles have a very short range in tissue. For
the highest energy natural alpha particles from 226Ra and its
decay products with an energy up to about 7.8 MeV, themaximum range is about 80 µm; for 5 MeV alpha particles
from 239Pu, the range is about 40 µm. These dimensions may
be compared with the dimensions of the cell nucleus, whichrange from about 5 to 10 µm in diameter. The dose that asingle alpha particle delivers crossing the cell nucleus,
considered to be the radiosensitive target, is highly variable.
It may range from very low doses for particles that graze thenucleus, to more than 1 Gy for particles crossing the diameter.
Thus the concept of average tissue dose is a considerable
simplification, and individual cells in a tissue will receive very
different doses. Furthermore, alpha-emitting radionuclides

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

may be deposited on the surfaces of organs within the body;this is the case, for example, for radon decay products
deposited in the lung and for plutonium isotopes accumulatedby the skeleton. There can therefore be a very heterogeneous
distribution of alpha particle dose within an organ (or tissue).
The specified dose depends on whether an average organ doseor the mean dose to a particular localized tissue volume iscalculated. In practice, average organ dose is usuallycalculated.
46. When it comes to choosing an appropriate RBE to use
for estimating the risk of tumour induction in organs and83

tissues, there are rather few data available. The maindifficulty is that comparable patterns of exposure are needed
for both the alpha-emitting and the reference radiation (x rays
or gamma rays). Although extensive data are available on
tumour induction for both of these radiations on their own,
their effects have been directly compared much lessfrequently. Published data relevant to the estimation of RBEm
are available for the induction of bone sarcomas and lungtumours in experimental animals, from studies on cells in
culture and, to a limited extent, from epidemiological studies
on human populations. Relevant data are summarized inTable 4.

Table 4
Estimated RBEm values for alpha particle irradiation and for gamma rays
Endpoint

RBEm

Ref.

Dogs
Mice
Dogs

26
25
5.4 (4.05.8)

[N6]
[N6]
[G16]

Various species
Dogs
Rats
Dogs

30 (640)
1018
25
36

[I5]
[B41]
[H30]
[H30]

1025

[B42]

up to 7.1 aup to 18

[C27]
[T16]

Chromosome aberrations

535[E12, P20]

Germ cell mutations (chromosome fragments, chromosome translocations, dominant lethals)

2224

[S37]

Bone tumours

Lung tumours

Cell transformation (C3H10T½)
Cell mutation
Human lung cells HF19
Chinese hamster cells V79aCompared with x rays (from [M18]).

3. Influence of track structure
47. As described above, it is commonly observed thathigh-LET radiations (neutrons and alpha particles) aremuch more effective per unit dose than low-LET (electronsand photons), in producing cellular effects such as
chromosome aberrations and mutations or for effects inanimals such as cancer and life shortening. Despite this,the number of DNA breaks produced per gray is not very
different for high- and low-LET radiations. Yet it is thesebreaks that lead to chromosomal and mutation events incells and eventually to cancer. The explanation lies either
in the difference in the efficiency/fidelity of double-strand
break repair after high- and low-LET radiation, or in the
difference between the spatial distribution of the initialphysical events (ionizations and excitations) which lead,
via double-strand breaks, to aberrations and mutations in
the cell. If the second explanation is true, there is somebiological relevance to the distribution of initial events ofenergy deposition around tracks of charged particles, i.e. totrack structure.

48. Computer programmes based on Monte Carlotechniques are now available to calculate on a scale ofnanometres or smaller the exact position of ionizations andexcitations in the track of charged particles [N9].
Examples for a 500 eV electron and a 4 MeV alphaparticle are given in Figure III [G10]. Electrons meander
by scattering and may travel in any direction. In contrast,
heavy charged particles (from protons to much heavierions) essentially travel in straight lines on a well-defined
path. They pass their energy on to secondary electrons,
which wander from the path of the ion. Generally, ions ofhigher velocity produce higher energy electrons that can
travel further from the path of the ion. As an example,
Figure IV shows calculations of the fraction of energy
deposited within a distance, r, of a track for protons ofenergy from 0.3 to 20 MeV. For 0.3 MeV protons, at least
99% of the energy is deposited within 30 nm of the centreof the track. For a 20 MeV proton, some 2% of the energy
is deposited more than 1 µm away from the path of theparticle track.

84

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

2 nm

Figure III. Simulated low-energy track (upper panel: initial energy 500 eV) and simulated short portion of an alphaparticle track (lower panel: initial energy 4 MeV). A section of DNA is shown to give a perspective on dimensions [G10].

49. By choosing particle type and energy, it is possible toselect particles of the same LET but markedly different
track structures, and when such experiments are done thendifferent biological effects are found [C24]. Clearly, track
structure is important for understanding how radiations ofdifferent quality cause differences in RBE, althoughopinions vary as to which particular features of track
structure and which objects and characteristic distances in
the cells relate to given biological endpoints. As an
example, Savage [S32] considers that for producing
chromosome aberrations, DNA breaks may exchange overdistances up to one or two hundred nanometers. Thus,
energy around a particle track deposited within volumes ofthe order of 100 nm may be important. Other dimensions

FRACTION OF ENERGY DEPOSITED

1.2

1.0

0.8

0.3 MeV
1 MeV
5 MeV
20 MeV

0.6

0.4

10 0

10 1
10 2
10 3
DISTANCE FROM THE TRACK, r (nm)

10 4

Figure IV. Fraction of energy deposited at distancesfrom a proton track less than a specified value, r [E16].

may applyto other biological effects. Some analytic models
employing volume sizes as fitted parameters have beenquite successful in representing quantitative relationships
between RBE and LET for biological endpoints such as cell
survival or transformation in irradiated mammalian cell
cultures [C25, C29, K24, K28].

C. CELLULAR DAMAGE
50. A range of assays has been developed for evaluatingradiation damage to cells either in vitro or in vivo. These
include survival curves, cell transformation, induction of
mutations, and chromosome aberrations. Various models havebeen developed to describe these radiation-induced cellulareffects and their dependence on dose, dose rate, and radiation
quality. Some of these models have been summarized by
Goodhead [G6, G7]. The near-consensus view of the criticaldamage to single cells that has resulted from these studies isthat single radiation tracks from ionizing radiations can leadto cellular damage. The various models that have beendeveloped assume inter alia that cellular damage arises fromDNA double-strand breaks, either singly or in pairs; pairs of
DNA single-strand breaks; localized clusters of radiation
damage in unspecified molecular targets or in DNA;
unspecified single or double lesions, probably in DNA, but
qualitatively similar and independent of radiation quality; ordamage to DNA and associated nuclear membranes [U3].
Such models indicate that a single track, even from low-LET
radiation, has a finite probability of producing one, or morethan one, double-strand break in DNA in a cell nucleus.
Hence the cellular consequences of a double-strand break, orof interactions between them, should be possible even at thelowest of doses or dose rates. This would not, however, be theANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

outcome if cellular repair processes for single or small
numbers of double-strand breaks were totally efficient, inwhich case a threshold for the response might be anticipated.
There is no evidence for repair being 100% efficient, however,
although experimental assays to test the hypothesis have
limited resolution.
51. The following Sections illustrate dose-response
relationships that have been obtained for chromosome
aberrations, for cell transformation, and for mutagenesis insomatic cells. Emphasis is placed on assessing effects at low
doses. Other cellular effects, including changes in gene
expression, are examined in Annex F, “DNA repair andmutagenesis” and noted in Chapter IV of this Annex. In anumber of cellular systems, an adaptive response has been
described in which a small initial radiation dose can modify
the effect of a subsequent larger dose. Some examples ofstudies demonstrating this response are given below.
1. Chromosome aberrations
52. The scoring of chromosome aberrations in humanperipheral blood lymphocytes provides a sensitive method
for biological dosimetry. It also provides a valuableapproach to assessing dose-response relationships for chromosome mutations. By scoring dicentric aberrations in the fullgenome of about 1,000 cells, average whole-body doses ofabout 100 mGy from x rays or gamma rays may be detected
and higher doses estimated. Calibration curves have been
prepared by a number of laboratories for a wide range ofradiations. All dose-effect curves for low-LET radiation
conform to equation (2) up to 5!10 Gy. At higher doses,
saturation of the curve can occur when yields of dicentrics
approach five per cell [E8].
53. The difficulty of experimentally demonstrating thepresence or absence of a threshold for single cellular events
can be illustrated by work on the induction of chromosome
aberrations in lymphocytes byx rays. In assessing radiation
exposure by the analysis of aberrations in blood lymphocytes, measurements are normally made of the incidence ofboth dicentrics and total aberrations. These are unstable
aberrations, and they will slowlydisappear from peripheral
blood. They are, however, more sensitive for the detectionof effects at low doses than are stable aberrations.
54. The background incidence of dicentric aberrations inblood lymphocytes observed at metaphase is about 1 in
1,000 cells. As radiation-induced aberrations arise at therate of about 4 per 100 cells per gray, the ability to detect
a dose of 100 mGy would require about 1,000 lymphocytes
to be scored, which would take about three man days.
Radiation damage at lower doses can be detected, but thisrequires the assay of proportionately more cells.
Investigations at doses much less than 100 mGy requirevery large numbers of cells to be assayed, which would be
likely to exceed the scoring capacity of any single
laboratory and cannot, as yet, be satisfactorily undertaken
on a routine basis, even with automated scoring techniques.

85

55. A number of in vitro studies of chromosome aberrations
published in the 1970s and 1980s gave data that could be
fitted with a linear dose-response relationship, although otherfunctions were also reported. Thus, Luchnik and Sevan’kaev[L13] reported a plateau in the dicentric response to gamma
rays at doses between 100 and 300 mGy (low-LET), andKuUerová et al. [K9] produced dicentric data for x rays, whichmight have indicated a threshold at about 150 mGy, although
the authors interpreted the data with a linear function. Lloyd
et al. [L9] found a linear response in the lower dose region for
x rays down to 50 mGy for both dicentrics and total aberrations. Wagner et al. [W1] found a linear-quadratic responseusing doses in the range 50!500 mGy from 220 kVp x rays.
One study by Pohl-Rüling et al. [P15] reported that 4 mGy of
200 kVp x rays produced a significant reduction in aberration
frequencies below the control value until doses of 20 mGy andabove were received. This was interpreted as evidence for the
stimulation of repair mechanisms at doses below a few tens of
milligrays. It was only when repair processes were overwhelmed that aberration yields rose following a linearquadratic response. It was subsequently noted that if thecontrol aberration yield from a similar experiment designed to
examine the effect of D-T neutrons (and in which one of thetwo controls was common to the x-ray study) [P23], then theyield for 4 mGy of x rays was no longer significant [E15].
56. To provide better data on the response at low doses, acoordinated project was carried out by scientists in six
laboratories They collaborated in a study to examine the yieldof unstable aberrations induced in peripheral blood
lymphocytes in vitro by x rays [L8]. The study covered doses
of 0, 3, 5, 6, 10, 20, 30, 50, and 300 mGy. Cells from 24
donors were examined, and a total of about 300,000
metaphases were scored. Aberration yields significantly inexcess of control values were seen at doses down to 20 mGy,and the dose-response data at low doses were consistent witha linear extrapolation from higher doses. The overall dose
response up to 290 mGy was fitted with a linear-quadratic
dose-response relationship of the form I = C + αD + βD2,
where I is the incidence of dicentrics, C is a constant equal to
the spontaneous dicentric yield, and α and β are thecoefficients for the linear and quadratic terms as a function of
the dose D (in gray). Values of C = 0.0012 ± 0.002, α = 0.027± 0.012, and β = 0.044 ± 0.042 (χ2 = 5.2 for 5 degrees of
freedom, df) were obtained as best fits to the data. At doses
below 20 mGy, the observed dicentric yields were generally
lower than background, but not significantly so. Excessacentric aberrations and centric rings, in contrast, were higher
than controls, although the increase was not statisticallysignificant. A number of uncertainties associated with thistype of analysis were described in the paper, including
differences in scoring by the participating laboratories, and it
was concluded that the statistical uncertainties were such that
it is unlikely that this technique would ever allow the response
for aberrations to be directly measured at doses much below
20 mGy.
57. The complete set of dicentric data published in thepaper have been subject to further analysis to determine theextent to which other models could fit the dose-responseANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

information obtained in the study [E2]. A threshold-linear
dose response of the form I = C + α(D – D0) has been used
for the analysis in which I is the incidence of dicentrics atdose D, in gray, and D0 is the threshold dose. With valuesof C = 0.0013 and α = 0.040, the best estimate of thethreshold, D0, was 0.0097 ± 0.0045 Gy (±1 SE) (χ2 = 4.0
for 5 df). It may be concluded, therefore, that while thedata can be reasonably fitted with a simple linear-quadratic
function, the possibility of a threshold for doses up to about
10 mGy cannot be excluded.
58. It has been found that the yield of aberrations
following a given radiation dose can be influenced by anearlier radiation exposure. Some of the earliest studies on
this so-called adaptive response were carried out in human
lymphocytes that had incorporated tritiated thymidine[O6]. The cells were exposed to chronic, low doses fromtritium in culture and were subsequently exposed at therelatively high dose of 1.5 Gy from x rays. Approximately
half as many chromosome aberrations were induced incells that had incorporated thymidine as in those that had
not. This observation was repeatable, and subsequent
experiments showed that exposure to tritium need not bechronic [W14] and that pre-exposure to 10 mGy of x rays
could also cause the lymphocytes to become adapted [S25].
Subsequent work showed that the response to low doses
took several hours to fully manifest itself [S26] and that it
depended on synthesis of proteins (possibly an enzyme),
which was inhibited by the addition of cycloheximilidine
4!6 hours after the 10 mGy priming dose of x rays [W15].
It has been postulated that stimulation of the synthesis of
enzymes responsible for DNA repair is the key factor inthis response [U2]. This type of an adaptive response has
been observed in other cellular systems, as for somatic
mutations, but it has been most comprehensively studied inhuman lymphocytes [U2, W13]. This issue is consideredfurther in Chapter IV.
59. The measurement of unstable aberrations in blood
lymphocytes has its limitations as a biological dosimeter,
because the incidence of dicentrics, rings, and otheraberrations decreases with time. In recent years stable
chromosome aberrations have been extensively studied, as
they provide a method for assessing exposures that
occurred some years previously. Some data have been
published on dose-response relationships for stable aberrations using fluorescent in situ hybridization techniques.
However, these aberrations have a higher backgroundyield, which increases and becomes more variable with age
and lifestyle of the individual [R21]. Cumulative
background radiation exposure accounts for only a smallpart of the increased frequency with age; clastogenic
physiological processes of normal ageing are more
important [H34, L50]. This higher and inherently morevariable background of stable translocations means that it
has not been possible to measure a significant increase in
response at doses below 200!300 mGy [G13, L38, N7].
Stable aberrations are, therefore, of little value at present
in obtaining information on the shape of the dose responseat low doses.

60. Recently chromosome aberrations have been used to
examine the effects of radiation in a high-backgroundradiation area (HBRA) in China and in a control area [J9].
The level of radiation in the high-background-radiation
area was 3 to 5 times higher than that in the control area.
Overall the cumulative doses in 39 individuals ranged from31 to 360 mGy for high-background-radiation area and6.0 to 59 Gy for the controls. The frequency of dicentrics
and ring chromosomes (unstable aberrations) increased inproportion to the cumulative dose in the high-backgroundradiation area group. Such a dose-response relationshipwas not clear for those in the control area. The increase in
the frequency of these aberrations at such an extremely low
dose rate suggested that there is no threshold dose for the
induction of chromosome aberrations. In contrast, in thecase of translocations any effect of radiation, at up to 3
times control levels, could not be detected against thebackground incidence [H33].
61. For high-LET radiation the dose response obtained
following exposure in vitro both to alpha particles and toneutrons is generally well fitted with a linear response up
to doses of around 1.5 Gy [E8] (Figure V). The lowest
doses used in these studies were about 50 mGy (high-LET).
1.5

Fission neutrons
14.9 MeV neutrons

DICENTRICS PER CELL

86

x rays1.0
Gamma rays

0.5

0
0

1

2

3

4

5

Figure V. Yield of dicentrics in human lymphocytes asa function of dose for some photon and neutronradiations [E8].

2. Cell transformation
62. Cell transformation systems in vitro have been widelyused to study the initial stages of oncogenesis. Celltransformation describes the cellular changes associated withloss of normal homeostatic control, particularly of cell
division, which ultimately results in the development of aneoplastic phenotype. They are considered to be the closest in
vitro model for carcinogenesis. The only biological endpoint
generally accepted as being definitive of oncogenictransformation is the growth of malignant clones in “nude” orimmunologically suppressed host animals. As it is notpracticable to screen every transformed cell in this way, otherendpoints are normally used, such as enhanced growth rate;

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

63. The most common cell transformation systems suchas the BALB/c3T3 and the C3H10T½ mouse-embryoderived lines are based on cell lines derived from rodent
fibroblasts. There are disadvantages to using such models
for carcinogenesis in humans, and these were reviewed by
the Committee in the UNSCEAR 1993 Report [U3]. Whentransformed C3H10T½ cells are innoculated into suitable
hosts, they form fibrosarcomas. These are not typical of thetumour types that arise in humans after exposure toradiation, which are mainly epithelial in origin. Ideally,
more relevant cell lines based on human epithelial tissues
are needed for studying the mechanisms of tumorigenesis
and dose-response relationships, but they have proven to bemuch more difficult to develop.
64. Both carcinogenesis and transformation are multi-stage
processes, although transformation in vitro is normallystudiedin cells that have already undergone one or more of thepossible steps involved, in particular, immortalization. Toomuch reliance on studies of rodent cell lines can, however,lead to errors in interpretation, if directly applied to humans.
Thus, a correlation between anchorage-independent growth
and the tumorigenic phenotype has been established in rodent
cells [F10, O1, S8], which has permitted the selection ofneoplastically transformed cells by growth in soft agar. This
does not, however, apply to cultured human cells, as normal
human fibroblasts are capable of anchorage-independent
growth when cultured in the presence of high concentrationsof bovine serum.

transformation studies on dose and dose-rate effects agree
closely with the results obtained with other cellular effects.
There are, however, limitations to the sensitivity at low doses
and dose-response data for low-LET radiation are generally
available down to doses of around 100 mGy(see, for example,[M35, M36]). Above 3 Gy, cell reproductive death starts topredominate over the transformation frequency per plated cell
[B2, B3, H1].
67. For transformation by low-LET radiation, various doseresponse relationships have been reported. A linear dose
response has been described by a number of authors (see, forexample, [B33, H19, H27]), while linear-quadratic or
curvilinear relationships have been described by others (see,for example, [B30, H19, H24, M15]). Balcer-Kubiczek andHarrison [B29] reported a linear dose response for the
induction frequency, IF, of cell transformation in C3H10T½
cells exposed to single doses of x rays (4 Gy min1) between
250 mGy and 2 Gy (Figure VI), described by IF = 2.50 ± 0.11
104 Gy1. The overall fits to the data were evaluated bycomparing χ2 values. The addition of a quadratic function was
found not to be justified by least-squares fitting. For
continuous exposures over 1 hour or 3 hours, linear responses
were also obtained but with a reduced transformation
frequency described by 1.5 ± 0.03 104 Gy1 and 0.87 ± 0.5
104 Gy1, respectively. In this study, no transformation wasobserved in unirradiated cultures. The laboratory estimate fortransformation was less than 0.81 ± 0.04 105 transformants
per viable cell; thus it is reasonable to assume that even thelowest transformation frequencyobtained at 250 mGywas due
to radiation exposure.
5
TRANSFORMATION FREQUENCY ( 10-4)

lack of contact inhibition and indefinite growth potential;
anchorage-independent growth; and the ability to grow in lessnutritious media. The specific criteria used to define
oncogenic transformation depend on the particular system thatis being used.

87

Continuous exposure over 1 hour
Continuous exposure over 3 hours
4 Gy min -1

0 4

65. Despite such limitations, a number of characteristics ofin vitro cell transformation have allowed their use as model
systems for studying the early stages of radiation carcinogenesis in vivo. These have been summarized by Little [L4]and include a high correlation between animals and celltransformation systems for carcinogenicityofmanychemicals;
the response of transformed cells to initiation and promotion
similar to two-stage carcinogenesis in the tissues of experimental animals; and the provision of quantitative informationon the conversion of normal to tumour cells. Cell-basedtransformation systems should be free of the influence ofhormonal and immunological factors, although cell-cellinteractions are still possible.
66. Both BALB/c3T3 and C3H10T½ cell lines have beenused to measure the oncogenic effects of ionizing radiation(see, for example, [H1, H18, M3]) and chemical agents (see,for example, [B25, R13]), as well as to screen for possible
carcinogenic agents (see, for example, [S9]). Dose-response
relationships for cell transformation following exposure to
low-LET radiation were reviewed in the UNSCEAR1986 and1993 Reports [U3, U5] and by Barendsen [B3]. The pattern of
response is verydependent on cell-cycle kinetics; nevertheless,in carefully controlled experiments, the results from

3

2

1

0
0

0.5

1
DOSE (Gy)

1.5

2

Figure VI. Transformation frequency in C3H10T½ cells
following exposure to a single or protracted doses of
x rays [B29]. Lines are fits to the data with a linear doseresponse function.

68. Little [L4] compared results from BALB/c3T3 andthe C3H10T½ cell lines. Following exposures between
100 mGy and 3 Gy, the dose response for the BALB/c3T3
cells was nearly linear but that for the C3H10T½ cells
could be represented by a linear-quadratic or quadratic
relationship (Figure VII).

88

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

TRANSFORMATION FREQUENCY ( 10-4)

120of the dose response at low doses of low-LET radiation
with the C3H10T½ transformation system [M14].
Considerable effort went into standardizing techniques inthe different laboratories and carrying out extensive
intercomparison exercises. One laboratory carried out all
the irradiations, and care was taken to ensure that transportconditions did not interfere with the assays. Dose-response
data were obtained for exposure to 250 kVp x rays at doseintervals from 0.25 to 5 Gy, and a total of 51,000 petri
dishes (of 55 cm2) were scored. In total, 759 transformedloci were obtained, far in excess of the numbers reported inany other study involving low-LET radiation and theC3H10T½ cell transformation system.

C3H10T½ cells
BALB-3T3 cells

10

8

6

4

2

0
0

1

2

3

DOSE (Gy)Figure VII. Transformation frequency in C3H10T½
cells and BALB-3T3 cells following exposure to single
doses of x rays [L4].

69. Miller et al. [M35, M36] measured the effect on
C3H10T½ cells of x-ray doses down to 100 mGy delivered
just 24 hours after seeding. They found a plateau in theincidence of transformants per surviving cell between
about 300 mGy and 1 Gy, which may have reflected thefact that the cells had not achieved asynchronous growth at
the time of exposure.

6

TRANSFORMATION FREQUENCY ( 10-4)-4

TRANSFORMATION FREQUENCY ( 10 )70. In a major study, six European laboratories collaborated
in a study that was specifically designed to address the issue(a) Cells viable for survival

0

4
3
2
1
0
1

2
3
4
5
ABSORBED DOSE (Gy)2.0(b) Cells at risk

01.8

5

0

71. The combined data are shown graphically in
Figure VIIIa. A regression fit to the data on transformationinduction frequency, IF, between 250 mGy and 5 Gy gave alinear fit of the form IF = (0.83 ± 0.08) 104 Gy1. A fit using
a linear-quadratic relationship resulted in a non-significant
value for the dose-squared term. The authors concluded thatthe data supported a linear dose-response relationship for celltransformation in vitro and that there was no evidence for a
threshold dose. A presentation of these data in terms of thenumbers of cells at risk might be more relevant. The datafrom Figure VIIIa are therefore replotted in Figure VIIIb,
showing the transformation frequency per cell at risk. This
follows the standard bell-shaped curve with a fall in frequency
at doses above about 2 Gy, reflecting the effect of cell killing.
At the lower end of the curve the response is similar to thatshown in Figure VIIIa.

6

1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0

1

2
3
4
5
ABSORBED DOSE (Gy)6

Figure VIII. Transformation frequencies in C3H10T½ cells
following exposure to 250 kVp x rays at 2 Gy min1 [M14].

72. These experiments were carried out under as nearidentical conditions as possible in the participating
laboratories and probablyrepresent the optimum conditions
that could be achieved for this type of experiment. The
results support a linear dose-response relationship for celltransformation in vitro at low doses and do little to support
the concept of either a threshold dose or an enhanced
supralinear response. Nevertheless, the lowest dose atwhich effects could be detected was 250 mGy, clearly
demonstrating the limitations of the technique forassessing dose-response relationships at low doses. Becauseof the large amount of scoring needed, it would not havebeen practicable to obtain information at appreciably lower
doses.
73. An adaptive response to low doses of gamma radiation that reduces the effectiveness of a subsequent challengedose in inducing spontaneous neoplastic transformation has
been reported for C3H10T1/2 cells [A20]. In a subsequent
study the same group reported that doses of 1!100 mGy
from gamma radiation resulted in a suppression of thetransformation frequency of C3H10T1/2 cells to levels belowthat seen for spontaneous transformation of unirradiated cells[A21]. Similar results have been obtained with HeLa x skin

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

89

fibroblast human hybrid cells [R22]. In the latter study thefrequency of transformation of unirradiated cultures wascompared with that of cultures irradiated with 10 mGy
from gamma radiation and either plated immediately orheld for a further 24 hours at 37EC prior to plating. Pooleddata from a number of studies indicated an adaptive
response in the case of post-irradiation holding, although
the results of four individual studies were quite variable.

traversals. While these results suggest a non-linear responseat low doses and that the risk is less than might be expected
for single-track traversals on the basis of a linear dose
response, these conclusion were based on only a single result
for each exposure condition and need further replication
before any confidence can be placed in them. Nevertheless,they have demonstrated a unique approach to examining the
carcinogenic potential of alpha particles at low doses.

74. Exposure to high-LET radiation results in a higher
transformation frequency than exposure to low-LET
radiation, with a general tendency towards a linear doseresponse relationship, but with a tendency to plateau andthen fall at high doses (see, for example, [H19, H25, M15,M16]). There is no tendency for the response per unit doseto decrease at low doses or low dose rates, although anumber of studies have shown an enhanced effect. As
described in the UNSCEAR 1993 Report [U3], the mainevidence for this enhanced effect is restricted to 5.9 MeV
or fission neutrons, and in more recent studies withmonoenergetic neutrons of various energies (see, forexample, [M37, M38]) the magnitude of this so-called
inverse dose-rate effect has been reduced from a factor ofaround 9 to a factor of 2 or 3 and has been shown to beradiation-quality dependent. A model has been developedthat can satisfactorily explain many experimental results
showing this enhanced dose-rate effect [B26, H20].

3. Mutagenesis in somatic cells

75. Recently, Miller et al. [M40] carried out a detailed
analysis of the effect of transformation of C3H10T½ cells byalpha particle irradiation using charged particle microbeams.
Cells in a monolayer in a cell culture dish were irradiated inturn under a highly collimated shuttered beam of alphaparticles. The technique permitted measurement of theoncogenic potential of a single or a fixed number of alphaparticles passing through a nucleus. The nucleus of each cell
was exposed to a predetermined exact number of alphaparticles with energy similar to that of radon decay progeny.
In parallel with these microbeam studies, “broad beam” alphaparticle exposures were also carried out such that cell nuclei
received different fluences of alpha particles with mean
numbers of 0, 1, 2, 6, and 8. In this case the actual number of“hits” of each cell was determined by Poisson statistics. Thus,
if the mean number of traversals of a cell is 1, then 37% are
not traversed at all, 37% once, and the remainder are
traversed by two or more alpha particles.
76. The authors reported that the measured oncogenicity
from exactly one alpha particle was significantly less thanfrom a Poisson distributed mean of one alpha particle,
implying that cells traversed by multiple alpha particles weremore likely to be subject to transformation. Transformation
frequencies for an exact or mean number of one alpha particle
per cell were 1.2 104 and 3.1 104, respectively. The
incidence of transformations in the exact single-traversal cells
was not significantly different from that in the zero-dose(sham) irradiated cells (0.86 104). The result was taken to
imply that the majority of the yield of transformed cells
following irradiation with a mean of one alpha particle percell must come from the minority of cells subject to multiple

77. The principal mechanism resulting in a neoplastic
initiating event is induced damage to DNA, which
predisposes target cells to subsequent malignant development(see Chapter IV). There is also strong evidence linking a
number of tumours to specific gene mutations. An
understanding of the dose-response relationships for thisinitial mutational change is relevant to an assessment of theeffect of low doses on tumour induction. The experimental
data have been reviewed by Thacker [T2] and are considered
in detail in Annex F, “DNA repair and mutagenesis”.
Considered here is information on dose-response relationships
for somatic mutation induction resulting from exposure toboth low- and high-LET radiations.
78. A range of mutation systems have been described in theliterature, but only a few are sufficiently well defined forquantitative studies. There are also a number of difficulties ininterpreting results from somatic cell systems. In particular,
the mutation frequency of a given gene is to some extent
modifiable, depending on the exact conditions of theexperiment. It may also be that a period of time is needed forthe mutation to manifest itself. Thus, the true mutation
frequency may be difficult to determine, and this can present
particular difficulties in studies of dose-response relationships.
Several established cell lines, derived from mouse, hamster, or
human tissue, have also been used to measure mutation
frequencies at different doses and dose rates. Because the cells
lines used experimentallycan have sensitivities that depend on
the stage of the cell cycle, to ensure as consistent a response
as possible, it is preferable to use a stationary culture inplateau phase in which only a limited number of the cells will
be cycling in the confluent monolayer.
79. The mutation of a single gene is a relatively rare event;the majority of experimental systems are therefore designed to
select out cells carrying mutations. Commonly used systems
employ the loss of function of a gene product (enzyme) that is
not essential for the survival of cells in culture. Thus, cells
may be challenged with a drug that they would normallymetabolize with fatal consequences. If mutation renders thegene product producing the specific enzyme ineffective, thecell will survive, and thus the mutation frequency can beobtained by measuring the survivors. Frequently usedexamples of such a system are those employing the loss ofthe enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT), which renders cells resistant to thedrug 6-thioguanine (6-TG), and of the enzyme thymidine
kinase (TK), which gives resistance to trifluorothymidine(TFT). HPRT activity is specified by an X-linked gene,ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

hprt, while TK is specified by an autosomal gene tk andtherefore has to be used in the heterozygous state.
80. Mutation induction in a human lymphoblastoid cellline (TK6) after acute x ray and continuous low-dose-rate
gamma irradiation was investigated using the hprt and tk
mutation assays [K20]. The TK6 cells are radiosensitive,
and increases in mutation rate for both 6-TK and HPRT
were obtained at acute x-ray doses from 250 mGy to 1 Gy,
with a response that could be fitted with a linear function
down to zero dose. At high doses (1.5 and 2 Gy), mutation
data were difficult to obtain because of a very low surviving
fraction (1%!4%). Mutation frequency after continuous
gamma irradiation could also be fitted with a linear
response and with mutation rates at both 27 mGy h1 and2.7 mGy h1 that did not differ significantly from those for
acute exposure.
81. Evans et al. [E7] examined the effect of dose anddose rate on the mutation frequency at both the tk and hprt
loci in two variants (LY-S1 and LY-R16) of mouse
lymphoma L5178Y cells. Mutation at the tk locus,
resulting from x-ray exposure, was dose-rate-dependent in
the LY-R16 variant but not in the LY-S1 variant. This wasthought to reflect the deficiency of DNA double-strand
break repair in LY-S1. In contrast, with the hprt locus,
mutation was dose-rate-independent in both strains. The
results suggested that mutation at the hprt locus is causedby single lesions, with dose-rate-independent repair,whereas for the tk locus, interaction of DNA damaged sites
is important. In both cases, however, an increased
incidence of mutations was obtained at doses down to about
500 mGy, the lowest dose tested. The data could be fittedwith a linear dose response, with no threshold up to 3 Gy
for LY-R16 cells and 2 Gy for LY-S1 cells.
82. Induction of mutation to 6-TG resistance was
examined in a radiation-sensitive mutant strain of mouse
leukaemia cells following gamma irradiation at dose ratesof 30 Gy h1, 200 mGy h1, and 6.2 mGy h1 [F14]. The
mutation frequency increased linearly with increasing dose
for all dose rates, with no significant difference between
the responses at any of the dose rates. The lowest dosetested was 250 mGy.
83. A particularly sensitive mutation assay has been
described using the pink-eyed unstable (pun) mutation in themouse [S23]. This causes a reduction in the pigment in coatcolour and eye colour as a result of a gene duplication andreverts to wild type by deletion of one copy. Reversion events
are assayed as black spots on the grey coat. The reversion
frequency of pun is at least five orders of magnitude greaterthan that of other recessive mutations at other coat colour loci.
Female mice, homozygous for the reversion, were irradiatedwith various doses of x rays between 10 mGy and 1 Gy andthe frequency of reversions measured. Even at a dose of
10 mGy, the incidence of black melanosome streaks wasincreased threefold. There was a linear dose response over thedose range examined, with no indication of a threshold( Figure IX).

20
HAIR WITH BLACK MELANOSOMES (%)

90

15

10

5

0
0

0.1

0.2

0.3

0.4
0.5
0.6
DOSE (Gy)

0.7

0.8

0.9

1.0

Figure IX. Dose response for x-ray induced reversionof pink-eyed unstable mutations in mice [S23].

84. Albertini et al. [A10] compared the effects of 137Cs
gamma rays and alpha irradiation from 222Rn on hprt
mutations induction in human T cells in vitro. For gamma-ray
doses between 500 mGy and 4 Gy, the dose response could be
represented by either a linear or linear-quadratic relationship,with a significant increase in mutations at the lowest dose.
The doubling dose for mutation induction was calculated to beabout 0.8 Gy. For alpha particle irradiation, a linear regression
gave a best fit to the data for doses between about 0.25 and
0.9 Gy. In this case the doubling dose was calculated to beabout 0.2 Gy. This would suggest a relative biologicaleffectiveness (RBE) of about 4 for alpha particle irradiationcompared with low-LET radiation.
85. Further comparative data on mutagenesis in human
lymphoblastoid cells have been published by Amundson et al.
[A11, A12]. Despite being derived from the same donor, twocell lines, WTK1 and TK6, have very different responses toradiation. Alpha particles from a 238Pu source produced aboutfour to five times more hprt mutants per gray in WTK1 cells
than did x rays. On the other hand, there was little difference
between the dose-response curves for the TK6 cells, althoughalpha particles were somewhat more effective. In contrast, thetk locus was only slightly more sensitive to alpha particles
than to x rays, although the WTK1 cells were considerably
more sensitive than the TK6 cells. The authors considered thatthe results suggested that WTK1 cells have an error-prone
repair pathway that is either missing or deficient in TK6, andthey further suggested that this pathway might be involved in
the processing of alpha-particle-induced damage. In all thesestudies the data could be fitted with a linear dose-response
function, with nothreshold, although the lowest exposure doseused was about 200 mGy for alpha particle irradiation.
86. Data on the response of TK6 cells exposed to the alphaemitter 212Bi have been reported by Metting et al. [M41]. The
radionuclide was added directly to the cell suspension as achelate complex. The incidence of mutations was a simple
linear function of the dose from 200 mGy up to 800 mGy.
Induced mutant frequencies were 2.5 105 Gy1 at the hprt
locus and 3.75 105 Gy1 at the tk locus.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

87. An adaptive response of the kind observed in blood
lymphocytes [W6, W15] has also been demonstrated to havean influence on mutation induction. Thus, experiments withthe hprt locus in human lymphocytes have shown that
exposure to tritiated thymidine [S27] or 10 mGy from x rays
[K21] can markedly decrease the number of mutations
induced by subsequent high doses of radiation. The response
to low doses from x rays eliminated about 70% of the effect ofa challenging dose of 3 Gy.
88. Similarly, when SR-1 mammary carcinoma cells were
irradiated with 10 mGy from x rays and subsequently
challenged 18 or 24 hours later with 3 Gy from x rays,
approximately one half as many mutations were induced aswhen the cells were irradiated only with 3 Gy [Z3].
Furthermore, the rate of repair of DNA double-strand breaks,
which are the lesions responsible for chromosomal breaks,
increased in cells that had been pre-exposed.
89. Wolff [W13] pointed out that the data on mutationinduction illustrate two aspects of adaptation that are
similar to that observed in blood lymphocytes: after the
initial dose it takes time for the induction to occur, andonce induced it disappears with time. The SR-1 data showthat in this system the effect takes more than 6 hours to
become effective and it then disappears if 48 hours elapse
between the two doses.
90. In addition to being studied in mammalian systems,
radiation-induced mutations have also been studied in plant
cells. Mutations can occur in stamen hairs in tradescantia that
result in the normal dominant blue colour being replaced byrecessive pink. This is a sensitive system for detecting effects
at low doses. Dose-response curves for pink mutations haveshown for 250 kV x rays a linear response between 2.5 mGy
and 50 mGy and for neutrons (0.43 MeV) a linear response
between 0.1 mGy and 80 mGy [S28]. In neither case wasevidence for a threshold obtained.

D. SUMMARY
91. Damage to DNA in the nucleus is considered to be themain initiating event by which radiation causes damage tocells that results in the development of cancer and hereditarydisease. Information on the effects of radiation on individual
cells can, therefore, provide insight into the fundamental
damage that may ultimately give rise to cancer or hereditarydisease. It can also provide information on the consequencesof damage to other cellular structures, such as the cellular
membrane and the cytoplasm, although damage here is lesssignificant in terms of long-term health effects.
92. Double-strand breaks in DNA are generally regarded
as the most likely candidate for causing the critical damage
to the nucleus that can subsequently manifest itself as amutation in somatic or germ cells. Single radiation tracks
have the potential to cause double-strand breaks and in theabsence of 100% efficient repair could result in long-term
damage, even at the lowest doses.

91

93. In examining the effects of radiation at low doses it has
been appropriate to consider how they should be defined. A
number of physical and biological approaches have been
examined for designating low doses and low dose rates.
Microdosimetric arguments suggest low doses will be lessthan 1 mGy. However, radiobiological experiments on cells in
culture suggest that acute doses of about 20 mGy are low,while epidemiological studies suggest that a low dose is of theorder of 200 mGy, whatever the dose rate. In addition, studies
oftumour induction in experimental animals suggest that dose
rates of about 0.1 mGy min1 are low, whatever the total dose.
94. A range of assays is available for evaluating radiation
damage to cells occurring either in vivo or in vitro and theform of the dose-response relationships. In the low-dose
region, damage from low-LET radiation can be considered
to be due to single tracks acting independently, whereas at
higher doses multi-track effects can occur, causing nonlinearity in the dose response. In the case of high-LET
radiation, the dose response is generally found to be linear.
95. The results of studies of the induction by low-LET
radiation of chromosome aberrations in blood lymphocytes, ofthe transformation of cells in culture, and of somatic mutations in mammalian cell systems at low doses all give resultsthat are somewhat variable. They depend on the experimental
design and the effort that went into assessing risks at doses ofless than about 1 Gy. In the most comprehensive studies theresults are consistent with an increasing incidence withincreasing dose at low to intermediate doses. Nevertheless,even very extensive studies, which have taken considerable
resources, have demonstrated that it is not practical to obtain
information on radiation effects at doses much below about
20 mGy for chromosome aberrations, 100 mGy for celltransformations, and 200 mGy for somatic mutations. The
exact form of the response for cellular effects at low doses
must therefore remain unclear. One exception is a particularly
sensitive system based on the assay of reversion events in agene mutation in pink-eyed unstable pun mutations in themouse, which cause a reduction in coat colour. A linear dose
response, with no indication of a threshold, was obtained overthe dose range for x rays from 10 mGy to 1 Gy. Similarly,
pink mutations have been induced in tradescantia stamen
hairs at doses down to 2.5 mGy from x rays and with a linear
response up to about 50 mGy.
96. For high-LET radiation the experimental data again
indicate a linear dose response down to the lowest doses thathave been tested. In the case of chromosome aberrations, thelowest neutron dose used was about 50 mGy. For mutationinduction, little information is available from mammaliancellular systems at doses much below 200 mGy.
97. The relative biological effectiveness (RBE) of highLET radiation compared with low-LET radiation varies
considerably depending on the biological damage and thedose range. In the case of deterministic effects, caused bycell killing, RBE values are generally less than 10. For
stochastic effects, values of RBE depend on the dose,reflecting an essentially linear dose response for high-LET

92

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

radiation and a linear-quadratic response for low-LET
radiation. The maximum value, RBEm, occurs at low doses.
For exposures to both neutrons and alpha particle irradiation,
values of RBEm depend on the biological endpoint but for
cytogenetic damage, cell transformation, and tumour induction are generally greater than 10.

small priming dose of low-LET radiation of about 10 mGy
can reduce the effect caused by a subsequent higher dose. This
adaptive response seems to take a few hours to manifest itself
and then lasts for up to about 40 hours. There are no
indications that this would modify the shape of the dose
response, although it could alter the magnitude of any effect.

98. A so-called adaptive response has been observed for anumber of indicators of cellular damage: a small radiationdose reduces the amount of cellular damage caused by a later
higher dose. For the induction of unstable chromosome
aberrations and of mutation, it has been demonstrated that a

99. The information on the lowest doses at which effects
from low-LET radiation have been detected in cellular
systems are summarized in Table 5. These are principallyfor endpoints arising from mutations.

Table 5Lowest doses at which chromosome aberrations and mutations have been detected in experimental
systems exposed to low-LET radiationab

SystemEndpoint

Radiation

Lowest dose a(mGy)Paragraph b

Ref.

Human lymphocytes
Human lymphocytes
C3H10T½ cells
Mouse
TK6 cells
Tradescantia

Unstable chromosomal aberrations
Stable chromosomal aberrations
Cells transformation
Pink-eye mutation
hprt and tk mutation
Pink mutation

x raysgamma rays
x and gamma rays
x raysx raysx rays

20
250
100
10
250
2.5

56
59
66/69
83
80
90

[L8]
[L38]
[M35]
[S23]
[K20]
[S28]

Acute exposures (minutes).
Text paragraph in which endpoints are further discussed.

II. ANIMAL EXPERIMENTS
100. Studies with experimental animals are important forpredicting the long-term effects of radiation in humans.
Provided the limitations of such studies are acknowledged,
considerable information can be obtained on bothradiation-induced cancer and hereditary disease. The mostdirectly relevant data come from studies with mammalian
species, with the majority of work relevant to assessing
effects at low doses and low dose rates having been carriedout with rodents and beagle dogs.
101. Studies with experimental animals are valuable for
examining the biological and physical factors that may
influence tumour induction by radiation. They can be used
to examine the form of dose-response relationships over awide range of doses; the effect of spatial and temporal
distribution of dose; and the influence of factors such as
sensitivity of individual organs and tissues, age at exposure,radiation quality, and dose protraction or fractionation on thetumour response. Animal models are also of increasing
value in understanding the molecular and cellularmechanisms underlying tumour response (Chapter IV).
Quantitative risk coefficients for radiation-induced cancer
in humans cannot, however, be based on the results ofanimal studies because there are differences in radiation
sensitivity between different mammalian species.

102. Experimental studies of genetic damage in the offspring of irradiated animals, mainly mice, have been used
to assess the hereditary effects of radiation. In the absenceof any clear evidence from observations in humans on therisks of radiation-induced hereditary disease, animalstudies provide information on dose-response relationships
as well as quantitative risk estimates. Studies of germ-cell
mutations are also relevant to understanding the doseresponse relationship for the initial damage to DNA that
could ultimately result in the development of cancer.

A. CANCER
103. Data on radiation-induced tumours in experimental
animals were extensively reviewed in the UNSCEAR 1977
and 1986 Reports [U5, U7], by NCRP [N1], by Upton[U22], and in a comprehensive monograph on radiation
carcinogenesis [U23]. The effect of dose rate on tumour
response was examined by NCRP [N1] and in the
UNSCEAR 1993 Report [U3]. As noted in the UNSCEAR
1993 Report, the experimental animals used in manystudies are inbred strains with patterns of disease that canbe very different from those found in humans. Very manystudies have used rodents as the experimental animal.
Different strains of mice and rats have varying susceptibilities

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

to both spontaneous and radiation-induced tumours; furthermore, within a given strain, there are frequently differences
between the sexes and ages in the incidence and time of onsetof specific tumour types. A number of tumour types for which
information is available are either not found in humans (e.g.
Harderian gland) or appear to require substantial cell killing
for their development and thus may exhibit a threshold in thedose response (e.g. ovarian tumour, thymic lymphoma). For
a number of other tumours there may be a human counterpart(e.g. myeloid leukaemia and tumours of the lung, breast,
pituitary, and thyroid), but even here there can be differencesin the cell types involved and in the development of thetumour. Although data for larger animals are not as extensive,
broadly similar findings are found for tumour induction indogs or other species.
104. There are also substantial variations in the rates ofturnover of cells and in the lifespan of most experimental
animals compared with humans. Furthermore, the development of tumours in both humans and animals is subject to themodifying influence of various internal and external environmental factors, all of which can potentially influence doseresponse relationships. Their development will also depend on
the genetic background, the physiological state, and theenvironmental conditions of the animals. All these factors
make it difficult to interpret the results of animal studies andto apply them to humans. Nevertheless, most tumours inlaboratory animals appear to arise as clonal growths and to93

develop, as do most human tumours, through stages of
initiation, promotion, and progression. They are therefore of
considerable value for helping to understand the form of thedose response for tumour induction in humans and thepotential for effects at very low doses.
105. While extensive data exist on tumour induction inlaboratory animals exposed either to external radiation or
to incorporated radionuclides, many of these studies werecarried out in the 1960s and 1970s. The ability to detect
radiation-induced cancer at low doses depends, as withepidemiological studies, on the number of animals in the
study, the spontaneous incidence of the disease, and theradiation sensitivity of the particular tumour type(s). Thisis illustrated in Table 6. Thus, in the CBA strain of mousewith a very low spontaneous incidence of acute myeloid
leukaemia (1 104) and a high sensitivity to induction byradiation (about 1 101 Gy1), only 300 exposed animals
and a similar number of controls would be needed to detecta significant increase (p=0.05) in tumour incidence at awhole-body dose of 100 mGy (low-LET). It would also be
possible to detect an effect of 10 mGy with groups of about
4,000 animals. In contrast, for the RFM mouse strain,
which has a much higher spontaneous rate of the disease(7 103) and a lower sensitivity (7 103 Gy1), the numberof animals needed to detect a significant increase in acute
myeloid leukaemia at 100 mGy would be 1.2 105, an
impractically high number of animals.

Table 6
Statistically determined sample sizes of irradiated and control mice needed to detect a significant increasein tumour risk a
Sample size
Mouse strain
RFM
RFM
CBA
a
b
c
d

TumourThymic lymphoma b
Myeloid leukaemia cMyeloid leukaemia d

1 000 mGy

100 mGy

10 mGy

1 mGy

1 300
1 700
30

1.2 105
1.2 105300

1.2 107
1.2 1074 000

1.2 109
1.2 109
1.3 105

p = 0.05.
Spontaneous incidence 1.3 10 -1; risk of 3 10 -2 Gy-1 assumed.
Spontaneous incidence 7 10 -3; risk of 7 10 -3 Gy-1 assumed.
Spontaneous incidence 1 10 -4; risk of 1 10 -1 Gy-1 assumed.

106. At low radiation doses the number of animals used andtheir sensitivity is thus important in determining the ability todetect any effect. Animal studies do not generally involve as
many individuals as there are in the more extensive epidemiological studies. They do, however, have the advantage thatthey are planned; the groups of animals exposed are, in
general, genetically homogeneous; and the numbers ofanimals allocated to various dose groups can be chosen to
maximize the information obtained. Laboratory animals are
exposed to sources of radiation under controlled conditions,and there is much greater certainty associated with thedosimetry. Information may also be available from studies ofanimals exposed at different dose rates. Data on irradiation oflaboratory animals can thus give information for a range oftumour types on the shape of dose-response relationships andprovide an estimate of the lowest dose at which a significant
effect on the induction of tumours from exposure to ionizing
radiation can be observed.
107. Despite a substantial number of research studies ontumour induction in experimental animals potentiallyavailable for analysis, there are in practice only a limited
number that can help to define the dose-response relationshipfor cancer induction down to low doses. The range of doseresponse relationships that have been obtained in experimental
animals and the effect of dose rate on tumour response werereviewed in the UNSCEAR 1993 Report [U3]. Accordingly,
only illustrative examples of dose-response relationships are
given here for tumour induction following exposure toexternal radiation and intakes of radionuclides.

94

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

1. Dose-response relationships
108. Tumour induction has been demonstrated in laboratory
animals exposed to both low- and high-LET radiation.
Information on dose-response relationships was previously
examined in the UNSCEAR1986 and 1993 Reports [U3, U5].
In the UNSCEAR 1986 Report, the Committee limited its
analysis to those models that appeared to be supported bygeneral knowledge of cellular and subcellular radiobiology.
Because most readily induced human tumours, such as thoseof the breast, thyroid, and lung, as well as leukaemia, did notindicate the existence of a threshold [U5, Annex B, paragraph108], analyses were confined to the linear no-threshold, thelinear-quadratic and the quadratic dose-response relationships.
It was considered that these three dose-response relationships
provided a general envelope for observation of tumour
induction in experimental animals as well as in humanpopulations. In the UNSCEAR 1993 Report [U3], emphasis
was placed on examining the effect of dose rate on the tumour
response, although information was also given on the doseresponse relationships.
109. Dose-response functions other than those adopted in1986 have also been proposed [E4, G1, U2]. Models thatincorporate a threshold assume that there is no response up tosome level of exposure, and that thereafter the response
increases with dose. Some animals models of tumour
induction show this type of response. Dose-response models
that incorporate an adaptive response have also aroused someinterest. These consider the possibility that stimulated repairof radiation damage as a result of the effect of a toxic agent,
including radiation, at low doses would reduce the influenceof subsequent, higher doses. The evidence for such anadaptive response was reviewed by the Committee in the
UNSCEAR 1994 Report [U2]. Much of the evidence for sucha response that is presently available comes from observationsof short-term effects in both plants and animals and fromstudies on cells in culture (Chapter I). Extensive data fromanimal experiments on dose-response relationships for cancer
induction and limited human epidemiological data on lowlevel exposures have, however, provided no firm evidence thatthe adaptive response decreases the incidence of late effects
such as cancer induction after exposure to low radiation doses.
Molecular and cellular studies have shown that DNA damage
in the form of double-strand breaks is repairable but that somedegree of misrepair is to be expected (Chapter IV). On thisbasis, it maybe concluded that the extent of damage caused byionization events resulting from exposure to low radiation
doses may be influenced by the stimulation of DNA repair
mechanisms, but even so, such repair can only be partiallyeffective and for many tumour types cannot entirely eliminate
the risk of tumour development following radiation exposure.
110. Published reports of dose-response relationships
obtained with various animal species have described
responses for different tumour types or life shortening (as asurrogate for tumour induction) using a wide range offunctions. Although in many studies dose-effect relationships
can be defined by a linear, linear-quadratic, or quadratic
response, the data are generally not well defined, particularly

at low doses, and alternative fits to the data are also possible.
Some animal models also indicate the presence of a thresholdfor a response. Extensive data are available on a wide rangeof tumour types including leukaemias and lymphomas arisingin haematologic tissue as well as tumours of solid tissues (e.g.
lung, liver, and bone). Examples of dose-response
relationships for exposures to both external radiation andinternally incorporated radionuclides are given below. The
studies have been chosen to illustrate the various patterns ofdose response that have been obtained with some emphasis on
those that give information at low doses.
(a) External radiation
111. Life shortening. Extensive studies in laboratory
animals have reported life shortening as a result of wholebody external irradiation. This is a precise biological
endpoint and reflects the early onset of lethal diseases, anincreased incidence of early occurring diseases, or a
combination of the two. At radiation doses up to a few gray(low-LET), life shortening in experimental animals
appears to be mainly the result of an increase in tumour
incidence. There is little suggestion that there is a generalincrease in other non-specific causes of death [G14, M39,
S38]. At higher doses, into the lethal range, a non-specificcomponent of life shortening becomes apparent owing tocellular damage to the blood vasculature and other tissues.
It was concluded in the UNSCEAR 1993 Report [U3] thatlife shortening at low to intermediate doses can be used asa basis for examining the factors that influence doseresponse relationships for tumour induction.
112. The majority of comprehensive studies that givequantitative information on dose-response relationships forlife shortening from exposure to low-LET radiation as well as
on factors such as the effects of age, dose fractionation anddose rate have used the mouse as the experimental animal.
Substantial differences in sensitivity have, however, been
noted between strains and between the sexes. A review of 10
studies involving about 20 strains of mice given single
exposures to x or gamma radiation showed that estimates oflife shortening ranged from 15 to 81 days Gy1, although the
majority of values (9 of 14 quoted in the review) were between
25 and 45 days Gy1, with an overall unweighted average of
35 days Gy1 [G14]. In general, in the range from about
0.5 Gy to acutely lethal doses, the dose response was either
linear or curvilinear upwards. In male BALB/c mice exposedto acute doses of 137Cs gamma rays (4 Gy min1), life
shortening was a linear function of dose between 0.25 and
6 Gy, with a loss of life expectancy of 46.2 ± 4.3 days Gy1[M39]. Similar data have been reported on B6CF3 mice
irradiated at 17 days before birth or at various times up to 365
days after birth [S38], although the lowest dose used was
1.9 Gy. The effects of acute single doses on life shortening inother species were summarized in the UNSCEAR 1982
Report [U6], although they are not as comprehensive as thedata for mice.
113. The effect of dose fractionation appears to be very
dependent on the strain of mouse and the spectrum of dis-

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

eases contributing to the overall death rate. Overall no clear
trend in the effect of dose fractionation on life shortening
could be found [U3], and the results from a number of studies
suggested that when compared with acute exposures, theeffects of dose fractionation are small and in some studies
have given either small increases or small decreases in lifespan. When the effects in mice of acute exposures to low-LET
radiation are compared with those of protracted irradiation
given more or less continuously, the effectiveness of theradiation decreases with decreasing dose rate and increasingtime of exposure. With lifetime exposures there is somedifficulty assessing the total dose contributing to the loss oflifespan. The results available suggest, however, that withprotracted exposures over a few months to a year, the effect onlife shortening is reduced by factors of between about 2 and 5,
compared with exposures at high dose rates.

95

at 10 ± 0.5 weeks of age [U16]. Groups of animals received arange of doses (0, 0.1, 0.25, 0.5, 1.0, 1.5, 2.0, and 3.0 Gy),
were followed for their lifespan, autopsied at death, anddiagnosed for various types of neoplastic disease. Doseresponse data were obtained for a range of tumour types. A
significant increase in the incidence, I (%), of acute myeloid
leukaemia was obtained at doses of 1.0 Gy and above. A
linear dose response of the form I = 0.63 + 1.4D, where D isthe dose in gray, adequately described the data, and the doses
were not high enough for a cell-killing term to have become
apparent. A linear-quadratic model, I = 0.69 + 0.86D +0.00227D2, also provided a fit to the data, although the dosesquared term was not significant. Ullrich and Storer [U18]published further data on myeloid leukaemia in female RFM
mice exposed under similar conditions. The results weresimilar to those published earlier, but with fewer exposure
points (0, 0.5, and 2.0 Gy).

114. The results of a number of studies on life shortening
as a result of exposure to high-LET radiation were
examined in the UNSCEAR 1993 Report [U3]. The data
were all reasonably consistent and suggest that the dose
response for life shortening is a linear function of dose, atleast for total doses up to about 0.5 Gy, and that neither
dose fractionation or dose protraction has much effect.

117. The information on myeloid leukaemia induction inmice for these two data sets has been combined in Table 7. An
analysis of the combined data carried out for this Annex
indicates that the incidence of myeloid leukaemia is increased
over controls at doses of about 0.5 Gy and above. The data
have been fitted with linear, linear-quadratic, and thresholdlinear dose responses. All three models give a good fit to thedata, and in the case of the threshold-linear model, a thresholdat about 0.22 Gy can be obtained (Figure X, Table 8). Thesestudies by Ullrich and Storer [U16, U18] involved a total ofnearly 18,000 mice, and yet the information at low doses is
equivocal because of the small numbers of acute myeloid
leukaemias occurring. Few other animal studies have been
carried out on such a scale, and this clearly illustrates thelimited ability of such animal studies to provide detailed
information on the effects of whole-body radiation at low
doses.

115. Tumour induction. In the late 1970s, Ullrich andStorer published a series of studies on tumour induction inmice (see, for example, [U16, U17, U18]). The data haveprovided comprehensive information on the effects of doseand dose rate on the induction of a range of neoplastic
diseases, including myeloid leukaemia and solid tumoursof the ovary, pituitary, lung, and thymus.
116. In a large study in female RFM mice, animals were
exposed to acute doses from 137Cs gamma rays (0.45 Gymin-1)

Table 7
Myeloid leukaemia incidence in female RFM mice exposed to acute doses of gamma rays [U16, U18]
Dose (Gy)

Number of animals

Incidence

0
0.1
0.25
0.5
1
1.5
2
3

4 763
2 827
965
1 918
1 100
1 054
1 099
4 133

0.72 ± 0.10
0.72 ± 0.15
0.84 ± 0.30
1.17 ± 0.26
1.60 ± 0.41
3.6 ± 0.76
3.22 ± 0.43
5.2 ± 0.51

Total

17 859

Table 8Model fits to data on myeloid leukaemia in mice exposed to 60Co gamma rays
Function

C

α

β

D0

χ2

DF

I = C + αD
I = C + αD βD2 b
I = C + α(D  D0) c

0.64 ± 0.09
0.69 ± 0.09
0.72 ± 0.08

1.39 ± 0.13
0.87 ± 0.39
1.55 ± 0.18

0.22 ± 0.15-

0.22 ± 0.14

4.7
2.7
2.6

6
5
5

a

a

Linear.

bLinear-quadratic.

c Linear-threshold.

96

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES
66(a) Linear relationship32
1

INCIDENCE (%)

4

4
3
2

0.5

1
1.5
2
2.5
ACUTE DOSE (Gy)

3

3
2

0

0
0

4

1

1

0

(c) Linear-threshold relationship55
INCIDENCE (%)

INCIDENCE (%)56(b) Linear-quadratic relationship

0

0.5

1
1.5
2
2.5
ACUTE DOSE (Gy)

0

3

0.5

1
1.5
2
2.5
ACUTE DOSE (Gy)3

Figure X. Dose-response relationships fitted to data on myeloid leukaemia in female RFM mice [U16, U18].

119. One of the most extensively studied tumours in themouse is that arising in the thymus. The dose response for the
induction of thymic lymphomas by acute whole-body
irradiation found in a number of studies has been of thethreshold type (Figure XI). Thus, Maisin et al. [M39] exposed
12-week-old male mice to single or fractionated doses of 137Cs
gamma rays (4 Gy min1) in the dose range from 0.25 to
6 Gy. The dose-response curve was of a threshold type; theincidence of thymic lymphomas rose above that in controls
onlyfollowing exposures at 4 Gyand above. Similarly, Ullrich
and Storer [U18, U24] studied the dose-response relationshipfor thymic lymphoma in female RFM/Un mice. Exposures
were at 0.45 Gy min1 and 0.06 mGy min1. For the highest
dose rate, the incidence of lymphoma up to 0.25 Gy increased
with the square of the dose, although a threshold for a
response up to about 0.1 Gy could not be excluded. Linearity
was rejected over this limited dose range. From 0.5 to 3 Gy

the increase in incidence with dose was nearly linear. At thelower dose rate the response was best described by a linearquadratic dose response with a shallow (perhaps zero) initial
linear slope, again allowing the possibility of a threshold at
low total doses.
40

30
INCIDENCE (%)

118. The induction of lung tumours has been compared infemale BALB/c mice given doses from 60Co gamma rays in
the range 0.5!2 Gy at two dose rates (0.4 Gy min1 and0.06 mGy min1) [U18, U24]. Tumour induction was less at
low dose rates than at high dose rates. After high-dose-rate
exposure, the age-correlated incidence, I (%), could be
represented by a linear function, [I(D) = 13.4 + 12D; p>0.5],
where D is the absorbed dose in gray. At low dose rates, alinear function also gave a good fit to the data [I(D) = 12.5 +4.3D; p>0.8]. The data were adjusted for differences in the
distribution of ages at death among the treatment groups, andthe authors indicated there were no changes with age over theperiod of irradiation. The differences in slope were taken to
indicate variations in effectiveness for tumour induction at thetwo dose rates. The data were subsequently extended toprovide additional information at the high dose rate
(0.4 Gy min1) in the dose range from 0.1 to 2 Gy [U14].
Although the tumour incidence data could again be fitted witha linear dose response [I(D) = 10.9 + 11D; p>0.70], a linearquadratic dose response [I(D) = 11.9 + 4D + 4D2; p>0.70]
would also give a fit. In this extended analysis, the linear term
was very similar to that obtained in the low-dose-rate study,
and it was concluded by the authors that the result wasconsistent with a linear-quadratic response in which the linear
term is independent of dose rate, at least for the dose rates
used in the study.

Single dose

20

Fractionated dose

10

0
0

1

2

3
DOSE (Gy)456

Figure XI. Incidence of thymic lymphoma as a function of dose for single or fractionated x rays [M39].

120. The induction of ovarian tumours in mice exposed tox rays or gamma radiation has also indicated the presenceof a threshold in the response for some strains, and this isreflected in a pronounced effect of dose and dose rate [U17,U18, U21, U24]. Thus, in SPF/RFM mice exposed at
0.45 Gy min1, a significant increase in tumour incidence
was obtained at doses from 0.25 to 3 Gy [U18]. The data
could be fitted with a linear-quadratic dose response witha negative linear component [I(D) = 2.3 + (!23)D + 1.8
D2; p>0.25] or by a threshold plus quadratic model [I(D) =2.2 + 2.3 (D!D*)2; p>0.75], where the threshold dose, D*,
was estimated to be 0.12 Gy. Linear and quadratic dose
responses were rejected. This pattern of response is considered to reflect the fact that ovarian tumour development in
the mouse seems to follow changes in hormonal status that
occur after substantial killing of oocytes. For low-dose-rate
exposures, cell killing is less effective, and as a consequence
there is a substantial reduction, by a factor of about 6, ofANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

121. A further substantial study in the mouse has
demonstrated that dose-response relationships for tumour
induction can vary in different organs and tissues [S39].
Groups of B6C3F1 mice were exposed to various doses
between 0.48 and 5.7 Gy low-LET radiation from 137Cs
gamma rays. The dose-response curves for tumour inductionin the liver, pituitary, ovary, and lungs were convex upwards
in the dose region examined, with a significant increase in
numbers of tumour at 0.48 Gy. The data suggested a
progressive increase with dose up to about 1 Gy. A subsequentgradual increase to the highest incidence obtained was seen
and then a declining incidence at doses above about 1.5 to
3 Gy, depending on the tumour type. The results could beinterpreted as showing an increasing risk with dose up to the
maximum incidence, although the lack of data below
480 mGy limited the ability to elucidate the dose response at
low doses. In contrast, the shape of the dose response for bonetumour induction was quite different from that for other solid
tumours: the initial slope was concave upwards, with thehighest incidence observed in the group given 3.8 Gy. Bonetumour incidence up to about 3 Gy was a function of thesquare of the dose, and the existence of a threshold could not
be excluded because the incidence of bone tumours in groups
irradiated with doses below 1.43 Gy was not significantly
increased.
122. Variations in sensitivity to radiation-induced mammary
cancers in different strains of mice and rats are well known,
although the reasons underlying these differences are not wellunderstood. Thus studies of mammary carcinogenesis in
Sprague-Dawley, WAG/Rij, and BN/BiRij rats have shownthat only in WAG/Rij rats was an appreciable number ofcarcinomas induced by radiation [V3]. Analysis of data onradiation-induced mammary tumours gave a linear doseresponse function for fibroadenomas in Sprague-Dawley ratsand for both fibroadenomas and carcinomas in WAG/Rij rats
after irradiation with either 0.5 MeV neutrons or x rays. In thecase of exposure to x rays, the lowest data point was at
200 mGy (Figure XII).
123. Studies of mammary tumours in mice by Adams et al.
[A13] have demonstrated that irradiation resulted in many
more transformed mammary cells than are ultimately expressed as tumours. A later study by Ullrich et al. [U26]examined possible reasons for differences in sensitivity insensitive BALB/c and resistant C57BL and B6CF1 hybrid
mice. Theydemonstrated that variations in sensitivitycould becorrelated with differences between strains in the sensitivity ofthe mammary epithelial cells to radiation-induced transformation. Differences in sensitivity could not, however, be
accounted for by differences in the number of sensitive cells orby systemic or cellular influences on progression. This observation of inherent differences in sensitivity to radiationinduced tumour initiation may be one approach to understanding the mechanism by which radiation induces cancer inthese different mouse strains and may have more generalapplication.

20

RELATIVE EXCESS INCIDENCE

effectiveness in inducing tumours at low dose rates. The
results suggested the possibility of a threshold up to about
0.115 Gy.

97

Neutrons
15

10

5
x rays00123
TOTAL ABSORBED DOSE (Gy)4

Figure XII. Relative excess incidence of carcinomas in
WAG/Rij rats after irradiation with x rays and 0.5 MeV
neutrons [B34].

124. A unique experimental system has been described by
Tanooka and Ootsuyama [O8, T13] in mice. The backs offemale ICR mice were irradiated with beta particles from90
Sr-90Y three times a week throughout life. At radiation
doses per exposure between 1 Gy and 11.8 Gy (low-LET),
the tumour incidence was 100%. At 0.75 Gy per exposure,
however, no tumours occurred in 31 mice over a period of
790 days from the start of irradiation. One osteosarcoma
did arise at 791 days and one squamous cell carcinoma at
819 days. This was despite the fact that the cumulative dose
was extremely high (305 Gy in 950 days). The appearance oftumours in irradiated mice depended on a fractionatedregime: no tumours occurred following single exposureswith doses up to 30 Gy. At such doses depilation andsevere skin damage occurred. The authors proposed that at
the lower dose fractionation regime efficient repair occurs,
resulting in an apparent threshold in the tumour response.
No histological information was reported, but it seems
likely that at these high doses deterministic damage would
occur, resulting in the development of a fibrotic response
preceding tumour development.
125. Tumour induction in rats and mice exposed to highLET neutron irradiation was described in the UNSCEAR
1993 Report [U3] and has also been summarized by Broerse[B34] and Fry [F15]. In general the experimental results
reviewed indicated that there are differences between tissues
in their tumorigenic response following either dose
fractionation or reductions in dose rate as compared withacute radiation exposures. Taken together, however, the effectsof dose rate and fractionation are small, and for the majorityof studies a linear dose response would give a good fit to thedata up to about 1 Gy. Exceptions are tumour types for whichcell killing seems to play a significant role in tumour
induction, as for thymic lymphomas, when a threshold dose
may be found. In many cases, however, information is notavailable down to low levels of exposure. For life shortening
at low doses, which has been shown to be the result of tumour
induction, again little effect of fractionation or dose rate has
been found [F15, U3].

98

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES(b) Internally incorporated radionuclides
126. In the case of intakes of radionuclides, many factors in
addition to those for external radiation exposure may
influence the dose response. For radionuclides such as 90Sr or
239
Pu with a long physical half-life and a long biological halftime in the body, radiation exposure after an intake will
generally be for the remaining lifespan of the animal, making
it difficult to relate tumour incidence to radiation dose.
Additional difficulties in interpreting dose-response data arise
from the heterogeneous distribution of dose between andwithin body organs and tissues as well as temporal changes inthe distribution of radionuclides, and hence dose, within the
body. A key factor in the calculation of the radiation dose is
the identification of the sensitive “target” cells at risk. Whena radionuclide is uniformly distributed throughout an organ or
tissue, as is the case for tritiated water or 137Cs, then thecalculation of average tissue dose is sufficient to assess thedose to these critical cells. In other cases, however, as with thebone-seeking radionuclides 239Pu and 241Am, the distributionof dose may be very heterogeneous, and then the calculationof dose to sensitive cells is essential in assessing dose-response
relationships.
127. Stem cells. The International Commission on Radiological Protection (ICRP) has developed a comprehensive
set of biokinetic and dosimetric models to enable thecalculation of organ doses from inhaled or ingested
radionuclides [B36, I9, I10]. These models take account of
the distribution and retention of radionuclides in individualorgans and the proportion of the energy of decay depositedin different organs. For penetrating photon radiation, it isnecessary to take account of crossfire between organs, but
in these cases the calculation of average energy absorbed ina tissue is sufficient. For non-penetrating alpha and beta
radiations, energy is taken to be deposited in the organ inwhich the radionuclide is retained. For these radiation
types it is necessary in some cases to take account of the
distribution of the radionuclide within the organ relative tosensitive target cells. This consideration has been addressedin models developed by ICRP for the respiratory andgastrointestinal tracts, the skin, and the skeleton. In thecase of other tissues (for example the liver, kidneys, andspleen), the average tissue dose is calculated on theassumption that sensitive cells are uniformly distributed
throughout them. In relation to intakes of radionuclides,
only the respiratory tract, the gastrointestinal tract and theskeleton are directly relevant. Radiation doses to thesensitive cells in the skin are, however, important in thecase of radionuclides deposited on the surface of the body.
128. Respiratory tract. The ICRP model for the humanrespiratory tract [I11] takes account of the distribution ofsensitive cells for cancer induction in the extrathoracic region
and the bronchial and bronchiolar regions of the lung. For theregion of the lung in which gaseous exchange occurs, thealveoli and terminal bronchioles, the dimensions of thestructures are considered to be sufficiently small for doses tobe calculated on the assumption that sensitive cells are
uniformly distributed.

129. The extrathoracic region of the respiratory tract (thenose, oropharynx, and larynx) are lined mainly with stratified
squamous epithelium. Excess nasal and laryngeal cancers
have been observed in luminizer workers and patients
receiving head and neck exposures [B37] but not in atomic
bomb survivors or patients treated for spondylitis [D12].
Sinonasal cancers were described in humans as a result ofsystemic contamination with radium [E11, F13]. Radiationinduced tumours were mainly carcinomas, including basal
cell, squamous cell, and epidermoid carcinomas, for which thecells at risk are assumed by ICRP [I11] to be the basal cells of
the epithelial layer with their nuclei at average depths of
40!50 µm.
130. The trachea, bronchi, and bronchioles are lined by apseudostratified, ciliated, columnar epithelium separated fromthe subepithelial connective tissue by a prominent basement
membrane. Radiation-induced lung cancers have been
documented in uranium miners, atomic bomb survivors, andtherapeutically irradiated patients [B38, I12, P16]. Lungcancers occur predominantly in the bronchial region; there isno evidence that radiation induces tracheal cancer. There are
four main classes of tumour observed: squamous cell carcinoma (most frequent), small-cell carcinoma, adenocarcinoma,
and large-cell carcinoma. It appears that these tumour types
share the same endodermic progenitors [M44, Y5]; the most
likely candidate cells for tumour induction were considered by
ICRP to be secretory cells [T15]. Basal cells may also be
involved, although their role may be limited [J7]. ICRP
therefore assumes for dosimetric purposes that the sensitive
cells in the bronchial region are secretory and basal cells, with
nuclei at average depths of 10!40 µm and 35!50 µm,
respectively[I11]. The sensitive cells in the bronchiolar region
are taken to be secretory cells, with nuclei at an average depth
of 4!15 µm.
131. Estimates of dose to the lung from short-range emitters,
particularly alpha emitters, depend on the assumptions maderegarding the depth and thickness of the sensitive layer in the
bronchi and bronchioles. For example, in a recent sensitivity
analysis of doses from radon progeny, a dose range that varied
by a factor of 2.6 resulted from consideration of sensitive cellparameters [M45].
132. Gastrointestinal tract. The current dosimetric modelof the gastrointestinal tract makes only a simple generalized
allowance for the position of sensitive cells relative to ingested
radionuclides [I9]. Doses are calculated separately for the
mucosal layer of each region modelled: the stomach, small
intestine, upper large intestine, and lower large intestine. For
penetrating radiations, the average dose to the wall of eachregion is used as a measure of the dose to the mucosal layer.
For non-penetrating radiations, the fraction absorbed by the
mucosal layer is taken to be equal to 0.5v/M, where M is themass of the contents of that section of the gastrointestinal tract
and v is a factor (between 0 and 1) representing the proportionof energy that reaches sensitive cells. The factor of 0.5 isintroduced because the dose at the surface of the contents will
be approximately half that within the contents for nonpenetrating radiations. For beta particles, v is taken to be 1.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

For alpha particles, v is taken to be 0.01. This value is basedon weak experimental evidence from an acute toxicity study
in rats in which the LD50 for ingested 91Y was estimated atabout 12 Gy while a more than 100 times greater dose to themucosal surface from 239Pu had no effect [S33].
133. This model is currently being revised. The new modelis expected to consider the location of sensitive cells in all
regions of the alimentary tract, from the mouth to the large
intestine. Radiation-induced cancer in human populations has
been documented for the oesophagus, stomach, and colon; thesmall intestine is not a significant site for cancer induction[B39]. The sensitive cells in the oesophagus are assumed to be
the basal layer of the stratified squamous epithelial lining.
This epithelium is quite thick (300!500 µm) and is protectedby a surface layer of mucus. At the gastro-oesophageal junction, it is abruptly succeeded by a simple columnar epithelium
with gastric pits and glands. In the stomach, the sensitive cells
for cancer induction are assumed to be the epithelial stem
cells, located within but towards the top of the gastric pits, at
a depth of about 75!100 µm from the surface. In the small
intestine, stem cells are located above the paneth cells, towards the base of the crypts. In the large intestine, stem cells
are situated at the very base of the crypts. These locations havebeen deduced from a variety of cell kinetic, mutational, andregeneration studies in mouse models [P17] and their positions are likely to be qualitatively similar in man. The numberof stem cells per colonic crypt in mice has been estimated to
be in the range 1!8, and as colonic crypts in man are aroundsix times as large as in mice, it is possible that the number ofstem cells per crypt may be greater in man. The depth of thestem cells, measured in human tissue samples, is about
100!150 µm in the small intestine and 200!400 µm in thelarge intestine [P18].
134. Skin. The skin is broadly divisible into two component
layers: the outer epidermis and the underlying dermis. The
epidermis arises from a single basal layer of cells, overlaid bylayers of cells with dead layers on the outer surface. The basal
layer is separated from the dermis by a basement membrane.
This boundary is not flat but undulates, with discrete points
known as rete pegs where the epidermis projects down into
the dermis. In addition, the basal layer extends around theskin appendages, notably the shaft and base of the hairfollicles, which project even deeper into the dermis. At somesites on the body, over 50% of the basal layer stem cells areassociated with the hair follicles. Thus, the depth of the basal
layer is highly variable. In most body areas it ranges from 20
to 100 µm in the interfollicular sites, but exceptionally (e.g.
the finger tips), it can be over 150 µm deep because ofincreased outer cornification [L46]. The deeper projections
associated with hair follicles result in basal cells being
situated more than 200 µm deep.
135. There is substantial evidence linking the incidence of
non-melanoma skin cancer (NMSC) with exposure toionizing radiation, including studies on irradiated children
and atomic bomb survivors [L46]. The two main types of
non-melanoma skin cancer are squamous cell carcinoma
and basal cell carcinoma, with the sensitive cells for cancer

99

induction assumed to be the basal layer of cells in each case.
This assumption is supported by animal data [A15, H28].
136. In calculating dose to the skin, ICRP has recommended that skin dose should be evaluated at an average
depth of 70 µm [I2]. However, when assessing dose incases of non-uniform exposure, it may be necessary to use
skin thickness values appropriate to the area of interest.
137. Skeleton. Biokinetic and dosimetric models for theskeleton take account of the two main types of bone, corticaland trabecular, and the behaviour of different bone-seeking
radionuclides as well as the location of sensitive cells for the
induction of bone sarcoma and leukaemia [I9, I10]. Corticalbone is the hard, dense bone that forms the outer wall of bones
and the whole of the shaft of long bones. Trabecular bone isa soft, spongy bone with a lattice-work structure that is foundwithin flat bones and in the ends of the long bones. The
endosteal layer of cells on the inner bone surfaces in corticaland trabecular bone is taken to be the sensitive cells for bonesarcoma and the red bone marrow is taken to be the sensitive
cells for leukaemia. It is assumed that all haemopoietically
active red marrow is confined to the spaces in trabecular bonein adults, with cortical bone containing inactive yellow
marrow. In children, a proportion of cortical marrow isassumed to be haemopoietically active and therefore a target
for leukaemia induction. In its 1979 Report, ICRP classified
bone-seeking radionuclides into two groups: bone-surface
seekers, including the actinide elements, and bone-volume
seekers, including the alkaline earth elements [I9]. Thus,
radionuclides were assumed to be retained either on endostealbone surfaces or uniformly distributed throughout the volume
of bone mineral. Absorbed fractions were calculated for the
proportions of alpha and beta energy emitted in each case thatwould be deposited in the sensitive regions of the endosteal
layer, taken to lie within 10 µm of bone surfaces and red
marrow. More realistic biokinetic models have since beendeveloped for the main bone-seeking radionuclides, isotopesof the actinides, alkaline earths and similar elements, whichallow for initial deposition on bone surfaces, movement intobone owing to bone remodelling and chemical exchange, andloss from bone, principally owing to bone resorption [I10].
For the actinides, transfer from bone to marrow is also
included.
138. An increased incidence of bone sarcomas has been
observed in populations exposed to alpha-emitting radiumisotopes, particularly in painters of luminuous dials, butalso radium chemists and people treated with radium salts
for the supposed therapeutic effect [M18]. Although the
ICRP assumption [I8] that the sensitive cells constitute a
10-µm-thick layer on endosteal surfaces gives reasonable
dose estimates, it has been suggested that all bone surfaces
may not be equally sensitive [P19] and that the sensitive
region may include cells at a greater depth into the marrow[G15]. Priest [P19] argued that the observed difference intoxicity between 226Ra (half-life = 1,600 years) and 224Ra(3.6 days) in animals and humans cannot be explained
simply in terms of a greater wastage of alpha dose from thelonger-lived 226Ra within bone mineral. He suggests that a

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

greater proportion of alpha dose from 224Ra may be delivered
to active trabecular surfaces and that these regions have agreater than average sensitivity.
139. Gössner et al. [G15] have reviewed the histopathologyof radiation-induced bone sarcomas, showing that there are oftwo main types, bone-producing osteosarcomas, and nonbone-producing sarcomas of the fibrous-histiocytic type. A
trend toa greater proportion of fibrous-histiocytic tumours was
identified at lower doses and shorter latency periods. The datasuggest that cells at risk are not only those committed to boneformation on the bone surfaces but multipotent marrowstromal cells located at some distance from the bone surface.
140. Excess leukaemia has been recorded in patients exposedto the alpha-emitting contrast medium thorotrast and in theatomic bomb survivors, but it is not a feature of exposure to
isotopes of radium [I9, M46]. Comparison of leukaemia
induction by thorotrast and external low-LET irradiationsuggests a low RBE for alpha-induced leukaemia. The
inability of 226Ra to induce leukaemia [R16] may be explainedby a low alpha RBE, but the distribution of sensitive cells in
the marrow may also be a contributory factor. While thecolloidal thorium oxide preparation thorotrast was retained inmacrophages throughout the marrow, radium on bone
surfaces delivers a dose only to peripheral marrow, and it may
be that sensitive cells are concentrated more towards thecentre of marrow spaces. Some evidence for this was provided
by studies using mice [L47]. It may be, therefore, that the
ICRP assumption that sensitive cells for leukaemia induction
are uniformly distributed throughout red marrow [I9] may
overestimate the risk of leukaemia from bone-seeking
radionuclides.
141. Tumour induction. A number of reviews and papers
have examined dose-response relationships for tumour
induction in animals exposed to either alpha emitters orbeta/gamma emitters (see, for example, [I5, L27, M11, N6,Y6]). Most information is available on the induction of bonetumours following the entry of radionuclides into the blood orlung tumours after inhalation of radioactive materials invarious chemical forms, although more limited data on otherorgans and tissues are also available. A wide range of doseresponse relationships has been obtained. These encompass
data that can be fitted with simple linear models up tointermediate levels of dose and other responses with clear
evidence of a threshold. The results of studies on tumour
induction from intakes of radionuclides are illustrated by dataon tumour induction in the lungs and skeleton.
142. Bone tumours. The incidence of bone tumours in
mice, rats, dogs, and pigs given graded doses of 90Sr was
examined by Mays and Lloyd [M11]. Although limited data
were available at low doses, and the various species had
different sensitivities to tumour induction, in all cases theincidence of bone tumours at the lowest doses examined wasless than would have been predicted on the basis of a linear
dose response. Thus, in beagle dogs with average skeletal
doses from 90Sr at 1 year before death of between 0.27 Gy and
111 Gy, no tumours were found in the three lowest dosegroups (1, 3.35, and 5.97 Gy), with an 8% incidence
occurring at 21.7 Gy [N6]. The numbers of dogs in eachgroup was, however, only about 12, and a small increase inincidence could not have been detected. Similar data havebeen reported for osteosarcoma induction by 90Sr in female
CF1 mice. In groups of about 100 animals with average bonedoses ranging from 0.26 to 120 Gy, no significant increase intumour incidence was found in animals with average doses
below about 10 Gy (1.3, 4.5, and 8.9 Gy) [M11, N6].
143. An extensive series of studies has examined tumour
induction in animals given various alpha emitters. Lloyd etal. [L27] examined the occurrence of skeletal tumours inyoung adult beagle dogs given single intravenous injectionsof monomeric 239Pu citrate. The relationship between theincidence of osteosarcoma and average dose to bone at thepresumed time of tumour initiation, taken to be at 1 year
before death, appeared to be linear below about 1.3 Gy(26 Sv, assuming an RBE for alpha radiation of 20)(Figure XIII). The observed tumour incidence, I (%), could
be approximated by the expression I = 0.76 + 75D, where
D is the average dose to bone in gray. Similar analyses ofdata from dogs given 226Ra also gave a linear response withthe expression I = 0.76 + 4.7D (for doses up to 20 Gy). The
ratio of the coefficients (75/4.7 = 16±5) shows that 239Pu is
more effective in inducing osteosarcoma than 226Ra. Thisis thought to be due to the tendency of plutonium to remain
longer on bone surfaces and to more effectively irradiate
the sensitive cells for tumour induction.
100
239

90

Pu

80
I = 0.76 + 75 D

70
INCIDENCE (%)

100

226

60
50

Ra
I = 0.76 + 4.7 D

40
30
20
10
0
0

2
4
6
8
10
12
14
16
18
20
AVERAGE SKELETAL DOSE AT 1 YEAR BEFORE DEATH (Gy)Figure XIII. Bone cancer incidence in beagle dogs
following a single injection of plutonium-239 or
radium-226 citrate at about one year of age [L14].
The ratio of plutonium to radium dose coefficients (75:4.7)
is 16.

144. Further data on bone tumour induction in animals givenalpha emitters were analysed byMays and Lloyd [M17]. They
found that although the induction of bone tumours appeared
to increase linearly with dose in some cases, in others it
followed threshold or sigmoid relationships. In CF1 female
mice injected intravenously at 70 days of age with 239Pu [F11],
no tumours were observed in groups of mice with averagebone doses of 0.01 Gy (N=99) and 0.22 Gy (N=96), whereasa linear response would have predicted some 5.3 cases. The

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES
10 4145. A linear dose response was found for osteosarcoma
induction in female CF1 mice given 226Ra by intravenous
injection at 70 days of age. In 1,436 mice with averagebone doses below 3 Gy (high-LET), 115 cases of osteosarcoma were observed, in good agreement with 92 cases
predicted using a linear dose response [F12, M17]. In
contrast, in beagle dogs given 228Ra and 228Th, the doseresponse data suggested the presence of thresholds at about
2 Gy and 0.5 Gy (high-LET), respectively [M17].

TIME TO DEATH (days)

probability of observing zero cases, if 5.3 cases is the true
number, is only 5%. At 0.4 Gy and above, bone tumour
incidence increased linearly with dose.

101

Controls

10 3

Bone cancer deaths
Radiation injury deaths
Other deaths
10 2146. More complex models have also been developed tointerpret dose-response relationships for bone tumour
induction. Raabe [R1] has described an example for predicting
risks associated with protracted exposure to ionizing radiationfrom internally deposited radionuclides. For long-lived
radionuclides such as 90Sr, 226Ra, or 239Pu, the radiation dose
will be delivered over the lifespan of the animal. Raabe et al.
[R1, R14] have interpreted the data from various lifetime
studies with beagle dogs exposed by injection, ingestion, andinhalation to either beta emitters or alpha emitters. The
cumulative absorbed dose required to give a specified level ofcancer risk was found to be less at lower dose rates than at thehigher dose rates, and the induction time required for tumours
to manifest themselves tended to be longer at lower dose rates
and could exceed the normal lifespan of the animal. The
authors interpreted the data to suggest that at the lowest dose
rates there is an effective threshold for the induction of fatalradiation-induced cancer.
147. For example, beagle dogs given eight fortnightly
injections of 226Ra in amounts from 0.099 kBq kg1 to 46.3
kBq kg1 received average lifetime skeletal doses from 0.9 ±0.2 Gy to 167 ± 44 Gy (±1 SD). Death in these dogs wasconsidered to be a function of three effects: (a) spontaneous
death arising from causes associated with the natural lifespan,(b) death associated with radiation-induced bone tumours, and(c) death from radiation-induced skeletal injury such asradiation osteodystrophy and bone fractions occurring at highdoses (Figure XIV). Mathematical three-dimensional doserate/time response models with log-normal probability
distributions were fitted to the lifespan data for the dogs. The
data plots indicated that bone cancer predominates as a causeof death at intermediate doses and is infrequent at low dose
rates (because of death associated with natural lifespan) andat high dose rates (because of deaths from acute radiation
injury). The cumulative dose required to cause bone cancer issmaller at the low dose rates; however at lower dose rates ittakes longer to reach any specified level of risk, perhaps
longer than the natural lifespan of the animal. This results ina lifespan effective threshold for cancer induction similar to
the “practical” threshold described by Rowland [R17] at acumulative lifespan alpha dose of about 1 Gy in man (seeChapter III). In practice, the lack of a significant effect during
the lifespan of the animals could also be taken to indicate arisk of cancer with a very low probability of occurrence at low
doses.

0

10 -4
10 -3
10 -2
10 -1
AVERAGE DOSE RATE TO SKELETON (Gy d-1)

Figure XIV.
Deaths from non-neoplastic radiation
injury, bone cancer and other causes in beagle dogs
injected with 226Ra. Initial intake occurred at 435 daysof age [R1].

148. Raabe et al. [R14] have also compared data on bonetumour induction in humans and CF1 mice with dataobtained in beagle dogs. When time was normalized withrespect to lifespan, the three species were found to havebone cancer dose-rate/time risk functions that were almost
identical and could be represented by one median
regression line.
149. Lung tumours. Extensive data have also been published on lung tumour induction in rodents and beagle dogs
exposed to internally incorporated alpha and beta/gamma
emitters. Studies conducted in the 1960s and 1970s wereconsidered by a Task Group of Committee 1 of ICRP [I5].
The data reviewed were from laboratories around the world
and from studies using a range of different protocols andmethodologies. One specific aim of the analyses was todetermine the relative effectiveness of alpha emitters andbeta/gamma emitters in causing lung damage, including
neoplastic development.
150. The Task Group commented on some of the difficulties
in ascertaining the dose response for lung tumour induction.
In studies with inhaled radionuclides it is impossible todeposit the same amount of activity in the pulmonary regionof different animals in a group. As a consequence, authors
have commonly shown dose ranges rather than a single value.
Further, researchers do not agree on how to express dose to
the types of tissue found in the lung. Cumulative doses may be
estimated for individual animals at death, at time of the first
tumour, or for the average lifespan of the group of animals.
Other variants have also been used. In all cases considered by
the Task Group, average lung dose was calculated.
151. The analyses of pooled data from studies with different
species generally used a probit model, as had commonly beenused in dose-response analyses, and the linear dose response,
which the Task Group considered to reflect conservatism.
Both linear and probit models gave an adequate description of
the incidence data for alpha emitters over the range ofANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

observed doses. For beta/gamma emitters, however, neither
model gave a good fit to the incidence data, which were rather
variable for similar doses. The linear dose response
considerably overestimated incidence at low doses. In general
the pooling of data from numerous species, although it is acomprehensive approach, does not readily permit detailed
comparison of any of the individual studies.
152. The results of a number of separate studies, mainly ofrodents exposed to both alpha and beta/gamma emittershave been published. Sanders and Lundgren [S11]compared lung cancer induction in F344 and Wistar rats
exposed to 239PuO2. In the F344 strain, significantly
increased lung tumour incidences were found at lung dosesof both 0.98 Gy (20%) and 37 Gy (34%) compared with
1.7% in controls. There were insufficient data to define adose-response function, but there was no evidence for a
threshold in the response. In contrast for the Wistar rats,
there was no significant increase in lung tumour incidence
in animals with an average lung dose of 0.75 Gy (0%)
compared with controls (0.1%), but for animals with a lungdose of 34 Gy the incidence was 68%. These datasuggested the presence of a threshold at doses somewhat
above 0.75 Gy.
153. The data on Wistar rats [S11] were similar to thosefound in a more comprehensive lifespan study [S12]. In
3,157 female Wistar rats that had inhaled 239PuO2 only
three adenomas were found in 1,877 rats at lung doses<1.5 Gy, for an incidence of 0.16%; tumour incidence
increased to 41% in 228 rats with lung doses >1.5 Gy.
Pulmonary squamous metaplasia was not seen in controls
and was first noted in exposed rats at lung doses >1 Gy.
All tumour types induced by inhaled 239PuO2 exhibited athreshold at lung doses >1 Gy. It was concluded that forlung tumours in Wistar rats resulting from inhaled 239PuO2,
plutonium particle aggregation is required to cause
proliferation of initiated cells and to promote the formationof premalignant and malignant lesions.
154. Similar results in Wistar rats were obtained by
Oghiso et al. [O5], although the study was not as extensive
as that by Saunders et al. [S12]. Dose-response relationships were compared among primarytumours, classified byhistological type, following a single inhalation exposure to239
PuO2. In this study there were 130 controls and 310
animals, separated into seven groups, exposed to 239PuO2.
Initial lung contents in the different groups varied between
about 97 and 1,670 Bq, giving average lung doses from 0.7
to 8.5 Gy. A differential tumour response was obtained. In
general, metaplasia and benign adenomas were induced atlower doses (<1 Gy), whereas malignant carcinomas were
induced at relatively high doses (>1.5 Gy) (Figure XV).
The peak incidence of adenomas occurred at a dose of
0.7 Gy, of adenocarcinomas at 2.9 Gy, and adenosquamousand squamous cell carcinomas at 5.4!8.5 Gy. These results
were considered by the authors to indicate a differential
dose response for pulmonary carcinogenesis, in which
metaplasia and benign adenomas were induced at lower
doses (<1 Gy) and malignant carcinomas were induced at

higher doses (>1.5 Gy). It was also noteworthy that thelifespan of the 0.7 Gy group (871 ± 105 days, ±1 SD) was
significantly longer than that of the control group (790 ±144 days, p<0.01). In the higher exposure dose groups,
lifespan was reduced.
100

Malignant tumours
Benign tumours

80

INCIDENCE (%)

102

60

40

20

0
0.001

0.01

0.1
LUNG DOSE (Gy)110

Figure XV. Benign and malignant lung tumours in rats
after inhalation of 239PuO2 aerosols [O5].

155. This threshold type of response did not seem to befound in Fisher 344 rats exposed to 244Cm as the oxide.
Groups of 100!200 male and female rats received averagelung doses from 244Cm2O3 between 0.2 and 36 Gy [L28]. In
general the prevalence of benign and malignant lungneoplasms increased with increasing average lung dose. For
lung tumours, a linear dose-response function adequatelyfitted
the data (I = 0.38 ± 0.04 Gy1). The response in rats exposed
to 239PuO2 (I = 0.70 ± 0.07 Gy1) was about twice the response
following exposure to 244Cm2O3.
156. Some information is available on tumour induction inrats exposed to the beta/gamma emitter 144Ce as the oxide[L39]. A total of 1,059 F344/N male and female rats (about
12 weeks of age) were exposed to graded levels of 144CeO2,
and a further 1,064 rats were maintained as controls (exposed
to stable CeO2). Groups of rats received mean lung doses of
3.6, 12, and 37 Gy. The incidence, I (%), of lung tumours
increased with increasing lung dose and could be represented
by a linear function of the form I = 0.13 + 0.51D Gy1, where
D is the dose in gray. Because the data are limited in extent,
more complex functions such as the linear-quadratic, exponential linear-quadratic, and Weibull functions also describedthe dose response adequately over the dose range of the study.
157. An extensive series of studies has been carried out inrodents exposed to radon and its decay products. They havedemonstrated that exposures at high doses can give rise toradiation-induced lung cancers. Experimental animal studies
have been valuable for understanding the consequences of
exposure at varying dose rates and the influence of otherenvironmental factors on the lung tumour response as theanimals can be exposed to a variety of agents under carefully
controlled conditions. Much of the information available on

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

103

exposure level. [Note: Working level (WL) is defined as any
combination of short-lived radon decay products in 1 litre of
air resulting in the ultimate emission of 1.3 105 MeV ofpotential alpha energy (1 WL = 2.08 105 J m3). Workinglevel month: exposure equivalent to 170 hours at 1 WL
concentration (1 WLM = 3.5 103 J h m3).]

experimental animals was reviewed by Cross [C22]. In
animals, exposure to radon has resulted in respiratory tract
tumours (adenomas, bronchiolar carcinomas or adenocarcinomas, epidermoid carcinomas, adenosquamous carcinomas,
and sarcomas). In addition, pulmonary fibrosis, pulmonary
emphysema, and life shortening have occurred at exposures
above about 1,000 WLM1 (3.5 J m3). Excess respiratorytract
tumours have occurred in rats at exposure levels well below
100 WLM1 (0.35 J m3), even at levels comparable to thosefound for typical lifespan exposures in homes. Further,tumours occurred in animals exposed to radon decay products
alone; thus indicating that exposure to other environmental
agents (uranium ore dust, cigarette smoke) is not necessaryfor
carcinoma development. Most (~80%) radon-induced lungtumours in rats are considered to originate peripherally and tooccur at the bronchiolar-alveolar junction. The remaining
20% are centrally located in association with the bronchi, theactual percent depending on exposure rate and possibly on

158. A notable finding in these animal studies has been that
longer duration of exposure at a lower dose rate induces morelung cancers than exposures for a shorter duration at a higher
dose rate. Table 9 compares the incidence of lung tumours inrats exposed at either 50 or 500 WLM per week. With
exposure levels between 320 and 5,120 WLM, in all except
the lowest exposure group there is a higher incidence oftumours in the groups exposed at the lower dose rate. The
decrease in exposure rate not only increased the tumour
incidence but specifically increased the incidence of
epidermoid carcinomas.

Table 9Percentage incidence of primary and fatal lung tumours in rats exposed to radon and decay products a[C22]
Exposure (WLM)
Cancer type
320

640

1 280

2 560

5 120

38
0
26
0
3
0
5
29

38
3
24
3
0
3
11
32

41
2
44
2
0
2
15
49

32
(22)
41
(13)
9
3
(22)
(66)

32
9
41
(47)
(9)
0
(50)
(69)

32
(22)
53
(44)
3
0
(44)
(75)

At 500 WLM per week
Number of animals examined
Adenoma
Adenocarcinoma
Epidermoid carcinoma
Adenosquamous carcinoma
Sarcoma
Fatal lung tumours
Animals with lung tumours (%)

70
3
7
0
0
0
1
10

131
5
8
1
0
0
2
15

At 50 WLM per week
Number of animals examined
Adenoma
Adenocarcinoma
Epidermoid carcinoma
Adenosquamous carcinoma
Sarcoma
Fatal lung tumours
Animals with lung tumours (%)
a

127
5
5
1
1
1
2
10

64
3
(20)
3
3
2
6
(28)

15 mg m 3 ore dust exposures accompanied radon and radon progeny exposures; data in parentheses at 50 WLM per week are significantly(p<0.05) higher than corresponding data at 500 WLM per week.

159. A series of studies has also been conducted in France on
the effects of radon exposure [G18, M48]. In theseexperiments more than 2,000 rats were exposed to cumulative
doses of up to 28,000 WLM of radon gas. There was anexcess of lung cancer at exposures down to 25 WLM(80 mJ h m3). These exposures were carried out at relatively
high concentrations of radon and its decay products (2 J m3).
Above 6,000 WLM, rats suffered increasingly from life
shortening due to radiation-induced non-neoplastic causes,
thus limiting tumour development. When the dose-response
data were adjusted for these competing causes of death, thehazard function for the excess risk of developing pulmonary
tumours was approximately linearly related to dose. Thissuggests that the apparent reductions in tumour induction
found at high doses may have been chiefly the result of acutedamage. Later experiments, however, found that chronic
exposure protracted over 18 months at an alpha energy of
2 WL (0.0042 mJ m3) resulted in fewer lung tumours in rats
(0.6%, 3/500 animals, 95% CI: 0.32!2.33) than similar
exposures at a potential alpha energy of 100 WL (2 mJ m3)
protracted over 4 months (2.2%, 11/500 animals, CI:
0.91!3.49) or over 6 months (2.4%, 12/500, CI: 1.06!3.74).
The incidence of lung tumours in controls was 0.6% (5/800,
CI: 0.20!1.49) [M48]. The confidence intervals are, however,wide, and the longer period of exposure (18 months) would in
itself have been expected to result in fewer lung tumours.

104

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

160. It has been suggested by Moolgavkar et al. [M13, M29],
based on a two-stage initiation-progression model for carcinogenesis, that extended duration of exposure allows time forproliferation of initiated cells and thus for a higher disease
incidence. The findings are that the first mutation rate is verystrongly dependent on the rate of exposure to radon progeny
and consistent with in vitro rates measured experimentally.
The second mutation rate is much less so, suggesting that thenature of the two mutational events is different. Furthermore,by incorporating cell killing into such a model, Luebeck et al.
[L12] proposed that the model that gave the best fit to the data
indicated that the initial increase in the proliferation rate ofinitiated cells depended on a second promotional step, which
may be due principally to the presence of ore dust and not toradon decay products. The inverse exposure rate effect may
thus be reduced in the absence of ore dust.
161. The main factors found to influence the tumorigenic
potential of radon exposure in laboratory rats include
cumulative exposure to radon progeny, exposure rate,
unattached fraction, and associated cigarette smoke exposure.
The respiratory tract cancer risk increases with the increase incumulative exposure to radon progeny and in the magnitudeof the unattached fraction. The increased risk with a highunattached fraction of radon progeny is particularly relevant
to indoor radon exposure, where the unattached levels are
generally much higher than in mines. The influence ofcigarette smoke has been variable, depending in part on thetemporal sequence of radon and cigarette smoke exposure.
162. Overall, the data on lung cancer risk resulting fromexposure to radon and its decay products show anincreasing risk with increasing exposure, although there
are strong indications of an inverse dose-rate effect that is
influenced by the presence of ore dust in the atmosphere.
The data are broadly similar to those obtained from followup studies on uranium miners (see Chapter III).
2. Cancer risks at low doses
163. An essential input to the analysis of dose-response
relationships is not only the shape of the dose response but
the extent to which a statistically significant effect ofradiation can be detected at low doses. It is informative to
examine a number of studies that have been concernedwith assessing risks at low doses.

over a period of 28 days (0.4!0.11 mGy min1). This strainof mice is exceptional, in that no case of myeloid leukaemia has been observed in more than 1,400 unirradiatedmale mice, so that every case occurring in irradiatedanimals can be regarded as radiation-induced. Total acutedoses were from 0.25 to 1.0 Gy for x rays and 1.5 to 3.0 Gy
for gamma rays.
166. Upton et al. [U21] used RFM mice of both sexes. For
acute exposures of female mice, a dose rate of 67 mGy min1
from 60Co gamma rays was used, giving doses between 0.25
and 4 Gy. For male mice, x rays at 800 mGy min1 were used,with doses from 0.25 to 3 Gy. Male and female mice werealso exposed chronically. Data are available on the inductionof myeloid leukaemia, thymic lymphoma, and ovarian
tumours.
167. Ullrich [U14] and Ullrich et al. [U15, U16, U17, U18,
U19] carried out experiments similar to those of Upton et al.
[U21] using RFM male and female mice acutely exposed (450mGy min1) to 137Cs gamma rays. Data were reported on thetumours examined by Upton et al. [U21], together with data
on Harderian gland and pituitary tumours.
168. Ullrich [U14], Ullrich and Storer [U16], and Ullrich
and Preston [U20] also used BALB-C female mice to obtainfurther data on dose-response relationships. Acute (450mGy min1) and chronic (0.06 mGy min1) exposures from137
Cs were given. Acute doses were between 0.01 and 1 Gy,and chronic doses were between 0.25 and 2 Gy. Tumours
showing a positive increase with dose were ovarian tumours
as well as mammary and lung adenocarcinomas.
169. Coggle [C6] reported data on the induction of lungadenocarcinomas in male and female SAS/4 mice acutely
exposed to x rays at 0.6 Gy min1. The dose range used was
0.25!3.0 Gy.
170. Covelli et al. [C7, C8] reported tumour induction inmale and female BC3F1 mice. They observed various typesof radiation-induced tumours following acute exposure of
113 mGy min1 (dose range males, 0.04!2.5 Gy; females,
0.5!5.0 Gy). The authors gave age-adjusted incidences oftumours and described tests showing which doses gavesignificant increases in cancer yield.
(b) Analysis(a) Studies
164. Laboratory animal studies that are most suitable fordetermining the lowest doses at which effects of radiation on
tumour induction can be detected have been carried out
predominantly with mice. Comprehensive data are, however,rather limited. Some of the more significant studies are briefly
summarized below and analysed in the following Section.
165. Mole and Major [M3] and Mole et al. [M4] reported
myeloid leukaemia incidence in male CBA-H mice acutely
exposed to x rays (0.5 Gy min1) and 60Co gamma rays
(0.25 Gy min1) and chronically exposed to gamma rays

171. To determine the lowest dose at which a significant
increase in tumour yield occurred in the various studies, thefollowing method was used. The tumour yield in control
animals was tested against the yield at the lowest dose used in
the study. If the difference in tumour incidence is statisticallysignificant, then that dose is taken as the lowest dose at which
a significant effect is found. If the difference is not significant,the data point with the next lowest dose is included and aweighted linear regression performed, either byweighted leastsquares or, where possible, by iteratively re-weighted leastsquares. This process is continued at progressively higher
doses until the linear regression coefficient becomes signifi-

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

cantly different from zero (p=0.05). The last dose added isthen taken to be the lowest dose to give a significant effect.
When calculating statistical significance, any lack of fit to astraight line is taken into account in computing uncertainties.
172. The exposure levels at which significant increases inrisks of leukaemia and solid cancers could be observed are105

given in Table 10. The lowest dose at which a significant
effect on tumour incidence could be determined is very
different from study to study. It depends on factors that
influence statistical power, such as the number of miceused and the spontaneous cancer rate, the cancer type, thelevel of radiation risk, the dose range used, and the periodof follow-up.

Table 10Lowest acute doses at which significant increases in cancers have been observed in mice
Cancer

Mouse strain

Sex

Irradiation

Dose (Gy)

Ref.

Myeloid leukaemia

RFM

Male
Male
FemaleFemaleMale
Male
Femalex raysGamma rays
Gamma rays
Gamma rays
x raysGamma rays
x rays

0.25
1
1
2
0.5
1.5
1

[U21]
[ U16, U17, U20]
[U15, U16, U17, U18]
[U21]
[M4]
[M3]
[C7, C8]

CBA-H
BC3F1

a
b

Thymic lymphoma

RFM

Male
Male
FemaleFemaleGamma rays
x raysGamma rays

1
3
1
2

[U16, U17, U20]
[U21]
[U15, U16, U17, U18]
[U21]

Lung
adenocarcinoma

BALB-cSAS/4

FemaleBothGamma rays
x rays

0.5
2.5

[U16]
[C6]

Mammary
adenocarcinoma

BALB-cFemaleGamma rays

0.2

[U14]

Ovarian tumour

BC3F1
BALB-c
RFM

FemaleFemaleFemalex raysGamma rays
Gamma rays

0.16
0.25
0.5

[C7, C8]
[U18]
[U21]

Harderian gland
tumourRFM

Male

Gamma rays

3

[U16, U17, U20]

All solid tumours

BC3F1

Femalex rays

1.3 a b4

[C7, C8][C7, C8]

Excluding ovarian tumour.
p=0.05.

173. For leukaemia induction in mice there was little
evidence for an increase in risk below 1 Gy, although twostudies indicated statistically significant increases at
0.25 Gy [U21] and 0.5 Gy [M3]. Most of the dose-response
data for acute exposures showed no significant departurefrom linearity. An exception was the study by Mole andMajor [M3], which showed a reduced effectiveness ofradiation, per unit dose, at 1 Gy. There was also asuggestion of a departure from linearity at high doses in
the results reported by Ullrich et al. in RFM mice [U16,U17, U20].
174. For solid cancers the overall results (Table 10) aresimilar to those for leukaemias, with significant increases
in tumour incidence occurring principally at acute doses of
1 Gy and above. Increases in risk were, however, seen atlower doses for ovarian tumours (0.16 and 0.25 Gy),
mammary adenocarcinomas (0.2 Gy), and lung adenocarcinomas (0.5 Gy). Although data are not given here, theuse of a lower dose rate consistently resulted in a lower risk
per unit of dose.

B. HEREDITARY DISEASE
175. In addition to inducing neoplastic changes in somatic
tissues, ionizing radiation may produce transmissible(heritable) effects in irradiated populations by inducing mutations in the DNA of male or female germ cells. These mutations, while having no direct consequences for the exposed
individual, may be expressed in subsequent generations as
genetic disorders of widely differing types and severity.
176. Studies of germ-cell mutations in vivo are not only
relevant for assessing dose-response relationships forhereditary effects but they also have value for assessing effects
on the primary lesion in DNA likely to be involved in tumour
initiation. As described in Chapter IV, subsequent tumour
expression will depend on the influence of manyother factors.
177. The evaluation of genetic hazards associated with theexposure of human populations to ionizing radiation is an
important area in which the Committee has been active
since its inception. To date, however, there has been a lack

106

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

of direct data that give quantitative information on genetic
effects leading to disease states in humans. The substantial
amount of data from other species indicates that radiation
can give rise to mutations in humans that will bemanifested as disease. So far there has been no alternative
but to use data from experimental animals as the mainbasis for predicting quantitative effects in humans.

of mutations. This independence of dose rate was shown overa range of doses differing by rather more than a factor of
1,000, and it was concluded that it was unlikely that a further
reduction in mutation frequency would be obtained at evenlower dose rates [R6]. This suggests that a substantial fractionof the damage to DNA that results in the induction ofheritable mutations is not amenable to effective repair.

178. In the UNSCEAR 1988 Report [U4], the Committee
summarized the principal assumptions thought to benecessary for extrapolating data on hereditary damage inmice and other animals to humans. The main considerations are the following:

182. Similar results for specific-locus mutations in male mice
were obtained in studies by Lyon et al. [L40]. For a gamma
ray dose from 60Co of 6.3 Gy given in 60 equal daily fractions
at 0.17 Gy min1, the mutation frequency (4.17 105 per locus)
was very similar to that obtained in mice chronically exposed
at 0.08 mGy min1 to a total dose of 6.2 Gy (3.15 105 perlocus). The mutation rate with fractionation was, however,about a third of that obtained for a single exposure to 6.4 Gy
at 0.17 Gy min1 (13.1 105 per locus).

(c)179. Studies in mice have provided the main basis forassessing the risks of hereditary disease in humans. The
doubling dose for hereditary disease that has been adopted bymost national and international organizations for chronic
exposure is 1 Gy (e.g. [C1, M18, U4]). Reviews of experimental data from mice generally give values in the range from
1 to 4 Gy and would therefore suggest that the value of 1 Gy
adopted for humans is conservative [M18, S13].
180. A series of studies have been reported on dose-response
relationships for the induction of germ-cell mutations in mice.
The most comprehensive information comes from studies inmale mice in which specific locus mutations were measured.
Russell et al. [R5, R6], for example, presented data on doseresponse relationships for male mice exposed to 0.72!0.9
Gy min1 for doses between 3 Gy and 6.7 Gy and#8 mGy min1for doses between 0.38 and 8.61 Gy. In bothcases the data could be fitted by a linear dose-response
relationship over the whole dose range examined. For chronic
exposure, I = (8.04 106 ± 1.19 106) + (7.34 106 ± 0.83
106)D; for acute exposure, I = (8.12 106 ± 1.19 106) + (2.19
105 ± 0.19 105)D, where I is the mutation frequency per
locus and D is the dose in gray. The difference in slope for thetwo exposure conditions, by a factor of about 3, reflects thedifference in the dose rates and opportunity for repair ofdamage at lower dose rates (Figure XVI). It might be expectedthat if lower doses had been used in the high-dose-rate study(0.72!0.9 Gy min1), the slope of the response at lower total
doses would approach that found for low-dose-rate exposures.
181. It was notable that although the incidence of mutations
fell by a factor of about 3 for a reduction in dose rate from
800!900 mGy min1 to 8 mGy min1, further reduction in thedose rate to 0.007 mGy min1 failed to further reduce the yield

24

Acute exposure
Chronic exposure

2210-5 per locus)(b)the amount of genetic damage induced by a given typeof radiation under a given set of conditions is the samein human germ cells and in those of the test species
used as a model;
the various biological and physical factors affect themagnitude of the damage in similar ways and to asimilar extent in the experimental species from which
extrapolations are made and in humans; andat low doses and low dose rates of low-LET radiation
there is a linear relationship between dose and thefrequency of genetic effects studied.

20

NUMBER OF MUTATIONS (

(a)

14


0

18
16

12
10
8
6
4
2
0
0

1

2

3

4

5

6

7

8

9

DOSE (Gy)Figure XVI.
Specific locus mutations in mouse
spermatogonia following radiation exposure [R6].

183. Searle [S10] reviewed data from a number ofpublications on specific-locus mutations in spermatogonia ofmice after chronic exposure to gamma rays. Data points froma number of authors, including those by Russell et al. [R5,R6], were obtained for doses in the range 0.38!8.6 Gy. A
linear relationship gave a good fit to the data on mutation
frequency: I = 8.34 106 + 6.59 106D (assuming 100
roentgens . 1 Gy). This fit does not differ significantly fromthat obtained byRussell and Kelly[R6]. With acute exposures,
a peak in the incidence of mutations was obtained with adecline in the incidence at doses between 6.7 and 10 Gy. The
reduction at high dose rates may be attributed to more
extensive killing of spermatogonia at doses above 6.7 Gy andto a lower mutational response in those more resistant spermatogonia that survive.
184. Searle [S10] also reviewed data on specific locus
mutations in mice both acutely and chronically exposed toneutron irradiation with dose rates varying between
0.79 Gy min1 and 0.01 Gy min1. A linear function fitted
essentially all the data points between 0.5 Gy and 2.1 Gy,
with the exception of a single value at 1.9 Gy. The fit had
the form I = 8.30 106 + 1.25 104D. There was no evidenceANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

of a dose-rate effect for neutrons. The ratio between theslopes for male mice chronically exposed to low-and highLET radiations gives an RBE of 19.
185. In female mice irradiated just before birth, there is amore pronounced dose-rate effect for mutational damage tooocytes than in those irradiated later. Using the specificlocus method, Selby et al. [S24] examined the effect ofdose rate on mutation induction in mouse oocytes. Female
mice were given 3 Gy of whole-body x-irradiation at dose
rates of 0.73!0.93 Gy min1 and 7.9 mGy min1 at 18.5
days after conception. The frequency of specific-locus
mutations measured in the offspring decreased from 8.7
106 to 6 107 mutations per gray per locus (a factor of
about 14) between the high-dose-rate and low-dose-rate
exposure. In practice, the mutation rate at the low dose rate
(7.9 mGy min1) did not differ significantly from that incontrols, indicating very effective repair. Similarcalculations based on the results of irradiating mature andmaturing oocytes at the same dose rates [R19, R20] suggest
an approximately fourfold drop in the induced mutation
frequencyin the adult. These results suggested that females
irradiated just before birth have a more pronounced doserate effect, although the confidence limits of magnitude ofthe dose-rate effect are too wide for firm conclusions to be
drawn.

C. SUMMARY
186. The results of animal studies contribute to the
database of information available for assessing thebiological effects of low doses of ionizing radiation anddose-response relationship. Because of differences inradiosensitivity between animals and humans, the results
obtained from animals cannot be used directly to obtain
quantitative estimates of cancer risks for human populations. Animal studies are, however, valuable for determining the shape of dose-response relationships as well as
for examining the biological and physical factors that may
influence radiation responses. They are also of use for
examining how factors such as age at exposure, radiation
quality, and dose protraction or fractionation can influence
the tumour response. Laboratory animals have theadvantage that they are a homogeneous population withminimal biological variability, and the influence of confounding factors can be eliminated. Although studies withlaboratoryanimals generallyinvolve fewer individuals thanepidemiological studies, they have the advantage that they
are carried out under controlled conditions with good
estimates of the radiation dose and with a known spontaneous cancer rate. In the case of radiation-induced
hereditary disease animal studies provide the principal
source of quantitative information.

107

187. Dose-response relationships for many tumour types invarious animal models following exposure to both low- or
high-LET radiation can be reasonably well represented by alinear or linear-quadratic function for the dose ranges
analysed. In many cases, however, alternative fits to the data
are also possible. Other model fits include the possibility of athreshold dose below which tumours will not occur, as well as
more complex functions in which the time for the tumour toappear is much later at low dose rates than at high dose rates
and thus can also suggest the presence of a threshold for a
response. For some lung tumours it has been demonstrated
that high local doses from alpha irradiation are required tocause proliferation of initiated cells and to promote thedevelopment of malignant lesions.
188. Analysis of a series of studies in mice has shown thatthe lowest dose at which a statistically significant (p=0.05)
increase in tumour yield is observed varies from study to
study. It depends on the number of animals used in eachexperiment, the radiation sensitivity of the species to specificcancers, and the spontaneous cancer rate. It also depends on
the range of doses used. From the animal data reviewed, thelowest single (acute) dose to give a significant (p=0.05) effecton tumour yield is of the order of 100!200 mGy (low-LET).
The higher values obtained in other studies can be attributed
to lack of sensitivity, high control incidence, or to small
numbers of animals. Values for the lowest dose to give asignificant increase in risk following continuous (chronic)
irradiation are generally higher than those for acuteirradiation. It may be concluded that animal studies provide
quantitative information on risks of radiation-induced tumour
induction at low to intermediate doses but do not, andprobablycannot, provide direct information at doses much lessthan about 100 mGy.
189. For radiation-induced hereditary disease, the mostcomprehensive information comes from measurements ofspecific-locus mutations in mouse spermatogonia. Data froma number of laboratories have demonstrated a dose-response
relationship for low-dose exposures from low-LET radiationthat is well fitted by a linear response. The lowest dose tested
in these studies was 380 mGy. Data from both male andfemale mice indicate a significant effect of dose rate. It wasnotable that although the incidence of mutations in male micefell by a factor of about 3 for a reduction in dose rate from
800!900 mGy min1 to 8 mGy min1, a further reduction in
the dose rate to 0.007 mGy min1 failed to further reduce theyield of mutations. This independence of dose rate occurred
over a range of doses differing by rather more than a factor of
1,000, and it was concluded that it was unlikely that a further
reduction in mutation frequency would be obtained at evenlower dose rates. This suggests that a substantial fraction of
the damage to DNA that results in the induction of heritable
mutations is not amenable to effective repair.

108

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

III. EPIDEMIOLOGY
190. The extent to which epidemiological studies canprovide information on the effect of ionizing radiation on
the induction of cancer at low doses is considered in this
Chapter. Although the role of radiation in inducing cancer
was recognized soon after the discovery of x rays by
Röntgen in 1895, up to the early 1950s only high doses
causing acute tissue damage were considered to beimportant. This view was reflected in the early recommendations of the International Commission on RadiologicalProtection (ICRP) and by national organizations. By 1959,
however, the stated aim of ICRP in setting dose limits was
to “prevent or minimize somatic injuries and to minimize
the deterioration of the genetic constitution of the population” [I6]. These recommendations reflected an increasingawareness of the effect of radiation in inducing cancer,
particularly leukaemia, at low doses and was largely the
result of information becoming available from the followup of the survivors of the atomic bombings and groups
exposed for medical reasons (see, for example, [L29]).
191. By the early 1970s it was known that radiation iscapable of causing tumours in many tissues of the body,although the frequency of appearance following a unit dose
varied markedly from one organ or tissue to another.
Information on the dose-related frequencyof tumour induction by radiation had become available from a number of
epidemiological studies of persons exposed to externalradiation or internally incorporated radionuclides. In the
UNSCEAR 1972 Report [U8], the Committee gave preliminary estimates of the risk of leukaemia and some solidcancers based on the survivors of the atomic bombings andother groups exposed at high dose rates. It also pointed outthat animal studies suggested that risks per unit dose atlower dose rates could be lower and that risk estimates
based on groups exposed at high dose rates would be overestimates for doses and dose rates received fromenvironmental sources.
192. The chief sources of information on the risks ofradiation-induced cancer were the survivors of the atomic
bombings exposed to whole-body irradiation at Hiroshima
and Nagasaki; patients with ankylosing spondylitis andother patients who were exposed to partial body irradiation
therapeutically, either from external radiation or frominternally incorporated radionuclides; and various
occupationally exposed populations, in particular uranium
miners and radium dial painters. Follow-up of these
populations had shown that there is a minimum period oftime between irradiation and the appearance of a radiationinduced tumour, although this “latency period” varies withage and from one tumour type to another. Some types ofleukaemia and bone cancer have latency periods of only a
few years, with most of the risk being expressed within
about 25 years of exposure. Many tumours of solid tissues(e.g. liver or lung) have latency periods of 10 years or
more, and it was not clear whether their incidence passes
through a maximum and subsequently declines with time

following exposure or whether the risk levels out or evenincreases indefinitely during the remainder of life.
193. To project the overall cancer risk for an exposed
population, it is therefore necessary to use empiricalmodels that extrapolate over time data based on only alimited portion of the lives of the individuals. Two suchprojection models have been generally considered:
(a)

(b)

the additive (absolute) risk model, which postulates
that radiation will induce cancer independently of the
spontaneous rate after a period of latency and thatvariations in risk due to gender and age at exposure
may occur; andthe multiplicative (relative) risk model, in which theexcess cancer risk (after latency) is given by a constantfactor applied to the age-dependent incidence of natural
cancers in the population.

Both models imply an increasing risk of cancer withincreasing radiation dose. In addition to these two models,alternative fits to the epidemiological data to assess lifetime risks have also been used such as a model expressing
excess relative risk as a function of attained age [K27].
Further information is given in Annex I, “Epidemiologicalevaluation of radiation-induced cancer”.
194. Most organizations assessing risks in the 1970s,
including UNSCEAR in its 1977 Report [U7], adopted the
additive model for the assessment of cancer risks, although
the Committee on the Biological Effects of Ionizing Radiation (BEIR I) [C17] of the United States National Academyof Sciences used both models for risk assessment. In a
major revision of its recommendations in 1977, ICRP, in
its System of Dose Limitation, considered it necessary tolimit the incidence of radiation-induced fatal cancers andsevere hereditary disease to a level accepted by society [I8].
Implicit in this approach for stochastic effects was thenecessity to use quantified risks of radiation-induced disease in setting limits on exposure. The risks of cancer andhereditary disease adopted by ICRP were derived mainly
from reviews by UNSCEAR [U7]. Organ-specific risks
were given for the red bone marrow, the lungs, cells onbone surfaces, and the thyroid and breast. No specific risks
were given for the other organs and tissues of the body,which were pooled in a risk factor for all the “remainder”organs and tissues.
195. During the 1980s new information progressively
became available from the Life Span Study in Japan, andthis necessitated a revision of the earlier risk estimates by
UNSCEAR. The data available from the extended followup of the survivors of the atomic bombings indicated thata multiplicative risk model now gave a best fit to data formost solid cancers [U4]. These new risk estimates, which
also allowed for improvements in dosimetry, were taken
into account by ICRP in its 1990 recommendations [I2].

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

196. Overall, the lifetime risks calculated in recent years by
various national and international organizations are not too
different (e.g. [C1, I2, M18, U4]). The estimates of risk have,
however, been obtained by direct extrapolation fromepidemiological studies. They are, therefore, appropriate forpopulations exposed at high doses and dose rates. To allow fora reduced effect of radiation in inducing cancer when
exposures are at low doses or low dose rates, mostorganizations have recommended the use of a reduction factor
to obtain risks for application in radiation protection. ICRP
[I2] applied a reduction factor of 2 (it called the factor a doseand dose-rate effectiveness factor (DDREF)) to give a risk
coefficient for radiation protection purposes. In the
UNSCEAR 1993 Report [U3], the Committee reviewed
epidemiological and experimental data relevant to the choiceof a reduction factor. It recommended that for tumour
induction, the DDREF adopted should, to be on the safe side,“have a low value, probably no more than 3”. Insufficient data
were available to make recommendations for specific tissues.
197. Epidemiological studies were recently reviewed in the
UNSCEAR 1994 Report [U2], and further studies and results
are reviewed in an accompanying Annex I, “Epidemiologicalevaluation of radiation-induced cancer”. In this Chapter thestatistical difficulties associated with obtaining quantitative
estimates of the risk of radiation-induced cancer fromepidemiological studies at low doses are first examined. The
available data from groups exposed at high dose rates, fromwhich dose-response relationships and quantitative risk
estimates are generally obtained, and from groups exposed at
low doses and dose rates are then reviewed. Also consideredis the choice of an appropriate value of the reduction factor forassessing risks at low doses and doses rates from studies ofgroups exposed at high doses and high dose rates.
198. There are no human data so far that can be applied indetermining quantitative dose-response relationships or
risk estimates for hereditary disease. Risk factors forhereditary disease have been considered in previous
UNSCEAR reports [U3, U4].

109

the absence of an effect, the probability of its occurrence isless than 1 in 20. If a large number of disease outcomes(e.g. different cancer types) are examined, however,possibly for each of several age groups and time periods, it
is quite likely that a “statistically significant” finding willarise simply by chance. It is therefore important to examine
the results of any epidemiological study in the context ofpossible dose-response relationships, other epidemiologicalstudies, and supporting experimental evidence.
201. As with animal studies, the statistical power of an
epidemiological study to detect an excess risk associated withionizing radiation exposure depends on a number of factors.
In Annex I, “Epidemiological evaluation of radiation-induced
cancer”, a procedure is described for assessing the power ofa study to detect an elevated risk of a disease before a study isconducted. The statistical precision of completed studies isalso examined. An example illustrates how the power of acohort study to detect an elevated risk depends on the relative
sizes of both the exposed and control populations, theirabsolute numbers, and the total numbers of cancers. These, inturn, depend on the baseline cancer rates, the length of followup, the radiation dose, and the specific radiation sensitivity ofthe organ(s) or tissue(s). Thus a study based on a very large
cohort may not be particularly informative if a rare cancer isunder investigation and the follow-up is short. Conversely, a
study based on a fairly small cohort may be quite informative
if a common cancer is being investigated and the follow-up islong. The distribution of the population and the number ofcancer cases between various exposure dose groups will also
influence the ability of a study to define a dose-response
relationship. More detailed information on statisticalconsiderations is given in the above-mentioned Annex.
202. The limitations of statistical power and the possibility ofbias or confounding will constrain not only the ability todetect small increases in the risk of cancer but also thedetermination of whether or not there is the potential for adose threshold for radiation carcinogenesis in specific tissues.
Some examples of dose-response relationships obtained fromepidemiological studies and the ability to detect risks at low
dose are illustrated below.

A. STATISTICAL CONSIDERATIONS
199. Making quantitative estimates of the risk of cancer
associated with low doses of ionizing radiation is complicated. Small epidemiological studies often have insufficient
statistical power to detect any increase in risks. If bias has
arisen in a study through, for example, failure to follow up alarge percentage of a cohort of persons or to allow for confounding factors, then spurious positive or negative findings
could occur. In low-dose studies where the excess risks are
predicted to be small, it is particularlyimportant to ensure thatthe potential for bias and confounding is kept as low as
possible, as this can create spurious results.
200. It is not, at present, possible to distinguish cancers
induced by ionizing radiation from those due to othercauses. A particular result of an epidemiological study isnormally considered to be “statistically significant” if, in

B. HIGH-DOSE AND HIGH-DOSE-RATEEXPOSURES
203. The primary basis for evaluating risks of cancer
associated with radiation exposure is the epidemiologicalstudy of human health in populations that include groups
exposed at high doses and generally at high dose rates. The
main features of the major high-dose-rate epidemiologicalstudies were considered in the UNSCEAR 1994 Report [U2]and are reviewed in Annex I, “Epidemiological evaluation ofradiation-induced cancer”. The Life Span Study of thesurvivors of the atomic bombings at Hiroshima and Nagasaki
by the Radiation Effects Research Foundation (RERF) is ofparticular importance in risk estimation. As well as involving
a population of all ages and both sexes, the Life Span Studyis based on large numbers of persons with a wide range of110

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

whole-body doses. Consequently, it has high statistical power
to examine any variation in cancer risk with dose. Theinterpretation of the dose-response data is, however,complicated by the fact that exposure was to both gamma rays
and neutrons. An RBE of 10 has generally been assumed
when fitting the dose-response data.
204. Other high-dose, high-dose-rate epidemiological studies
are more limited in terms of the sex and age structure of the
exposed population or in terms of the organs irradiated.
However, they do provide additional information on risks forparticular organs or for exposures at particular ages.
205. As discussed in Annex I, “Epidemiological evaluationof radiation-induced cancer”, the statistical power of
epidemiological studies to assess risks depends on the rangeof doses received by the study population and the spontaneouscancer rate. Analyses based on a restricted set of data for
exposures in the low-dose region would have much reduced
statistical power to detect risks. However, it may still bepossible to detect raised risks in some circumstances.
Furthermore, analysis of the dose-response relationship over
the whole range of doses can be informative in making
inferences about risks at low doses, when interpreted inconjunction with the mechanistic and computationalmodelling approaches described in Chapters IV and V.
1. Dose-response relationships(a) Survivors of atomic bombings
206. Various analyses of the dose-response data for the Life
Span Study have been reported, and with increasing length offollow-up the quality of the information available has
improved considerably. Pierce and Vaeth [P1] examinedmortality data from the follow-up of the Life Span Study to
1985, based on the most recent published DS86 dosimetry. In
their analyses, those persons with shielded kerma estimates inexcess of 4 Gy were excluded, in view of an apparent
levelling-off in the dose response that may be associated witherrors in the estimates of such high doses or with cell killing.
The authors concluded that for all cancers other than
leukaemia the data could be well fitted by a linear doseresponse model, although a linear-quadratic model would not
be inconsistent with the data.
207. Shimizu et al. [S1] assessed the slopes of the doseresponse curves for the survivors of the atomic bombingsin various low-dose regions. Over the lowest dose range(0!0.49 Gy) with a statistically significant trend (p<0.05),
the value of the excess relative risk per gray for all cancers,
other than leukaemia, was 0.38. This is similar to the value
obtained for the whole dose range (0.41), in line with theanalysis by Pierce and Vaeth [P1], suggesting a linear
dose-response relationship.
208. For leukaemia mortality, the data up to 1985 on
survivors of the atomic bombings suggested that a linear
dose-response model did not provide a good fit and that a
linear-quadratic model would be preferred [P1]. In theanalysis of Shimizu et al. [S1], the excess relative risk pergray of leukaemia mortality in the dose region 0!0.49 Gy
was 2.40 (p<0.05), which is about half of the value over thewhole dose region (0!6 Gy) of 5.21 (p<0.001). This
supported the conclusions of Pierce and Vaeth [P1] that a
linear-quadratic dose-response model better fits the data.
209. Errors in the estimates of dose in the Life Span Study
can substantially alter the shape of dose-response relationships. The problem of random dosimetry errors for the RERF
data on the Life Span Study has been investigated by anumber of authors [G2, J3, P1, P7]. Pierce and Vaeth [P1]found that after adjustment for dosimetric errors there were
non-significant indications of upward curvature in the doseresponse function for mortality from all solid cancers, while
for leukaemia the evidence for curvilinearitybecame stronger.
210. The evidence for possible curvilinearity in the dose
response for leukaemia and for solid cancers in the mostrecent cancer incidence data [P3] has been examined [L7,
M19]. A variety of relative risk models have been fitted tothe data, including those that allow for a possible dosethreshold. Errors in estimates of doses were also allowed
for, as these can substantially alter the shape of the doseresponse relationship.
211. For solid cancers taken together, a variety of models
provided little evidence for curvilinearity. A significant
positive dose response was found for all survivors receiving
doses less than 0.5 Sv but not for doses less than 0.2 Sv(assuming an RBE of 10 for neutrons). A threshold-linear
relative risk model fitted to the data gives no support for a
threshold above about 0.2 Sv, and the data are consistent
with the absence of a threshold. For most solid cancers taken
separately, the data on cancer incidence are also consistent
with a linear dose-response relationship [T4] (also see
Annex I, “Epidemiological evaluation of radiation-induced
cancer”). These findings are in accord with previous analysesof the dose response for all solid cancers taken together.
212. In contrast, the latest data on non-melanoma skincancer incidence indicate substantial curvilinearity, consistent
with a possible dose threshold of about 1 Sv to the skin orwith a dose response in which the excess relative risk (ERR)
is proportional to the fourth power of dose, with a decrease in
the response at high doses (>3 Sv) [L30] (Figure XVII).
Supporting epidemiological data on the shape of the dose
response for non-melanoma skin cancer are, however, limited.
For example, Ron et al. [R15] found no evidence for curvilinearity in the dose response in a group of children in Israel
who had been treated with large therapeutic doses of radiation
for tinea capitis (ringworm of the scalp). However, the dosesin this study were generally much higher than those received
by survivors of the atomic bombings. Thus there are nopatients with doses less than 5 Gy (low-LET) in the Israeli
data set. The only other information on the shape of the dose
response for skin cancer comes from animal experiments.
Some evidence of a threshold has been obtained in studies
with mice and rats [A5, B23, O3, P8], although a linearexponential form of induction curve was obtained for betairradiated male SAS/4 mice [W8].

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES
15
INCIDENCE RATE (10 5 person a)-1

INCIDENCE RATE (10 5 person a)-1

100

111

80

60

40

20

0

10

5

0
0

2
4
6
8
SHIELDED KERMA (Sv)

10

0

0.1
0.2
0.3
0.4
0.5
SHIELDED KERMA (Sv)

0.6

Figure XVII. Observed incidence of non-melanoma skin cancer in survivors of atomic bombings (CI: 90%)
compared with fourth-power exponential (solid line) and linear-exponential threshold (dotted line)
models of dose response [L30].
The diagram on the right shows the low-dose region in detail.

213. In contrast with solid cancers, the analysis by RERF
and other groups of the dose-response relationship forleukaemia incidence in the Life Span Study cohort foundquite a marked upward quadratic component, i.e. significant upward curvature [P2, P3], with the evidence fornon-linearity being strongest for acute myeloid leukaemia[P3]. For the three main radiation-inducible leukaemia
subtypes analysed together (acute lymphatic leukaemia,
acute myeloid leukaemia, and chronic myeloid leukaemia),
there is a significant increase in the risk of leukaemia if
the dose responses for all survivors with doses less than
0.5 Sv are considered together [L7, M19]. Thissignificance vanishes, however, if doses less than 0.2 Sv
are considered.

215. Recent analyses by Kellerer and Nekolla [K25] andLittle and Muirhead [L52] of the tumour incidence andmortality data demonstrate that if account is taken of
possible systematic errors in the Hiroshima DS86 neutron
dose estimates, then there is evidence of appreciable
upward curvature in the dose response for solid tumours in
the Life Span Study data. This is particularly marked ifanalysis is restricted to the 0!2 Gy dose range rather thanthe 0!4 Gy dose range that has been used for most analysesof dose response in the Life Span Study. Over the 0!2 Gy
dose range, the low-dose extrapolation factor (LDEF) forall solid tumour incidence is 1.43 (95% CI: 0.97!2.72),
and so is comparable with the LDEF for leukaemia
incidence, 1.58 (95% CI: 0.90!10.58) [L52].

214. Analysis of leukaemia incidence among the Japaneseatomic bomb survivors by Little and Muirhead [L7]
showed that incorporation of a threshold in the linearquadratic dose response yielded an improvement in fit atborderline levels of statistical significance [best estimate ofthreshold for a linear-quadratic-threshold model was
0.12 Sv (95% CI: 0.01!0.28; two-sided p=0.04)]. This
analysis takes account of random dosimetric errors, but notpossible systematic errors in dose estimates. The fits of alinear-quadratic-threshold dose response to the recently
released leukaemia mortality data [P2] and that takes
account of random dosimetric errors, demonstrated that thethreshold was not significantly different from zero [bestestimate of threshold for a linear-quadratic-thresh-oldmodel was 0.09 Sv (95% CI: <0.00!0.29; two-sided
p=0.16)] [L44]. Similar findings have been reported by
Hoel and Li [H26] in analyses that do not take account of
dosimetric error. Comparison of the incidence andmortality data by Little and Muirhead [L44] and Little[L49] demonstrates the essential similarity of theleukaemia incidence and mortality data. Little andMuirhead [L44] concluded that the most probable reason
for the difference between the findings in the incidence andmortality data sets was the finer subdivision of dose groups
in the mortality data set. (There are 14 dose groups in themortality data sets in their publicly available form,
compared with 10 dose groups in the incidence data sets.)

216. Recent data on the mortality of the atomic bomb
survivors was reported by Pierce et al. [P2]. The follow-up
covers the period to 1990 and includes an extra 10,500
survivors for whom DS86 dose estimates have been
calculated. The total cohort comprises approximately
86,500 persons, 60% of whom received doses in excess of
5 mSv. Of the total population, 44% had died by 1990,
including 8,827 who died of cancer. The shape of the doseresponse curve for all solid cancers is essentially linear up
to 3 Sv, beyond which there is an apparent decrease in risk.
This may be attributed both to cell killing and toimprecision in the estimates of high doses (Figure XVIII).
217. As discussed in Annex I, “Epidemiologicalevaluation of radiation-induced cancer”, the dose-response
relationships for mortalityfrom manyspecific tumour types(stomach, colon, lung) are consistent with a linear response
although generally based upon the analysis of a restricted
number of cases. For leukaemia, the dose response over therange 0!3 Sv can be fitted with a linear-quadratic doseresponse relationship (Figure XVIII).
218. While the Life Span Study provides information on
cancer risks in a number of tissues, there are others forwhich there is either very little or no evidence for an effect.
These include, for example, the bone, cervix, prostate,
testes and rectum.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSESEXCESS RELATIVE RISK

1.2

EXCESS ABSOLUTE RISK PER PERSON

112(a) Solid cancers

1.0
0.8
0.6
0.4
0.2
0
0

1
2
3
4
WEIGHTED COLON DOSE (Sv)

5

0.06

(b) Leukaemia

0.05
0.04
0.03
0.02
0.01
0
0
1
2
3
4
5
WEIGHTED BONE MARROW DOSE (Sv)

Figure XVIII. Dose response for mortality from solid cancer (males of 30 years of age at exposure)and leukaemia in survivors of atomic bombings in Japan [P2].

(b) Other groups exposed to low-LET radiation
219. Additional data on dose-response relationships forgroups exposed to low-LET radiation are available from anumber of other studies. For Canadian tuberculosis patients
given fluoroscopies, Miller et al. [M2] showed that a linear
dose-response relationship gave a good fit to the data on
breast cancer among patients in Canadian provinces otherthan Nova Scotia. For patients from Nova Scotia, who
generallyreceived higher doses, thedose-response relationshipwas also consistent with linearity, but it had a steeper slope
than for other Canadian provinces. Howe and McLaughlin[H31] have given further results from an extended follow-up
of this population. The data on breast cancer mortality could
again be fitted with a linear dose-response relationship. As
before, the slope of the dose trend was greater for patients in
Nova Scotia than for patients in other provinces.
220. Dose-response analyses have also been performed forsome other groups with medical exposures. Boice et al. [B6]studied the relationship between the risk of breast cancer anddose for women in Massachusetts (United States) given
multiple chest x-ray fluoroscopies. For this study, doses were
mostly in the range 0!3 Gy. A linear dose-response model
was found to provide as good a fit to these data as a linearquadratic model, whereas a purely quadratic model did not fit
well. Among women given radiotherapy for cervical cancer,
the risk of leukaemia increased with dose up to 4 Gy, in amanner consistent with linearity, although the data were also
consistent with a quadratic dose response; beyond 4 Gy therisk decreased, probably as a result of cell killing [B7]. At
lower doses, Ron et al. [R8] found that the risk of thyroid
cancer among children in Israel irradiated for tinea capitis wasconsistent with a linear dose-response relationship, based on
doses that were mostly less than 0.15 Gy.
(c) Groups exposed to high-LET radiation
221. Information on dose-response relationships that departfrom the conventional linear or linear-quadratic response has
been obtained for bone tumours arising from alpha particle
irradiation of bone following the deposition of isotopes ofradium. Extensive epidemiological information is available ongroups of persons exposed, principally by ingestion, to 226Ra
and 228Ra in the 1920s and 1930s. The most comprehensive
data relate to female radium dial painters. The data on tumour
induction in this population have been the subject of extensive
analysis over the last 40 years (e.g. [E1, F13, H21, R4]). Afterthe radium programme at the Argonne National Laboratory
finished in the early 1990s, Rowland brought together all thedata collected in this long-term study [R16]. His most recent
analysis considered all female radium dial painters with bodycontent measurements and who had entered the study prior to1950, a total of 1,530 women [R17]. In this cohort, 46 women
had bone sarcomas and 19 had head sinus carcinomas; 3
women had both a bone sarcoma and a head sinus carcinoma.
The analysis incorporated revised estimates of systemic intake,which took into account the magnitude of the original intake.
This has been shown to influence the retention kinetics andhence the cumulative doses [K15, R7]. The intakes by thevarious members of the cohort covered several orders ofmagnitude. The 46 bone sarcomas had appearance times
ranging from 7 to 63 years. The lowest systemic intake associated with a bone sarcoma was 3.7 MBq (100 µCi). Thismalignancy, diagnosed in 1981 and resulting from an intake
in 1918, was thus detected 63 years later.
222. Various forms of a general incidence!systemic intake
expression(3)
I  (αSI  βSI 2) e γSI
were fitted to the data and tested with a χ2 statistic. In theequation, I is the incidence of bone tumours, α, β, and γ areconstants, and SI is the systemic intake. No acceptable fit
to the equation was found. However, when a constant, C,
was included in a general function of the form
I  C  kSI β e γSI(4)

in which k and β are constants, a good fit to the data could
be obtained with C = !1.44 104, k = 2.14 1015, β = 3.15,
and γ = 7.06 105. With the incidence I equal to zero, thisgives an intercept at 2,920 kBq. This fit to the data, shownANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

in Figure XIX, gives good evidence for a threshold dose forthe induction of bone tumours. The dose-response data
were further analysed by Thomas [T12], who suggested thedata were consistent with a threshold for tumour inductionin the range 3.96.2 Gy high-LET (average bone dose). He
proposed a rounded value of 10 Gy (average bone dose) asa “practical threshold” below which there should be little
cause for concern.
1339 cases
No sarcomas

191 cases
46 sarcomas

40,00030,00020,00010,000tumours. In a follow-up study, a similar spectrum of tumours
was obtained in persons occupationally exposed to 226/228Ra,
patients given external irradiation and other so-called secondary bone tumours arising at sites of pre-existing bone lesions
as had been obtained in the 224Ra patients [G17].
226. The authors of the review [G17] concluded that disturbance of the local cellular system caused by deterministic radiation damage and repair resulted in the unexpectedly highproportion of fibrosarcoma-MFHs. It was considered that thedevelopment of the tumours reflected the cell types involvedin a disturbed remodelling process in the skeleton. The
reactive proliferation of the predominantly fibroblastic tissue
at the site of tissue damage could be the presumptive origin of
this special type of radiation-induced bone sarcoma. As afibrotic response would be likely to arise as a consequence ofdeterministic radiation damage, the fibrosarcoma-MFH typeof tumour might well arise only at doses above a limiting
threshold.
(d) Groups exposed to radon

0
10

100

1,000
10,000SYSTEMIC INTAKE (kBq)

100,000

Figure XIX. Bone sarcoma incidence in female radium
dial painters [R17].
Systemic intake is kBq 226Ra plus 2.5 × 228Ra activities.

223. Various forms of the general dose-response expression
were also fitted to the data on head sinus carcinoma. In contrast to the data on bone sarcomas, linear, linear-exponential,
and dose-squared-exponential functions all provided acceptable fits. Models that included a threshold would also fit thedata, but the threshold value was not statistically significant.
224. It was concluded that the tumour induction data forosteosarcoma induction show a very steep dose response[R16]. Whether this actually demonstrated a threshold or
simply showed a very low probability of osteosarcoma
induction at intakes below about 3,000 kBq could not be
determined. For head sinus carcinoma, the data did notsuggest the presence of a threshold, although various model
fits to the data were possible, reflecting the paucity of data,
which prevented discrimination between alternative functions.
225. Further information on bone tumour pathology inpersons exposed to external radiation or internally incorporated radionuclides may explain some of these observations. A
review of bone tumour pathologyin patients treated with 224Ra
revealed an unexpectedly high proportion of bone sarcomas of
the fibrous connective tissue type, including the first case of
malignant fibrous histiocytoma (MFH) of bone described after
internal irradiation [G17]. Out of 46 bone tumours in the 224Ra
patients, osteosarcoma was the most common histological type
(48% of cases), but 30% of these were fibrosarcoma-MFHs
and the remainder were chondrosarcomas, malignant lymphomas, myelomas, and malignant chordomas. The 30% of
fibrosarcoma-MFHs substantially exceeds the usual prevalence of this disease, which is 8%11% in spontaneous bone

227. Radon has been extensively studied as a humancarcinogen. Epidemiological studies are reviewed in Annex I,“Epidemiological evaluation of radiation-induced cancer”
and are summarized here only briefly. The results of a seriesof cohort studies of miners in countries throughout the world
have provided the basis for estimates of the risk of lung cancer
associated with exposure to radon and its decay products.
These data, although subject to some uncertainties, haveallowed characterization of exposure response relationships[L51]. The exposure response relationship in the various
studies of radon exposed miners is consistent with linearity,
but the slope appears to be higher at lower exposure rates(Figure XX). As discussed in Annex I, this apparent inverse
exposure-rate effect does not imply that low exposures carrya greater risk than higher exposures; rather it suggests that fora given total exposure, the risk is higher if the exposure isreceived over a long rather than a shorter period of time. This
3.0

RR
RR

( x 1 0 0 )( x 5 0 )= 1 + 0.008 x WLM= 1 + 0.012 x WLM

2.5

RELATIVE RISKSARCOMA INCIDENCE (105 person a)-1

50,000

113

2.0

1.5

1.0

RR = 1

0.5

0
0

10

20

30

40

50

60

70

80

90

100

WORKING LEVEL MONTHS

Figure XX. Relative risks of lung cancer from pooleddata for miners, restricted to <100 WLM exposure andalso to <50 WLM exposures [L53].

114

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

could reflect some cell killing at high exposure rates. Casecontrol studies of residential radon exposure and lung cancer
have also been conducted in various countries. Although thesehave also been informative, the generally lower exposures ofpeople and methodological difficulties have meant the powerof these studies is less than that of the occupational studies.
However, the estimates of lung cancer risk based upon arecent meta-analysis of these eight studies are in close agreement with the risk predicted on the basis of miner data [L53].
228. The Committee on Biological Effects of Ionizing
Radiations (BEIR VI) considered the data on 11 minercohorts exposed to radon previously analysed by Lubin et al.
[L48]. The most recent data available were used indeveloping the Committee*s risk models for radon exposure[C26]. The Committee recognized that care is needed in
combining data from different cohorts of underground miners
around the world. The levels of exposure to radon and otherrelevant covariates, such as arsenic and tobacco smoke,
differed appreciably among groups of miners. The
completeness and quality of the data available on relevant
exposures also differed notably among the cohorts.
Information on tobacco consumption was available for only 6of the 11 cohorts; of these 6, only 3 had information on
duration and intensity of exposure to tobacco smoke. Lifestyleand genetic factors that influence susceptibility to cancer
might also account for heterogeneity among cohorts.
229. Despite those differences, the Committee concluded thatthe best possible estimate of lung cancer risk associated with
exposure to radon and its decay products would be obtained by
combining, in a judicious manner, the available informationfrom all 11 cohorts. The Committee used statistical methods
for combining data that both allowed for heterogeneity amongcohorts and provided an overall summary estimate of the lungcancer risk. Confidence limits for the overall estimate of risk
allowed for such heterogeneity.
230. The Committee*s risk models described the excess
relative risk as a simple linear function of cumulative
exposure to radon, allowing for differential effects of exposure
during the periods 5!14 years, 15!24 years, and 25 years ormore before death from lung cancer. The most weight wasgiven to exposures occurring 5!14 years before death fromlung cancer. The Committee examined two types of risk
models in which the excess relative risk was modified either
by attained age and duration of exposure or by attained age
and exposure rate. The excess relative risk decreased withboth attained age and exposure rate and increased withduration of exposure. For cumulative exposures below
0.175 J h m3 (50 WLM), a constant-relative-risk model
without these modifying factors appeared to fit the data as wellas the two models that allow for effect modification.
2. Minimum doses for a detectable increasein cancer risk
231. It is important to examine the lowest levels of dose atwhich a significantly elevated level of radiation-induced
cancer has been observed in human populations. Relevant

information from epidemiological studies is available fromthe follow-up of the atomic bomb survivors, from otherstudies of thyroid cancer in infants, children, and adults,
and from studies of the risk of cancer in children following
radiation exposure in utero.
(a) Survivors of atomic bombings
232. The analysis by Pierce et al. [P2] of the atomic bomb
survivor mortalitydata set finds a statisticallysignificant (twosided p<0.05) trend in mortality risks in the 0!50 mSv range
for all solid cancers combined, based upon follow-up to 1990
(assuming an RBE for neutrons of 10). This finding is basedon the fitting of a linear relative risk model to the 0!50 mSv
data, but using fixed adjustments for sex and age at exposure
based on fits of a model to the full data set. Pierce et al. [P2]pointed out that without these adjustments for sex and age, thesignificance of the trend in dose in the 0!50 mSv groupwould be lost.
233. As discussed by Little [L11], this procedure is statistically problematic. Little [L11] and Pierce et al. [P11] proposed modified forms of the one-degree-of-freedom test fortrend in the low-dose region, using nested models thatincorporate sex and age adjustments but that do not rely on
fixed modifications fitted to the whole dose range. Wheneither of these modified tests is used, the finding of asignificant increasing (two-sided p<0.05) trend with dose in
the low-dose region (0!50 mSv) remains valid [P11].
234. Notwithstanding these statistical considerations,
Pierce et al. [P2, P11] were cautious in their interpretationof this finding, which is at variance with the findings in
the latest atomic bomb survivor solid tumour incidence
data in which a significant excess risk of solid cancers isonly seen down to doses of 200!500 mSv [T4]; they
indicated that the finding in the 0!50 mSv group might beartefactual, resulting from the differential misclassificationof cause of death in the lowest dose groups. Further
information on recent data from the atomic bomb survivors
is given in Annex I, “Epidemiological evaluation ofradiation-induced cancer”.
(b) Thyroid cancer incidence
235. Information on the risks of radiation-induced thyroid
cancer is described in Annex I, “Epidemiological evaluationof radiation-induced cancer”. Studies of thyroid cancer
incidence following radiation exposure were reviewed by
Shore [S6], and a combined analysis of seven studies was
performed by Ron et al. [R9]. Among various cohorts withexternal low-LET exposures, the excess relative risk per graytends to be higher for thyroid cancer than for most other solidcancers. Furthermore, the excess relative risk is higher for
those irradiated at young ages than for adults. Studies ofcohorts with low-dose, external irradiation of the thyroid inchildhood are therefore of value for examining risks at low
doses. The risks of thyroid cancer following exposure to 131I
are less well understood, as discussed in Annex I,“Epidemiological evaluation of radiation-induced cancer”.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

236. A study of about 10,800 children in Israel given x-ray
treatment for tinea capitis was reported by Ron et al. [R8].
The total dose was given in five daily fractions to fivetreatment fields on the scalp. While the dose to the scalp
was of the order of several gray, the average total thyroid
dose was calculated to be only about 100 mGy. An analysis
over the range 0!0.5 Gy showed a statistically significant
trend of increasing risk of thyroid cancer with increasing
thyroid dose. In addition, the trend in relative risk per unit
dose was greater for those irradiated at ages under five
years than for those irradiated at older ages, in line withthe general observation of an increasing relative risk withdecreasing age at exposure, as well as being consistent witha study of thyroid cancer in young persons irradiated forenlarged tonsils [P4]. Among the tinea capitis patients lessthan five years old at exposure, the relative risk at about
0.1 Gy (100 mGy) was approximately 5 and wassignificantly greater than 1. This finding was, however,based on a fairly small number of cases, although it arose
among those persons for whom the risk would be predicted
to be greatest.
237. Thyroid cancer in a cohort of 2,657 infants in New
York State given x-ray treatment for a purported enlarged
thymus gland and followed for an average of 37 years has
been reported by Shore et al. [S2]. Estimated thyroid doses
ranged from 0.03 to more than 10 Gy, with 62% receiving
less than 0.5 Gy. The dose-response relationship for thyroid
cancer was fitted by a linear dose-response relationship, withno evidence of a quadratic dose component. An analysis
restricted to the range 0!0.3 Gy showed a statisticallysignificant trend with dose (p=0.002), although based on justfour thyroid cancer cases with non-zero doses. The estimate ofabsolute excess risk per unit dose over this dose range wassimilar to that from the Israeli tinea capitis study [R8].
238. Ron et al. [R9] conducted a combined analysis of datafrom seven studies of thyroid cancer after exposure to externalradiation. The range of doses varied considerably between the
different studies. For exposure before age 15 years, linearity
was considered to best describe the dose response, even down
to 0.1 Gy. The estimated excess relative risk per gray of 7.7(95% CI: 2.1!28.7) is one of the highest values found for any
organ.

115

239. Thyroid cancer in a cohort of 4,404 children of whom
2,827 were given x-ray treatment for cancer in childhood andfollowed for an average of 15 years has been reported by de
Vathaire [D14]. Estimated thyroid doses ranged from 0.001
to 75 Gy, with 41% receiving less than 0.5 Gy. The doseresponse relationship for thyroid cancer was best fitted by alinear dose-response relationship, with no evidence for aquadratic component. A standardized incidence ratio of 35(90% CI: 10!87, p<0.01) was found to be associated with adose of 0.5 Gy to the thyroid.
(c) Exposures in utero240. A number of studies have been published that haveexamined the risks of cancer in childhood following
exposures in utero. These studies have particularadvantages for detecting risks of cancer at low doses
because of the low spontaneous cancer rate in childhood.
241. Information on cancer risks following radiation
exposure in utero are available from studies of those withprenatal diagnostic x-ray exposures, as well as those
irradiated as a consequence of the atomic bombings ofHiroshima and Nagasaki. The largest study of childhood
cancer following prenatal x-ray exposure is the Oxford
Survey of Childhood Cancers (OSCC), which is a national
case-control studyof childhood cancer mortalitycarried out
in the United Kingdom. Information is also available fromother studies of prenatal x-ray exposure that have been
carried out in North America and elsewhere [B28].
242. The Oxford Survey of Childhood Cancers was started in
the mid-1950s. Up to 1981, mothers of 15,276 cases and thesame number of matched controls had been interviewed[K10]. During the late 1950s, the study investigators reported
a doubling in the risk of childhood cancer associated withprenatal x-ray exposure [S7]. Later analysis covering a longer
period indicated a falling risk with time and an average raisedrisk of about 40% (95% CI: 31!50) [B8, K10].
243. Data on doses to the embryo and fetus are available
for the Oxford Survey of Childhood Cancers, although
there is some uncertainty in these values. Table 11 shows
the mean number of films per x-ray examination in the

Table 11
Doses from prenatal x rays in the Oxford Study of Childhood Cancers

Birth year

1943-1949
1950-1954
1955-1959
1960-1965

Mean numberof films
per examination
1.9
2.2
1.9
1.5

Survey according to calendar period, together with estimatesof the average dose per film made by the Committee in the
UNSCEAR 1972 Report [U8] and by Stewart and Kneale

Mean dose per film according to reference (mGy)
[U8]

[S21]

18
10
5
2

4.6
4.0
2.5
2.0

[S21]. UNSCEAR estimated that the average dose per
examination was 10!20 mGy (low-LET) during the 1950sand decreased over time. Stewart and Kneale’s dose estimates

116

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

were about half of the UNSCEAR values. Based on the
UNSCEAR dose estimates, Muirhead and Kneale [M8]estimated the absolute radiation-induced risk for the incidenceof all cancers up to age 15 years to be about 0.06 Gy1 (lowLET) (95% CI: 0.04!0.10). A similar risk estimate wascalculated by Mole [M6] based on a national survey in the
United Kingdom of doses from obstetric radiography
performed in 1958, for which the average dose was about
6 mGy.
244. There has been concern that owing to the retrospective
nature of the Oxford Survey of Childhood Cancers, which
relied at least partially on mothers’ memories, some bias may
have been introduced. The results of the follow-up were
supported by a study in the United States [M9, M10] ofcontemporary records of x-ray exposures of children born in
hospitals in the north-eastern United States. In an initial study[M9] of 734,243 children born between 1947 and 1954, 556
children were identified as having died from cancer between
1947 and 1960. Prenatal x-ray exposure was associated withan increased risk of cancer, with relative risks for leukaemia
of 1.58 and for solid cancers of 1.45. These increases werevery similar to those in the Oxford Survey. In an extension of
the study, however, with a further 695,157 children born up
to 1960 and having 786 additional cases of cancer, the relativerisk for leukaemia was similar to the first phase (1.48) but thatfor solid cancers was appreciably lower (1.06). The overallvalues of relative risks for leukaemia (1.52) and solid cancers(1.27) do not differ significantly when compared directly(p=0.4).
245. Bithell [B28] reviewed a number of studies thatexamined the risk of childhood cancer following in utero
radiation exposure. None of the studies on their own had
the statistical power of the Oxford Survey of Childhood
Cancers, but a total of 12 studies, when taken together,gave a weighted average of an increase in relative risk of
1.37 (95% CI: 1.26!1.49). Including the Oxford Survey ofChildhood Cancers data gave a relative risk of 1.39 (CI:
1.33!1.45). While the individual study designs andmethods of analysis were very different, the overall finding
lends support to the results of Childhood Cancers.
246. Doll and Wakeford [D3] reviewed the evidence fromepidemiological studies on the risk of cancer in childhoodfrom exposure of the fetus in utero from diagnostic radiology.
They also considered the limited studies in experimental
animals. They concluded that while information is available
from a number of epidemiological studies, the most significant
comes from the Oxford Survey. It was concluded that there isstrong evidence for a causal relationship, with radiation doses
to the fetus of the order of 10!20 mGy giving increases in therisk of childhood leukaemia and solid cancers of about 40%.
Because of the low risk of cancer in childhood, the calculated
absolute risk coefficient was approximately 6% Gy1. The
analysis supports the view that small doses of radiation arepotentially carcinogenic. The possibility still exists that there
may be some as yet unidentified confounding factor in the
Oxford Survey affecting both the probability of the fetus being
irradiated in utero and the risk of subsequent cancer. A featureof the data from the Oxford Survey that remains unexplainedis that the increase in risk for both leukaemia and solidcancers following exposure in utero is essentially the same,with a relative risk of about 1.4. Most other human andanimal studies consistently indicate different sensitivities ofleukaemia and solid cancers [B45].
247. Several cohort studies of in utero exposures have notshown evidence of excess risk. Those studies, however, were
small in size. Among those exposed to atomic bomb radiationin utero [J1], no childhood leukaemia cases have been
observed. For 1,263 children irradiated in utero and followedfrom birth, two cases of cancer arose up to 15 years of age,compared with 0.73 expected from Japanese national rates[Y2]. The resulting upper limit on the 95% confidence
interval for the absolute radiation-induced risk is 2.8 102 Gy1(low-LET). Continued follow-up showed an excess of adultcancers among those exposed to atomic bomb radiation inutero. Based on the follow-up to 1988, the relative risk at
1 Gy was estimated to be 3.77 [Y2], which is similar to thatseen among survivors of the atomic bombings irradiated in thefirst 10 years of life [S1]. Further follow-up to the end of 1989
suggested a subsequent decrease in the relative risk [Y1], inline with the pattern indicated by the earlier follow-up of those
exposed post-natally at ages under 10 years.
248. More recently, Delongchamp et al. [D2] reported
cancer mortality data in atomic bomb survivors exposed inutero for the period October 1950 to May 1992. Only 10
cancer deaths were reported among persons exposed inutero. Although there were only two leukaemia deaths, thiswas higher than in a control group (p=0.054). Mortality
from solid cancers at ages over 16 years was in excess of
expected (ERR = 2.4 Sv1; 90% CI: 0.3!6.7); all the deaths
occurred in females.
249. Thus, although there is some consistencyin case-control
studies in showing a raised risk of childhood cancer, theabsence of clear confirmation in cohort studies leaves someuncertainty in establishing a risk estimate. For the Oxford
Survey of Childhood Cancers, however, an increase inchildhood cancer risk by about 40% is associated with dosesof about 10!20 mGy (low-LET). A number of other studies,taken together, support this finding.
3. Effect of dose and dose rate
250. As explained above, quantitative information on the riskof cancer in human populations comes largelyfrom epidemiological studies of population groups exposed at intermediateand high doses and dose rates. For the assessment of the riskof cancer from environmental and occupational exposure toradiation, a reduction factor, frequently termed a dose anddose-rate effectiveness factor (DDREF), has normally beenused to assess risks at low doses and low dose rates. The
choice of reduction factors was reviewed most recently in the
UNSCEAR 1993 Report [U3] and has also been reviewed by
ICRP [I2] and by a number of other international bodies. In
the UNSCEAR 1993 Report [U3], the Committee examined
cellular studies, data from experimental animal studies, andANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

117

information from epidemiological studies that would allow
judgements to be made on an appropriate reduction factor.
The judgements made in that report remain valid and aresummarized here only briefly.

information from human populations exposed at low dose
rates suggests risk coefficients that are not very different fromthose obtained for the atomic bomb survivors, although therisk estimates have wide confidence intervals.

251. The dose-response information on cancer induction in
the survivors of the atomic bombings in Japan provided, forsolid tumours, no clear evidence for a reduction factor muchin excess of 1 for low-LET radiation. For leukaemia, the dose
response fits a linear-quadratic relationship, and a best
estimate of the reduction factor is about 2. Analyses by Little
and Muirhead [L52] of the latest cancer incidence data thattake account of possible random errors and possible systematic
errors in DS86 dose estimates show that there is little
indication of upward curvature in the dose response for solid
tumours over the 0!4 Gy dose range, although over the
0!2 Gy dose range and after adjustment of Hiroshima DS86
neutron dose estimates the upward curvature is more
pronounced. There is marked upward curvature in the dose
response for leukaemia over the 0!4 Gy dose range, whichbecomes less pronounced if attention is restricted to thosereceiving less than 2 Gy [L52]. If adjustments are made to theHiroshima DS86 neutron dose estimates, then over the
0!2 Gy dose range the LDEF for all solid tumours is 1.43
(95% CI: 0.97!2.72), and so is comparable with the LDEF forleukaemia, 1.58 (95% CI: 0.90!10.58) [L52]. There is only
limited support for the use of a reduction factor from otherepidemiological studies of groups exposed at high dose rates,
although for both thyroid cancer and female breast cancersome data suggest a value of about 3 may be appropriate.

254. In the UNSCEAR 1993 Report [U3], the Committee
concluded that, when taken together, the available
epidemiological and experimental data suggested that fortumour induction, the reduction factor adopted should, to beon the safe side, have a low value, probably no more than 3.
Insufficient data were available to make recommendations for
specific tissues [U3]. For high-LET radiation, a reduction
factor of 1 was indicated on the basis of experimental data thatsuggested little effect of dose rate or dose fractionation on
tumour response at low to intermediate doses. It was notedthat a value of somewhat less than 1 is suggested by somestudies, but the results are equivocal, and cell killing may be
a factor in the tissue response [U3].

252. The results of studies in experimental animals conducted over a dose range that was similar to, althoughgenerally somewhat higher than, the dose range to which thesurvivors of the atomic bombings in Japan were exposed, andat dose rates that varied by factors between about 100 and
1,000 or more, give reduction factors from about 1 to 10 or
more, with a central value of about 4. Some of the tumour
types for which information is available have a humancounterpart (e.g. myeloid leukaemia and tumours of the breastand lung) while others do not (e.g. Harderian gland in themouse) or require for their development substantial cell killing
and/or changes in hormonal status (ovarian tumour, thymic
lymphoma). Similar results to those obtained with animal
tumour models have been obtained for somatic mutations andfor transformation of cells in culture, although the reduction
factors obtained have not been as large. In a number of theexperimental studies on tumour induction, linear functions
would give a good fit to both the high- and low-dose-rate datain the range from low to intermediate doses. This indicatesthat even if the cellular response can, in principle, be fitted bya linear-quadratic dose response, in practice it is not always
possible to resolve a common linear term for exposures atdifferent dose rates.
253. If human response is similar to that in experimental
animals, then it can be envisaged that at lower dose rates thanwere experienced in Hiroshima and Nagasaki, a reduction
factor greater than the value of about 1.5 that is suggested byanalysis of the dose-response data could be obtained. However,255. In the case of hereditary disease, the adoption of areduction factor of 3 was supported by experimental data
in male mice, although a somewhat higher value has beenfound with one study of female mice.

C. LOW-DOSE-RATE EXPOSURES
256. Information from studies of groups exposed to lowdose rates is potentially of more direct relevance to risk
estimates. However, studies of low-dose-rate exposure
generally involve low doses and, because of the probably
low excess risks, are likely to be hampered by a lack ofstatistical power and possibly also by confounding factors.
Examination of the results of low-dose-rate studies can,however, provide a check on the risks derived byextrapolation from high-dose-rate studies.
1. Occupational exposures257. Several studies have been conducted of nuclear industry
workers. In the United States, Gilbert et al. [G3] performed ajoint analysis of data for about 36,000 workers at the Hanford,
Oak Ridge, and Rocky Flats weapons plants. Neither for thegrouping “all cancers” nor for leukaemia was there any
indication of an increasing trend in risk with dose. However,the upper limit of the 90% confidence interval for the excess
relative risk per unit dose was several times greater than thecorresponding value for the survivors of the atomic bombingsin Japan in the case of all cancers other than leukaemia andslightly greater than the value from Japan in the case ofleukaemia.
258. The first analysis of the National Registry for Radiation
Workers (NRRW) in the United Kingdom examined cancer
mortality in relation to dose in a cohort of over 95,000
workers [K3]. The mean lifetime dose received was 33.6 mSv;
however, over 8,000 workers had a lifetime dose in excess of
100 mSv. For all malignant neoplasms, the trend in therelative risk with dose was positive but was not statisticallysignificant (p=0.10). Based on a relative risk projection model,

118

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

the central estimate of the lifetime risk based on these data
was 10% Sv1, which is 2½ times the value of 4% Sv1 citedby ICRP [I2] for risks associated with the exposure of workers(based on applying a DDREF of 2 to the Japanese data). The
90% confidence interval for the NRRW-derived risk ranged
from a negative value up to about six times the ICRP value.
For leukaemia (excluding chronic lymphatic leukaemia, which
does not appear to be radiation-inducible), the trend in risk
with dose was statistically significant (p=0.03). Based on a
projection model as used by BEIR V [C1], the central estimateof the corresponding lifetime leukaemia risk was 0.76% Sv1,
which is 1.9 times the ICRP [I2] value for a working
population (0.4% Sv1), with 90% confidence limits ranging
from just above zero up to about six times the ICRP value.
259. A second analysis of the NRRW cohort was publishedin 1999 [M47] and covered a total of 124,743 workers. For
leukaemia, excluding chronic lymphatic leukaemia, there wasa marginally significant increasing risk with dose. The central
estimate of excess relative risk per sievert, 2.55 (90% CI:!0.03!7.16), is similar to that estimated for the Japaneseatomic bomb survivors at low doses (2.15 Sv1, 90% CI :0.43!4.68); the corresponding 90% confidence limits weretighter than in the first analysis, ranging from just under fourtimes the risk estimated at low doses from the Japanese atomic
bomb survivors to about zero. For all malignancies other than
leukaemia, the central estimate of the trend with dose,
0.09 Sv1 (90% CI: 0.28!0.52), was closer to zero than in thefirst analysis and smaller than the Japanese atomic bomb
estimate of 0.24 Sv1 (90% CI: 0.12!0.37) (without theincorporation of a dose-rate reduction factor). Also, the 90%
confidence intervals were tighter than before and include zero.
Overall, the second NRRW analysis provides stronger
evidence than the first on occupational radiation exposure andcancer mortality; the 90% confidence interval for the risk perunit dose now excludes values that are more than four times
those seen in the atomic bomb survivors, although they arealso consistent with there being no risk at all.
260. The NRRW therefore provides some evidence of anelevated risk of leukaemia associated with occupational
exposure to radiation and, like the combined study of workers
in the United States, is consistent with the risk estimates for
low-dose/low-dose-rate exposures derived by ICRP [I2] fromthe data on the survivors of the atomic bombings in Japan.
261. A cohort study of occupational radiation exposure has
been conducted using the records of the National Dose
Registry of Canada [A17]. The cohort consisted of 206,620
individuals monitored for radiation exposure between 1951
and 1983, with mortality followed up to the end of 1987. A
total of 5,425 deaths were identified by computerized record
linkage with the Canadian Mortality Database. A trend ofincreasing mortality with increasing cumulative radiation
exposure was found for all causes of death in both males andfemales. In males, cancer mortality appeared to increase withradiation exposure without any relationship to specific types.
Unexplained trends of increasing mortality due tocardiovascular diseases (males and females) and accidents(males) were also noted. The excess relative risk for radiation-induced cancer was calculated to be 3.0% per 10 mSv (90%
CI: 1.1!4.8) for all cancers combined and was significantly
higher than the comparable risk estimate for survivors of theatomic bombings. However, the very low SMR for all-cause
mortality suggests that record linkage procedures between the
Canadian National Dose Registry and the Canadian Mortality
Database may have been imperfect and that there could havebeen some confounding of the dose response.
262. In the UNSCEAR 1994 Report [U2] information wasgiven on the association of leukaemia and radiation exposure
among workers at the Mayak facility in the RussianFederation, some of whom received substantial exposuresseveral decades ago [K26]. Risk coefficients for radiationinduced leukaemia were similar to those given by the ICRP
[I2] for workers, although no confidence interval wasprovided. Limitations in the study were that 15% of theoriginal cohort had been lost to follow-up, and bone marrowdoses from plutonium remained to be evaluated.
263. An international studyof cancer risk among radiation
workers in the nuclear industry was coordinated by theInternational Agency for Research on Cancer (IARC)[C20, I1]. It consisted of a combined analysis of mortality
data for nearly 96,000 workers in Canada, the United
Kingdom, and the United States. The groups of workers
studied were the subject of individual analyses that had
been published in 1988 or earlier. The United Kingdom
component of this study was the Nuclear Industry
Combined Epidemiological Analysis (NICEA) [C3], basedon workers at BNFL Sellafield, the United Kingdom
Atomic Energy Establishment, and the Atomic Weapons
Establishment. The other groups studied were workers at
the three United States Department of Energy plants
referred to earlier (Hanford, Oak Ridge, and Rocky Flats)[G3] and workers at Atomic Energy Canada Ltd. [G4].
264. Analysis of the combined cohort of radiation workers
showed a statistically significant trend in the risk ofleukaemia (excluding chronic lymphatic leukaemia) withexternal dose. This finding is similar to that reported in thefirst analysis of the NRRW [K3], although the results are notindependent, since many of the workers in the NRRW werealso in the IARC study. The central estimate of risk per unit
dose corresponded to 0.59 times the value estimated from theatomic bomb survivors based on a linear dose-response modeland 1.59 times the value based on a linear-quadratic model
fitted to the atomic bomb survivor data; the corresponding
90% confidence interval ranged from about zero up to fourtimes the value from the linear-quadratic atomic bomb
survivor model. The evidence for a trend with dose wasparticularly strong for chronic myeloid leukaemia, as was also
reported in the large study of workers in the United Kingdom[M47], some of whom were included in the international
study.
265. For all cancers other than leukaemia, the central
estimate of the trend in risks with dose was negative, but theupper 90% confidence limit corresponded to about twice thevalue arising from a linear extrapolation to low doses ofANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

results for the survivors of the atomic bombings in Japan, i.e.
about four times the estimate for low-dose-rate exposures
based on a reduction factor of 2.
266. The authors of the IARC study concluded that theiranalysis provides little evidence that the risk estimates thatform the basis of current radiation protection standards areappreciably in error. Since most of the workers studied arestill alive, however, they recommended further follow-up ofthese and other workers to increase the precision of risk
estimates. To further address the issue of effects at low doses,
IARC is now coordinating an enlarged International
Collaborative Study of Cancer Risk among Radiation
Workers in the Nuclear Industry [C2]. This study will containadditional workers from countries such as France and Japan,
and the combined cohort should number several hundred
thousand.
2. Environmental exposures
267. Studies of exposures to natural background radiation(other than radon) or to environmental contamination fromman-made sources have generally involved examining geographical correlations in cancer rates. Such studies can bedifficult to interpret, owing to the effect of confounding
factors such as sociodemographic variables and other factorsthat vary geographically, together with the lack of
information on doses.
268. Sources of natural background radiation include
terrestrial gamma rays and cosmic radiation, which varyconsiderably with geographical location. Many attempts havebeen made to correlate radiation exposure with cancer
mortality or incidence in different populations. While thiswould in principle give information on exposures at relatively
low radiation doses, such attempts are subject to considerable
difficulties, as was described in the UNSCEAR 1994 Report[U2]. Interpreting the data is made difficult by uncertainties inthe doses actually received, geographical variation in theaccuracy of cancer diagnoses, and confounding with thenumerous other environmental factors. Furthermore, whendifferent geographical areas are compared, exact matching ofcontrol groups or groups exposed at different levels can bedifficult. As a consequence, studies that have tried to compare
cancer risks from natural background radiation in different
geographical locations are subject to considerable uncertainty
and must be interpreted with care [C1].
269. Darby [D10] has made some estimates of the proportionof deaths from various cancers that might be caused byexposure to natural background radiation based on models
developed by the BEIR V Committee [C1]. These models
were based on the data from the survivors of the atomic
bombings in Japan. They predict that about 11% of deaths
from leukaemia might be caused by post-natal exposure tonatural background sources, excluding radon. For othercancers the estimate was 4% or less. The interval between
exposure and the development of the disease is shorter forleukaemia than for most other tumours. There is a higher
relative risk for leukaemia, and the influence of other119

environmental factors on leukaemia risk is less than for manyother types of cancer. It might be expected, therefore, that any
effect of variations in natural background would be more
readily detectable for leukaemia than for other cancer types.
As described in the UNSCEAR 1994 Report [U2], however,well designed studies conducted in a number of countries findno significant association between natural backgroundradiation and leukaemia (excluding chronic lymphatic
leukaemia) (e.g. [I7, T10, U2, W11]).
270. Few of the studies examining cancer incidence inrelation to exposure to natural background radiation have
tried to obtain realistic dose estimates that take into accountdifferences between indoor and outdoor exposure and theeffects of population movement. One exception is a Chinese
study [W12] that compares leukaemia mortality in two
neighbouring regions having quite different levels of exposure
as a result of the high thorium content in monazite sands.
Yangjiang is a high-background-radiation area and Taishan/
Enping is a control area. In both regions there was a highly
stable population, and considerable effort went into measuring radiation exposure both indoors and outdoors. In the highbackground area the radiation dose calculated to the red bonemarrow by age 50 years would have been about 60 mSv
greater than that for someone living in the low-backgroundarea. During 1970!1985, the age-adjusted mortality rates forleukaemia in females were 2.21 and 3.56 105 PY1 in thehigh- and low-background areas, respectively, while in males
the rates were 3.32 and 3.82 105 PY1, where PY stands for
person-years. The differences were not significant; ifanything,
they suggested a lower risk in the more highly exposed
population [W12]. The study had low statistical power todetect an effect, if one existed, as the relative risk expected
was about 1.2 in the highly exposed group, and effects of thismagnitude are very difficult to detect epidemiologically.
271. An extension to this study covering 1987 to 1990 has
also been reported [T9]. The later study covered a fixed cohort
with 78,614 persons in the high-background-radiation areaand 27,903 in the control area at the start of 1987. Doseestimates were obtained by measurements using environmental gamma-ray dose-rate measurements and individual
TLDs. The cohort was added to that monitored previously togive a total population of 64,070 subjects in the highbackground-radiation area and 24,876 in the control area atthe beginning of 1979. In total, the study covered 949,018
person-years (PY) during 1979!1990 (696,181 in the highbackground-radiation area and 252,837 in the control area).
The relative risks (the high-background-radiation area
compared with the control area) for all cancers and for all
cancers except leukaemia in each of three dose subgroups in
the high-background-radiation area did not differ significantly
from 1. The relative risks for site-specific cancers of the lungs,
liver, and stomach were generally less than 1, while fornasopharyngeal cancer and leukaemia they were greater than
1. It is noteworthy that the result for leukaemia was thereverse of that found in the earlier study [W12]. The authors
concluded that even for the combined data the sample size ineach group was not large enough to come to any definite
conclusions.

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ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

272. A further extension to the study has also beenreported [T17] covering a total of 125,079 subjects with
1,698,350 PY (10,415 cancer deaths) followed from 1979
to 1995. The population was separated into controls andhigh, medium and low dose groups. Despite higher death
rates in the males than in the females no significant
difference was found between the persons from the highbackground-radiation area and the controls; if anything,
the death rates in the high-background-radiation area werelower.
273. It may be concluded from this and other studies
reviewed in the UNSCEAR 1994 Report [U2] thatcomparative studies of groups exposed to differing levels ofnatural background gamma radiation have not demonstrated
any significant effects on cancer incidence.
274. Some studies of environmental exposures haveexamined the temporal trends in cancer rates. For example,
Darby et al. [D1] examined temporal trends in childhood
leukaemia in the Nordic countries in relation to fallout
from atmospheric nuclear weapons testing during the1950s and the 1960s. They concluded that there was someevidence of a raised risk associated with the “high”
exposure period, when children would have received a dosefrom fallout of about 1.5 mSv, compared with the adjacent“medium” exposure period, when the dose received would
have been about 0.5 mSv (relative risk for ages 0!14 years
is 1.07; 95% CI: 1.00!1.14). These data are consistent
with a relative risk of 1.03 predicted with the BEIR V
leukaemia model [C1], although the central estimate fromthis study is larger than the BEIR V value, a difference that
may be explained bythe different follow-up times on which
the two values are based (0!7 years and 5!15 years,
respectively).
275. Studies have been reported of a population in the
East-Urals that was exposed to radioactive materials
following an accident at the Mayak reprocessing plant in
September 1957 [K29]. A total of 7,854 persons who
received radiation doses estimated to be between 40 and
500 mSv have been followed. No statistically significant
changes in causes of death, mortality or reproductivefunction have been found compared with control values
from the province and USSR data. Although this study is
to be continued, it illustrated the difficulties in conducting
carefullycontrolled epidemiological studies, which requirea defined control group and accurate dose estimates.

D. SUMMARY
276. Epidemiological studies provide direct quantitative dataon the risks of cancer in humans following radiation
exposure. The main source of information is the Life Span
Study of survivors of the atomic bombings in Hiroshima andNagasaki in 1945. Substantial information is also available
from studies of people occupationally or medically exposed
either to external radiation or to internally incorporated
radionuclides.

277. The Life Span Study is important, as it gives
information on the effects of whole-bodyirradiation following
exposure at different ages. The interpretation of the doseresponse data is, however, complicated by the fact that
exposure was to both gamma rays and to neutrons. An RBEof 10 has generally been assumed when fitting the doseresponse data. The data show a pattern of increasing risk withincreasing dose for both leukaemia and most solid cancers.
The most recent analyses of the data suggest that the numbersof solid cancers induced in the population depends on the
spontaneous cancer rate, and that at least for those exposed in
adulthood the absolute level of the radiation-induced riskincreases with age over the period of follow up. The follow-up
study indicates a significant (p=0.05) increase in the risk ofradiation-induced fatal solid cancers over the dose range of
0!50 mSv (assuming an RBE for neutrons of 10). Caution isneeded in interpreting this finding, however, as an increased
incidence of solid cancers is seen only at doses down to
200!500 mSv, suggesting the possibility of bias at the lower
dose range.
278. The data on mortality from leukaemia are best fitted
by a linear-quadratic dose response, while for all solidcancers taken together, a linear dose response provides abest fit for dose-response data up to doses of about 3 Sv.
However, while a linear dose response can also be fitted tothe data for a number of individual tumour types, in thecase of non-melanoma skin cancer there is substantial
curvilinearity in the dose response, consistent with apossible dose threshold of about 1 Sv or with a dose
response in which the excess relative risk is proportional to
the fourth power of dose. It is notable that if analyses are
restricted to the dose range up to 2 Gy and account takenof possible systematic errors in the Hiroshima DS86 data,then there is evidence of appreciable upward curvature of
the dose response for solid tumours. It has become clearthat further follow-up and improved information on thedoses received will be needed before the shape of the dose
response at low doses for both morbidity and mortality canbe determined with confidence at doses below about
100!200 mSv. While the Life Span Study has shown
elevated cancer risks in a number of tissues, there areothers for which there is either very little or no evidence
for an effect. These include, for example, the bone, cervix,
prostate, testes and rectum.
279. Information on cancer risks is also available from anumber of studies of patients irradiated for medical
reasons. Many of the patients in these studies received highdoses to particular organs, often 1 Gy or more, althoughsome received much lower doses. Patients were generally
given acute exposures, although women treated withfluoroscopy for tuberculosis were given highly fractionateddoses. As with solid cancers in the Life Span Study, thedose-response data from many of these studies are
generally consistent with a linear dose-response
relationship at low to intermediate doses. Results fromseveral studies have suggested a statistically significant
increase in the risk of thyroid cancer at doses of about
100!300 mGy received in childhood.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

280. In contrast, the best fit to the data on bone tumour
induction in radium dial painters exposed to 226/228Ra can beobtained with a model indicating a “practical threshold” for a
response at an average bone dose in the range 3.9!6.2 Gy(high-LET). This observation might also reflect the extent of
the data available at low doses. For head sinus carcinomas in
the radium dial painters, linear, linear-exponential, or dosesquared exponential functions all provided acceptable fits tothe data. Data are also available on the risk of bone sarcomasin patients given 224Ra. Recent analysis of the pathology ofthese tumours has shown that a high proportion of them(30%) are malignant fibrous histiosarcomas, which is higher
than would have been expected in sarcomas occurring
spontaneously (8%!11%). It has been proposed that thesetumours can only be expected to arise in tissue withdeterministic radiation damage and so would be expected toappear only above a threshold dose. Similar conclusions havebeen drawn for the bone tumours arising in the radium
workers.
281. Extensive data are available on cohorts of miners
occupationally exposed to radon and its decay products.
These studies have provided information on the risk ofradiation-induced lung cancer. The most recent analyses ofthe data examined a range of risk models. However, for
cumulative exposures below 0.175 J h m3 (50 WLM), a
constant-relative-risk model without anymodifying factors,
such as attained age and exposure rate, appeared to fit thedata well.
282. A number of studies have provided information on therisk of childhood cancer following obstetric radiography. In
the Oxford Survey of Childhood Cancers, a statisticallysignificant 40% increase in the childhood leukaemia rate (upto 15 years of age) has been seen following doses of
10!20 mGy (low-LET). Similar results have been obtained ina number of other, smaller studies of the effects of obstetricradiography. Although there may be some increase in sensitivity to radiation at this early stage of development, there is121

no reason to believe the mechanisms involved in tumour
induction will be fundamentallydifferent from those in adults.
The number of cells at risk would, however, be different. The
principal reasons for being able to determine this increase inrisk, which in absolute terms is small, is the low backgroundincidence of leukaemia in childhood and greater sensitivity toradiation. A feature of the data from the Oxford Survey thatremains unexplained is that the increase in risk for bothleukaemia and solid cancers following exposure in utero isessentially the same, with a relative risk of about 1.4. Most
other human and animal studies consistentlyindicate different
sensitivities of leukaemia and solid cancers.
283. More recently, direct information on the effects of lowdose, chronic exposure has become available from studies ofradiation workers. The estimation of cancer risks associatedwith exposure to low doses poses particular problems. The
predicted level of excess risk associated with such exposures
is lower than that for high-dose exposures, and consequently
the size of the study population required to detect a raised risk
is usually much larger than that required for the high-dosestudies. The information available to date is generally
consistent with information on the risks of cancer obtained
from the high-dose-rate studies, although having wide
confidence intervals, and would also be consistent with there
being no risk at all. A long period of follow-up and pooling ofdata from different studies will be necessary if statisticallyuseful data are to be obtained.
284. A number of studies have been published that haveexamined the risks of cancer in areas of high natural
background. Comparative studies on groups exposed todifferent levels of natural background radiation do not,however, have the statistical power to detect significant effects
on cancer incidence. There are difficulties in interpreting thedata as a result of uncertainties in the doses actually received,
geographical variation in the accuracy of cancer diagnoses,
and confounding by other environmental factors.

IV. MECHANISMS AND UNCERTAINTIES IN MULTI-STAGE
TUMORIGENESIS
285. The development and application of modern
molecular methods has, in recent years, substantially
increased the understanding of the mechanisms oftumorigenesis. At the same time, there has been an
equivalent increase in the understanding of the action ofradiation on cellular DNA, control of the reproductive cell
cycle, and the mechanisms of DNA repair and mutagenesis.
286. Mechanisms of radiation oncogenesis were reviewed
by the Committee in the UNSCEAR 1993 Report [U3] andare considered further in Annex F, “DNA repair andmutagenesis”; and Annex H, “Combined effects ofradiation and other agents”. Accordingly, the aim of this
Chapter is to provide an updated view of the mechanismsof tumorigenesis in order to relate them to data on doseeffect relationships. Emphasis will be placed on current
uncertainties surrounding the mechanisms of radiation
tumorigenesis, with a view to exploring their importancefor the development of biologically based computationalmodels that seek to describe radiation cancer risk at low
doses and low dose rates (Chapter V).

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ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

A. MULTI-STAGE PROCESSES IN
TUMORIGENESIS
287. In accord with earlier proposals on spontaneously
arising neoplasia [F1, F5, V1], UNSCEAR supports a multistage model as a conceptual framework for describing
radiation tumorigenesis [U3]. A generalized model of thisform is illustrated in Figure XXI. In this model, radiation
tumorigenesis is imprecisely subdivided into four phases:
neoplastic initiation, promotion, conversion, and progression.
This operational framework, while subject to considerable
uncertainty, may be used to illustrate the critical cellular andmolecular processes that direct neoplastic change.

expression of the initial mutation being dependent not only oninteraction with other endogenous mutations but also on
factors that may transiently change the patterns of specificgene expression, e.g. cytokines, lipid metabolites, and certain
phorbol esters. As a consequence, there may be an enhancement of cellular growth potential and/or an uncoupling of theintercellular communication processes that act to restrict
cellular autonomy and thereby coordinate tissue maintenanceand development [T5, U3]. In this way, tumour-initiated cells
can receive a supranormal growth stimulus and begin toproliferate in a semi-autonomous manner, allowing for theclonal development of pre-neoplastic lesions in tissues, e.g.
benign papillomas, adenomas, or haemopoietic dysplasias.
3. Neoplastic conversion

Damage to chromosomal DNA
of a normal target cell
Failure to correct
DNA damage
Appearance of specificneoplasia-initiating mutation
Promotional growthof pre-neoplasm
Conversion to overtly
malignant phenotype
Malignant progression
and tumour spread
Figure XXI. A simple generalized scheme for multistage oncogenesis.

290. Neoplastic conversion of pre-neoplastic cells to a statein which they are more committed to malignant development
is believed to be driven by further gene mutations accumulating within the expanding pre-neoplastic cell clone. Evidence
is accumulating that the dynamic cellular heterogeneity thatis a feature of malignant development may in many instances
be a consequence of the early acquisition of gene-specificmutations that destabilize the genome. Mutations of the TP53
gene or one of a set of DNA mismatch repair genes provide
examples of such destabilizing events in neoplasia [F3, H6,
H17, L10]. There is also evidence that mutations resulting inenhanced chromosomal non-disjunction may also contributeto oncogenic change [L3].
291. An elevated mutation rate established relatively earlyin tumour development may, therefore, provide for thehigh-frequency generation of variant cells within a premalignant cell population. Such variant cells having thecapacity to evade the constraints that act to restrict
proliferation of aberrant cells will tend to be selectedduring tumorigenesis.
4. Progression of neoplasia

1. Initiation of neoplasia
288. Neoplastic initiation may be broadly defined as
essentially irreversible changes to appropriate target somatic
cells, driven principally by gene mutations that create thepotential for neoplastic development [C9, U3]. Such tumour
gene mutations can have profound effects on cellularbehaviour and response, e.g. dysregulation of genes involved
in biochemical signalling pathways associated with the control
of cell proliferation and/or disruption of the natural processesof cellular communication, development, and differentiation.
Although the full expression of such neoplasia-initiating
mutations invariably requires interaction with other laterarising gene mutations and/or changes to the cellularenvironment, the initiating mutation creates the stablepotential for pre-neoplastic cellular development in cells withproliferative capacity.
2. Promotion of neoplasia
289. Neoplastic development is believed to be highly influenced by the intra- and extracellular environment, with the292. The progression of neoplastic disease may be dependent
on metastatic changes that facilitate (a) the invasion of local
normal tissues, (b) the entry and transit of neoplastic cells inthe blood and lymphatic systems, and (c) the subsequentestablishment of secondarytumour growth at distant sites [H4,
T1]. It is the metastatic process and tumour spreading that are
mainly responsible for the lethal effects of many common
human tumours. Again, it is believed that in many cases genemutations are the driving force for tumour metastasis, with thedevelopment of tumour vasculature an important element indisease progression [F4].

B. MUTATIONAL EVENTS MEDIATING
THE TUMORIGENESIS PROCESS
293. Although models such as that shown in Figure XXI are
most useful in placing clinical, histopathological, andcellular/molecular experimental data in the context of ageneralized mechanism of tumour development, important

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

uncertainties remain. Identification of these uncertainties
should help to guard against over-interpretation of data
relating to radiation tumorigenesis, particularly with respect
to biological modelling.
294. The gene-specific determinants of the initiation processthat is believed to operate to allow entry of normal somatic
cells into a given neoplastic pathway are incompletely
understood, although for some organs there are strong
associations with specific tumour gene mutations [U3, V1]. A
similar degree of uncertainty attaches to the molecular events
that determine the other cellular transitions noted in
Figure XXI. While it is accepted that, in general, target cells
for tumorigenic initiation will reside in the stem cellcompartment of most tissues, the specific identityand locationof these cells is poorly understood. As noted earlier in the
Annex, this represents a significant uncertainty in some areasof radiation tumorigenesis, particularly with respect to alphaparticle irradiation.
295. In general, the concept of stepwise interaction between
loss-of-function mutation of tumour-suppressor genes andgain-of-function mutations of proto-oncogenes [U3] is still
believed to apply. Further tumour-specific gene mutations
have been identified, and there is much new information on
the biochemical interactions between tumour gene mutations,which maydestabilize the genome, compromise control of cellsignalling, proliferation, and differentiation, and interfere with
the normal interaction of cells in tissues (see [K1, S3]).
296. In the UNSCEAR 1993 Report [U3], the Committee
concluded that the somatic genetic changes to cells that inter-123

mediate multi-stage tumour development potentially involve
sequential mutation of different classes of genes, i.e. protooncogenes, tumour-suppressor genes, genes involved in cellcycle regulation, and genes that play roles in maintaining
normal genomic stability. It should be recognized, however,that the above classification serves principally as a frameworkfor discussion and that there is substantial functional overlap
between these classes.
1. Proto-oncogenes
297. Proto-oncogenes may be broadly defined as tumourassociated genes that can sustain productive gain-of-function
mutations that result in over-expression or more subtle
functional abnormalities in a wide range of cellular proteins.
These proteins normally serve to control or effect cellularsignalling and the temporal maintenance of growth anddevelopment [H9, L14, M20, W3]. Indeed, the known protooncogene proteins perform an extraordinary range of specificcellular functions, many of them interacting with each otherin biochemical signalling cascades that, for example, target
mitogenic processes, apoptotic activity, cell-to-cell interactions, and cytoskeletal functions. The capacity to effecttranscriptional or post-translational activation of such pathways is a common theme for many such genes.
298. Thus, mutations resulting in altered proto-oncogene
activity/specificity can lead to profound and constitutively
expressed cellular effects; the close linkage between manycellular signalling pathways means that these effects are
frequently pleiotropic. Table 12 gives a convenient scheme forclassifying proto-oncogenes, along with a few examples.

Table 12Classification scheme for proto-oncogene products
Designation

Product

Class 1
Class 2Growth factors
Receptor and non-receptor protein tyrosine kinases (RPTK and
NRPTK)
Receptors lacking protein kinase activity
Membrane-associated G proteins
Cytoplasmic protein-serine kinases
Cytoplasmic regulators
Nuclear transcription factors
Cell survival factors
Cell cycle genes

Class 3
Class 4
Class 5Class 6Class 7
Class 8Class 9299. Numerous and often multiple proto-oncogene activation events characterize different tumours; some of thesewere discussed in the UNSCEAR 1993 Report [U3]; others
are noted in a series of reviews [B9, L14, M20, M21, V1]
and are also mentioned later in this Annex.
300. In the context of this Annex, a very important issue isthe nature of the mutational events that characterize protooncogene activation. Although there has been a large gain in
the biochemical understanding of proto-oncogene action, little
has changed since 1993 in respect of mutational activation
mechanisms [U3]. In essence, human proto-oncogenes maybe

ExamplesPDDGF-β chain (sis) and FGF-related growth factor (hst)src (NRPTK) and erbB (RPTK); also ret
Angiotensin receptor (mas)Ras family
raf-1 and mos
SH2/SH3 protein (crk)
myc, myb, jun, fos
bcl-2
PRAD1 (cyclin D1)

activated by point mutation (e.g. RAS), by gene amplification(e.g. MYC), or by chromosomal rearrangement (e.g. ABL)
[W3].
301. There has, however, been a rapid increase inknowledge of the range of proto-oncogene activation events
via chromosomal rearrangement and their often early rolein tumorigenesis. These advances have been reviewed [R2],and cytogenetic data relevant to human tumorigenesis havebeen subject to detailed analyses [M23].
302. In brief, an increasingly wide range of chromosomal
rearrangements associated with many subtypes of human

124

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

lympho-haemopoietic neoplasms have been characterized at
the molecular and biochemical levels. More than 30 geneactivation events resulting from proto-oncogene juxtaposition
with T-cell receptors and immunoglobulin loci are known in
T- and B-cell neoplasms, respectively. In the case of specificgene fusion by chromosome translocation/inversion, morethan 25 examples have been characterized, with myeloid neoplasms the predominant carriers. Table 13 provides examples
ofchromosome translocations in human lympho-haemopoietic
neoplasms.

Table 13
Examples of human tumour-suppressor genes
Gene

Chromosome
map location

Cancer type

Product location

APC
DCC
NF1
NF2
p53
RB1
VHL
WT-1
p16
BRCA-1
PTCH
TSC2

5q21
18q21
17q21
22q12
17p13
13q14
3p25
11p13
9p21
17q21
9q
16p13

Colon carcinoma
Colon carcinoma
Neurofibromas
Schwannomas and meningiomas
Multiple
Multiple
Kidney carcinoma
Nephroblastoma
Multiple
Breast carcinoma
Skin (basal cell)
Multiple303. It has also become apparent that although protooncogene juxtaposition/fusion is most commonly observed
in lympho-haemopoietic tumours, such events are alsocharac-teristic of certain solid tumours. For example,
Ewing’s sarcoma frequently carries a chromosomallymediated FLI/EWS gene fusion [R2], and in some papillary
thyroid cancers the RET proto-oncogene can be activatedby a set of specific chromosome rearrangements [Z1].
304. Since the cytogenetics of solid tumours are often
complex and difficult to resolve accurately, it may be that
proto-oncogene activation via chromosomal rearrangement
is being underestimated. New methods of cytogenetic
analysis by FISH are now available to approach thisproblem [S14].

Cytoplasm
Membrane
Cytoplasm
Inner membrane
Nucleus
Nucleus
Membrane
Nucleus
Nucleus
Nucleus?
?

Mode of action

Transcription regulator
Cell adhesion/signalling
GTPase-activator
Links membrane to cytoskeleton?
Transcription factor
Transcription factor
Transcription factor
Transcription factor
CDK inhibitor
Transcription factor/DNA repair
Signalling protein?

2. Tumour-suppressor genes
305. Tumour-suppressor genes are defined as genes that can
act as negative regulators of cellular processes such as signaltransduction, gene transcription, mitogenesis, and cell
development/ differentiation [H10, H11, L15, W3]. As noted
later, some genes that act to regulate cell-cycle progression,
apoptosis, and various aspects of DNA processing may also be
included in this category. Consequently, all cancer-associatedgenes that act via a loss-of-function mechanism may bedescribed as tumour suppressors even though, universally,
they may not have true tumour-suppressing activity [H11].
The loss of function of tumour-suppressor genes characterizes
a broad range of human neoplasms; some examples discussed
in this Annex are listed in Table 14.

Table 14
Examples of chromosome translocations in lympho-haemopoietic neoplasia
Chromosome translocation

DiseaseTranslocation

Genes involved

Involving T-cell receptors

T-cell acute lymphatic leukaemiat(1;7)(p32;q34)t(1;14)(p32;q11)t(1;7)(p34;q34)t(7;9)(q34;q32)t(7;9)(q34;q34)

TCRβ-TCL5
TCRδ-TCL5
TCRβ-LCKTCRβ-TAL2TCRβ-TAN1

Involving immunoglobulin

Burkitts lymphoma/B-cell acute lymphatic leukaemiat(8;14)(q24;q32)t(2;8)(p11;q24)t(8;22)(q24;q11)t(2;14)(p13;q32)t(5;14)(q31;q32)

IgH-MYC
Igk-MYC
Igl-MYC
IgH-REL
IgH-IL-3t(1;19)(q23;p13)t(6;9)(p23;q34)t(9;9)(q34;q34)t(8;21)(q22;q22)t(9;22)(q34;q11)

E2A-PBXσσDEK-CANSET-CANAML1-ETO
BCR-ABL

B-cell chronic lymphatic leukaemia
Pre-B-cell acute lymphoma
Involving fusion gene sequences

Pre-B-cell acute lymphoma
Acute myeloid leukaemia

Chronic myeloid leukaemia /B-cell acute lymphatic leukaemia

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

306. Unlike activated proto-oncogenes, which arefunctionallydominant, most tumour-suppressor genes require
mutation of both autosomal copies to occur, often via
intragenic point mutation of one copy and complete deletionof the other [U3]. Thus, the genomic location of potential
tumour-suppressor genes is often revealed by the presence ofconsistent, region-specific DNA losses in a given tumour type.
As noted in the UNSCEAR 1993 Report [U3], there are,however, examples where the mutation of one copy of such agene can result in a change in cellular phenotype via
intragenic mutations that result in so-called dominant negative
effects. There are other examples where it seems that one genecopyis mutated conventionallyand the other silenced byDNA
methylation; there are also cases where effects from gene copy
number have been found [H11]. Changes in chromosome
complement (ploidy) are common during the development ofmany tumours, and it seems likely that some specificnumerical chromosome changes in neoplasia relate to the lossof tumour-suppressor functions.
307. Overall, the loss of function that is characteristic of therole of tumour-suppressor genes means that the responsible
mutational events can vary greatly, i.e. there can be intragenic
point mutation/deletion, interstitial chromosome segment
deletion, whole chromosome loss, or epigenetic silencing.
Much will depend on the capacity of the target cell to remain
viable, particularly with the deletion of large segments ofDNA. This position contrasts with proto-oncogene activation,
which demands relativelyhigh DNA sequence specificitywith
respect to both intragenic point mutation and gene-specificjuxtaposition or fusion. Few such gain-of-function mutations
are expected to involve large DNA losses.
3. Genes involved in cell-cycle control andgenomic stability
308. Abrogation of normal control of the cell cycle andmaintenance of genomic stability is frequently observed inneoplasia. These phenotypes are sometimes closely linked,
and recent advances have led to the consensus view that
mutations leading to cell-cycle defects and mutator phenotypes can be critical for neoplastic development [H12, L10].
309. An example of the effect of tumour-suppressor mutations on cell-cycle control and genomic stabilityis provided by
the TP53 gene, which is mutated in a high proportion oftumours of various types [G8, L41]. The p53 protein is known
to bind DNA and can act on a transcriptional regulator withpotential effects on cell-cycle progression, DNA repair/recombination, and apoptosis [B43, H13, O1].
310. The half-life of the p53 protein in cells is short but
increases in response to cellular stress, including DNA
damage. Through mechanisms that remain uncertain, theincrease in p53 protein serves to check the cell cycle in G1/S
or sometimes in G2/M. It is believed that such cell-cycle
checkpoints promote cellular recovery from stress, including
the facilitation of DNA repair [H6, H14].
311. According to these proposals, when TP53 is appropriately mutated, cell-cycle control and its checkpoints for

125

repair are compromised, and during subsequent cellulardevelopment, errors of DNA replication and damage repair
accumulate. Failure to adequately effect apoptotic death indamaged cells is also believed to be a feature of TP53deficiency that contributes to neoplastic development [O1].
Other protein products of tumour-suppressor genes that
impinge on cell-cycle control include pRb, p16, p27, and p85,
and a complex series of cascade interactions involving
tumour-suppressor and proto-oncogene proteins, together withcytokines, is believed to maintain close control of cell
replication and apoptosis (see [K11, M22, N2]). It can be seenthat many of the mutations that accumulate during neoplastic
development do so because of the need for cooperation inorder to fully compromise normal proliferative control.
312. In this context it has been argued for many years thatthe accumulation of the series of gene-specific clonal mutations that are believed to drive tumorigenesis would be
improbable ifnormal genomic stabilitywas maintained. Thus,
recent findings regarding the spontaneous development ofgenomic instability in tumours has come as no great surprise.
In addition to the TP53-mediated effects noted above, otheraspects of somatically acquired genomic instability have been
debated widely [H6, L10, L17]. The most important of theseis the role of defects in DNA mismatch repair.
313. Mismatches in DNA base pairing occur at a relatively
high spontaneous rate through replication errors (RER)
and spontaneous oxidative/hydrolytic damage; these mismatches are corrected at high fidelity by a repair systemthat is highly conserved across species [F3]. Following thefinding of a high frequency of replication errors in shortmicrosatellite repeat sequences (the RER+ phenotype) in avariety of tumours, some form of DNA mismatch repair
defect was suspected.
314. Subsequently, many RER+ human tumours, particularly
those of the gastrointestinal (GI) tract, were shown to harbour
mutations in DNA mismatch repair genes, principallyhMSH2
and hMLH1 [A2, F3, H17, S15]. It is clear, however, that
“instability” genes other than TP53 and those associated with
DNA mismatch repair are somatically mutated in humantumours. For example, the onset of aneuploidy is often afeature of the transition from pre-neoplastic to malignant
phenotype, but the genes participating in the control of ploidy
remain poorly understood. Recently, however, a dominantly
expressing gene in this category has been revealed by a
combination of FISH cytogenetics and somatic cell fusion
techniques applied to a panel of colorectal cell lines [L3]; thisgene appears to be functionally independent of TP53 status.
315. Some progress is also being made with respect tosomatic tumour genes that have a known or suspected role
in DNA damage recognition and processing. The 11q22encoded ATM gene of human ataxia-telangiectasia (A-T)
and its role in the cellular and biochemical response toradiation damage are described in Annex F, “DNA repair
and mutagenesis”. With the knowledge that ataxiatelangiectasia patients are genetically predisposed to thedevelopment of neoplasms of the T-lymphoid lineage, a126

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

search has been conducted for somatic ATM mutation insporadic T-prolymphocytic leukaemia (T-PLL) [S16]. This
investigation revealed that 11q22 losses and biallelic ATM
mutations were present in a high proportion of sporadic
T-PLL, suggesting a tumour-suppressor-like role for thisgene in target T-cell precursors. It may be speculated that
this is associated with its role in controlling genomic stability,
particularly with respect to T-cell receptor sequences. Also
noted in Annex F, “DNA repair and mutagenesis”, is thegrowing recognition that the breast cancer suppressor genes
BRCA1 and BRCA2 play a role in the recognition/repair ofdamage to cellular DNA. Although the specific functions ofthese genes with respect to genomic stability remain to beresolved, their importance to heritable and sporadic breast
cancer is well established.
4. Early events in multi-stage tumorigenesis316. In the UNSCEAR 1993 Report [U3], the Committee
recognized the difficulties of identifying the specific genes
that, in mutant form, act at the initiation phase of tumorigenesis. For some lympho-haemopoietic neoplasms,
specific chromosomally mediated proto-oncogene eventswere suggested to occur early in neoplastic development,
and the Mll1 gene data outlined later strengthen this view.
Equally, however, many human myeloid neoplasms arecharacterized by region-specific chromosome deletions[M23], some of which are believed to arise early.
317. In the case of human solid tumours of certain tissues,
there is growing evidence that those genes that act early arealso represented as rare germ-line determinants of heritable
cancer; the principal examples of this association are the RET
gene in thyroid cancer, the APC gene in colorectal cancer, theVHL gene in renal cancer, the PTCH(patched) gene in basal
cell carcinoma, and the RB1 gene in retinoblastoma/
osteosarcoma [H11, S17, W3].
318. Although the BRCA1 and BRCA2 genes of breast
cancer may be exceptions, a concept of early tumour
development is evolving from the above associations. The
concept requires a relatively tissue-specific “gatekeeper”
gene to be mutated in order for stem-like cells to enter aphase of inappropriate clonal expansion [K12, S17]; thisexpansion then allows for the accumulation of further
mutations. According to the concept, the accumulation ofother mutations in the neoplastic pathway in the absence ofgatekeeper defects will only infrequently result in theclonal development of recognizable tissue lesions. In
essence, the temporal order of mutational events is likely
to be important for productive neoplastic growth with lossof specific gatekeeper genes as critical early events.
319. In the UNSCEAR 1993 Report [U3], the Committee
drew heavily on evolving models of colorectal
carcinogenesis to support its views on the mechanisms thatdrive the genesis of solid tumours. In the same way, further
data relating to this tumour type, while not necessarily
fully representative of all solid cancers, may also be used
to support the gatekeeper hypothesis.

320. A key element in this hypothesis as it relates tocolorectal cancer is that the first consistent mutation intissue lesions should be monoclonal mutation of the APC
gatekeeper gene, which acts as a transcriptional regulator[N4]. In the main, the data to be discussed later [K12]support this, but a recent investigation of the temporalsequence of gene mutations adds considerable weight to theargument.
321. Using tumour microdissection and allelotyping methods,
the sequence and tempo of allelic losses in a series ofcolorectal cancers at different stages of development was
followed [B10]. The principal losses that were tracked were
those associated with deletion of APC (5q21), TP53(17p13), and DCC (18q21). In brief, loss of heterozygosity(LOH) via allelic loss was not recorded in normal tissue
surrounding colorectal tumours. However, 5q but not 17p
losses arose abruptly and consistently at the transition fromnormal tissue to benign adenoma; a proportion of adenomas
also showed 18q losses. Losses to 17p occurred equallyabruptly and consistently at the adenoma to carcinoma
transition border, and in highly advanced and invasive
carcinomas, there was a high level of allelic variation
indicative of clonal heterogeneity due to genomic instability.
322. Thus, commencing with APC loss from cells innormal tissue, the development of colonic tumours ischaracterized by abrupt waves of clonal expansion, with
TP53 loss and chaotic allelic variation being criticalwatersheds in the evolution of the fully malignant
phenotype. Considering these and other molecular genetic
observations with colorectal cancer, a temporal model ofneoplastic initiation and malignant development has been
proposed [B10]. This is illustrated in Figure XXII.
323. Although critical evidence in support of the gatekeeper
gene hypothesis remains to be gathered, the hypothesis does
account for many key observations made with respect totumour genetics. Assuming for the moment that the
hypothesis realistically reflects the processes of tumour
initiation and subsequent development, then the spontaneousor induced mutation of rate-limiting and tissue-specific genes
will be critical. Albeit less forcefully, the data discussed in this
Chapter also imply that induced mutation of other genes in
the neoplastic pathway for a given tissue will tend to be of lessimportance. This would be particularly true if a mutator
phenotype, as described earlier, were to arise relatively earlyin the development of a malignancy; evidence for such early
development of genomic instability is accumulating [S34].
With such mutator phenotypes, secondarymutations might be
expected to arise in a developing neoplastic clone at a
sufficiently elevated spontaneous rate for exogenous DNAdamaging agents at low doses to have no great effect on
subsequent tumour development. Some caution is needed,
however, before concluding firmly that the majority of
induced neoplasms will spontaneously acquire genomic
instability during malignant development. In this context it
has been argued that current models of tumorigenesis place
too much emphasis on the elevation of mutation rates and that
cellular selection of evolving clones is more critical [T8].

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES
Gradual accumulation of mutations (K-ras, p53)
and loss of heterogeneity (DCC)

NormalColonic epithelium
Muscularis mucosae

Loss of heterogeneity(p53)

AdvancedadenomaInactivation of APC(mutation and loss of heterogeneity)
Early
adenoma127

High grade
dysplasia

Submucosa

Carcinoma

Muscularis propria
SerosaInvasion

Multiple lossof heterogeneity
events;genomic chaos

Regional
lymph nodemetastases

Periods of stableclonal domination

Figure XXII. A model of the sequence of genetic events in neoplastic
development in the human colon [B10].
Initial mutations of loss of heterogeneity at the APC locus of a colonic epithelial cell is followed by adenoma
development involving k-ras mutation and DCC loss.
Loss of p53 from advanced adenoma marks the transition between benign and malignant disease characterized inturn by the development of genomic instability, multiple gene losses and invasive behaviour/metastasis to regional
lymph nodes.

5. Non-mutational stable changes
in tumorigenesis324. It has been recognized for some years that nonmutational but stable changes to cellular genomes cancontribute to neoplastic development [C10, F2, U3]. In thelight of knowledge of the role of specific genes in the tumorigenic process, the central questions are whether the activation/silencing of such genes can be identified in neoplasms
and what mechanisms are involved. Such non-mutational
mechanisms are broadlytermed epigenetic and are believed to
involve DNA methylation, genomic imprinting, and changes
in DNA-nucleoprotein structure. As will be seen, thesemechanisms are not mutually exclusive.
325. The DNA methylation status is believed to be one of theprincipal determinants of gene expression, and numerous
studies have revealed widespread changes in the methylation
patterns of the genomes of neoplastic cells [B11]. According
to one theory, these changes contribute to the epigenetic
modulation of gene expression, while another theory statesthat increased abundance of5-methylcytosine serves to elevatespontaneous mutation rates in affected genomic domains.
There is some evidence that both processes can occur, butattention will be given here to gene expression effects.
326. The promotor regions of genes are often rich inislands of CpG dinucleotides. These islands are normally
free of methylation, irrespective of the state of expressionof the genes in question [C10]. Studies with a wide rangeof neoplastic cells have revealed that de novo methylationof CpG islands is frequently acquired, e.g. [D4, J2, J3,
M24]. Such effects have been recorded, for example, in asignificant fraction of sporadic retinoblastoma and renal
tumours for the RB1 and VHL genes, respectively. Recently
the p16 gene (various cancers) and oestrogen receptor gene(colonic cancers) have been shown to be similarly

methylated, sometimes at an early stage of tumorigenesis[I3, I4, M1]. In essence, methylation-mediated epigenetic
changes in somatic gene expression appear to be analternative route to mutation for the inactivation of tumoursuppressor genes.
327. Cytosine methylation of CpG dinucleotides is also
known to be involved in the process of genomic imprinting,whereby specific genes are marked during gametogenesis for
subsequent differential somatic expression [B12, C10]. These
imprints, which inactivate one gene copy of sets of autosomal
genes throughout the genome, are retained throughout
development in spite ofa wave of genome-wide demethylation
during embryogenesis. Since certain genes involved inneoplastic development are believed to lie within imprinted
genomic regions, the remaining active copy will be exposed,
and a single somatic mutation can therefore result in the full
expression of a mutant cellular phenotype [F2, U3].
328. Evidence of the involvement of this gametic form of
imprinting on tumorigenesis was outlined in the
UNSCEAR 1993 Report [U3] and has been discussed in
depth elsewhere [F2]. Although the original geneinactivating role ascribed to gametic imprinting withrespect to the RB1, IGF2, and H19 genes may be correct incertain instances, an alternative process may also operate.
According to this second hypothesis, genomic imprinting
serves principally to repress the expression of one somatic
copy of growth-promoting genes. Loss of imprinting (LOI)
during tumorigenesis acts to de-repress this normally silent
copy, thereby increasing gene dosage and contributing to the
deregulation of cellular growth and development. The datathat support this second hypothesis include N-MYC gene
amplification in neuroblastoma, loss of imprinting incolorectal cancer, and certain aspects of H19 gene activation
and BCR-ABL gene fusion, together with the overall pictureof DNA demethylation in neoplasia [F2, R10, M31].

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329. However, a considerable degree of uncertainty attaches
to the contribution of genomic imprinting in tumorigenesis.
Overall it appears that classical region-specific genomic
imprinting established during gametogenesis may not play alarge role in the development of common tumours. On theother hand, somatic changes in gene expression that do
involve changes in the methylation status of critical genes may
be widespread in common tumours and contribute significantly to their development.

333. Thus it is becoming clearer that region-specificchanges in the heterochromatic state of chromosome
regions can have profound effects on gene expression.
Given the accepted role of gene expression changes inneoplasia, it would be surprising if the acetylation-related
MOZ-CBP fusion noted above proved to be an isolated
example of oncoprotein involvement, and recent studies
point towards a more general role in neoplasia of the genes
involved in modifying chromatin [K18].

330. The third stream of knowledge concerning epigenetic
changes in gene expression derives from relatively recent
findings in yeast concerning the nature of DNA-nucleoprotein
interactions and its relationship to chromatin structure. Socalled mating-type switches in yeast depend on the silencing
ofHM mating loci bytrans-acting factors [R11]. The silencingof HM loci has been shown to occur via the action of silent
information regulator (Sir) proteins; these also act on silent
genes close to chromosome termini. Current evidence favours
a role for a complex of Sir and other regulators in sequencespecific binding to silent target genes, with the acetylation ofneighbouring histone proteins as a critical factor; CpG island
methylation may also be important.

6. Summary

331. Overall it seems that gene silencing demands theformation of tightly folded nucleoprotein configurations inchromatin (heterochromatization), with the Sir proteinsplaying a role in establishing the necessary pattern of histone
deacetylation. In subsequent studies, a mutant form of theyeast gene SAS2 was found to enhance the loss of genesilencing; the yeast and mammalian forms of this gene havebeen shown to have sequence homology with several known
acetyltransferase genes. Again, a role in the formation ofheterochromatin structure is implied [R11]. These and othermechanisms that link gene expression with chromatin
structure have been considered in depth [E4]. Knowledge ofthe biochemistry and genetics of Sir proteins also provides
evidence that their diverse functions include regulation of thecell cycle and repair of DNA double-strand breaks. It has beensuggested that such regulation may involve the provision ofheterochromatic sites for the storage of DNA repair andreplication proteins [G19]. DNA strand break repair isconsidered in depth in Annex F, “DNA repair andmutagenesis”.
332. An association between these gene-silencing
observations in yeast and tumorigenic processes in man
was established by the finding that MOZ, a humanhomologue of yeast SAS2, was a partner in a fusion gene
(MOZ-CBP) generated by the primaryt(8;16) chromosometranslocation in human myeloid leukaemia [B13].
Although critical evidence is lacking, it seems feasible thatthe fusion protein could act to redirect MOZ acetylation
function to an inappropriate set of genomic domains. In
this way normal patterns of heterochromatization and geneactivation/silencing would be compromised. Further tothis, there is now evidence of a synergy between DNA
demethylation and inhibition of histone deacetylase in there-expression of genes silenced during tumorigenesis[C18].

334. Proto-oncogenes and tumour-suppressor genes
control a complex array of biochemical pathways involvedin cellular signalling and interaction, growth, mitogenesis,
apoptosis, genomic stability, and differentiation. Mutationof these genes can, in an often pleiotropic fashion, compromise these controls and contribute to the multi-stage
development of neoplasia. Mutant proto-oncogenes disturb
cellular homeostasis in a dominant gain-of-function
manner, whereas for tumour-suppressor genes sequential
loss-of-function mutation of both autosomal copies isusually, but not always, required. Thus, proto-oncogene
mutations are invariably subtle, while the mutations oftumour-suppressor genes can range up to gross DNA
deletion.
335. On the basis of accumulating knowledge it is argued
that early proto-oncogene activation by chromosomaltranslocation is often associated with the development oflympho-haemopoietic neoplasia. In contrast, for many solid
tumours there is a requirement that tissue-specific tumoursuppressor genes that act as gatekeepers to the neoplastic
pathway must undergo mutation; some of these mutations
directly or indirectly affect control of the cell cycle andapoptosis. On the basis that solid tumour initiation is mostfrequently associated with tumour-suppressor gene mutation,it has also been proposed that the subsequent onset of
spontaneous genomic instability via further clonal mutation isa critical event in neoplastic conversion from a benign to amalignant phenotype. Loss of apoptotic control is believed to
be an important feature of neoplastic development and is
described in more detail later in this Annex. Apoptosis as a
response to radiation is also discussed in Annex F, “DNArepair and mutagenesis”.
336. In spite of continuing gains in knowledge, it isimportant to recognize that much of the information
available on multi-stage tumorigenesis remains incomplete,thus limiting the predictive power of mechanistic models
that seek to describe these complex cellular processes.
Although the concept of sequential and interacting genemutations as the driving force is more firmly established,
there is a lack of understanding of the complex physiological interplaybetween these events and its consequences
for cellular behaviour and tissue homeostasis. It is also
important to stress that the concepts outlined in thisSection derive from detailed studies in a somewhat limited
set of tumour types; there is an inherent danger in applying
a single mechanistic concept to all or many tumour types.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

337. Uncertainty also surrounds the degree to which nonmutational (epigenetic) changes to the genomes of neoplastic
cells contribute to tumorigenesis. Increases in the methylation
status of critical tumour-suppressor genes is known to be analternative to mutational inactivation in a range of neoplasms,
and loss of methylation imprints may also serve to increase
the activity of some growth-promoting genes. DNA methylation is also believed to be involved in the genomic imprinting
processes occurring during gametogenesis, but these may not
make a major contribution to tumorigenesis. New evidence
also implicates histone acetylation in genomic heterochromatization and gene silencing. It is suggested that suchgene silencing may make a potentially important contribution
to epigenetic change.
338. An important feature of recent studies has been theclarification of the role of specific gene mutations in tumoursthat serve to destabilize the genome, thereby allowing for the
rapid spontaneous development of clonal heterogeneity andtumour progression. Although critical evidence is lacking, it
is possible to envisage that after this transition point isreached, tumour development may be relatively independentof exogenously induced DNA damage.

C. CELLULAR AND MOLECULARTARGETS FOR TUMOUR INITIATION
339. In the UNSCEAR 1993 Report [U3], the Committee
reviewed data for appraising the cellular targets that are or
might be involved in tumour initiation. The critical question
posed was whether the mutation of single genes in a single
normal target cell in tissue could, in principle, divert thatmutant cell into a potentially neoplastic pathway. At the timeof the UNSCEAR 1993 Report, the evidence available broadly
supported this view. Uncertainties on this issue were, however,recognized, and since that time there have been further
developments, which are summarized in the following
paragraphs.
1. Monoclonal origin of tumours
340. The critical features of human and animal tumours that
lend support to the single-cell (monoclonal) origin of tumours
are that they exhibit (a) consistent and characteristic
chromosomal and/or gene mutations in all neoplastic cells, (b)clonality with respect to the expression of X-chromosomeencoded genes in tumours of females, and (c) characteristic
monoclonal restriction enzyme polymorphism of known andanonymous DNA sequences. It has also been noted thatmolecular analysis of human tumours associated with
exposure to chemical carcinogens and ultraviolet radiation,
together with that of tumours arising in genetically predisposed individuals, adds weight to the concept that the
majority of tumours are of single-cell origin [U3].
341. It was recognized, however, that because such analyses
are performed on macroscopic neoplasms, monoclonality
might, in some circumstances, be due to cellular selection via
proliferative advantage, i.e. initially neoplasms are pre-129

dominantly polyclonal but become increasingly monoclonal
during early growth. Although this issue remains somewhat
problematical, a number of recent observations allow further
comment.
342. The first observation concerns tumours that are believed
to have their origins in utero. Some leukaemias arising inmonozygotic twin children have, in the past, been shown toshare the same primary chromosomal anomaly, implying, but
not proving, that they arose in a monoclonal fashion from anearly precursor cell population present when the two fetuses
shared a common (in utero) blood supply. This interpretation
has been greatly strengthened by the finding that suchleukaemia in monozygotic twins can have identical molecularrearrangements of a proto-oncogene termed Mll1 [F6].
343. The monoclonality of childhood solid cancers is alsostrongly supported by the finding that a specific tumoursuppressor-gene-associated chromosome loss/reduplication
event in early embryogenesis can lead not only to mosaicism
in normal tissue but also to the development of monoclonal
Wilms’ tumour [C11].
344. A second line of evidence concerns further
developments in the understanding of multi-stage colon
carcinogenesis [U3]. Mutation/loss of the tumour-suppressor
gene APC has for some time been believed to be a critical
early event in the development of human colon cancer. Up toabout 70% of early colonic adenomas show apparently
monoclonal structural/functional loss of this gene [P5], and acritical role in tumour initiation seems likely [B10].
345. With use of a mouse (Min) model of intestinal carcinogenesis, this view of monoclonal tumour initiation has beenstrengthened. In essence, aberrant crypts, the earliest
intestinal lesions detectable microscopically, have been
microdissected from Min mice and shown to be monoclonalwith respect to Apc loss [L18, L19] (but see also para. 284].
This and another mouse model of myeloid leukaemia,
described below, have also been used to provide evidence ofearly monoclonal events associated with radiation tumorigenesis. There are also data describing early events in thymic
lymphomagenesis [M33].
346. Given the recent evidence outlined in this Annex andthat previously reviewed by the Committee [U3], it seems
likely that the vast majority of tumours in humans andanimals arise from mutation of single target stem-like cells intissues. This view continues to find the support of mostcommentators but has been debated widely [A3, F7, F8, R12].
347. The Committee also noted publications where tumour
monoclonality has been questioned [U3], and a striking
contribution to the debate was recently published [N3]. The
basic finding was that a large proportion of intestinal
adenomas in the gastrointestinal tracts of human familial
adenomatous polyposis patients, who were also XO/XY in
genotype and therefore mosaic for the Y-chromosome, wasapparently polyclonal. In this study polyclonality was judged
by the presence within single adenomas of a mixed

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population of cells with respect to the Y-chromosome
sequence. By this measure, up to 76% of adenomas werepolyclonal. However, early Y-chromosome loss and field
effects creating tumour clustering and collision [U3] might
contribute to this finding.
348. Tumour clustering may also explain new data on theapparent polyclonality of a proportion of spontaneously
arising intestinal adenomas in Min mice as assessed bygenetic features other than Apc loss [D9]. Thus, polyclonality
may be acquired during adenoma development rather thanarising de novo at the time of initiation. For example, fusionof independent Apc-deficient microclones may allow forcooperative growth. These data illustrate some of the problems
that remain in resolving the early molecular events andcomplex cellular interactions of tumour development. In spiteof these uncertainties there remain experimental data on
intestinal tumorigenesis that forcibly support monoclonality
for induced neoplasms [G11].
2. Molecular targets for radiation
tumorigenesis
349. Following its review of the mechanisms of mutagenesis,
oncogenesis and the data available on molecular targets, theCommittee suggested that loss of critical tumour-suppressor
genes via DNA deletion might be the principal mechanism bywhich radiation damage might contribute to tumour development [U3]. It was suggested that proto-oncogene activation via
point mutation or chromosomal rearrangement played a lesscritical role overall but might be important for certain
tumours.
350. Direct human data relating to these issues remain,
however, fragmentary. As noted in Annex F, “DNA repair
and mutagenesis”, more data have emerged on TP53 genemutations in radiation-associated human tumours, particularly lung tumours. Unfortunately the interpretation of thesedata remains highly problematical, and at present it is notpossible to judge whether intragenic mutation of this gene isan earlyradiation-associated event in anyhuman tumour type.
TP53 gene mutations have also been studied in liver tumours
arising in excess in patients treated with the radiographic
contrast agent thorotrast, which contains alpha-emitting
thorium oxide [I14]. These studies comment more on
secondary TP53 mutation than on early radiation-associatedevents in liver tumorigenesis.
351. In the case of human thyroid cancer, chromosomallymediated rearrangement of the RET proto-oncogene is acommon but not invariate feature; such events are believed tooccur early in the genesis of the papillary form of this tumour[Z1]. RET proto-oncogene rearrangements have been found insome cases of papillary childhood thyroid cancer arising inareas contaminated by the Chernobyl accident. Since threedifferent forms of RET rearrangement are present, overall,among spontaneously arising papillary thyroid cancer cases,
it is possible that in radiation-associated cases one particularform will predominate. A recent commentary [W4] on one
data set suggests that the spectrum of these rearrangements in

Chernobyl-related papillary thyroid cancer is unremarkable,
although in other studies [B14, K13] one type (RET/PTC3)
appeared to be more frequent than expected. A causal
relationship between RET rearrangement and radiation
remains, therefore, a matter ofsome uncertainty. Nevertheless,the finding of RET rearrangement following experimental
high-dose irradiation of human thyroid cells [M25] suggeststhat specific and rare RET proto-oncogene rearrangements
associated with the genesis of these tumours can be induced byradiation.
352. The study of second cancers after radiotherapy [C12,C28] provides another direct approach to the problem.
Investigations of gene-specific mutations in such tumours
have yet to be particularly informative, and at this stage ofknowledge cytogenetic approaches may prove to be moreproductive. Studies that include cytogenetic evaluation oftherapy-related sarcoma, meningioma, and rectal carcinoma
provide some evidence that chromosomally complex monoclonal tumours having hypodiploid karyotypes with multiple
deletions may be most common [C13, C28]. The number oftherapy-related tumours characterized in this way remains,
however, too small to make these findings conclusive.
353. With respect to target DNA regions and genes forradiation tumorigenesis, more rapid progress is being madethrough the use of experimental models of tumorigenesis inrodents. Regarding as yet uncharacterized molecular targets,
some studies with breast and thyroid clonogens provide
evidence of an apparently high frequency of tumour-initiating
events that, it is argued, may reflect the involvement of nonmutational processes [C19, U3]. Uncertainties attaching tothe
status of these events are discussed later in this Annex, andhere attention will focus on mouse models that more
specifically suggest genomic targets for tumorigenesis. Three
examples of this work are given below.
354. In a mouse genetic model of germ-line p53-deficiency(p53+/), quantitative studies of tumorigenesis (principally
lymphomas and sarcomas) showed these mice to be extremely
sensitive to tumour induction by an acute dose of 4 Gy fromgamma rays [K2]. Of particular note was the shortening of thetumour latency period after irradiation. Molecular studies ofthese tumours revealed that complete loss of wild type p53
was a consistent event; these data, together with those fromquantitative studies, provided good evidence that p53 andsurrounding sequences could act as a direct target forradiation.
355. A somewhat unexpected finding was that in almost allcases of p53 loss from induced tumours there had been
duplication of the mutant p53 gene. A likely reason for thiswas provided by subsequent in vivo analyses of post-irradiation cytogenetic damage in the haemopoietic system ofmurine
p53 genotypes [B4]. These studies showed that although p53deficiency had only marginal effects with respect to thefrequency of structural chromosomal rearrangements afterradiation, there were substantial effects on whole chromosome
loss and gain (aneuploidy). This enhancement of radiationinduced aneuploidy appeared to be driven by a p53-associatedANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

defect in a G2/M cell-cycle checkpoint. Thus, it was suggested
that loss of wild type p53 occurred through loss of the wholeof the encoding chromosome (chromosome 11). For the
purposes of establishing genetic balance and viability, there
was strong selection for those cells that had duplicated theremaining chromosome 11, which accounts for the duplicationof mutant p53. On this basis it may be seen that in somecircumstances the molecular target for radiation oncogenesis
may be as large as a whole chromosome.
356. Somewhat similar studies have been undertaken using
the Min mouse (Apc+/) model of intestinal carcinogenesis[S18]. Using F1 hybrid mice carrying the Apc mutation, a
2 Gy whole-body dose from x rays has been shown to double
the spontaneous incidence of intestinal adenomas [E5]. In
hybrid genetic backgrounds it is possible to determine throughthe use of polymorphic microsatellites whether spontaneousand radiation-associated adenomas arise through early loss ofwild type Apc and, if so, the type/extent of the mutation
involved. Published studies [H32, L20] reveal that complete
loss of wild type Apc is characteristic of the majority of bothspontaneously arising and radiation-associated early adenomas. These mutational events may involve whole chromosome loss or interstitial deletions, but deletion events tend to
predominate in radiation-associated adenomas [H32]. Again,
therefore, a tumour-suppressor gene appears to be acting as adirect target for radiation, with gene losses usually being
associated with gross deletion events.
357. The third example of informative animal data concerns
the induction by radiation of acute myeloid leukaemia (AML)in certain strains of mice. It has been known for some years
that these acute myeloid leukaemias are consistentlyassociatedwith early arising deletions from chromosome 2 [B15, H15].
More recently, however, cytogenetic studies of bone marrowcells of irradiated mice [B16] have revealed that characteristic
chromosome 2 deletions are apparent within the first few days
following in vivo irradiation. Carrier cells of stem-like origin
remain, however, relativelyindolent in bone marrow for many
months until unknown secondary events trigger them into
rapid clonal expansion prior to the development of overt
monoclonal acute myeloid leukaemia. The identityof the geneloss from mouse chromosome 2 that initiates acute myeloid
leukaemia development remains unknown, but the criticalchromosomal region encoding an acute myeloid leukaemia
suppressor gene has been narrowed to around 1 cM (~106 basepairs) [C5, S40]. Thus, data on the mechanisms of radiationinduced murine acute myeloid leukaemia point to tumourinitiating loss of gene function from stem-like cells in bonemarrow, followed by the accumulation of spontaneous
secondary events that trigger initiated cells into a pathway
leading to monoclonal leukaemia development.

3. Summary
358. On the basis of a large body of data it may be judged
that, in the main, tumours appear to have their origin in
gene/chromosomal mutations affecting single target stem-likecells in tissues. It is recognized, however, that there are131

circumstances where earlyphases oftumour development may
be bi- or even polyclonal and that monoclonal selection occurs
later.
359. Direct evidence on the nature of radiation-associatedinitiating events in human tumours is sparse, and rapid
progress in this area should not be anticipated. By contrast,
good progress is being made in resolving early events inradiation-associated tumours in mouse models. In the case oftumours induced in p53 and Apc heterozygously deficient
mice, radiation appears to target the remaining wild typetumour-suppressor gene via gross chromosomal deletion.
Radiation-induced deletion ofa specific chromosomal segment
also appears to act as an initiating event for mouse acute
myeloid leukaemia. These molecular observations lend further
support to the view expressed in the UNSCEAR 1993 Report[U3] that radiation-induced tumorigenesis will tend toproceed
via gene-specific losses from target stem cells.

D. CELLULAR FACTORS THAT COUNTERONCOGENIC DEVELOPMENT
360. Somatic cells employ a series of measures to protect
against the development of abnormal and potentiallyneoplastic phenotypes. In essence, a certain proportion ofthese barriers has to be breached by the cell before it
becomes committed to malignant development. Thus, theprocess of multi-stage oncogenesis may be viewed as the
stepwise acquisition of cellular properties that allow
evasion of these protective functions [U3].
1. Control of cellular proliferationand genomic stability
361. The ordered replication of DNA during the
reproductive cell cycle, the equal sharing of the replicatedgenome to the daughter cells, and the close control of mitotic
activity is an essential element of normal tissue development
and maintenance [U3]. Under normal circumstances cells can
respond to specific mitogenic stimuli, continue proliferation
while that stimulus is maintained, and fall to a resting state
when it is removed. Such normal somatic cells are also
believed to have a finite lifespan, and as a consequence of aninternal genetic programme, after completing a given series ofreproductive cycles, they cease proliferative activity and entera degenerative senescence phase. There is also a strong
requirement for phase controls within the reproductive cycle
itself, such that DNA replication is appropriately initiated andcompleted before genomic segregation to daughter cells andthat in the event of non-optimal cellular conditions, the cell
cycle is checked until the problem is rectified. Cell-cycle
checkpoints may be particularly sensitive to induced DNA
damage, and there is some evidence that the presence of very
few DNA double-strand breaks can lead to cell-cycle arrest[H23].
362. In recent years much has been learned of the control ofcellular proliferation [N2], the process of cellular senescence[H3, H5, S4], and the importance of cell-cycle checkpoint

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ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

control [H7] for maintenance of genomic stability. As thisinformation accumulates, it has become evident that all of theabove normal controls are potentially subject to mutationalchange during multi-stage oncogenesis and therefore require
some consideration in the modelling of tumour development.
363. The information discussed previouslybythe Committee[U3] and in Section B of this Chapter includes a number ofexamples ofhowgene/chromosomal mutations and sometimes
epigenetic events in tumours can compromise control of thecell cycle (e.g. RB1, TP53, and p16) and/or decrease genomic
stability, e.g. TP53, DNA mismatch repair genes, and ATM.
As noted earlier, the resulting abnormal patterns of cellular
proliferation and the generation of clonal heterogeneity are
sentinel features of neoplastic growth, representing the escapefrom normal cellular constraints. Thus, precise control of thecell cycle and high-fidelity DNA damage recognition/repair
are clearly important protective factors against tumour
development. Also associated with proliferative control andgenomic stability are the DNA sequences present at
chromosome termini (telomeres).
364. The characteristic hexamer repeat sequences(TTAGGG) at mammalian telomeres erode via incomplete
replication during each cell cycle. Since the majority ofhuman somatic cells lack expression of the enzyme telomerasethat adds these hexamer repeats to telomeres, it has beensuggested that the process of erosion acts as a “molecular
clock” (see [H16, K14]). In this way the replication potentialof somatic cells is limited, and there is direct evidence that thesenescence of human cells is, in some part, determined by theabsence of telomerase [B35, J4].
365. During the senescence process as measured in vitro,
there is a tendency for cells to become chromosomallyunstable, with many of the resulting aberrations being centredon chromosome termini [C14, K14]. Thus, telomeric erosion
during senescence renders chromosomes prone to end-to-end
association and subsequent cycles of breakage and fusion.
366. It is believed that telomere-sequence-mediated cellularsenescence is one of the means whereby cells may beeliminated from neoplastic pathways. It follows, therefore, thatin human cells the stabilization of telomeres and thegeneration of immortal or lifespan extended phenotypes islikely to be a critical step in tumorigenesis [G9, K14]. In
accord with this proposition many, but not all, human tumours
have been shown to carry stabilized telomeres via reactivationof telomerase or utilization of alternative pathways oftelomere
maintenance (see [B17, K14]).
367. Although good progress continues to be made in thiswhole area, a simple relationship between senescence,
immortalization, telomerase, and tumorigenesis should not
be inferred [B17, J4, L21].
2. Programmed cell death and gene expression
368. Programmed cell death, also termed apoptosis, plays
an important role in restricting the growth of many normal

cell lineages and is an important element in the regulationof organ development and maintenance [R3]. Apoptotic
processes are believed to be controlled by the interaction of
intra- and extracellular factors with the signalling machineryof the cell. These signals, or in some cases their absence, cantrigger the recipient cell into a characteristic biochemicalsuicide pathway that usually involves genomic degradation.
Importantly, apoptotic responses can also accompany
exogenous insult, induced by ionizing radiation, genotoxic
chemicals, and other sources of stress; in some cellular
systems these responses have been associated with priorperturbation of the cell cycle. The biochemistry and genetics
of apoptosis are becoming much better understood, andadvances in the whole area have been reviewed extensively[C16, H22, K6] and have received comment with respect toradiation protection [S22]. A detailed description of thesemechanisms is beyond the scope of this Annex, but a brief
outline is appropriate.
369. The process of cellular apoptosis may be divided
conveniently into three phases: initiation, effector, anddegredation (nucleolytic and cellular). The initiation phasediffers according to cell type and the source of stress, while
the effector and degradation phases, although regulated, tend
to be more uniform [K7]. As noted in Section B, a range of
proto-oncogenes and tumour-suppressor genes participate in
the intracellular signal cascades that can initiate apoptosis.
Here, information on two apoptosis-related genes, TP53 [E3]and Bcl2 [K7], will be presented.
370. Biochemical studies indicate that almost all productive
mutations of TP53 compromise the ability of the protein tobind to gene-specific DNA sequences and to regulate transcription; in general, the cellular consequences are alterations
in growth arrest or apoptosis. Although p53 protein response
is apparent under a range of stresses, recent studies suggest acommon root. A series of findings (see [K5]) imply that intracellular oxidative stress is a critical trigger for p53-mediatedapoptosis. Together with p85 and perhaps Abl protein, p53 isbelieved to regulate the redox state of the cell [K5, Y3], and
it is this state that may be a common determinant of apoptosis
or survival. Other aspects of p53-dependent and developmentally regulated apoptosis including the role of ceramide
are discussed in Annex F, “DNA repair and mutagenesis”.
371. The second example concerns the genes in the Bcl2
family [K6, K7]. These include Bcl XL, Bcl-w, and Blf-1(death antagonists) and Bax, Bak, Bcl X5, and Bad (death
promotors). The protein products of these genes participate
in a complex network of biochemical reactions that differ
between cell types. These pathways may be linked withRaf, MEK, and Jun amino terminal kinase (JNK) protein
and also, via Ras protein, with the p85 pathway noted
above [K7, P6].
372. In a broad sense it is believed that it is the balance
between pro- and anti-apoptotic factors that determines cellfate [K7]. Thus, apoptotic signals via, for example, cellsurface receptors, redox changes, reactive oxygen species,
and Ca++ ion concentration (initiation phase) are sensed by

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

the Bcl-2 regulatory complex, resulting in changes inmitochondrial permeability (effector phase). According tocurrent proposals, the degredative and nucleolytic phases
then proceed as a consequence of the release of directly
apopto-genic factors, e.g. caspases, superoxide anions, andendo-nucleases from mitochondria into the cellular cytosol[K7].
373. Overall, it may be seen that cells possess a highly
developed system for detecting stress, eliciting biochemical
responses, and, in essence, deciding on the basis of biochemical balance whether to survive or to proceed towards
cell death.
374. The potential of these stress-related apoptotic pathways
to reduce tumorigenic risk, although not formally established,is strongly indicated. There appear to be at least two principal

133

points of action of apoptosis during tumorigenesis. There isevidence for at least three stress-related pathways in cells that
respond to genotoxic insult, including that from ionizing
radiation, i.e. those pathways centred on Abl, JNK, and p53
proteins [C4]; the p53 pathway has been judged to be the“universal sensor” of damage in normal cells. A variety ofother cellular genes have also been shown to be up- or downregulated in response to radiation. While a comprehensive
review of such studies is beyond the scope of this Annex,
Table 15 provides, by way of example, a summary of the dataobtained after neutron or gamma-ray exposure of Syrian
hamster cells [W10]. These results were obtained after
exposures of 0.21!2.0 Gy of neutrons or 0.96!3.0 Gy of
gamma rays at low and high dose rates. These data should
not, however, be taken as representative of mammalian cells
in general, and cell type dependency in induced gene
expression should be expected.

Table 15
Radiation effects on gene expression in Syrian hamster cells[W10]
Effect on expression a
GeneInterleukin-1
β-actin
γ-actin
β-PKC
Rp-8
c-fos
c-myc
α-tubulin
fibronectinInterleukin-6
Proliferating cell nuclear Ag (PCNA)
Superoxide dismutasec-jun
RbH4-histonep53
aFunction
Neutrons

Gamma rays

Increase
Decrease
Increase
No change
Increase
Decrease
No change
Increase
Decrease
Increase
Increase
Increase
Decrease
Decrease
No change

Increase
Decrease
Increase
Increase
No change
Increase
No change
Increase
Increase
Increase
Increase
Increase
Increase
Increase
No change

Cytokine
Cytoskeleton
Cytoskeleton
Signal transduction
Apoptosis
Transcription factor
Nuclear protein
Cytoskeleton
Cellular matrix
Cytokine
Transcription factor/repair
Scavenger
Transcription factor
Nuclear protein
Nuclear protein
Nuclear protein

All changes evident within the first four hours following radiation exposure. Neutron dose rates: 1 mGy min -1 and 140 mGy min -1; gamma raydose rates: 10 mGy min -1 and 120 mGy min -1.

375. The development of high-throughput screening
technologies promises to greatly increase the power ofresolution of studies on such radiation-associated changes ingene expression in mammalian cells. For example, using
these new techniques a linear non-threshold dose response forthe transcriptional induction of the stress-related genes
CIP1/WAF1 and GADD45 has been demonstrated for gamma
ray doses in the range 20!500 mGy [A18]. The consequencesof such induced stress responses for low-dose tumorigenesis
remain a matter for speculation. Nevertheless, an association
between the in vitro induction of PBP 74 gene transcription by
250 mGy radiation and human cell hypersensitivity to cell
inactivation might be explained by some form of damage
threshold for the enhancement of DNA repair [A18, S41].
One speculation is that if such a hypersensitive mechanism forcell inactivation were to dominate at low doses (say, up toaround 100 mGy), mutation induction rates would be

depressed, leading to a non-linear and, perhaps, a thresholdtype relationship for radiation cancer risk [J8]. If, however,this increased sensitivity to cell inactivation were to beaccompanied by increased cell mutation rates, no such
threshold would be expected. The data available do not allow
these two possibilities to be distinguished, although some ofthe data discussed in Annex F, “DNA repair andmutagenesis”, suggest a direct dose-effect relationshipbetween cell inactivation and gene mutation. Stress-related
cellular responses are also discussed in Annex F, “DNArepair and mutagenesis”, which draws attention to new workthat associates the appearance of specific novel proteins withcellular stress.
376. In the absence of complete DNA repair fidelity, thewhole organism gains a large advantage by promoting thedeath of damaged and potentially neoplastic cells. However,134

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

the true effectiveness of apoptotic pathways in removing such
aberrant cells cannot be judged at this time. The mere fact thatthe frequency of gene/chromosomal mutations increases incell populations surviving genotoxic insult argues against an
extremely high capacity for apoptotic surveillance of mutagenic damage in all cell types. In the context of ionizing
radiation, the shape of the low dose response for the inductionof apoptosis in different cell types remains very uncertain, andequal uncertainty surrounds the influence of dose rate.
Accordingly, for the purposes of modelling tumorigenic risk,
judgements on the balance between mutagenesis and apoptosis
at low doses cannot be made with confidence. The
radiobiological factors that influence the induction of
apoptosis vary with cell type, and there is also somedependency on the mechanisms involved [B27, S29]. In
general, doses greater than 0.5 Gy of low-LET are necessary
to obtain statistically significant increases in apoptotic rates;
a plateau in the dose response is frequently seen at doses>5 Gy. In the well-studied human lymphocyte system there isevidence that the induction of apoptosis is largelyindependentof LET and dose rate, implying that in these cells, initial DNA
damage is more important than its repair [V4]. DNA doublestrand lesions are believed to be one of the determinants ofapoptotic response, but some have suggested that damage toplasma membranes may also act as an apoptotic signal [O4].
There is also evidence of linkage of the signalling of apoptosis
and cell-cycle arrest; for example, a protein known as survivin
has been implicated in the control of both apoptosis and a
mitotic spindle checkpoint [L37]. Additional aspects ofapoptotic response are discussed in Annex F, “DNA repair
and mutagenesis”.
377. Apoptosis is also believed to play an important rolein tumour cell survival during post-initiation clonal
expansion. At a critical point during clonal expansion, theoxygen supply to the neoplasm begins to become limiting[F4, U3]. It has been proposed that under these circumstances the redox stress placed on tumour cells triggers anapoptotic response, with cell death being most pronounced
in the regions most distant from vascular supply [K5, N5].
Thus, during this phase in tumorigenesis, apoptosis will beplaying a crucial role in limiting in situ growth andinvasive behaviour.
378. Given the scenarios noted above, it is not surprisingthat a broad array of tumour types carry a variety of mutant
genes that directly or indirectly uncouple stress response
and apoptosis. Resistance to apoptosis may be viewed as
the means wherebycell survival is favoured over cell death,and under conditions of in vivo stress, this phenotype willtend to be strongly selected. The p53 pathway appears to be
the universal sensor of cellular stress, and it is this featurethat may make loss-of-function TP53 mutations so
prominent in human tumorigenesis.
379. Overall, it is judged that apoptotic suicide of cells
provides an important protective mechanism against aberrant
cell growth and neoplasia. However, via gene-specificmutation, a number of potential bypasses or mechanisms oftolerance are available.

3. Cellular differentiation and othercellular interactions
380. The stepwise accumulation of genetic/epigenetic events
demands continuing growth potential in cells that havesustained a neoplasia-initiating event. Running counter to thisis the normal process of terminal cellular differentiation,
whereby uncommitted progenitor cells assume specialized
functions in tissues and no longer retain proliferative
potential. Thus, a developing subpopulation of cells maycarrya tumour-initiating mutation that, for example, deregulates
cellular proliferation but in the absence of further phenotypicchange will complete a quasi-normal programme of terminal
differentiation mediated by cellular interactions.
381. In this way neoplasia-initiated cells will, in the absenceof other changes, exit the pathway to malignancy. Thus, theantiproliferative process of terminal differentiation will tend
to be rate-limiting with respect to overt malignancy and may
be evidenced by the accumulation of benign lesions in tissues.
There are numerous examples of associations between protooncogene/tumour-suppressor gene functions and cellulardifferentiation/development [K1, L1, R2, S3]; here it will be
sufficient to give only a few examples.
382. In the case of the lympho-haemopoietic system, thedevelopment of the different cell lineages is known to depend
on a complex interplay between cell-cell interaction, cytokines, and intracellular signalling cascades [O2]. The genes
AML1 and tal/SCL have been implicated in haemopoietic
stem-cell differentiation and, in mutant form, are known tocontribute to the genesis of certain types of leukaemia. Other
examples of leukaemia/lymphoma-associated genes with roles
in normal differentiation processes include genes of the Hox
and Pax families, RBTN2, RARA, and Mll1 [O2, R2]. As
noted in Section B, the functional development of the T- andB-haemopoietic cell lineages is highly dependent on therecombination of immune gene sequences, and specific misrecombination of these sequences makes a major contribution
to T- and B-cell neoplasia. Recent evidence also links
downstream signals from these recombinogenic processes
with subsequent clonal growth and differentiation; that is,normal growth and differentiation of cells may be blocked
if recom-bination does not proceed normally [W5]. In
general, it may be concluded that many mutations inlympho-haemopoietic neoplasia serve to compromise theclosely controlled process of cell-lineage-dependent
differentiation.
383. Similar evidence exists with respect to solid tumours[L14]. For example, via its interaction with Ras proteins,the protein product of the NF1 tumour-suppressor geneplays a role in regulating the normal growth anddifferentiation of neural cells, and the Ras/Raf/MAP kinase
intracellular signalling pathway appears to play a moregeneral role in the regulation of cellular differentiation. In
addition, there are data that support a role for tumourassociated catenin/APC, Rb1, and DCC proteins intranscriptional regulation/cellular signalling processes that
control cell-lineage-dependent growth and differentiation.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

384. Thus, manyforms ofproto-oncogene/tumour-suppressor
gene mutations, often in combination and via perturbation ofcellular signalling, will have dual effects on cellular growthand differentiation. The maintenance of constitutive growth ofstem-like cells having differentiation defects maybe viewed as
an important element in the early phases of tumour
development. In essence, the homeostatic imbalance createdby these mutations will tend to promote the clonal expansionof cells having limited potential for terminal differentiation.
Alone, however, such clonal growth may not be productive,
since the cells will be potentially subject to senescence andapoptosis, both of which serve to limit the opportunity toaccumulate the further mutations necessary for overtly
malignant development. Nonetheless, as noted earlier, each of
the processes of senescence and apoptosis may itself becompromised by gene mutation, e.g. by telomerase
deregulation and TP53 gene mutation, respectively. In
principle, therefore, extended clonal growth can be achieved.
Further to this, and in accord with previous discussion, theearly appearance of genome-destabilizing mutations may
dramatically accelerate neoplastic development.
385. Overall, the frequencywith which the genes involved innormal cellular differentiation are mutated in tumours
testifies to the protective function offered by terminal cellulardifferentiation to a non-proliferative state. It is judged,
however, that alone, such aberrant differentiation is usually
insufficient for full malignancy and that cooperating
mutations that further extend clonal lifespan and/or destabilize
the genome are likely to be required.
386. Intercellular transmission of biological signals followed
by intracellular biochemical cascades is believed to be an
integral component of the differentiation of cells. Not surprisingly, cell-to-cell communication has alsobeen implicatedin the expression of neoplastic phenotypes, and more recently,
cellular communication has been shown to influence radiation
response.
387. The UNSCEAR 1993 Report [U3] reviewed the role ofcellular communication via gap junctions in neoplastic
development. In summary, it is believed that theestablishment of such intercellular communication can lead tothe suppression of neoplastic features byneighbouring normal
cells. During clonal evolution, however, many tumour cells
lose the capacity to communicate with normal cells and in thisway become less receptive to tissue regulation, i.e. they
become increasingly autonomous. Mechanistic links between
gap junction processes, tumour promotion, and cell cycle
control were also discussed in the UNSCEAR 1993 Report[U3]. In respect of radiation response, the term “bystander
effect” has been coined to describe a range of in vitro
responses occurring in unirradiated cells that are close
neighbours of others receiving a given radiation dose, usuallyfrom single ionizing particles.
388. The effects seen in bystander cells include changes ingene expression [A19], lethality [M49], sister chromatid
exchange [D13], chromosome breakage [P21], and genemutation [N11]. The mechanisms involved are not well135

understood but are believed to involve the transfer of factors
from irradiated cells via the extracellular medium or via
intercellular communication [A19, M50]. Such effects have
yet to be demonstrated in vivo, and their consequences fortumour risk cannot be judged. In the context of this Annex,
the most important data set concerns apparent alpha-particleinduced bystander effects on gene mutation [N11]. Thesestudies imply that at a low fluence of alpha particles, thefrequencyof gene mutations arising in bystander cells exceeds
by up to fivefold that in cells intersected by single particles. At
higher particle fluences, the bystander contribution to
mutation rates decreases, and in this way the dose response formutation induction is supralinear, with a steep rise at doses
below ~50 mGy. Assuming that there is a direct relationshipbetween mutation rate and tumour risk, the data noted above
imply that per unit dose of alpha particles, tumour risk atdoses below ~50 mGy is substantially greater than that at
higher doses. Whether these data represent an importantsource of uncertainty in high-LET radiation risk estimates
must await replication of the study and the establishment of its
generality, particularly in the in vivo situation. Other data onradiation response implicate cellular interactions in the
induction of genomic instability [G20, M51] and in adaptive
responses [I15]. Some studies in this general area also imply
that cellular DNA may not always be the principal target forradiation effects, particularly those that may have transient
epigenetic components. Although it remains difficult to
integrate such data intoa mechanistic framework for assessing
tumour risk at low doses, the findings noted above caution
against a dogmatic view in the modelling of dose-response
data on the basis of DNA damage alone.
4. Cellular surveillance
389. Following review of epidemiological, animal, andcellular studies, the Committee concluded that conventionalT- and B-lymphocyte-mediated immune response was not aparticularly effective protective mechanism against thedevelopment of most human tumours [U3]. Although these
immune responses appear to be able to target the specific nonself antigens presented by oncogenic viruses or theirassociated neoplasms, the common radiogenic tumours seemunable to effectively initiate timely immune responses or arecapable of efficiently evading surveillance [B18, U3]. The
Committee did, however, recognize some uncertainty
surrounding the potential protective role of certain classes ofcytotoxic T-lymphocytes (CTL), including natural killer (NK)cells [U3]. A number of novel approaches have been used toresolve some of these uncertainties.
390. One area of recent study [L23] has been to determine
whether the poor immunogenicity of most tumour types isdue to the lack of signals for co-stimulation of full CTL
activity; this is believed to be mediated by specialized
antigen-presenting cells. A study with mice using anantibody to block CTLA-4, a negative regulator of CTL
activation, showed that such a blockade resulted in therejection of transplanted and pre-existing human tumour
cells and also the development of resistance to a second
challenge. It seems, therefore, that with fully malignant

136

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

cells, an effective anti-tumour response can be elicitedprovided that specific immune regulators are manipulated.
391. Another approach to the problem has been to seek
mutational signatures in tumours indicative of evasion of
immune surveillance. In one such study [B19], it was argued
that the RER+ mutator phenotype of certain colorectal
carcinoma cell lines might generate a sufficientlydiverse arrayof mutant protein neo-antigens to elicit a strong CTL
response. If this is the case, inefficient antigen presentation via
the loss of beta-2-microglobulin (β2M) would be strongly
selected in the resulting tumour cell population. Suchcorrelation was observed in a study of 37 cell lines, where thefour mutator lines were the only examples in which β2M
expression was lost.
392. Beta-2-microglobulin associates with polymorphic
heavy-chain glycoproteins in cell membranes for the
purposes of antigen presentation by the resulting class I
major histocompatibility complex (MHC). Intracellularantigens are believed to be transported to the MHC byproteins of the TAP family [E6]. Not only do a substantial
fraction of human tumours lack expression of the class I
MHC, but there is some evidence of the involvement of
TAP gene mutation in tumours. Other strategies adopted bytumour cells in order to evade immune surveillance include
the expression of decoy receptors, the Fas-mediatedinactivation of CTL/NK cells at membrane surfaces, andthe secretion of factors that inhibit or inactivate CTL [V5].
393. In general, these observations, while providing somecorrelative support for the view that evasion of CTL-mediatedsurveillance may be of some importance in tumour
development, provide no information on the effectiveness of
this surveillance in reducing tumour risk. It is, however,intriguing to note emerging evidence of a possible two-way
interaction between genomic instability in neoplasia and CTL
response. On the one hand, the RER+ mutator phenotype may
serve to generate sufficientlystrong tumour antigen signals for
CTL response; on the other, it can provide an enhanced
mutational capacity to evade the resulting T-cell surveillance.
For potentially anti-tumorigenic CTL response, there is also
some evidence to support a model whereby normal cells thatengulf apoptotic tumour cells can migrate to lymph nodes,
where, in principle, they can invoke a response to tumour
antigens [A8].
394. With respect to NK cells, it is now well establishedthat this class of cytotoxic cells can, in principle, exert adegree of anti-tumour activity via the release of factors
such as interferon γ, tumour necrosis factor, and Fas
ligand. There is also some evidence of an additional
antitumour mechanism involving NK attack on tumour
vasculature [B24, F9]. In spite of numerous studies, there
is, however, no convincing evidence of a correlation
between NK abundance/function in vivo and tumour
development or prognosis (see [F9]). In general, this areaof study remains most controversial, and with current
knowledge it is not possible to judge the extent to which
NK cells act to protect against non-viral human cancers.

395. Overall, the role of immune surveillance in protecting
against common neoplasms has yet to be adequately
described, and some studies tend to argue against this
proposition [B18, U3]. Gains in fundamental knowledge willprobably contribute to the debate. For example, the complement protein system is an important determinant of humoral
immune surveillance and is believed to target certain
malignant cells. In accord with this, a novel stress-related
protein has been revealed that appears to participate in thediscrimination of malignant cells byhomologous complement[M26]. The recent observation that the proto-oncogene PMLof human myeloid leukaemia plays a role in the regulation of
antigen presentation in cells also implies the need for somedeveloping haemopoietic neoplasms to evade cellularsurveillance [Z2]. Equally, however, the tumorigenic expression of Apc-deficiency in Min mice is not enhanced by adefect (scid) in immune function [S18]. Thus, recent findings
can be used to both support and question the true role ofcellular surveillance in tumour defence. Studies of low-dose
stimulation of immune functions, e.g. [L58, M54] havepreviously been reviewed by the Committee [U2] and a few
additional studies have been published (e.g. [H36, K30, S42,S43]) in more recent years. Doubts were expressed as towhether the immune system plays a significant role in any
cancer-related adaptive processes at low doses.
5. Summary
396. Through a better understanding of the processes thatmediate multi-stage tumorigenesis it has become evident
that neoplastic development is subject to a number ofcellular constraints. The main constraints are control ofcellular proliferation/genomic stability, the induction ofprogrammed cell death, tumorigenic suppression by cellcell communication, and terminal differentiation to a nonproliferative cellular state. In addition, for at least certain
tumour types there is evidence that immunosurveillancemechanisms can recognize and restrict the growth ofneoplastic cells.
397. These protective mechanisms are believed to provide
a high level of protection against neoplastic growth anddevelopment. In spite of this, there is growing evidence
that during the evolution of tumours, resistance to or
tolerance of all these countermeasures can be developed via
gene-specific mutation. Thus a substantial proportion of
consistent mutations in tumours may be linked directlywith cellular strategies aimed at maintaining viability andgrowth, avoiding terminal differentiation and immune
recognition, and promoting genomic instability such thata wide range of clonal variants are available for the fulldevelopment of malignancy. On the basis of current
molecular genetic knowledge, there seems no good reason
to suppose that different modes of in vivo constraint apply
to spontaneously arising and carcinogen-induced tumours.
Evidence is also accumulating in support of the view that
cellular communication can also influence early in vitro
radiation responses, with possible effects on cellular
recovery, genomic stability, and mutation rates. The
present state of knowledge does not allow for extrapolation

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSESof these findings to tumorigenesis in vivo, but some recent
alpha-particle mutation data, if confirmed, may be ofimportance.

E. DNA REPAIR AND TUMORIGENESIS
398. For the purposes of relating mechanisms of radiation
tumorigenesis to mechanisms that are believed to apply tospontaneously arising disease, it is important to consider ingreater depth the evidence on the role of DNA repair andthe uncertainties that attach to this association.
1. DNA repair as a determinant of
oncogenic response
399. Data relating to the influence of DNA repair onmutagenic and other cellular radiation responses are discussed in Annex F, “DNA repair and mutagenesis”. Criticalto the role of DNA repair in radiation tumorigenesis is the
now unambiguous evidence that heritable human deficiency
in genes controlling DNA repair and maintenance of genomic
stability is frequently associated with an increased incidenceof spontaneously arising neoplasms.
400. Thus, such DNA processing functions in normal
somatic human cells must play a critical role in protecting
against spontaneous neoplastic development. As discussed
in the UNSCEAR 1993 Report [U3], these data also
provide important support for the mutational origin ofneoplasia via failures in repair of DNA damage.
401. With respect to tumours associated with human
exposure to exogenous genotoxic agents, studies with two
categories of genetic disorders, xeroderma pigmentosum(XP) [K8] and Li-Fraumeni syndrome (LFS) [H8], provide
evidence that defects in DNA damage processing are also
important to oncogenic development after ultraviolet andionizing radiation, respectively.
402. In the case of xeroderma pigmentosum, there isunambiguous evidence that the inherited deficiency inrepair of DNA photoproducts is associated with an excess
of cancer in regions of skin receiving significant solar
exposure [K8]. Unexposed skin of XP patients shows anunremarkable frequencyof these neoplasms, indicating thecritical importance of DNA photoproduct induction forskin carcinogenesis and the high level of protection
afforded by high-fidelity DNA repair processes.
403. The cancer-prone genetic disorder Li-Fraumeni
syndrome (LFS) is frequently, although not always,
characterized by a deficiency in the TP53 tumoursuppressor gene that normally plays a role in DNA damage
sensing, cell-cycle control, and apoptosis (see Sections
IV.B and IV.D). Although the data are less compellingthan those for ultraviolet radiation exposure of XP patients,
there is evidence that LFS and LFS-like patients exhibit, inchildhood, an elevated risk of tumour induction afterradiotherapy [H8, S19]. Thus, inherited human defects in

137

DNA damage processing can be reflected in an increased
risk of carcinogen-induced as well as spontaneouslyarisingtumours.
404. In addition to these important human studies,
experimental animal data relating to the role of DNA
repair in radiation tumorigenesis are also beginning toemerge, largely through studies of mice that have been
genetically manipulated to be deficient in specific genes
involved in DNA repair and genomic stability [W2].
405. These animal data are discussed in Section IV.C. In
brief, studies with p53-deficient mice give evidence ofenhanced tumorigenic radiosensitivity associated withabrogation of a G2/M cell-cycle checkpoint for chromosomal repair; radiation-induced p53 gene loss via increased
sensitivity to the induction of aneuploidy appeared to be theprincipal mechanism involved.
406. The ATM gene of human ataxia-telangiectasia isdiscussed in Annex F, “DNA repair and mutagenesis”,
together with recent data relating to the radiosensitivity ofmice manipulated to be deficient in this critical DNA
damage response gene. In brief, Atm-deficient mice arehighly radiosensitive, and while radiation tumorigenesis
studies have yet to be reported, the animals, like their
human counterparts, are prone to the spontaneous development of lymphoma and specific lymphoma-associatedchromosomal rearrangement in haemopoietic cells [B1]. It
may be anticipated that radiation tumorigenesis studies
with these and other relevant genetically manipulated
animals, e.g. those deficient in BRCA1, BRCA2, andRad51, will be informative on the further relationships
between DNA damage repair and tumorigenesis.
2. Implications and uncertainties
407. The Annex F, “DNA repair and mutagenesis”,
provides evidence that certain forms of DNA double-strand
lesions are the principal biologicallyrelevant event inducedby radiation in mammalian cells. Since current data imply
that these lesions are usually repaired via a process ofillegitimate rather than homologous recombination, there
will be an inherent degree of error proneness in DNA
repair after radiation exposure. Such misrepair events may
be represented bygross chromosomal abnormality(deletionor rearrangement) or subchromosomal and intragenic
events. Judging from molecular analyses of radiationinduced somatic cell mutants, these misrepair events takethe form of DNA base-pair substitutions, frameshift
mutations, or, more frequently, DNA deletions of varying
size. Some data are suggestive of changes in mutational
spectra with radiation dose rate and LET, but this issue
remains controversial.
408. Also noted in Annex F, “DNA repair and mutagenesis”, is the growing evidence that DNA repairfunctions are important determinants of dose, dose rate,
and radiation quality effects in mammalian cells. In brief,
there is evidence that the extent and fidelity of repair

138

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

strongly influence the initial and final slopes of low-LET
dose-effect relationships and the progressively steeper
slopes of these relationships with increasing LET. It is
concluded that RBE-LET effects are largely dependent on
the repairability of initial DNA damage. To what extent arethese in vitro data reflected in current knowledge ofradiation tumorigenesis in vivo?

lity in the mammary-tumour-susceptible BALB/c mouse wasshown to be genetically associated with changes in expressionof repair-related DNA PK protein as well as with reduced
DNA double-strand break repair. From these studies it ispossible to indirectly implicate late-expressing genomic
instability in radiation tumorigenesis in certain genetic
settings, but whether a causal relationship applies is uncertain.

409. The data discussed in the UNSCEAR 1993 Report[U3] and in this Annex are broadly consistent with thesingle-cell mutational origin of most tumours. Loss-offunction mutations of critical gatekeeper genes appear to beearly events in the genesis of many human solid tumours,
and gain-of-function chromosomal events occur early inleukaemias and lymphomas (see Section B). There isevidence from animal studies, described earlier, thatradiation-associated gene and chromosomal loss/deletion
can act as an initiating event for tumorigenesis and thatDNA damage processing is a crucial protective factor forin vivo radiation response. Given the obvious parallels
between in vitro and in vivo data, it becomes possible toconsider a general mechanistic framework within which tomodel dose-effect/RBE-LET relationships for radiation
tumorigenesis and the protective functions that may
operate. These modelling approaches will be discussed anddeveloped later in this Annex.

412. Overall, a general link between such induced instability
and radiation tumour risk remains to be established. Indeed,
the cytogenetic findings of in vivo studies relating tomechanisms of radiation-induced lymphomagenesis in p53deficient mice and myeloid leukaemogenesis in CBA micetend to argue against a significant contribution from induced
and persistent genome-wide instability [B4, B20]. However,a specific and persistent clonal feature of chromosomal
instability that has been closely associated with thedevelopment of human neoplasia is the so-called segmental
jumping translocation. These events are not uncommon in
spontaneous human lymphohaemopoietic tumours [U3] andwere reported recently in myeloid neoplasms arising in
irradiated survivors of the atomic bombings [N8]. It may beconcluded that certain elements of radiation-associatedpersistent genomic instability probably do play a role intumorigenic processes, but there is much uncertainty as to theoverall importance of that role and therefore as to how it
might be taken into account in the modelling of radiation risk.

410. There are, however, aspects of the data discussed in
Annex F, “DNA repair and mutagenesis”, that caution
against seeking oversimplistic correlations between in vitro
cellular response data and tumorigenesis in vivo. First is
the issue of novel mechanisms of genetic change inmammalian cells. In addition to epigenetic changes suchas imprinting and gene silencing noted in this Annex, it
has also been speculated that radiation may induce unknown
cellular pathways that promote untargeted mutation. In somestudies the activity of these pathways has been shown topersist for many post-irradiation cell generations, leading toan apparent elevation of the spontaneous mutation rate. As
discussed in Annex F, “DNA repair and mutagenesis”, suchfindings have been made with respect to lethal mutation, genemutation, and unstable chromosomal damage.
411. Given the emphasis in this Annex on the early
development of genomic instability in tumour development, it
is possible to speculate that any persistent genomic instability
induced by radiation in target somatic cells in vivo mightmake a significant, late-expressingcontribution totumorigenic
risk [L24]. The cellular processes underlying this induced
instability remain uncertain, however, and no single
mechanism seems capable of explaining the various andsometimes inconsistent manifestations referred to in Annex F,“DNA repair and mutagenesis”. A collection of recent papers[M32] addresses various aspects of this developing field; ofparticular note is the finding of a possible genetic associationin mouse strains between the post-irradiation expression of
persistent chromosomal instability and susceptibility toradiation tumorigenesis [U25]. The authors were, however,cautious about the implications of these initial findings. In
follow-up studies [O7], late-expressing chromosomal instabi-413. A second source of uncertainty discussed in the
UNSCEAR 1994 Report [U2] and in Annex F, “DNArepair and mutagenesis”, is the cellular phenomenon ofadaptive response to DNA damage; the data describing
such responses were reviewed recently [W6]. In brief, incertain experimental systems a small priming dose ofradiation (or of some other genotoxic agents) can result in
the development of partial resistance to a challenge by asecond, higher dose. The radiobiological endpoint mostfrequently employed in cellular and animal systems has
been cytogenetic damage, but there are also some data withrespect to gene mutation and cell survival. In addition,some mechanistic studies have been undertaken of thepossible role of activation/induction of novel proteins andcell-cycle perturbation. In principle, inducible DNA repair
might underlie some manifestations of adaptive response,
but as noted in Annex F, “DNA repair and mutagenesis”,
evidence for the up-regulation of relevant repair genes isfragmentary. There is, however, growing evidence forsubtle post-irradiation modification of repair-related
protein complexes and the induction of stress-response
genes. More detailed information on adaptive responses tocytogenetic and lethal cellular damage in mammalian cells
are provided in Annex F, “DNA repair and mutagenesis”.
414. These studies lend credibility to the true existence ofadaptive responses, but they also draw attention to the factthat the response is transient, not usually robust, andfrequently lacking a clear mechanistic basis. For example,
at the cellular level, adaptive responses have rather
uncertain dose and dose-rate dependency and when
expressed lead to only a modest decrease in sensitivity toANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

the subsequent radiation challenge. In addition, at thecytogenetic level this response is not consistently
expressed, with cells from some humans and mouse strains
failing, for unknown reasons, to show adaptive responses.
Although some novel proteins have been detected in“adapted” cells, cell-cycle-related changes are not obvious,and the relationships between cytogenetic adaptive
responses and known stress-related biochemical signalling
pathways remain to be clarified [W6].
415. In the UNSCEAR 1994 Report [U2], the Committee
reviewed animal studies of life-shortening and tumour
induction that were relevant to the possible role of adaptive
responses, but these studies did not report convincing
evidence of such effects. Subsequently there have beenreports of possible adaptive effects in relation to radiation
lymphomagenesis [B21] and life-shortening in mice [Y4].
Of particular note is a recent report on a possible gammaray-induced adaptive response in respect of acute myeloid
leukaemia (AML) induction in the mouse [M52]. This
study reported that a chronic priming dose of 100 mGy did
not influence the incidence of acute myeloid leukaemia
following a chronic challenge dose of 1 Gy; the lifespan ofmice without acute myeloid leukaemia was similarly
unaffected. However, the animals receiving the priming
dose showed modestly increased latency for induced acute
myeloid leukaemia, implying that the later stages of theleukaemogenic process had been modified. The
mechanistic basis of this unexpected result remains highly
uncertain, but the authors speculate that the increased
tumour latency might reflect the triggering of some formof persistent stress response.
416. Although the relevance of adaptive responses tohuman tumorigenesis should not be discounted, in theabsence of a consistent body of in vivo tumorigenesis data
and with current uncertainties on cellular mechanisms, itwould be most difficult to include adaptive response
parameters in mechanistic models of low-dose radiation
tumorigenesis.
417. A third area of uncertaintysurrounding DNA damage
and repair concerns the relationships between spontaneousand radiation-induced damage and their implications for
low-dose tumorigenic risk. Debate on these issues has been
conducted for some years [A4, B22, C15, W7], and themain areas of contention may be outlined as follows.
418. In spite of its critical information-carrying role in thecell, genomic DNA has limited chemical stability. Via
hydrolysis, oxidative attack, and chemical methylation
processes, cellular DNA is constantly modified by its
endogenous environment irrespective of the influence ofexogenous agents such as electrophilic chemicals,
ultraviolet radiation, and ionizing radiation [L25].
419. Endogenous damage to the mammalian genome may
take the form of hydrolytic depurination and deaminationof DNA bases, oxidative attack on DNA bases and thesugar-phosphate backbone, and non-enzymatic DNA

139

methylation of certain bases. For largely technical reasons,estimates of the rate of accumulation and abundance ofsuch endogenous DNA lesions vary considerably [L25].
420. Less uncertainty surrounds the general form that thisDNA damage takes [L25]. By their very nature, hydrolytic,
oxidative, and methylation events are random andunclustered, affecting chemical moieties on one or the otherstrands of the DNA duplex; examples are the formation ofabasic sites due to hydrolytic depurination, 8-hydroxyguanine
formation via hydroxyl radical attack, and uracil formation via
deamination of 5-methylcytosine. DNA single-strand breaks
as a consequence of base loss, oxidative attack, and as repair
intermediates also arise spontaneously.
421. The evidence concerning the type, abundance, andrepair of endogenously arising spontaneous DNA damage issummarized in Annex F, “DNA repair and mutagenesis”.
The Annex emphasizes the technical uncertainties surrounding estimates of the abundance of such DNA damage.
422. The general conclusion that may be reached fromthese data is that while it is difficult to make precise
quantitative comparisons, there are differences between the
spectra of DNA damage types arising spontaneously andthose induced by radiation; there are also differences intheir repair characteristics (see also [C15, L25, W7]).
423. This view of endogenous damage and its consequences may be set against the following theoretical
proposition: since the cell is able to repair a very high levelof endogenous DNA damage without frequent mutagenic
consequences, a further small increment of DNA damage
from low doses of radiation will not impose significant
risk; that risk only becomes significant at relatively highdoses, when, at a given level of genomic damage, DNA
repair capacity is exceeded.
424. The fundamental scientific uncertainty surrounding
this proposition is that it assumes that the nature andreparability of spontaneous and radiation-induced DNA
damage are essentially equivalent [C15, W7]. The data in
Annex F, “DNA repair and mutagenesis”, provide evidence
that although there are some similarities between the DNA
lesion types arising spontaneously and those induced byradiation, DNA double-strand breaks almost certainly makea substantially greater relative contribution after radiation
exposure. More important, however, is the evidence
accumulating on the chemical nature of radiation-induced
DNA double-strand breaks and other double-strand lesions.
425. Through a combination of cellular, biophysical,
biochemical, and molecular approaches, it has become
apparent that a high proportion of radiation-induced DNA
double-strand breaks and related lesions are chemicallycomplex and/or part of multiply damaged DNA sites. Thisfeature stems from the requirement for local clustering ofenergy loss events from a given radiation track to effectcoincident damage to both sugar-phosphate backbones of
the DNA duplex [G5, G10]. This chemical complexity of140

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

DNA double-strand breaks is apparent after low-LET
radiation but will increase with LET.
426. As noted in Annex F, “DNA repair and mutagenesis”,
the correct repair of such complex damage is difficult becauseof multiple and coincident damage to coding on both DNA
strands. In most instances such repair is believed to proceed
via illegitimate recombination, which is inherently errorprone, and it is this reparability factor that will principally
distinguish spontaneous and radiation-induced DNA lesions.
Accordingly, excess dicentric chromosomes have beenrecorded in human lymphocytes in vitro at low-LET doses ofaround 20 mGy, while the spontaneous rate of generation ofthese events is very low (~1 per 1,000 cell generations) [L8].
427. Stated simply, the relative abundance of complex andpoorly repairable DNA lesions after radiation exposure isjudged to be very much greater than that of lesions thatarise spontaneously. Therefore, it is this feature rather thanlesion abundance overall that should guide judgements on
the role of DNA repair in low-dose response and radiation
quality effects. Accordingly, the proposition stated inparagraph 423 runs counter to advances in fundamental
knowledge and therefore has no obvious role in themodelling of tumorigenic risk.
3. Summary
428. A large body of information points to the crucial
importance of DNA repair and other damage-response
functions in tumorigenesis. Not only do these DNA damageprocessing functions influence the appearance of initial eventsin the multi-stage process, but they also serve to reduce theprobabilitythat a benign neoplasm will spontaneouslyacquire
the secondary mutations necessary for full malignant
development. Thus, mutations of DNA damage-response
genes in tumours play important roles in the spontaneousdevelopment of genomic instability. Various forms ofradiation-induced persistent genomic instability have beenrecorded in experimental cellular systems. With the possible
exception of instability at certain chromosomal translocation
junctions, these phenomena are not well understood, and theirassociation with in vivo tumorigenesis has yet to beestablished.
429. With respect to radiation damage to DNA, it isconcluded that the repair of sometimes complex DNA doublestrand lesions is inherentlyerror-prone and is most likelyto be
an important determinant of dose, dose rate, and radiation
quality effects for the induction of tumorigenic lesions.
Uncertainties remain on the significance for tumorigenesis ofadaptive responses to DNA damage; the mechanistic basis ofsuch responses has yet to be clarified. In contrast, recent
scientific advances provide clear evidence of the differences incomplexity and reparability between spontaneously arisingand radiation-induced DNA lesions. In the modelling ofradiation tumorigenesis these data argue against basing
judgements about low-dose response on uncriticalcomparisons between overall lesion abundance and repair
capacity. Overall, the general concepts linking DNA damage

repair and tumorigenesis that were summarized previously in
the UNSCEAR 1993 Report [U3] remain valid. However, thedata discussed here and in Annex F, “DNA repair andmutagenesis”, provide a far more robust scientific framework
to support these concepts than was available to the Committee
in 1993.

F. BIOLOGICAL MODELLING OF
TUMORIGENIC RESPONSES
1. Implications of current data
430. As knowledge of the fundamental basis of multi-stage
tumorigenesis continues to advance, it becomes possible toidentify critical features of the process and the uncertainties
that may attach to the development of biological models to
describe risk at low doses.
431. The earliest phase of tumour development (initiation)
appears, in the main, to involve loss- or gain-of-function
mutation of single genes in single target stem-like cells intissue. In the case of solid tumours, a set of tissue-/cell-typespecific gatekeeper genes in the tumour-suppressor category
may be the principal loss-of-function gene targets. For
lympho-haemopoietic tumours, both loss-of-function
mutations and gain-of-function chromosomal translocations
are likely to be important. In biological modelling, these two
types of tumorigenic event may require different forms oftreatment. Not only do mutational mechanisms differ but also
gain-of-function mutations in leukaemia/lymphoma can havemore profound cellular effects than loss of a single tumoursuppressor gene.
432. An important source of uncertainty is provided,
however, by the non-mutational (epigenetic) events thatcan, for certain genes, substitute for gene mutation. The
overall contribution of, for example, gene silencing andloss of imprinting to human tumorigenesis, although very
difficult to quantify, seems likely to be significant.
433. Some uncertainties about the probability of accumulation of multiple genetic events in tumorigenesis have been
reduced by the characterization of the spontaneous development of genomic instability at a relatively early point inneoplastic development. Some of the mutator genes thatserve to drive tumorigenesis have been characterized, andfrom a modelling standpoint it seems most appropriate toview the development of the mutator phenotype as marking
the transition between benign and malignant disease.
Although critical evidence is lacking, the early appearanceof a strongly expressing mutator phenotype with respect tospontaneously arising DNA damage may mean that low
doses of exogenous carcinogens such as radiation will
make a relatively small contribution to later phases oftumour development compared with those phases occurring
before the spontaneous onset of genomic instability. Thismight serve as a mechanistic explanation for theobservation that radiation usually acts only weakly on
tumour promotion and progression [U3].

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

434. Evidence is growing, however, that in somecircumstances, the expression of radiation-induced DNA/
chromosomal damage in cells may be a persistentphenomenon. Secondary chromosomal change centred on
primary exchange junctions, e.g. jumping translocations, is
not unexpected and has been recorded in cellular systems andhuman/animal lymphohaemopoietic neoplasms [B44, G21,
N8, U3]. The other cellular features of induced genomic
instability discussed in this Annex and in Annex F, “DNArepair and mutagenesis” are more difficult to relate directly
to tumorigenesis, so for this reason it may be premature toattempt to integrate them specificallyinto mechanistic models
of tumour risk. The same general problem applies to bystander
effects of radiation on cell inactivation and mutagenesis,
although the alpha-particle data [N11] discussed earlier
deserve some attention. That said, the Committee recognizes
recent signs of progress in resolving the mechanistic
uncertainties associated with the role of stress-related
processes in cellular response to radiation and anticipates thatmuch better informed judgements will be possible within a
few years.
435. By contrast, many sources of data on tumorigenesis
point toward the crucially important protective role playedby high-fidelity DNA repair. In the light of information on
DNA repair capacity in relation to the high flux of
spontaneous DNA damage in mammalian cells, it has beensuggested by some that low doses of radiation would be
expected to contribute little to tumour risk. If true, thiswould have important implications for biological modelling.
Uncertainties on this contentious issue have been reduced by
the growing evidence that an important fraction of the DNA
double-strand lesions induced by radiation is chemicallycomplex, extremely difficult to repair correctly, and only very
rarely occurs spontaneously. In the context of biologicalmodels of tumorigenesis, it is judged that overly simplistic
analysis of data on DNA lesion abundance and repair can bemost misleading. New data on the role of DNA repair incellular dose and dose-rate effects and the associations
between DNA lesion complexity and RBE-LET effects do,
however, have implications for mechanistic models.
436. Other important protective factors in tumorigenesis
include apoptotic cell death, cellular senescence, terminal
differentiation of cells to a non-proliferating state, and theelimination of neoplastic cells by immunosurveillancemechanisms. Through their capacity to remove cells fromneoplastic pathways, these processes collectively serve toprovide a high level of protection against tumours arisingspontaneously or induced by carcinogens. Critically, however, molecular genetic studies have provided compelling
evidence of gene-specific neoplastic mutations that serve toblock these processes. Thus, a given overtlymalignant tumour
will have succeeded via mutation or epigenetic change inevading or gaining tolerance to each of the protective
challenges it has faced during development.
437. There are few quantitative data on which to basejudgements on the relative magnitude or efficiency of theprotective factors noted above. Indeed, it seems likely that141

this relative magnitude will vary from one tumour type toanother, and the differences between virally associated andother tumour types with respect to immunological protection
may be evidence of this. Thus, with limited knowledge, themodelling of protective factors in tumorigenesis can, at best,
be only empirical.
438. In spite of this, a critically important question iswhether the extent or effectiveness of such protective factors
might be different for tumours arising spontaneously andthose induced by radiation. As noted in the UNSCEAR 1993
Report [U3], exposure to high doses of radiation, where cellkilling becomes important, would be expected to influence
final tumour yield not only by initially reducing target cellnumbers but also by subsequently mobilizing quiescent stem
cells for tissue repopulation. High-dose suppression of
immune functions might also be important for certain tumour
types, principally those associated with oncogenic viruses.
439. At low doses and dose rates, where cell killing is not
significant, there is, however, no specific reason to anticipate
profound and long-term effects of radiation on the function ofprotective mechanisms. Transient changes in the activity ofthese systems resulting from stress-related effects on cellularbiochemical signalling might be anticipated, but with current
knowledge it is not possible to relate these to final tumour
yields.
440. The possible exception to this are the adaptive
responses to radiation noted in this Annex and discussed in
depth in the UNSCEAR 1994 Report [U2]. These would
include not only adaptive DNA damage responses but also
possible stimulating effects on immune system and otherpotentially protective functions.
441. Stated simply, if low doses of radiation could be shownto enhance profoundly, over an extended period, the antitumour activities outlined in this Annex, then radiationinduced tumours would be expected to be subject to greatersuppression than those arising spontaneously. Under these
theoretical circumstances the shape of the dose-effect relationship for tumorigenesis would not be expected to be simple andmight well depart from those for related radiobiological
endpoints in single cells, i.e. the induction of chromosome
aberrations, gene mutation, or cell transformation.
442. Although current knowledge does not exclude thepossibility of this scenario, the data available on adaptive
responses in cells or animals are judged to be insufficiently
well developed for the purposes of biological modelling.
Accordingly, the existence or otherwise of an adaptive
response for radiation tumorigenesis remains a continuingsource of debate.
2. Basic premises
443. Although much knowledge has been gained on the
cytogenetic, molecular, and biochemical processes involvedin the development of neoplasia, considerable uncertainty
remains, particularly with respect to the quantitative

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ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

aspects of multi-stage tumour development. For this reason
any attempts to include such data in the modelling ofradiation tumorigenesis demands a set of simplifying
judgements.
444. In full recognition of the uncertainties discussed inearlier Sections of this Annex, the following premises,
based on the weight of current evidence, may be stated:
(a)

(b)

(c)

(d)

The principal role of radiation in tumour development is to generate the DNA damage that can giverise to gene-specific mutation in critical target cells.
Repair of that damage may be enhanced by cell-cycle
checkpoints and, possibly, adaptive repair processes,
but there is no specific expectation of wholly errorfree repair, even at low doses and dose rates. Equally,
the elimination of radiation-damaged cells byapoptotic processes is very unlikely to be complete;
The vast majority of spontaneous and induced tumours
have their origin in single specific mutations in single
target cells in tissues. The probability of such a mutatedcell progressing to overt malignancy is, however, very
low because of the defences afforded by protective
processes such as apoptosis, terminal differentiation,
senescence, and cellular surveillance. Further mutation
during pre-neoplastic clonal development can serve to
bypass these defensive measures, and none are likely tobe wholly protective or to be consistently enhanced bylow doses of radiation;
Although both loss-of-function (tumour-suppressor) andgain-of-function (proto-oncogene) gene mutations cancontribute to multi-stage tumorigenesis, the DNA
deletion mechanism characteristic of radiation will tend
tomake loss-of-function events the predominant process
at all doses of radiation; andRadiation acts principally at the early stages oftumorigenesis by inducing specific mutations innormal stem-like cells. During protracted radiation
exposures a contribution to the later stages of tumour
development is possible, but during these later phases
the acquisition of a mutator phenotype and/or one
associated with epigenetic gene silencing/activation
may be the primary driving force for neoplastic
selection and progression.

445. Assuming these premises to be correct, it would seemthat the dose-response parameters for radiation tumorigenesis
at low doses are determined principally by factors that apply
to the induction of the specific gene/chromosomal mutationsin the target cells in question; the abundance and kinetics ofthese target cells will also be important determinants of the
response. Stated simply, these radiation-induced mutations
would be adding in a dose-dependent manner to the in vivo
pool of tumour-initiating mutational events contributed byspontaneous processes and other genotoxic exposures.
Thereafter, it seems reasonable to assume that all such events
will be subject to the same variable sets of cellular andhumoral factors that serve to suppress or enhance malignant
development. On this basis, significant departure from a
simple dose-response relationship would demand a dose-

dependent change in the kinetics of one or more of these postirradiation modifying processes. For example, if there were tobe persistent post-irradiation elevation of error-free DNA
repair, apoptosis, terminal differentiation, cellular senescence,or immunosurveillance, then the radiation cancer risk might
be depressed. Conversely, ifpost-irradiation mutation rates are
persistently high (as a form of induced genomic instability),
tumour development might be enhanced.
446. Although it is possible to speculate on the roles that theabove processes might play in determining tumorigenic
responses, any such hypotheses currently lack criticalexperimental support and plausible mechanisms that might
operate after low doses. It is not, however, difficult toenvisage substantial modification of cellular/tissue behaviour
during the long-term cellular repopulation required for tissue
regeneration after high doses; these processes may well haveconsequences for local tumour development.
447. At low doses, transient changes in biochemical
equilibria and cell kinetics should be expected, but theseare probably part of the normal cellular damage response
pathways associated with the cell-cycle checkpoint, DNA
repair, and mutagenesis functions already discussed. Thereis, however, no reason to believe that induced transientchanges are unique to radiation.
448. The overall judgement that, on mechanistic grounds,
cancer risk at low doses will increase as a simple functionof dose is, however, subject to a number of importantcaveats. Some of these have already been rehearsed in thisAnnex, but two deserve additional attention.
3. Error-free DNA repair at low doses
449. Recombinational repair involving fully homologous
DNA sequences may be regarded as the sole source ofpotentially error-free repair, particularly with respect to
DNA double-strand breaks/lesions that involve complexity
of DNA damage at single sites, so-called multiply damagedsites. It has been argued that such homologous recombination is not the predominant mode of repair after
ionizing radiation and that the majority of the relevant
double-strand lesions are processed via error-prone
pathways involving non-homologous recombination. The
data that underpin this judgement have, however, beenobtained largely through cellular and molecular studies
conducted after relatively high radiation doses. For
example, as discussed earlier, the formation of dicentricchromosome aberrations probablyreflects such error-prone
repair processes, and for these aberrations, the lowest doseat which an excess has been reproducibly obtained isaround 20 mGy (low-LET). Human epidemiologicalstudies, also outlined in this Annex, reveal evidence ofexcess cancer risk at somewhat higher doses. Below thesedoses there must be complete dependence on anunderstanding of mechanistic processes and, critically, on
the contention that error-prone DNA repair processes
remain in place in cells down to single-track intersectionsof DNA.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

450. Although there is no specific reason to depart fromthis position, it remains possible that at low doses, below,
say, 10 mGy(low-LET), where radiogenic DNA lesions arefew, error-free homologous recombination predominates.
In this hypothetical situation, error-prone repair would bea secondaryresponse that applied onlywhen the abundanceof DNA lesions increased above some critical level. Under
these conditions the form of the dose-response relationshipfor mutational/tumorigenic risk would be expected to havea threshold-like component at very low doses.
451. Formal experimental approaches to this problem arebeyond the resolution of current quantitative techniques incellular radiobiology. One particular set of observations
argues, however, against the proposition of wholly error-free
repair at very low abundance of radiogenic DNA lesions.
452. As noted in this Annex and in Annex F, “DNArepair and mutagenesis”, spontaneously arising DNA
double-strand breaks have a relatively low abundance inmammalian cells. In spite of this, dicentric chromosome
aberrations, a manifestation of DNA break misrepair, occur
at a reproducibly measurable spontaneous rate of about 1
per 1,000 cell generations [L8]. These observations argue
that such DNA misrepair processes are not solely a productof a high incidence of DNA lesions and therefore thaterror-free repair at low doses is unlikely. Althoughsomewhat less certain, a proportion of the DNA deletions/rearrangements that characterize some spontaneously
arising gene mutations in mammalian cells may also arise
as a consequence of DNA break misrepair mechanisms (seeAnnex F, “DNA repair and mutagenesis”).
453. Finally, although in a repair context homologous
recombination is potentially error-free, it may carry its own
risk in that it can serve to duplicate genes from theundamaged homologous chromosome of a given target cell.
These recombinational processes are well-recognized mechanisms for the unmasking of variant heterozygous genes that cancontribute to tumorigenic development, i.e. homologous
recombination as well as DNA deletion results in the loss ofheterozygosity in DNA that characterizes many human andanimal tumours [V2]. Thus, it may be that there is no risk-free
way in which complex DNA double-strand lesions can beprocessed in the cells of genetically heterozygous organisms
such as humans.
4. Epigenetic events in tumorigenesis454. Stable epigenetic effects on gene activity such as genesilencing via heterochromatization and gene activation via
loss of imprinting have been described in this Annex as being
involved in tumour development. Whereas there is a wealth of
information on dose-response relationships for the inductionof gene/chromosomal mutations, it remains most uncertain
whether radiation damage contributes directlyto the establishment of these stable epigenetic events and whether there isany form of dose response. In general, the implications for
low-dose cancer risk remain a matter of speculation, but anumber of issues may be raised.

143455. First and most simply, in some instances the translocation of intact genetic material from one genomic
location to another can lead to changes in gene activity, socalled position effects (see [P12]). The dose response forsuch effects should follow that of chromosomal exchange,
so there should be no major uncertainty. Cellular targets
for loss of imprinting and/or changes in the status of DNA
methylation and/or heterochromatin in situ remain obscure,
however.
456. Second, some in vivo/in vitro studies with rodents
noted in the UNSCEAR 1993 Report [U3] and in thisAnnex imply that the rate of induction by radiation of earlytumour-associated events in clonogens of thyroid andbreast tissue is too high to be explained by conventional
mutational mechanisms. These observations, together withthose made in certain cellular transformation systems, have
led to suggestions that induced epigenetic processes may
play an important role in radiation tumorigenesis [C19].
457. The cellular and molecular basis ofthese high-frequency
events remains unresolved. Nevertheless, the finding of
induced frequencies for tumour-initiated clonogens of around102 cannot possibly be explained by a gene-specific deletionmechanism such as that which applies to HPRT in cultured
cells, where induced frequencies after low-LET irradiation
rarely exceed a value of 104. As noted in Annex F, “DNArepair and mutagenesis”, induced mutation frequency is,however, influenced strongly by genetic context, withtolerance ofDNA loss an important factor; recent observations
concerning tumorigenic mechanisms in mice, outlined below,
suggest that extreme forms of such tolerance may not be
unusual and may be misleading with respect to gene targets.
458. In studies of radiation tumorigenesis in p53-deficient(+/!) mice, Kemp et al. [K2] noted the very high frequency
at which tumour-initiating events appeared to be induced
and suggested that radiation-induced persistent genomic
instability with respect to wild type p53 might beresponsible. Follow-up studies [B4] suggest, however, that thishigh frequency of initial events can be explained by amechanism involving whole chromosome gain and loss
(aneuploidy), where the target size for p53 gene loss fromcritical cells may be orders of magnitude greater than the geneitself. In the same way, loss of wild-type Apc during the early
development of intestinal neoplasia in Apc+/ mice frequently
involves loss of the whole of the encoding chromosome [L20,
S18] and is also likely to be a much more frequent event thansingle gene deletion. Thus, alone, frequency of tumour
initiation may not be a reliable indicator of epigenetic
involvement in radiation tumorigenesis. In this respect theextrapolation of mechanistic data from rodent experimental
systems to human tumorigenesis should be undertaken withcaution.
459. Nevertheless, these findings indicate that in seeking
to model multi-stage radiation tumorigenesis, it is notalways necessary to be constrained by conventional valuesof induced mutation frequency that apply to single genes incells irradiated in vitro. Very large chromosomal deletions

144

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

and specific forms of aneuploidy can be tolerated andselected during in vivo tumour development, and the rateof many forms of gene/chromosomal mutation is often
enhanced following the spontaneous development of mutator
phenotypes. In addition, the suppression of apoptosis during
tumorigenesis may allow the proliferation of genomicallyaberrant cells that would otherwise have been eliminated. In
effect, the loss of apoptotic processes can further enhance therate at which viable mutant cells appear within evolving
neoplastic clones.

G. SUMMARY
460. Current evidence suggests that the biologicalmodelling of radiation tumorigenesis might best proceedon the initial assumption that at low doses radiation acts
primarily as a mutational initiator of neoplasia. The
situation regarding protracted low-dose irradiation isbiologically more complex, and mechanistic studies have
yet to comment specifically upon the extent to whichradiation may influence the later stages of tumorigenesis.
The possible existence of error-free DNA repair in target
cells that might generate a low-dose threshold for tumour
induction is recognized but judged on fundamental grounds
to be unlikely. Other cellular and humoral factors would
need to be modulated in a dose-dependent fashion to
specifically change the initial slope of the dose response fortumour induction. At present, conclusive evidence for theseradiation-specific modulations operating at low doses islacking, but such effects would not be unexpected after
high-dose irradiation. An additional uncertainty is thebalance between mutational and epigenetic contributions
to induced neoplasia, particularly the role and possible
dose response for epigenetic effects. Epigenetic effects
following radiation could, in principle, impact all stages oftumour development. Transient biochemical stress responses
and bystander effects are most likely to influence themutagenic and apoptotic aspects of tumour initiation, while

induced and persistent genomic instability may be envisaged
to impact pre-neoplastic clonal evolution and malignant
development. Although fundamental knowledge is increasing
rapidly, the extent to which such processes specificallydetermine low-dose tumorigenic response remains largely amatter of speculation.
461. Overall, evidence on the fundamental aspects ofradiation action and its relationship to tumour induction
provide no firm scientific reasons to believe that at low doses
the form of cellular dose response is related in a complex
fashion to increasing dose. Employing the principle ofparsimony, it is therefore suggested that low-dose cellularmutagenic risk and, by implication, that for tumorigenesis
rises from the zero-dose baseline as a simple function of dose.
The linear form is the simplest of these responses and is notinconsistent with the majority of the quantitative data
discussed in this Annex. Irrespective of future scientificdevelopments, however, it may well be impossible to provide
formal scientific proof of linearity or any other form of lowdose radiation response for tumorigenesis in vivo.
462. In addition, for a complex multifactorial response suchas in vivo tumorigenesis, the expression of initial in vivo
cellular events may in some circumstances be subject to highdose modification. Accordingly, caution needs to be exercisedin interpreting dose-response data for in vivo tumorigenesisthat encompass wide dose ranges, say, 0!5 Gy low-LET.
463. The observation of apparently simple forms of in vivo
dose response over such a dose range can, in principle,
disguise competing dose-dependent elements in the tumorigenic process. For example, at high doses, the suppressive
effects of initial inactivation of target cells may compete withsubsequent tumour promotion in damaged normal tissue. For
the purposes of modelling the biological elements of radiation
tumorigenesis, it is not possible at present to quantify suchcompeting effects, but as knowledge accumulates these
problems will demand increasing attention.

V. BIOLOGICALLY BASED MODELLING OF RADIATION CARCINOGENESIS
464. The principal aims of this Chapter are (a) to review
general aspects of the computational models that seek tointerpret epidemiological data on cancer risk, (b) to describe
the empirical and mechanistic models that have beendeveloped, and (c) to illustrate and compare the predictive
features of empirical and mechanistic models with emphasis
on risk at low doses. Computational models of cancer risk canalso play a role in describing the possible interactions between
radiation and other agents. This complex issue is explored in
Annex H, “Combined effects of radiation and other agents”.
465. In Chapter IV the review of fundamental data onradiation tumorigenesis allowed proposals to be made on how
evolving knowledge might guide the development of
mechanistic models of radiation risk. At this early stage ofunderstanding, much of the guidance remains to be
implemented, but three main principles can be considered: (a)radiation will tend to act at the earliest stage of tumorigenesis(initiation), (b) in general, no low-dose threshold should be
expected, and (c) time-constant relative risk is suggested on
the basis that radiation-induced and spontaneously arisingtumorigenic events will be subject to the same host andenvironmental modifications although this is recognized as
being somewhat simplistic.
466. The biological uncertainties noted in earlier Chapterssuggest, however, that dose-dependent differences between
tumour types with respect to their induction and themechanisms involved should be expected. Accordingly,
general comment will be provided on the predictive value ofANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

biologically based models with respect to the projection of risk
with time and dose response for radiation tumorigenesis. At
the outset it is however important to stress that although anumber of valuable mathematical and statistical tools havebeen developed, the outcome of cancer risk modelling is often
dependent on the initial biological assumptions made. Even
using the same data sets, different groups of workers canarrive at different optimal mathematical/statistical solutions
depending on these assumptions. This is clearly a significant
source of uncertainty.

A. GENERAL ASPECTS OF THE PROBLEM
467. One of the principal uncertainties that surround thecalculation of cancer risks from epidemiological data resultsfrom the fact that few radiation-exposed cohorts have been
followed up to extinction. For example, 50 years after theatomic bombings of Hiroshima and Nagasaki, about half ofthe survivors were still alive [P2]. In attempting to calculate
lifetime population cancer risks it is therefore important topredict how risks might vary as a function of time afterradiation exposure, in particular for that group for whom theuncertainties in projection of risk to the end of life are mostuncertain, namely those who were exposed in childhood.
468. One way to model the variation in risk is to use
empirical models incorporating adjustments for a number ofvariables (e.g. age at exposure, time since exposure, sex) andindeed this approach was used by the BEIR V Committee[C1] in its analyses of data on the Japanese atomic bomb
survivors and various other irradiated groups. Recent analysesof solid cancers for these groups have found that the radiationinduced excess risk can be described fairly well by a relativerisk model [I2]. The time-constant relative risk model assumes
that if a dose of radiation is administered to a population,
then, after some latent period, there is an increase in thecancer rate, the excess rate being proportional to theunderlying cancer rate in an unirradiated population. For
leukaemia, this model provides an unsatisfactory fit,consequently a number of other models have been used forthis group of malignancies, including one in which the excess
cancer rate resulting from exposure is assumed to be constanti.e. the time-constant additive risk model [U4].
469. It is well known that for all cancer subtypes (including
leukaemia) the excess relative risk (ERR) diminishes withincreasing age at exposure [U2]. For those irradiated inchildhood there is evidence of a reduction in the excess
relative risk of solid cancer 25 or more years after exposure[L6, L33, P2, T4]. For solid cancers in adulthood the excess
relative risk is more nearly constant, or perhaps evenincreasing over time [L32, L33], although there are someindications to the contrary [W9]. Clearly then, even in thecase of solid cancers various factors have to be employed tomodify the excess relative risk.
470. Associated with the issue of projection of cancer riskover time is that of projection of cancer risk between two
populations with differing underlying susceptibilities to145cancer. Analogous to the relative risk time projection model
one can employ a multiplicative transfer of risks, in which theratio of the radiation-induced excess cancer rates to theunderlying cancer rates in the two populations might beassumed to be identical. Similarly, akin to the additive risktime projection model one can use an additive transfer ofrisks, in which the radiation-induced excess cancer rates in thetwo populations might be assumed to be identical. The datathat are available suggests that there is no simple solution to
the problem [U2]. For example, there are weak indications
that the relative risks of stomach cancer following radiation
exposure may be more comparable than the absolute excess
risks in populations with different background stomach cancer
rates [U2]. The breast cancer relative risks observed in themost recent analysis of the Japanese atomic bomb survivor
incidence data [T4] are rather higher than those seen invarious other data sets, particularly for older ages at exposure[B6, M2, S20]. The observation that sex differences in solid
tumour excess relative risk are generally offset by differences
in sex-specific background cancer rates [U2] might suggestthat absolute excess risks are more alike than excess relative
risks. Taken together, these considerations suggest that invarious circumstances relative or absolute transfers of risk
between populations may be advocated, or indeed, the use ofsome sort of hybrid approach such as that which has been
employed by Muirhead and Darby [M12] and Little et al.
[L56].
471. The exposed populations that are often used for deriving cancer risks e.g. the Japanese atomic bomb survivors, were
exposed to ionizing radiation at high doses and high dose
rates. However, it is the possible risks arising from low doseand low dose-rate exposure to ionizing radiation which arecentral to the setting of standards for radio-logical protection.
The ICRP [I2] recommended application of a dose and doserate effectiveness factor of 2 to scale cancer risks from highdose and high dose-rate exposure to low dose and low doserate exposure on the basis of animal data, the shape of thecancer dose response in the bomb survivor data and otherepidemiological data. Although the linear-quadratic doseresponse model (with upward curvature) found for leukaemia
is perhaps the most often employed departure from linearity
in analyses of cancer in radiation-exposed groups [P1, P2],
other shapes are possible for the dose-response curve [U3].
While for most tumour types in the Japanese data linearquadratic curvature adequatelydescribes the shape of the doseresponse curve, for non-melanoma skin cancer (NMSC) thereis evidence for departures from linear-quadratic curvature.
The non-melanoma skin cancer dose response in the Japanesecohort is consistent with a dose threshold of 1 Sv [L7, L30] orwith an induction term proportional to the fourth power of
dose, with, in each case, an exponential cell sterilization term
to reduce non-melanoma skin cancer risk at high doses(>3 Sv).
472. Arguably, models which take account of the biologicalprocesses leading to the development of cancer can provide
insight into these related issues of projection of cancer riskover time, transfer of risk across population and extrapolationof risks from high doses and dose-rates to low doses and dose-

146

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

rates. For example, Little and Charles [L32] have demonstrated that a variety of mechanistic models of carcinogenesis
predict an excess relative risk which reduces with increasingtime after exposure for those exposed in childhood, while forthose exposed in adulthood the excess relative risk might beapproximatelyconstant over time. Mechanistic considerations
also imply that the interactions between radiation and thevarious other factors that modulate the process of carcinogenesis may be complex [L2], so that in general one would
not expect either relative or absolute risks to be invariant
across populations. Some of the general features of interaction
between radiation and other factors are described in Annex H,“Combined effects of radiation and other agents”.

B. EMPIRICAL AND MECHANISTIC
MODELS
1. Armitage-Doll multi-stage model
473. Mechanistic models of carcinogenesis were originallydeveloped to explain phenomena other than the effects ofionizing radiation. One of the more commonly observed
patterns in the age-incidence curves for epithelial cancers isthat the cancer incidence rate varies approximately as Ctβ forage t and some constants C and β. At least for most epithelial
cancers in adulthood, the exponent β of age seems to lie
between 4 and 6 [D5]. The so-called multi-stage model of

ARMITAGE-DOLL MULTISTAGE MODELSC

Mutation

Mutation

SC1

M(0)(t)Mutation

SC2

M(1)(t)M(2)(t)SCk-1

MutationM(k-1)(t)SCM

TWO-STAGE MVK MODEL
Death/differentiation
D(1)(t)MutationSC

Mutation

SC1

M(0)(t)SC1

SCM

M(1)(t)G(1)(t)SC1SC1Cell division
GENERALIZED MVK MODEL
Death/differentiation
D(1)(t)SC

MutationD(2)(t)SC1
Mutation

SC1

M(0)(t)Death/differentiation

M(2)(t)

SCk-1

SCk-1

MutationM(k-1)(t)SCM

G(2)(t)G(1)(t)SC1

D(k-1)(t)SC2

Mutation

SC2

M(1)(t)Death/differentiation

SC1

Cell division

SC2

SC2

Cell division

SCk-1

SCk-1

Cell division

Figure XXIII. Empirical/mechanistic models of multi-stage carcinogenesis.
SCM: Malignant stem cell
SC: Initial stem cellSCi: Stem cell of stage iM(i)(t): Mutation rateG(i)(t): Stem cell rateD(i)(t): Death/differentiation rate
at stage i and age t
at stage i and age t
at stage i and age t

carcinogenesis of Armitage and Doll [A1] was developed inpart as a way of accounting for this approximately log-log
variation of cancer incidence with age. The model supposesthat at age t an individual has a population of X(t) completely
normal (stem) cells and that these cells acquire one mutation
at a rate M(0)(t). The cells with one mutation acquire a second
mutation at a rate M(1)(t), and so on until at the (k ! 1)th

stage the cells with (k ! 1) mutations proceed at a rate M(k !
1)(t) to become fully malignant. The model is illustrated
schematicallyin Figure XXIII. It can be shown that when X(t)and M(i)(t) are constant, a model with k stages predicts acancer incidence rate that is approximately given by theexpression Ctk1, with C = M(0) M(1)...M(k ! 1)/(1 × 2 ... (k! 1)) [A1, M27].

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

474. In developing their model, Armitage and Doll [A1]were driven largely by epidemiological findings and, inparticular, by the age distribution of epithelial cancers. In theintervening 30 years, substantial biological evidence has beengathered that cancer is a multi-step process involving theaccumulation of a number of genetic and epigenetic changes
in a clonal population of cells. This evidence was reviewed in
the UNSCEAR 1993 Report [U3], and subsequent data andconcepts are outlined in Chapter IV of this Annex. However,there are certain problems with the model proposed byArmitage and Doll [A1] associated with the fact that toaccount for the observed age-incidence curve Ctβ with βbetween 4 and 6, between five and seven stages are needed.
For colon cancer there is evidence that six stages might berequired [F1, U3]. However, for other cancers there is, atpresent, insufficient evidence to conclude that there are as
many rate-limiting stages as this. BEIR V [C1] surveyed
evidence for all cancers and found that two or three stages
might be justifiable, but not a much larger number. To thisextent, the large number of stages predicted by the ArmitageDoll model appears to be verging on the biologically unlikely.
Related to the large number of stages required by the
Armitage-Doll multi-stage model are the high mutation rates
predicted bythe model. Moolgavkar and Luebeck [M28] fitted
the Armitage-Doll multi-stage model to data sets describing
the incidence of colon cancer in a general population and inpatients with familial adenomatous polyposis. Moolgavkar andLuebeck [M28] found that Armitage-Doll models with five or
six stages gave good fits to these data sets, but that both ofthese models implied mutation rates that were too high by atleast two orders of magnitude. The discrepancy between thepredicted and experimentally measured mutation rates might
be eliminated, or at least significantly reduced, if account istaken of the fact that the experimental mutation rates arelocus-specific. A “mutation” in the sense in which it is definedin this model might result from the "failure" of any one of anumber of independent loci, so that the “mutation” rate would
be the sum of the failure rates at each individual locus.
475. Notwithstanding these problems, much use has been
made of the Armitage-Doll multi-stage model as a framework
for understanding the time course of carcinogenesis,
particularly for the interaction of different carcinogens [P10].
Thomas [T3] has fitted the Armitage-Doll model with one andtwo radiation-affected stages to the solid cancer data in theJapanese Life Span Study 11 cohort of bomb survivors.
Thomas [T3] found that a model with a total of five stages, ofwhich either stages one and three or stages two and four wereradiation-affected, fitted significantly better than models witha single radiation-affected stage. Little et al. [L5, L35] also
fitted the Armitage-Doll model with up to two radiationaffected stages to the Japanese Life Span Study 11 data set andalso to data on various medically exposed groups, using aslightlydifferent technique to that of Thomas [T3]. Little et al.
[L5, L35] found that the optimal solid cancer model for theJapanese data had three stages, the first of which wasradiation affected, while for the Japanese leukaemia data thebest fitting model had three stages, the first and second ofwhich were radiation affected. A version of the Armitage-Doll
has also been fitted to the Life Span Study solid tumour

147

incidence data by Pierce and Mendelsohn [P22], who foundthat a model with five or six stages gave the best fit to thisdata.
476. Both the paper of Thomas [T3] and those of Little et al.
[L5, L35] assumed the ith and the jth stages or mutation rates[M(i !1), M(j ! 1)] (j > i) in a model with k stages to be(linearly) affected by radiation and the transfer coefficients(other than M(i !1) and M(j !1)) to be constant, as is thestem-cell population X(t). In these circumstances, it can beshown [L5] that if an instantaneously administered dose ofradiation d is given at age a, then at age t (>a) the cancer rate
is approximately as follows:µ t k1  α d a i1 t ki1 (5)

β d a i1 t kj1  γ d 2 a i1 t kj1
for some positive constants µ, α and β, and where γ isgiven byγ 

α β Γ(ki) Γ(j)if j  i  1
2 µ Γ(k)and(6)

 0 if j > i  1

and Γ(i) is the gamma function [A6].
477. The first term (µtk1) in expression (5) corresponds tothe cancer rate that would be observed in the absence ofradiation, while the second term (αdai1tki1) and the third
term (βdai1tkj1) represent the separate effects of radiationon the ith and jth stages, respectively. The fourth term
(γd2ai1tkj1), which is quadratic in dose, d, represents theconsequences of interaction between the effects of radiationon the ith and the jth stages and is only non-zero when thetwo radiation-affected stages are adjacent (j = i + 1). Thus
if the two affected stages are adjacent, a quadratic (doseplus dose-squared) relationship will occur, whereas therelationship will be approximatelylinear if the two affectedstages have at least one intervening stage. Another way of
considering the joint effects of radiation on two stages isthat for a brief exposure, unless the two radiation-affectedstages are adjacent, there will be insignificant interaction
between the cells affected by radiation in the earlier andlater of the two radiation-affected cell compartments. Thisis simply because very few cells will move between the twocompartments in the course of the radiation exposure. If
the ith and the jth stages are radiation-affected, the resultof a brief dose of radiation will be to cause some of thecells that have already accumulated (i -1) mutations toacquire an extra mutation and move from the (i - 1)th tothe ith compartment. Similarly, it will cause some of thecells that have already acquired (j - 1) mutations to acquire
an extra mutation and so move from the (j - 1)th to the jthcompartment. It should be noted that the model does not
require that the same cells be hit by the radiation at the ith
and jth stages, and in practice for low total doses, or
whenever the two radiation-affected stages are separated byan additional unaffected stage or stages, an insignificant

148

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

proportion of the same cells will be hit (and mutated) by
the radiation at both the ith and the jth stages. The result
is that unless the radiation-affected stages are adjacent, fora brief exposure the total effect on cancer rate isapproximately the sum of the effects, assuming radiation acts
on each of the radiation-affected stages alone. One interesting
implication of models with two or more radiation-affectedstages is that as a result of interaction between the effects ofradiation at the various stages, protraction of dose, in general,
results in an increase in cancer rate, i.e. an inverse dose-rate
effect [L5]. However, it can be shown that in practice theresulting increase in cancer risk is likely to be small [L5].
478. The variant of the Armitage-Doll model fitted by Pierce
and Mendelsohn [P22] is unusual in that it assumes thatradiation equally affects all k mutation rates in the model
except the last. (In the last stage, radiation is not assumed tohave any effect.) This assumption distinguishes their use ofthis model from the approaches of Little et al. [L5] or Thomas[T3], both of which assumed that radiation affected at mosttwo of the mutation rates (and neither of which constrained
the effects of radiation to be equal in these stages). There aresome technical problems with the paper of Pierce and
Mendelsohn [P22] arising from the authors’ failure to takeaccount of interactions between the effects of radiation on the(k ! 2) pairs of adjacent stages. These interactions contributesignificantly by adding a quadratic term in the dose response
and cannot be ignored, even to a first-order approximation.
The fact that in general there is little evidence for upwardcurvature in the solid cancer dose response in the Life Span
Study [P1, L7, L44] argues that if proper account had beentaken of these interaction terms, the model of Pierce and
Mendelsohn [P22] would not fit the data well. Moreover, one
implication of the model of Pierce and Mendelsohn [P22] isthat the excess relative risk will be proportional to 1/a, i.e. theinverse of attained age. However, this is known to provide apoor description of the excess relative risk of solid cancer,
even within the Life Span Study cohort [L56, L57].
479. The optimal leukaemia model found by Little et al. [L5,
L35], having adjacent radiation-affected stages, predicts a
linear-quadratic dose response, in accordance with thesignificant upward curvature which has been observed in theJapanese data set [P1, P2, P3]. This leukaemia model, andalso that for solid cancer, predicts the pronounced reductionof excess relative risk with increasing age at exposure which
has been seen in the Japanese atomic bomb survivors andother data sets [U2]. The optimal Armitage-Doll leukaemia
model predicts a reduction of excess relative risk withincreasing time after exposure for leukaemia. At least for
those exposed in childhood, the optimal Armitage-Doll solidcancer model also predicts a reduction in excess relative risk
with time for solid cancers. These observations are consistent
with the observed pattern of risk in the Japanese and otherdata sets [L33, U2]. Nevertheless, there are indications that
the Armitage-Doll model may not provide an adequate fit to
the Japanese data [L6]. For this reason, and because of theother problems with the Armitage-Doll model discussed
above, a slightly different class of models need to beconsidered.

2. Two-mutation models
480. In order to reduce the biologically implausible numberof stages required by their first model, Armitage and Doll
[A7] developed a further model of carcinogenesis, which
postulated a two-stage probabilistic process whereby a cell
following an initial transformation into a pre-neoplastic state(initiation) was subject to a period of accelerated (exponential)
growth. At some point in this exponential growth a cell fromthis expanding population might undergo a second
transformation (promotion) leading directly to thedevelopment of a neoplasm. Like their previous model, it
satisfactorily explained the incidence of cancer in adults, but
was less successful in describing the pattern of certain
childhood cancers.
481. The two-mutation model developed by Knudson [K16]to explain the incidence of retinoblastoma in children tookaccount of the process of growth and differentiation in normal
tissues. Subsequently, the stochastic two-mutation model ofMoolgavkar and Venzon [M7] generalized Knudson'smodel, by taking account of cell mortality at all stages aswell as allowing for differential growth of intermediate
cells. The two-stage model developed by Tucker [T7] isvery similar to the model of Moolgavkar and Venzon but
does not take account of the differential growth of intermediate cells. The two-mutation model of Moolgavkar,
Venzon, and Knudson (MVK) supposes that at age t there
are X(t) susceptible stem cells, each subject to mutation to
an intermediate type of cell at a rate M(0)(t). Theintermediate cells divide at a rate G(1)(t); at a rate D(1)(t)they die or differentiate; at a rate M(1)(t) they aretransformed into malignant cells. The model is illustrated
schematically in Figure XXIII. In contrast to the case ofthe (first) Armitage-Doll model, there is a considerable
body of experimental biological data supporting thisinitiation-promotion type of model (see, e.g. [M5, T6]).
The model has been developed to allow for time-varying
parameters at the first stage of mutation [M30]. An
additional generalization of this model (to account for
time-varying parameters at the second stage of mutation)
was presented by Little and Charles [L32], who also
demonstrated that the excess relative risk predicted by themodel when the first mutation rate was subject toinstantaneous perturbation decayed at least exponentially fora sufficiently long time after the perturbation. Moolgavkar etal. [M29] and Luebeck et al. [L12] and Heidenreich et al. [H2]used the two-mutation model to describe the incidence of lungcancer in rats exposed to radon, and in particular to describe
the inverse dose-rate effect that has been observed in these
data. Other groups have also modelled lung tumour risk inrats exposed to radon [H35, L2], and in this experimental
system the modelling suggests effects of radiation on thelater stages of tumorigenesis. Moolgavkar et al. [M13] andLuebeck et al. [L16] also applied the model to describe theinteraction of smoking and radiation as causes of lungcancer in the Colorado Plateau uranium miner cohort.
More recently the two-mutation model has been utilized tomodel lung, stomach, and colon cancer in the atomic bomb
survivor incidence data [K17].

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

482. Moolgavkar and Luebeck [M28] have used models
with two or three mutations to describe the incidence ofcolon cancer in a general population and in patients withfamilial adenomatous polyposis. They found that bothmodels gave good fits to both data sets, but that the modelwith two mutations implied biologically implausibly low
mutation rates. The three-mutation model, which predicted
mutation rates more in line with biological data, wastherefore somewhat preferable. The problem of implausibly
low mutation rates implied by the two-mutation model is
not specific to the case of colon cancer, and is discussed at
greater length by Den Otter et al. [D6] and Derkinderen etal. [D7], who argue that for most cancer sites a model withmore than two stages is required.
3. Generalized Moolgavkar-Venzon-Knudson(MVK) multi-stage models
483. A number of generalizations of the Armitage-Dolland two- and three-mutation models have been developed[L31, T6]. In particular, two closely related models havebeen developed, whose properties have been described by
Little [L31]. The first model is a generalization of the twomutation model of Moolgavkar, Venzon, and Knudson andso will be termed the generalized MVK model. The second
model generalizes the multi-stage model of Armitage andDoll and will be termed the generalized multi-stage model.
AGE AT EXPOSURE < 15 years
Solid cancers

For the generalized MVK model it may be supposed that at
age t there are X(t) susceptible stem cells, each subject tomutation to a type of cell carrying an irreversible mutation
at a rate of M(0)(t). The cells with one mutation divide at
a rate G(1)(t); at a rate D(1)(t) they die or differentiate.
Each cell with one mutation can also divide into an
equivalent daughter cell and another cell with a second
irreversible mutation at a rate M(1)(t). For the cells withtwo mutations there are also assumed to be competing
processes of cell growth, differentiation, and mutation
taking place at rates G(2)(t), D(2)(t), and M(2)(t),respectively, and so on until at the (k ! 1)th stage the cells
that have accumulated (k ! 1) mutations proceed at a rateM(k ! 1)(t) to acquire another mutation and become
malignant. The model is illustrated schematicallyin Figure
XXIII. The two-mutation model of Moolgavkar, Venzon,
and Knudson corresponds to the case k = 2. The
generalized multi-stage model differs from the generalized
MVK model only in that the process whereby a cell isassumed to split into an identical daughter cell and a cellcarrying an additional mutation is replaced by the process
in which only the cell with an additional mutation results,
i.e. an identical daughter cell is not produced. The classicalArmitage-Doll multi-stage model corresponds to the casein which the intermediate cell proliferation rates G(I)(t)and the cell differentiation rates D(i)(t) are all zero.

100
EXCESS RELATIVE RISKEXCESS RELATIVE RISK

20

15

10

5

0

Leukaemia

80

60

40

20

0
0

10
20
30
40
YEARS AFTER EXPOSURE

50

0

10
20
30
40
YEARS AFTER EXPOSURE

1.0

50

2-mutation model
3-mutation model

AGE AT EXPOSURE > 15 years
20

Solid cancers
EXCESS RELATIVE RISKEXCESS RELATIVE RISK

149

0.8

0.6

0.4

0.2

0

Leukaemia

15

10

5

0
0

10
20
30
40
YEARS AFTER EXPOSURE

50

0

10
20
30
40
YEARS AFTER EXPOSURE

50

Figure XXIV. Comparison of generalized MVK models fitted to the observed excess relative risk Sv1 (CI:90%) in survivors of the atomic bombings in Japan [L36].

150

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

484. It can be shown [L31] that the excess relative risk foreither model following a perturbation of the parameters
will tend to zero as the attained age tends to infinity. One
can also demonstrate that perturbation of the parameters
M(k ! 2), M(k ! 1), G(k ! 1), and D(k ! 1) will result inan almost instantaneous change in the cancer rate [L31].
485. Generalized MVK models have been fitted to theatomic bomb survivor mortality data [L36]. For bothleukaemia and solid cancers, the only models with a single
radiation-affected parameter that give even slightly
satisfactory fits are those in which radiation is assumed toaffect M(0) [L36]. For both leukaemia and solid cancer,
generalized two- and three-mutation MVK models fit equally
well. For leukaemia, the three-mutation model provides asatisfactoryfit onlywhen M(0) and M(1) are assumed affectedby radiation. For solid cancer and leukaemia there areindications of lack of fit to the youngest age-at-exposure groupfor the three-mutation model; there is also some lack of fit of
the optimal solid cancer three-mutation model to this age-atexposure group (Figure XXIV).
486. For solid cancer, only M(0) is (linearly) affected byradiation for two- or three-mutation generalized MVK
models. In contrast to the solid cancer models, both leukaemia models assume a linear-quadratic dose dependence of theM(i). The non-linearity found in the leukaemia M(i) dose

487. Despite the indications of lack of fit discussed above, thevariation of excess relative risk with time since exposure andage at exposure predicted by the optimal two- and threemutation models for solid cancer (Figure XXIV) is inqualitative agreement with the variation seen in the Japanesebomb survivors and in other irradiated groups [U2]. In
particular the optimal models demonstrate the progressive
reduction in excess relative risk with increasing age at
exposure seen in many data sets [U2], together with themarked reduction in excess relative risk with increasing timesince exposure observed in various groups exposed inchildhood [L42, P2].
488. Figure XXIV reinforces the theoretical predictions of an
earlier paper by Little [L31] and shows that immediately after
perturbing M(0) in the two-mutation model, the excess
relative risk for solid cancers and leukaemia quicklyincreases.
However, there are no data in the first five years of follow-up
in the survivor cohort [P2], so that it is difficult to test thepredictions [L31] in respect of the variation in risk shortly
after exposure using that data set.

ANKYLOSING SPONDYLITIS
Solid cancers

1.0

0.5

0

-0.5

0

10
20
30
YEARS AFTER EXPOSURE

5

0

40

0

CERVICAL CANCER
Solid cancers

2.0

2.0

EXCESS RELATIVE RISKEXCESS RELATIVE RISK

10

-5

-1.0

2.5

Leukaemia

15
EXCESS RELATIVE RISKEXCESS RELATIVE RISK

1.5

response reflects known curvature in the leukaemia dose
response in the atomic bomb survivor data [C1, P1]. There issome evidence, e.g. for chromosome aberrations, that the
mutation induction curve is linear-quadratic at least for lowLET radiation, although linearity is generally observed forhigh-LET radiation [L34].

1.5
1.0

0.5
0

10
20
30
YEARS AFTER EXPOSURE

40

Leukaemia

1.5
1.0
0.5
0

-0.5

-0.5

-1.0
0

10
20
30
YEARS AFTER EXPOSURE

40

0

10
20
30
YEARS AFTER EXPOSURE

40

Figure XXV. Excess relative risk (CI: 90%) of solid cancers and leukaemia in Ankylosing Spondylitis Study inUnited Kingdom and the International Study of Cervical Cancer [B5, D8].

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

489. There is a suggestive increase in the excess relativerisk of cancers other than leukaemia and colon cancer in
the UK ankylosing spondylitis patients <5 years after firsttreatment (the first two datapoints in the top-left panel of
Figure XXV), but the authors caution against interpreting
this as the effect of the x-irradiation [D8]. There are nostrong indications of an elevation in risk in the first five
years after radiotherapy for cancers other than leukaemia
and of the reproductive organs in a study of womenfollowed up for second cancer after radiotherapy forcervical cancer [B5]. This corresponds to the first two
datapoints in the bottom panel of Figure XXV. (Lungcancers are also excluded from the International Radiation
Study of Cervical Cancer (IRSCC) data shown in the lower
left panel of Figure XXV because of indications of aboveaverage smoking rates in this cohort [B5].) In general there
are no strong indications of an elevation in solid cancer
risk soon after irradiation in other exposed groups [U2]. To
this extent there are indications of inconsistency for solidcancers between the predictions of the two-mutation modeland the observed variation in risks shortly after exposure.
490. In their analysis of the Colorado uranium miners
data, Moolgavkar et al. [M13] partially overcame theproblem posed by this instantaneous rise in the hazard after
perturbation of the two-mutation model parameters byassuming a fixed period (3.5 years) between the appearanceof the first malignant cell and the clinical detection ofmalignancy. However, the use of such a fixed latency
period only translates a few years into the future the sudden
step-change in the hazard. To achieve the observed gradualincrease in excess relative risk shortly after exposure, astochastic process must be used to model the transition
from the first malignant cell to detectable cancer; such a
process is provided by the final stage(s) in the three- or
four-mutation generalized MVK models used in theanalysis of Little [L36]. In particular, an exponentiallygrowing population of malignant cells could be modelled
by a penultimate stage with G(k ! 1) > 0 and D(k ! 1) = 0,
the probability of detection of the clone being determinedby M(k ! 1). In their analysis of lung, stomach, and coloncancer in the atomic bomb survivor incidence data, Kai etal. [K17] did not assume any such period of latency,
perhaps because of the long time after the bombings (12.4
years) before solid cancer incidence follow-up began in theLife Span Study.
491. The evidence with respect to the variation in excess
relative risk shortly after exposure for leukaemias is rather
different from that for solid cancers. In the United
Kingdom ankylosing spondylitis patients [D8] there issignificant excess risk even in the period <2.5 years after
first treatment (first datapoint in top-right panel of Figure
XXV). The IRSCC data [B5] shows a significant excess
risk for acute non-lymphocytic leukaemia in the period 1!4years after first treatment (the second datapoint in thelower-right panel of Figure XXV), and this pattern isobserved in many other groups [U2]. More detailed
analysis of UK leukaemia incidence data indicate that theage-incidence curves for all subtypes of lymphocytic

151

leukaemia can be adequately modelled by two- and threemutation generalized MVK models [L26, L36]. However,
the two-mutation models for acute lymphocytic leukaemia(ALL) imply a very small number of stem cells (<104 cells)
if the model is not to yield implausibly low mutation rates[L26].
492. Little [L55] fitted various generalized MVK models
to the three main radiogenic leukaemia subtypes, namely
acute myeloid leukaemia (AML), chronic myeloid
leukaemia (CML) and acute lymphocytic leukaemia (ALL)in two incidence data sets, one relating to a subset of the
population of the United Kingdom recently assembled by
the Leukaemia Research Fund (LRF) [C21] and the second
the Japanese atomic bomb survivors [P3]. The results ofthis model fitting are illustrated by Figures XXVI andXXVII. Figure XXVI shows that the optimal two-mutation
models adequatelydescribe the background incidence of all
three leukaemia subtypes in the United Kingdom
Leukaemia Research Fund data [L55]. The optimal twomutation model for AML assumes a step change in thenumbers of susceptible stem cells and a simultaneouschange in the intermediate cell proliferation parameters,
G(1)(t) and D(1)(t). The optimal two-mutation model for
CML assumes a step change in the numbers of susceptible
stem cells and a simultaneous change in the number ofsusceptible stem cells and a simultaneous change in theintermediate cell growth parameter, G(1)(t), although thecell death or differentiation rate, D(1)(t), is constant. The
optimal two-mutation model for ALL assumes a susceptible
stem cell population of the form X = X0 exp [X1 t + 1[1>T]X+] and a step change in the intermediate cell proliferation
parameters, G(1)(t) and D(1)(t). As can be seen from
Figure XXVI, three-mutation models provide a rather
worse fit for all leukaemia subtypes, particularly for ALL[L55]. For ALL, two-mutation models which assumed
ionizing radiation acts to elevate mutation rates for life
fitted the Japanese atomic bomb survivor incidence datarather worse than models which assumed ionizing
radiation acts to elevate the first mutation rateinstantaneously [L55] (see Figure XXVII, lower panel).
For CML, two-mutation models which assumed ionizing
radiation acts to elevate mutation rates for life fitted theJapanese atomic bomb survivor incidence data rather better
than models which assumed ionizing radiation acts toelevate the first mutation rate instantaneously [L55] (seeFigure XXVII, center panel). For AML (Figure XXVII,
upper panel), both sorts of two-mutation models fitted
equivalently well [L55].
4. Multiple pathway models
493. Little et al. [L6] fitted a generalization of the ArmitageDoll model to the Japanese atomic bomb survivor and IRSCC
leukaemia data which allowed for two cell populations atbirth, one consisting of normal stem cells carrying nomutations, the second a population of cells each of which has
been subject to a single mutation. The leukaemia risk
predicted by such a model is equivalent to that resulting froma model with two pathways between the normal stem cell

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSESANNUAL INCIDENCE RATE (10 -6 a-1)

152
20

AML

Males

15

15

10

10

5

5

0

ANNUAL INCIDENCE RATE (10 -6 a-1)

Females

0
20

0
6

40
AGE (years)

60

CML

0

80

Males

20

40
AGE (years)

60

6

5

5

4

4

3

3

2

2

1

1

0

80

Females

020

0

ANNUAL INCIDENCE RATE (10 -6 a-1)

20

10

40
AGE (years)

60

ALL

0

80

Males

20

40
AGE (years)

60

10

8

8

6

6

4

4

2

2

80

Females

0

0
0

20

40
AGE (years)

60

80

0

20

40
AGE (years)

60

80

Figure XXVI. Fit of optimal two-stage and three-stage generalized MVK models
to Leukeamia Research Fund data for acute myeloid leukaemia (AML), chronic myeloid leukaemia (CML),
and acute lymphocytic leukaemia (ALL) [L55].
Observed risks are shown with 95% CI.
Observed
Two-stage model
Three-stage modelcompartment and the final compartment of malignant cells,
the second pathwayhaving one fewer stage than the first. Thismodel fitted the Japanese and IRSCC leukaemia data sets
significantly better, albeit with biologically implausible
parameters, than a model which assumed just a single
pathway [L6]. The findings of Kadhim et al. [K4], namelythat the exposure of mammalian haemopoietic stem cells toalpha particles could generally elevate mutation rates to verymuch higher than normal levels, imply (if they are at all
relevant to tumorigenesis) that there might be multiple
pathways in the progression from normal stem cells tomalignant cells (discussed in Chapter IV). The mutation rates
and indeed the number of rate-limiting stages might besubstantially different in these two or more pathways. A
number of such models are described by Tan [T6], who also

discusses at some length the biological and epidemiologicalevidence for such models.

C. DOSE-RESPONSE RELATIONSHIPS
494. The shape of the cancer dose response is largely driven
by assumptions made about the shape of the dose-response
curve for the initiating lesion or lesions. In particular, if alesion induced by a single acutelydelivered dose D of ionizing
radiation at age α has a dose response given by the function
F(D,a) and this is assumed to act on a single stage (notnecessarily the first) in the multi-stage process of carcinogenesis and assuming also that the dose is low enough to
avoid saturation effects, then it can be shown [L5, L31] that

ANNUAL INCIDENCE RATE (10 -6 a-1)

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES
15

AML

Age at exposure < 25

40

30

30

20

20

10

10

Age at exposure > 45

50
0

ANNUAL INCIDENCE RATE (10 -6 a-1)

Age at exposure 25-45

10

0

20

20
30
10
40
YEARS AFTER EXPOSURE

CML

50

Age at exposure < 25

0
0

20

10
20
30
40
YEARS AFTER EXPOSURE

050

Age at exposure 25-45

20

15

15

15

10

10

10

5

5

5

0

0
0

ANNUAL INCIDENCE RATE (10 -6 a-1)

40

153

14

20
30
10
40
YEARS AFTER EXPOSURE

ALL

50

Age at exposure < 25

14

10
20
30
40
YEARS AFTER EXPOSURE

Age at exposure 25-45

14
12

10

10

10

8

8

8

6

6

6

4

4

4

2

2

2

0
30
20
40
10
YEARS AFTER EXPOSURE

50

Age at exposure > 45

0

50

12

0

50

0
0

12

0

20
30
40
10
YEARS AFTER EXPOSURE

20
30
40
10
YEARS AFTER EXPOSURE

50

Age at exposure > 45

0
0

10
20
30
40
YEARS AFTER EXPOSURE

50

0

20
30
40
10
YEARS AFTER EXPOSURE

50

Figure XXVII. Fit of optimal two-stage MVK models
to Japanese data for acute myeloid leukaemia (AML), chronic myeloid leukaemia (CML),
and acute lymphocytic leukaemia (ALL) [L55].
Observed risks are shown with 95% CI.
Japanese exposed
Japanese unexposed
MVK unexposed
MVK exposed, M(0) elevated instantaneously
MVK exposed, M(i) elevated lifetimethe dose-response curve for carcinogenesis i years after
exposure is given by w0 (t,a) + F(D,a) ! w1(t,a) for somefunctions w0(t,a) and w1(t,a). In other words, the dose response
for cancer has the same shape as the dose response for initiallesion production. In particular, if the initial lesion production
is a linear-quadratic function of dose D, F(D) = σ0 + σ1 ! D+ σ2 ! D2, then the cancer dose response will also be linearquadratic, with the same ratio of quadratic to linear
coefficients.
495. It is a crucial assumption underlying this invariance in“shape” of the dose-response curves for the production of

initial lesions and cancer that the radiation-induced lesions act
only at a single “stage” in the carcinogenic process. If, forexample, there is a quadratic term in the dose response
resulting from interactions between the (linear) effects ofradiation on different “stages” (e.g. adjacent “stages” in theclassical multistage model of carcinogenesis) then the ratio of
the quadratic to the linear coefficients for the cancer dose
response would change with time after exposure [L5]. It has
been hypothesized that the quadratic term in the dose-response
curve for chromosome aberrations results from interactions
between the effects of two radiation tracks through the cell
nucleus [E10, M42].

154

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

496. Comparison of the shape of dose-response relationships
for tumour induction with that of in vitro cellular endpoints
such as chromosome aberration induction is notstraightforward. Chapter IV provides evidence of the cellularcomplexity of multistage tumour development, and some
distortion of dose-response parameters for initial events insingle cells seems likely. Accordingly, such dose-responsecomparisons need to be made with some caution.
497. Nevertheless, the assumption made here is thattumorigenic dose response is determined largely by the dose
response for the production of initial lesions. The shape of thedose response for the various candidate lesions that might beassociated with cancer has been discussed in earlier Chaptersof this Annex. There is some information on this question thatcan be obtained from the epidemiological data, although thisis generally obtained from moderately high dose studies (withtotal dose up to 5 Gy). There is very little reliable epidemiological data relating to total doses less than 20 mGy. In mostanalyses of epidemiological data, linear dose-response models
are used, and give satisfactory fit. In particular this is the casefor most solid tumours in the Japanese atomic bomb survivors[P2, T4] and in many other radiation-exposed groups [U2].
The linear-quadratic dose response (with upward curvature)that is found for leukaemia is perhaps the most often
employed departure from linearity. However, in analyses ofthe shape of the cancer dose-response curve in radiationexposed groups [P1, S1], there are various other possible
shapes to the dose-response curve [U3]. For the class ofdeterministic effects defined by the ICRP [I2], it is assumed
that there is a threshold dose, below which there is no effect[E9]. Such a form of dose response has also been employed inanalyses of brain damage among those exposed in utero to theatomic bombings in Hiroshima and Nagasaki [O9, O10].
Arguments have been put forward that sufficiently small
doses of radiation induce either no increase in cancer risk (i.e.
a dose threshold), or a reduction in cancer risk (i.e. hormesis)[L45, K19, K22, P9], although these interpretations have beenchallenged [C15, U3].
498. Recently there have appeared a number of assessmentsof possible threshold-type departures from linear-quadratic
curvature in the cancer dose-response curve in the Japaneseatomic bomb survivor tumour incidence and mortality data.
These data are noted earlier in the Annex and are discussed
in detail below.
499. Analysis by Little and Muirhead [L7, L43] and by Hoeland Li [H26] of the Japanese atomic bomb survivor incidence
data demonstrated a significant improvement in fit to theleukaemia incidence data when a threshold is incorporated ina linear-quadratic relative risk model, albeit at borderline
levels of statistical significance (two-sided p=0.04). Little andMuirhead [L7, L43] examined the three radiogenic leukaemia
subtypes (AML, ALL, CML), as well as the principal solidcancer sites in the incidence data, and found that, apart fromleukaemia, only for non-melanoma skin cancer was there
evidence of a threshold (at about 1 Sv); this last finding was
reinforced by a more detailed examination of this cancer type
[L30]. The evidence for there being a significant excess riskof non-melanoma skin cancer at relatively low doses (<1 Gy)
in other (Caucasian) populations [S31] may indicate thelimited relevance of these findings in the Japanese data toWestern populations with respect to this cancer type.
Paralleling the analysis of Little and Muirhead [L7, L43].
Hoel and Li [H26] also examined the fit of linear-thresholdmodels to a number of solid cancer sites in both the Japaneseincidence and mortality data and found that for none of thecancer sites was there evidence that incorporation of athreshold significantly improved the fit.
500. It is well recognized that errors in the estimates ofdose can substantially alter the shape of the dose-response
relationship and hence the evidence both for a dose
response and also for any possible curvature in that dose
response. The problem of random dosimetric errors for the
Radiation Effects Research Foundation (RERF) data has
been previously investigated by Jablon [J5], Gilbert [G2],
Pierce et al. [P7] and Pierce and Vaeth [P1]. Such random
errors in doses were taken into account in the analysis of
the tumour incidence and mortality data by Little andMuirhead [L7, L32, L44], but not by Hoel and Li [H26].
The issue of dosimetric errors in epidemiological data is
considered in Annex I, “Epidemiological evaluation ofradiation-induced cancer”.
501. There are certain technical problems associated withuse of threshold models. In general, the asymptotic (χ2)
distribution of the deviance difference statistic used forsignificance tests is not guaranteed, because of a lack ofsufficient smoothness in the likelihood as a function of thedose threshold parameter [S30]. This problem is circumvented by the likelihood-averaging techniques used to takeaccount of dosimetric errors in the analysis by Little andMuirhead [L7, L43, L44]; this problem is not addressed in
the analyses of Hoel and Li [H26].
502. Other subtle problems affect the interpretation of theresults of both Hoel and Li [H26] and Little and Muirhead[L7, L43, L44]. These problems are connected with the useof the grouped form of the data, and in particular thegrouped dose categories, in the publicly available forms ofboth the Japanese incidence and mortality data sets. A
likelihood-averaging technique used to take account of
dosimetric errors [L7, L43, L44] is one possible way aroundthis problem. Little and Muirhead [L7, L43, L44], following
the methodology of Pierce et al. [P7], evaluated the average,
for a given “nominal” dose, d, of the relative risk, RR(i,D),
evaluated at the “true” dose D: Avg[RR(i,D)*d]. The data set
used was in grouped form, the strata being defined in eachcase by the variables city, sex, age at exposure, time since
exposure, and dose. (In the mortality data set [P2], there isadditional stratification by attained age.) For each suchstratum, i, the average “nominal” dose was available for the
persons in that stratum Avgi[d]. Ideally one should calculate
for each strata Avgi[Avg[RR(i,D)*d]], i.e. the average of
Avg[RR(i,D)*.] over all individuals in the stratum i. It was notpossible to calculate this quantity using the grouped data thatwere publicly available, so that in the analyses of Little andMuirhead [L7, L43, L44] the quantity Avg[RR(i,D)*Avgi[d]]ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

155

Table 16
Leukaemia cases and deaths in various dose groups in the Japanese atomic bomb survivor incidence andmortality data
Number of cases
Bone marrow dose group a
b

a
b
c

Incidence [P3]

Deaths c [P2]0.0-0.1
0.1-0.2
0.2-0.5
0.5-1.0
1.0-1.5
1.5-2.0
2.0-3.0
3.0-4.0
4.0

128
8
27
24
19
8
17
2
4

125
11
24
22
16
9
18
7
4

Total

237

236

In the incidence data a neutron relative biological effectiveness (RBE) of 1 is used to calculate the neutron component of bone marrow dose(in Gy), for the mortality data a neutron RBE of 10 is used to calculate dose (in Sv).
All leukaemia cases over the years 1950-1987.
All leukaemia deaths over the years 1950-1987.

was evaluated, i.e. the value of Avg[RR(i,D)*.] evaluated at
the average “nominal’ dose (Avgi[d]) within stratum i. Even
for linear dose-response models there can be differences
between Avg[RR(i,D)*Avgi[d]] and Avgi[Avg[RR(i,D)*d]]. As
shown by Little and Muirhead [L43], at least when 35%
dosimetric errors were assumed, this approximation did not
introduce appreciable errors for the optimal linear-quadraticthreshold model for leukaemia; errors were at most 5% [L43].
503. Although the errors introduced bythis approximation
were small, nevertheless they may be sufficiently great toquestion the analyses of Little and Muirhead [L7, L43] andHoel and Li [H26]. As discussed previously, one of themain findings of the analysis of leukaemia incidence
among the Japanese atomic bomb survivors by Little andMuirhead [L7] was that incorporation of a threshold in thelinear-quadratic model yielded an improvement in fit at50borderline levels of statistical significance (best estimate ofthreshold for a linear-quadratic-threshold model was 0.12
Sv, 95% CI: 0.01!0.28; two-sided p=0.04). In contrast, thefits of a linear-quadratic-threshold model to the mortality
data by the same authors demonstrated that the threshold
was not significantly different from zero (best estimate ofthreshold for a linear-quadratic-threshold model was 0.09
Sv, 95% CI: <0.00!0.29; two-sided p=0.16) [L44].
Comparison of the leukaemia incidence and mortality data
in Figure XXVIII and Table 16 demonstrates theirsimilarity. Little and Muirhead [L44] concluded that themost probable reason for the difference between thereported findings in the incidence and mortality data sets
was the finer subdivision of dose groups in the mortality
data set. (There are 14 dose groups in the mortality data
sets in a publicly available form, compared with 10 dosegroups in the incidence data sets.)
3

Mortality
Incidence

Mortality
Incidence

RELATIVE RISKRELATIVE RISK

40

30

20

2

1

Relative risk = 1

10
3
0

see
details0

Relative risk = 1

1
2
3
DOSE IN BONE MARROW (Sv)

0
4

0

0.1
0.2
0.3
0.4
DOSE IN BONE MARROW (Sv)

Figure XXVIII. Relative risk of leukaemia in survivors of the atomic bombings [L44].
The diagram on the right shows the low-dose region in detail.

504. Substantial low-dose curvilinearity in dose response has
been observed for skin cancer in some human populations[L30] (although not in all [S31, R15]) and in CBA/CaH mice[P8], which is consistent either with a threshold or with apower of dose substantially greater than 2. Low-dose curvature, consistent with a quadratic-exponential dose response,
has also been observed for myeloid leukaemia in CBA mice
[M4, D11]. For a specific endpoint a threshold dose response,

156

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

or something approximating to it (e.g. a dose response
proportional to some high power (>2) of dose), might be
expected if radiation must hit a large number of targets
relevant to that endpoint, for example in order to inactivate acritical number of cells in a particular tissue [E9]. In
particular, there are grounds for believing that this might be
the case for induction of cataract, sterility, in utero severe
mental retardation and various other deterministic effects[E9]. There is substantial evidence that oncogenesis arises
from damage to a single cell, and in particular from damage
to the genetic material in a cell, as reviewed by UNSCEAR[U3], in Chapter IV of the present Annex and discussed by
Little and Muirhead [L7]. Given the evidence that single
tracks of all types of ionizing radiation can induce a variety ofdamage, including DNA double-strand breaks [C15, G10,
G12], a dose threshold for cancer induction is judged to beunlikely but cannot be excluded formally. These mechanistic
issues have been discussed in depth in Chapter IV. The
finding of a significant excess leukaemia risk in various
occupationally exposed groups [C20], in which total doses
generally are administered in an episodic manner, and also
among those exposed to small doses (<0.02 Sv) of
x-irradiation in utero [K23], provide further evidence thatargues against a low dose threshold in the leukaemia dose
response.
505. In conclusion, although there is evidence at borderline
levels of statistical significance for threshold departures fromlinear-quadratic curvature for leukaemia incidence in theJapanese atomic bomb survivor data, the grouped nature of theJapanese data make inferences on a possible dose thresholdproblematic. Based on the most current analysis of themortality data [L44], there is no evidence for a thresholddeparture from linear-quadratic curvature for leukaemia in theJapanese atomic bomb survivor data, nor is there for anyothercancer type, with the possible exception of non-melanoma
skin cancer. In arriving at these conclusions the Committee
recognizes the uncertainties that attach to current modelling
approaches to cancer risk and the shape of the dose-response
relationship.

D. SUMMARY
506. The classical multi-stage model of Armitage and Dolland the two-mutation model of Moolgavkar, Venzon, andKnudson, and various generalizations of them also, arecapable of describing, at least qualitatively, many of theobserved patterns of excess cancer risk following ionizing
radiation exposure. However, there are certain inconsistencies

with the biological and epidemiological data for both themulti-stage and two-mutation models. In particular, there areindications that the two-mutation model is not totally suitablefor describing the pattern of excess risk for solid cancers thatis often seen after exposure to ionizing radiation, althoughleukaemia may be better fitted by this type of model.
Generalized MVK models which require three or more mutations are easier to reconcile with biological and epidemiological data relating to solid cancers. Firm statements on therelative validity of different biologically based models ofradiation tumorigenesis must await further developments. In
general, however, it appears that those models retaining
biologically realistic parameters while providing satisfactory
fits to the data tend to require radiation action at the earlystage of tumorigenesis. This feature is consistent with theconclusions reached in Chapter IV following review of
mechanistic data. At the same time it is recognized that the
optimal solutions in such modelling can often depend on
initial assumptions made on the role of radiation-induced
damage in complex multi-stage tumorigenic processes. Some
influence of radiation on the later stages of tumorigenesis
should be anticipated, particularly perhaps with respect toprotracted exposures.
507. Although there is evidence at borderline levels of
statistical significance for threshold departures from linearquadratic curvature for leukaemia incidence data in theJapanese atomic bombings, the grouped nature of theJapanese data make inferences on a possible dose thresholdproblematic. Based on the most current analysis of themortality data, there is no evidence for a threshold departurefrom linear-quadratic curvature for leukaemia in the Japaneseatomic bomb survivor data, nor is there for any other cancer
type, with the possible exception of non-melanoma skincancer. Thus, as is the case for data from animal, cellular andmolecular studies, evidence from the modelling of
epidemiological data tends to favour the view that, in general,cancer risk at low doses rises as a simple function of dose. It
is recognized, however, that, at present, the descriptions of
dose-effect relationships that have been published are
principally qualitative in nature and the choice of models forthe quantitative estimation of risk remains to be satisfactorily
resolved. Substantial uncertainties attach to the true form of
these dose-response relationships and the extent to which they
are determined by biological assumptions. The Committee is
supportive of further work aimed at the further development
and validation of these biologically-based models; it isbelieved that they will have an important role in the future
work of the Committee.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

157

SUMMARY AND CONCLUSIONS
508. A number of considerations are important in determining the risks of exposures to radiation at low doses and low
dose rates. These include (a) analysis of epidemiological andexperimental studies to determine the lowest doses at which,for statistical and methodological reasons, radiation effects are
directly observable; (b) examination of the shape of the doseresponse relationships in the low-dose region using available
epidemiological and experimental data; and (c) assessment of
the possibilities for extrapolation to lower levels of dose basedon an understanding of the mechanisms involved in radiation
response. The aim of this Annex has been to provide an
overview of the data available on the relationship between
radiation exposure and the induction of cancer and hereditarydisease, with emphasis on the limits of detection of effects at
low doses of low-LET radiation and the associated uncertainties. This information, coupled with the understanding todate of mechanisms of damage to cells and tissues, provides
a basis for reasoned judgements to be made about the likely
form of the dose response at exposures below those at which
direct information is available.
509. DNA damage. It is generally recognized that damage
to DNA in the nucleus is the main initiating event by whichradiation causes long-term damage to organs and tissues ofthe body. Double-strand breaks in DNA are generally
regarded as the most likely candidate for causing the criticaldamage. Single radiation tracks have the potential to cause
double-strand breaks and in the absence of 100% efficient
repair could result in long-term damage, even at the lowest
doses, although with a low probability. Damage to othercellular components (epigenetic changes) may influence thefunctioning of the cell and progression to the malignant state.
510. Direct observations. Studies of cellular systems, animal experiments and human epidemiological investigations
provide direct and relatively consistent evidence of linear or
linear-quadratic dose-response relationships at high tointermediate levels of dose and dose rate. However, all suchstudies are hampered by statistical limitations in providing
clear indications of effects at acute doses much less than about
100 mGy(low-LET). Epidemiological studies at low doses arealso subject to uncertainties due to methodological issues
related to bias and confounding that can limit interpretationof the data. Some exceptions are the induction of cancer
following irradiation in utero for which an increase in risk has
been observed at doses of about 10!20 mGy, experimental
data on mouse hair mutations at 10 mGy, and unstable
chromosomal aberrations at 20 mGy. In the case of high-LET
radiation, the experimental data on cellular damage generallyindicate a linear dose-response relationship.
511. For the induction of unstable chromosome aberrations
and mutations, a small priming dose of low-LET radiation cansometimes reduce the effect caused by a subsequent higher
dose. This adaptive response seems to be a consequence ofstimulating the expression/production of genes/proteins incells involved in DNA damage response and takes a few hours

to become effective. Such adaptive responses appear to betransient and have also been observed for cell transformation.
512. Animal studies are valuable for determining the shapesof dose-response relationships and examining how thebiological and physical conditions of exposure may influence
radiation responses. For manytumour types, the dose response
following exposure to both low- or high-LET radiation can bereasonably well represented by a linear or linear-quadratic
function. In many cases, however, alternative fits to the data
are also possible. Other model fits include the possibility of athreshold dose below which tumours do not occur, as well as
more complex functions in which the time for the tumour toappear is much later at low dose rates, which can also suggest
the presence of a threshold for response. Animal studies donot, and probably cannot, provide direct information at acutedoses much less than about 100 mGy. Values for the lowest
doses to give a significant increase in tumour yield following
chronic irradiation are generally higher than those for acuteirradiation.
513. For radiation-induced hereditary disease, the mostcomprehensive information comes from measurements ofspecific locus mutations in mouse spermatogonia. The
dose-response relationship for low-dose exposures fromlow-LET radiation is well fitted by a linear response. The
lowest dose tested in these studies was 380 mGy (lowLET). The incidence of mutations in male mice falls by afactor of about three for a reduction in dose rate from
800!900 mGy min1 to 0.007 mGy min1. This suggeststhat a substantial fraction of the damage to DNA that
results in the induction of heritable mutations is not
amenable to effective repair.
514. Epidemiological studies provide a substantial amount of
direct quantitative data on the risks of cancer in humans
following radiation exposure. The main source of informationis the Japanese Life Span Study (LSS), which gives
information on the effects of whole-body irradiation following
exposure at different ages. The follow-up study indicates asignificant (p=0.05) increase in the risk of radiation-induced
fatal solid cancers in the 0!50 mSv dose range.
515. The dose-response relationship for mortality fromleukaemia has been fitted bya linear-quadratic function, while
for all solid cancers taken together, a linear dose response
provides a best fit for the data for doses up to about 3 Sv. A
linear dose response can also be fitted to the data for a numberof individual tumour types. There are a number of cancers thathave not been significantly increased, including those of therectum, bone, prostate and testes. Further follow-up and better
information on the doses received will be needed before theshape of the dose response for both morbidity and mortality
can be determined with confidence at doses below about
100 mSv.
516. Dose-response data from a number of otherepidemiological studies can also be fitted with a linear or

158

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

linear-quadratic dose response at doses up to a few gray, butalternative relationships have also been obtained. Thus, inradium dial painters exposed to the alpha emitters 226/228Ra, thebest fit to the data on bone tumour induction can be obtainedwith a model indicating a “practical threshold”. Data are alsoavailable on an increased risk of bone sarcomas in patients
given 224Ra. It has been proposed that some of these tumours
would be expected to arise only in tissue with deterministic
radiation damage and only above a threshold dose. Similar
conclusions have been drawn for the bone tumours arising in
the radium dial painters. For exposure to radon and its decay
products a constant-relative-risk model without anymodifying
factors, such as attained age and exposure rate, appear to givea good fit to the data at low doses.
517. Data on patients irradiated for medical reasons are
generally consistent with a linear dose-response relationshipat doses below a few gray. Results suggest a statisticallysignificant increase in the risk of thyroid cancer at externalradiation doses above about 100 mGy received in childhood.
518. A number of studies provide information on the risk ofchildhood cancer following obstetric radiography at low
doses. A statistically significant, 40% increase in the relativerisk of leukaemia and other childhood cancers (up to 15 yearsof age) has been seen following doses in the 10!20 mGy(lowLET) range. The principal reason for being able to determine
this increase in risk, which in absolute terms is modest, is the
low background incidence of cancer in childhood.
519. Data on the effects of low-dose, chronic exposure inradiation workers are generally consistent with results
obtained from the high-dose-rate studies on leukaemia
induction, although having wide statistical uncertainties. A
longer period of follow-up and pooling of data from different
studies will, however, be necessary if information on the slopeof the dose-response relationship is to be obtained.
520. Some data are available on the risks of cancer in areasof high natural background. Comparative studies of groups
exposed to different levels of natural background radiation do
not have the statistical power to detect predicted effects on
cancer incidence. Generally, there are substantial difficulties
in interpreting the data because of uncertainties in the doses
actually received, geographical variation in the accuracy ofcancer diagnoses, and confounding by environmental factors.
521. Mechanistic considerations. Proto-oncogenes andtumour-suppressor genes control a complex array of biochemical pathways involved in cellular signalling and interaction, growth, mitogenesis, apoptosis, genomic stability, anddifferentiation. Mutation of these genes can, in an often
pleiotropic fashion, compromise these controls and contributeto the multi-stage development of neoplasia.
522. On the basis of accumulating knowledge it is argued
that early gain-of-function proto-oncogene activation bychromosomal translocation is often associated with thedevelopment of human lympho-haemopoietic neoplasia,
although gene loss is not infrequent. For many solid tumours

there is a requirement for loss of function mutation of tissuespecific tumour-suppressor genes that act as cellular
gatekeepers. It has also been proposed that the subsequent
onset of spontaneous genomic instability via further clonal
mutation is a critical event in neoplastic conversion from abenign to a malignant phenotype. Loss of apoptotic control isalso believed to be an important feature throughout neoplastic
development.
523. Much information on multi-stage tumorigenesis still
remains to be learned. Although the concept of sequential andinteracting gene mutations as the driving force for neoplasia
is more firmly established, there is insufficient understandingof the complex physiological interplay between these eventsand the consequences for cellular behaviour and tissue
homeostasis.
524. Uncertainty also surrounds the degree to which nonmutational (epigenetic) changes to the genomes ofneoplastic cells contribute to tumorigenesis. Increases in
the methylation status of critical tumour-suppressor genes
is known to be an alternative to mutational inactivation ina range of neoplasms, and loss of methylation imprints
may also serve to increase the activity of some growthpromoting genes. DNA methylation is also believed to be
involved in genomic imprinting processes. Loss of such
imprinting may be important in a number of tumour types.
New evidence also implicates histone acetylation in
genomic heterochromatization and gene silencing; thisprocess is suggested to be a potentially important contributor to epigenetic change. Epigenetic processes (bystander effects and induced genomic instability) have beenshown to influence certain aspects of cellular response invitro. The relevance of these poorly understood processes
to in vivo tumour induction at low doses of radiation
remains to be established.
525. Studies have clarified the role of specific genemutations in tumours that serve to destabilize the genome,
therebyallowing for the accelerated spontaneous developmentof clonal heterogeneity and tumour progression. Althoughcritical evidence is lacking, it is possible to envisage that after
this transition point is reached, tumour development may berelatively independent of exogenously induced DNA damage.
Cellular selection during neoplastic development is judged to
be of crucial importance at all stages of tumorigenesis.
Overall it is judged that most tumours have their origin in
gene/chromosomal mutations affecting single target stem-likecells in tissues.
526. Direct evidence on the nature of radiation-associatedinitiating events in human tumours is sparse, and rapid
progress in this area should not be anticipated. By contrast,
good progress is being made in resolving early events inradiation-associated tumours in mouse models. Thesemolecular observations strengthen the view expressed in
the UNSCEAR 1993 Report [U3] that radiation-induced
tumorigenesis will tend to proceed via gene-specific losses;
a contribution from early arising epigenetic events should
not, however, be discounted.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

527. Neoplastic development is subject to a large number ofcellular constraints, which provide a high level of protection
against neoplastic growth and development. Principal of these
are control of cellular proliferation/genomic stability, theinduction of apoptosis, and terminal differentiation to a nonproliferative cellular state. For at least certain tumour types
there is evidence that immunosurveillance mechanisms canrecognize and restrict the growth of neoplastic cells. In spiteof these constraints, resistance to or tolerance of all thesecountermeasures can be developed via gene-specific mutation.
On the basis of current molecular genetic knowledge, different
modes of in vivo constraint are unlikely to apply tospontaneously arising and radiation-induced tumours.
528. Much information points to the crucial importance of
DNA repair and other damage-response functions intumorigenesis. DNA damage response functions influence
the appearance of initial events in the multi-stage process,and reduce the probability that a benign neoplasm will
spontaneously acquire the secondary mutations necessary
for full malignant development. Thus, mutations of DNA
damage-response genes in tumours play an important rolein the spontaneous development of genomic instability.
529. The repair of sometimes complex DNA double-strand
lesions is largely error-prone, and is an important
determinant of dose, dose rate, and radiation quality effects incells. Uncertainties continue to surround the significance totumorigenesis of adaptive responses to DNA damage; themechanistic basis of such responses has yet to be wellcharacterized although associations with the induction ofbiochemical stress responses seems likely. Recent scientificadvances highlight the differences in complexity andreparability between spontaneously arising and radiationinduced DNA lesions. These data argue against basing
judgements concerning low-dose response on comparisons ofoverall lesion abundance rather than their nature.
530. Biological uncertainties and dose-response models.
Evidence suggesting the predominance of error-prone repairof radiation damage to cellular DNA has grown, implying that
mutational/ tumorigenic risk should be expected at low doses.
Important uncertainties remain, however, on whether errorfree DNA repair might applyat very low doses, although there
are some arguments against it.
531. There are also uncertainties about whether radiationinduced non-mutational (epigenetic) events, such as induced
genomic instability, contribute significantly to tumour risk.
The dose-response characteristics of such events are obscure,and there is no way to judge the ensuing risk at low doses, if
indeed it exists. The involvement of such processes cannot be
inferred solely on the basis of the frequency of phenotypic
effects after radiation.
532. Since tumorigenic processes are highly complex,
attention is drawn to the problems of judging the shape of the
low-dose response on data sets that are over-reliant on highdose estimates of effect. Apparently simple dose-response
relationships may disguise competing processes that havedifferent dose dependencies.

159

533. In spite of these uncertainties the weight of evidence
from fundamental studies favours the mutagenic action ofradiation acting primarily at a very early stage oftumorigenesis (initiation), with risk rising as a function ofdose. Thus the risk of developing malignant tumours should
follow the dose response for initiating lesions unless there are
dose-dependent effects on the later phases of tumorigenesis.
534. Computational modelling of tumorigenesis. The
different characteristics of empirical and biologically basedmodels of radiation tumorigenesis have been considered by
the Committee. The classical multi-stage model of Armitage
and Doll and the two-mutation model of Moolgavkar,
Venzon, and Knudson, and various generalizations of both,are capable of describing, at least qualitatively, many of theobserved patterns of excess cancer risk following ionizing
radiation exposure. However, different solutions have beenobtained by different investigators and there are certain
inconsistencies with the biological and epidemiological datafor both the multi-stage and two-mutation models.
Generalized MVK models that require three or moremutations are easier to reconcile with biological andepidemiological data relating to solid cancers.
535. Although there is evidence at borderline levels of
statistical significance for threshold departures from linearquadratic curvature for leukaemia incidence in the Japaneseatomic bomb survivor data, the grouped nature of the Japanesedata make inferences on a possible dose thresholdproblematic. The most current analysis of the mortality data
provides no evidence for a threshold departure from linearquadratic curvature for leukaemia in the Japanese atomic
bomb survivor data, nor is there evidence of this for any othercancer type, with the possible exception of non-melanoma
skin cancer.
536. Conclusions. DNA is the principal target for the
initiation of radiation-induced cancer and for radiationinduced hereditarydisease. Experimental studies of the effectsof ionizing radiation on cellular systems, including theinduction of chromosome aberrations, cell transformation andsomatic mutations are of value for providing information on
damage to DNA. The data obtained have been generally
consistent with a linear or linear-quadratic dose response at
exposures below those at which cell killing becomessignificant (a few gray). In general, significant radiation
effects can be detected at doses of about 100 mGy (low-LET)and above, although there are some experimental systems forwhich effects at lower doses have been observed. In the caseof high-LET radiation, the experimental data generallyindicate a linear dose-response relationship in the absence ofcell killing.
537. For most tumour types in experimental animals and in
man a significant increase in risk is only detectable at doses
above about 100 mGy. An exception is for human exposuresin utero when a significant increase in tumour induction inchildren has been found for doses in the 10!20 mGy range(low-LET). No such excess was observed in the studies ofJapanese atomic bomb survivors irradiated in utero.

160

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

538. In both experimental animals and in humans the doseresponse data for tumour induction can be frequently fitted bya linear or linear-quadratic dose response at doses below a few
gray. There is evidence though that for some cancer types thisform of response does not apply and there may be a practicalthreshold for a response. Other forms of dose response canalso be fitted for the induction of some tumour types.
539. With respect to direct observations of radiation effects,
which all carry statistical and/or methodological uncertainty,
there are no circumstances where it is scientifically valid toequate the absence of an observable biological effect with theabsence of risk.
540. Although mechanistic uncertainty remains, studies on
DNA repair and the cellular/molecular processes of radiation
tumorigenesis provide no good reason to assume that there
will be a low-dose threshold for the induction of tumours ingeneral. However, curvilinearity of the dose response in the
low-dose region, perhaps associated with biochemical stressresponses and/or changing DNA repair characteristics, cannot
be excluded as a general feature. The mechanistic modellingof radiation tumorigenesis is at a relatively early stage ofdevelopment, but the data available tend to argue against adose threshold for most tumour types.
541. Until the above uncertainties on low-dose response areresolved, the Committee believes that an increase in the riskof tumour induction proportionate to the radiation dose isconsistent with developing knowledge and that it remains,accordingly, the most scientifically defensible approximationof low-dose response. However, a strictly linear dose response
should not be expected in all circumstances.
542. The dose response for the induction of heritable diseasecarries fewer low-dose biological uncertainties than that ofmulti-stage tumorigenesis, but the same uncertainties
surrounding DNA damage response remain; an increase in

the risk of germ-cell mutation that is proportionate to radiation dose is judged to be a scientificallyreasonable approximation for the induction of heritable effects at low doses.
543. The Committee recognizes that ongoing and futurestudies in epidemiologyand animal sciences, while remainingof great importance for quantitative risk assessment, will not
resolve the uncertainties surrounding the effects in humans of
low-dose radiation. Accordingly, there will be an increasingneed for weight-of-evidence judgements based on largely
qualitative data from cellular/molecular studies of thebiological mechanisms that underlie health effects; theprovision of such judgements demands strong support from
biologically validated computational models of risk. With
ever-improving experimental technology, fundamental
knowledge will continue to grow. On this basis, theCommittee emphasizes the need for further work on themechanisms of DNA damage response/cellular stress andstudies of the consequences of these responses for neoplastic
development. Current uncertainties on the role of epigenetic
factors, such as bystander effects and induced genomic
instability, are expected to be reduced, but it may remaindifficult toestimate their overall contribution to risk. However,the development of mechanistic models of radiation risk
demands more than a simple improvement in the understanding of cellular/molecular processes. Issues such as targetcell identity/multiplicity; the kinetics of pre-neoplastic clonal
development; and rates of cell mutation, clonal proliferation,differentiation, and apoptosis, as well as the pattern of energy
deposition in critical cellular targets, all need to be better
understood in order to define biological parameters for use in
the biological modelling of tumorigenesis. These are difficult
areas of research, and it is not easy to anticipate the rate ofprogress. In spite of such experimental difficulties, theCommittee believes that advances in computational modellingof the physical and biological aspects of radiation
tumorigenesis will provide an essential tool for estimatingradiation risk at low doses and low dose rates.

ANNEX G: BIOLOGICAL EFFECTS AT LOW RADIATION DOSES

161

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