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REPORT OF THE

UNITED NATIONSSCIENTIFIC COMMITTEE
ON THE
EFFECTS OF ATOMIC RADIATIONGENERAL ASSEMBLY

OFFICIAL RECORDS : TWENTY-FOURTH SESSIONSUPPLEMENT No. 13 (A/7613)

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UNITED NATIONSNew York, 1969

NOTE
Symbols of United Nations documents are composed of capital letters
combined with figures. Mention of such a symbol indicates a reference to a
United Nations document.

Annex C
RADIATION.11\j"DUCED CHROMOSOME ABERRA.TIONS IN HUMAN CELLSCONTENTS
Parc11raph.r

Paragraphs

I. I 1'."TRODUCTIOXll.

D. Co11c/usio11s . . . . . . . . . . . . . . . . . . . . . . . . . . . • 264-273

THE TYPES OF ABERRATION'S PRODUCED, THEIRSTRUCTURE .-I.ND BEHAVIOUR ••••••••...•.•••.

13-104

A. The general pattern of response ...... .

13-23

B. Chronwsome-f)•Pe aberrations .......... .

24-50).{ATERIALS AND :METHODS OF STUDY •..•....••

ii-84
90-91
92-104105-144.......................... .

105-112113-123

C. Culture sampling time ............... .

124-13i

D. Conclusio11s ........................... .

138-144

THE
RELATIONSHIPBETWEENABERRATIONYIELD AND DOSE • • • • • • • . • • • • • • • • • • . . . • • • • • •145-2i3

A. l11troduction. . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. In vitro studies . . . . . . . . . . . . . . . . . . . . . . . .
1. X rays and gamma rays . . . . . . . . . . . .
2. Fast neutrons . . . . . . . . . . . . . . . . . . . . . .
3. The variation in response between
blood samples obtained from different
individuals . . . . . . . . . . . . . . . . . . . . . . . . . .

V.

85-89

B. Tlte peripheral blood culture teclmiq1tes ..

A. Introduction

JV.

i5-76

1-12

C. Chromatid-type aberrations ............•
D. Subchromatid-type aberrations ......... .
E. Ane11pJoidy ............................ .
F. Polyploidy •.•..........................
G. Eiuloreduplication .................... .
H. Co11clusio11s

III.

51-i4

C. In vivo studies . . . . . . . . . . . . . . . . . . . . . . . . .
1. Clinical exposure . . . . . . . . . . . . . . . . . . .
2. Occupational exposure . . . . . . . . . . . . . .
3. Accidental exposure . . . . . . . . . . . . . . . .
4. Nuclear explosion . . . . . . . . . . . . . . . . . .
5. Time of sampling after radiation exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

••••••••••..•••.••••••••••.•••

VI.

POSSIBLE BIOLOGICAL SIGNIFICAXCE OF THE
ABERRATIONS • • • • • • • • • • • • • . • • • • • • • • • • • • • . • . •184-263
185-215
216-222223-232
233-251
252-263274-325

A. l11trod11ction . • . . . • . . . . . . . . . . . . . . . . . . . . .
B. Abe1·rations fo germ cells . . . . . . . . . . . . .
C. Aberrations i1~ somatic cells . . . . . . . . . .
1. Somatic mutation and metabolic effects
2. Somatic mutation and cell killing . . . . .
3. Somatic mutation and life-shortening..
4. Somatic mutation and neop!asia . . . . . .
5. Somatic mutation and immunologicaldeficiency . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Conclusions

316-320321-325

CoNCLUSIONS

326-360• •• •• • ••• •• • • ••• • •• ••••• . ••• • •2i4-2i6
27i-284285-320286-288289-290291-293294-315

A. Application of aberration l'ields for biological dosimetry . . . . . . . . . . . . . . . . . . . . . . . 326-340
B. Assessment of risks . . . . . . . . . . . . . . . . . . 341-343
C. Reco-m111c11datio11s for f11rther study . . . . 344-360

145-153154-183154-170
171-17iPao<
TABLES •.•••••••...••••••••.•••..••••...•.•

liS-183

REFERENCES

I. Introduction•••••••.••••••••.•..•••••••••..

140
143

portance, and a considerable amount of work has been
devoted to studying the mechanisms of their induction
by radiation. their behaviour at mitosis and meiosis andtheir genetic consequence.
3. Up until relatively recently, most of this work
had been carried out on species particularly well suited
to the demands of cytological study (that is, species
having a relatively small number of rather large chromosomes) and on organisms amenable to use in breeding experiments ( 1958. 1962 and 1966 reports1 · 3 of theCommittee). This work has provided. and will continue to provide. fundamental knowledge on the actionsof radiations on chromosomes and also information on
the genetic hazards of radiation exposure in the particular species chosen for study. However, extrapolation from these species to man is beset with difficulties. particularly in the absence of any comparable
information on the radio-sensitivitv of human chromosomes that could serve as a reference point. Quantitative estimates of the radiation hazard to man ·s chromosomes haYe, therefore, been fraught with uncertainties.

1. It has been known for a long time that aberrations of chromosome structure (chromosome aberrations
or chromosome structural changes) and alterations inchromosome number arise spontaneously at a low rate
in somatic and germ cells of plants and animals andthat the frequency of such aberrations increases following exposure to ionizing radiations. These aberrations
may. in fact. comprise the major component of thegenetic damage resulting from radiation exposure, but
in many instances the genetic consequences of certain
kinds of aberrations are so disastrous as to result in
the early death of the cells containing them.
2. Although a considerable fraction of induced
chromosome aberrations mav behave as dominant lethal
events. all aberrations that do not result in an "immediate·' loss of viability are mutational changes which
may be transmitted to descendant cells and to theoffspring of the irradiated individual. Chromosome
aberrations are clearly, therefore, of great genetic im-98

4. In the late 1950s. routine cytogenetic studies on
mammalian chromosomes became possible as a consequence of the development of simple and reliable
methods of culturing mammalian cells in 7-'1.tro and oftechniques similar to those previously used in plant
cytogenetics. Refinements of these techniques•-• opened
the way for cytological studies on the response of
mammalian chromosomes to radiation exposure.
5. In the original work carried out by Bender 8 andby various other authors 9 -11 in the United States, studies
were made on the effects of x radiation on chromosomes in human epitheloid and fibroblast cell populations cultured in vitro. and comparisons were latermade between the responses of human cells and ofcultured cells obtained from spider monkeys andChinese hamsters. 12 At about the same time, Fliedneret al. 13 reported that chromosome aberrations could be
detected in cells from bone-marrow samples takenfrom a number of persons accidentally exposed to amixed neutron-gamma-ray beam.
6. Prior to all these observations, a large numberof earlier reports had shown that the chromosome;iberrations induced by the irradiation of mammaliancells were similar to the aberrations induced in otheranimal and plant cells.
7. At the same time that these developments in
mammalian radiation cytogenetics were occurring, thegeneral field of human cytogenetics was rapidly emerging. The initial work in this field soon confirmed thatthe kinds of chromosome aberrations already well known
to occur in plants and animals also arose spontaneously
in man, and demonstrated that, in man, these aberrations
were responsible for a number of very important harmful traits (1962 and 1966 reports~• 8 of the Committee).
These advances in human cytogenetics were given
further impetus by the development14 in 19t50 of asimple and reliable technique for obtaining preparations of mitotic cells from cultured peripheral blood
leucocytes. As a result of these developments. information on the spontaneous frequency and on the generalconsequences of chromosome aberrations in man has
been continually accming.
8. The advent of the peripheral blood culture technique afforded an opportunity to examine, by means ofa simple and painless procedure. the response of humanchromosomes in individuals exposed to ionizing radiations. Moreover, since large numbers of mitotic cells
could be obtained from only a few millilitres of blood.
frequent cell samples could be taken from an individual
at various time intervals after exposure. The firststudies of this kind were carried out by the Edinburgh
group1 5 in the United Kingdom. and in the last nine
years a great deal of information has been obtained on
chromosome damage and the potential hazards of radiation to man's genetic materials.
9. Observations have been made on chromosome
aberrations induced in 7->ivo in persons x-rayed for
diagnostic reasons. in personnel subjected to low-dose
occupational e..xposure (either externally or internally,or to a mixture of both e..xternal and internal radiation), in patients exposed to therapeutic radiation andin individuals accidentally exposed to radiation. In
addition to this. information is also available on members of the surviving populations at Hiroshima andNagasaki. Much of the information on patients given
therapeutic doses has come from partial-body irradiation studies. and here the data are somewhat difficult

99

to interpret since accurate physical dosimetry, particularly in relation to the cells sampled. is difficult toobtain. More recently, a little information has beengathered from a few ·patients exposed to low doses ofwhole-body radiation. 16
10. Although in terms of application of our knmvledge we are clearly most interested in results obtained
from in vivo studies, a great deal of information canbe and is being obtained from in vitro studies. Here.
cultured cells can be exposea to accurately measured
radiation doses, and accurate information on doseresponse kinetics, etc. can be obtained. Such knowledge
forms an important background to the in vivo work,
and it has been generally thought it may well prove
possible to extrapolate directly from the in vitro state
to the in vivo state, provided certain requirements aremet.
11. In view of the developments in this field over the
past few years. the Committee decided that an appraisalof the progress made in this area was necessary. The
time seemed particularly opportune for two reasons.
First, a number of laboratories have made use ofchromosome aberration yields as a method of estimating absorbed dose in individuals accidentally e.."Cposed to radiations. and a considerable amount of information on the relation between dose and aberration
yield from both in vivo and in vitro exposure has been
accumulating. Second, as a consequence of the developments in human cytogenetics, there has been an increasing amount of information on the importance of
certain aberrations as causal factors in human congenital abnormalities and also on the possible association between certain kinds of chromosome aberrations
in somatic cells and the development of neoplastic
disease.
12. As the Committee is primarily concerned withevaluating risks and with reviewing pertinent scientific data, information on the genetic consequencesof chromosome aberrations in man and on the possibility of using the levels of chromosome-aberration
yield following radiation e..xposure as a measure of doseis particularly relevant. In the present report, therefore, emphasis has been placed on somatic cell damage.
and attention has been centered on methodology. thepossible application of aberration yields in dosimetry,
their biological significance and their possible use in
the assessment of risk.
Il. The types of aberrations produced,
their structure and heha'\'iour

A. THE GENERAL PATTER.~ OF RESPOXSE
13. The types of chromosome aberrations inducedin human cells are identical in structure and behaviour
with the aberrations induced in other animal and plant
cells having similarly organized monocentric chromosomes. These aberrations are usually considered to
be of two basic types-the simple deletion, ·which may
be the result of a single break in the chromosome thread,and the e.."Cchange. which involves at least two breaks
and an exchange of parts either between different
chromosomes (interchange) or between different partsof the same chromosome ( intrachange).
14. The detailed mechanisms of formation of theaberrations are not fully understood. and two hypotheses are currently in vogue (see reference 17). The
more generally accepted classical theory, which wasdeveloped principally by Sax15 •20 and later by Lea andCatcheside (see reference 21), proposes that x-ray
induced simple deletions are a consequence of single
breaks in the chromosome produced through the action
oi a single electron track. whereas e..'Cchange events area consequence of the aberrant rejoining of breaks produced through the action of one or more (usually two)
separate electron tracks.
15. On this classical theory. the evidence obtained
from dose. dose-rate and dose-fractionation studies isinterpreted to indicate that broken chromosome ends
remain available for rejoining with themselves (thusrestituting the original chromosome structure) or withother broken ends (thus giving rise to an exchangeaberration) for only a limited time period (rejoiningtime) of around thirty minutes. 18• 21 This timing. however, is very dependent upon conditions. 23 Since theexchange aberrations can only be produced if the two
breaks involved are closely associated spatially21 • 22 andare produced close together in time, it follows that, on
this theory, the yield of simple deletions should increase linearly with increasing x-ray dose, and twobreak exchanges should increase as approximately thesquare of the dose, when exposure times are short
relative to the rejoining time (see references 17 and21).
16. On the exchange hypothesis of Revell, 24 allaberrations, including the so-called simple chromatid
deletions, are believed to be a consequence of exchange.
On this hypothesis a proportion of the simple deletions could result from the interaction of the effectsof two separate electron tracks. The deletions are believed to be the consequence of an incomplete exchange
between two regions within a chromosome so that thedeletion is associated with an inversion or duplicationof a short length of the chromosome at the point of"failed union". Thus, on this hypothesis, simple deletions can show either a negligible or a significant''dose-squared" component in their rate of increase
with increasing x-ray dose. 25 On both hypotheses, withhigh LET radiations all aberration types increaselinearly with increase in dose.17 • 23
17. In general, although there are certain exceptions,proliferating somatic cells spend by far the majority of
their lifetimes in an interphase state and pass relatively
rapidly through the division process of mitosis. The
duration of interphase may range from the life spanof the individual in a non-dividing differentiated cell,to a number of years in a mitotically quiescent cell or
to a period of less than one day in the case of anactively proliferating cell. In all cases, however. theduration of the mitotic phase is usually, at most, anhour or two and is. therefore, short in relation tointerphase. Thus. although the chromosome aberrations
produced in irradiated cells are only observed when thechromosomes appear at mitosis (or at meiosis in the
gonads). on the average almost all the aberrations
produced are a consequence of damage sustained in aninterphase state.
18. The aberrations observed in dividing cells arethus yisible manifestations of radiation damage sustained at an earlier point in time. A number of cellular( enzymic) processes may, therefore, intervene between
the initial radiation exposure and the final development of an aberration. Thus. for a given cell type,radiation dose, quality. etc .. the final yield of aberrations may be modified by physiological as well as
physical factors. The influence of such modifying factors was considered in some detail in the Committee's
1962 and 1966 reports.2, a100

19. The types of aberrations induced following
radiation exposure fall into three groups according
to the unit of breakage or exchange that is involved.
Aberrations which involve both chromatids of a chromosome at identical loci are generally referred to as
chromosome-type aberrations, whereas those in which
the unit of aberration formation is the half-chromosome
or chromatid are termed chroniatid-type aberrations.
The third category of aberrations known as subchromatid-type aberrations appear to involve breakage andexchange of subunits of a chromatid.
20. Which of the three basic types of chromosome
aberrations are obsen•ed at mitosis (or at meiosis)
depends upon the stage of development of the cell at
the time of irradiation. In a mitotically proliferating
cell, the interphase period of the cell cycle can bepartitioned into three phases :26 the pre-DNA synthesis
or G1 phase of early interphase; the DNA synthesis or
S phase; and the post-DNA synthesis or G2 phase oflate interphase. \Vith few exceptions. cells not actively
proceeding through a mitotic cycle usually rest in the
Gi phase (for example, .the peripheral blood small
lymphocyte in normal healthy individuals), and suchcells are sometimes referred to as being in a Go state. 27
However, there are e..xceptions to this general rule, andcertain types of mitotically inactive cells (for example,
certain epidermal cells in the mouse ear) 28 may rest ina G2 phase.
21. Irradiation of all resting cells and of the majority of proliferating cells in a G1 phase results inthe production of chromosome-type aberrations. At thevery end of the G1 phase29 • 30 there is a transition
from the chromosome-type aberration to the chromatidtype, and this transitional phase extends from late G1
into early S (figure 1). Thus, most of the cells irradiated while in S and all the cells exposed while in G2
yield chromatid-type aberrations. Subchromatid-type
aberrations are only produced in cells irradiated in theearly prophase of mitosis (or mid-prophase of meiosis),and cells exposed to radiation at the metaphase or laterstages of mitosis yield chromosome-type aberrations attheir ne..xt mitosis (see figure 1 and reference 17).
22. In addition to aberrations involving changes inchromosome structure, certain kinds of damage may
also result in alterations in chromosome number andyield aneuploid or polyploid cells (paragraphs 77-89).
Such changes in chromosome number are the result oferrors (non-disjunction) in chromosome or chromatid
segregation at meiosis or mitosis, and these errors areoften, although not invariably, a consequence of thepresence of chromosome structural changes.
23. It should be emphasized here that all of thevarieties of chromosome structural changes and ofchanges in chromosome number that are to be observedin irradiated cells are also to be found in cells e.."'\:posed
only to natural background irradiation. The frequencyof such spontaneous aberrations in unirradiated cells
from normal healthy individuals is, of course, e>..-tremely
low (table I), but the kinds of changes found are
precisely the same as those induced as a consequenceof radiation exposure.

B. CHRO:lfOSO:l!E-TYPE ABERRATIO~S
24. Chromosome-type aberrations, in which both
chromatids of a chromosome are broken or e..'Cchanged
at the same locus and in an identical fashion. are theaberrations that have been most frequently studied inhuman cells. This is because most of the work on man

CHROMOSOME-TYPE

CHROMA TIO-TYPE.Ji,

MITOSISINTERPHASE

DEVELOPMENT

Figure 1. Relation between type of aberration induced by radiation and stage in cell cycle attime of irradiation

25. Studies on metaphase somatic cells reveal thatseyen kinds of chromosome-type aberrations can be
distinguished cytologically (figure 2). Aberration types( i) to ( v) are produced within single chromosomes

has been carried out on peripheral blood leucocytesthat were irradiated while in a resting G1 phase andexamined at a mitotic metaphase following the stimulation of development of these cells in culture.
TERMINALNORMAL

DELETION

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DELETIONFRAGMENTACENTRIC

P!:RICENTRIC

RING

INVERSION

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DICENTRIC AND FRAGMENTen

SYMMETRICAL.
IN.TERCHANGE

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Figure 2. Chromosome-type aberrations that can be distinguished cytologically at mitosis

and are referred to as illtrachangcs, whereas types (vi)and (vii) involve an exchange of parts between different chromosomes and are. therefore, interchanges.
26. (i) Terminal deletio11s are paired acentric fragments which have the appearance of resulting from a
simple break across the chromosome and are not associated with an exchange aberration such as a ring orinterchange (paragraphs 28. 33-34). Some authors
refer to terminal deletions simply as free acentric
fragments.
27. (ii) iV!imctes (interstitial, isodiametric or dot
deletions) are pairs of acentric fragments, smaller insize than terminal deletions. characteristically appearing as paired spheres of chromatin, hence the terms.
dot or isodiametric deletions. These deletions areusually not terminal but intercalary and are the consequence of two closely juxtaposed transverse breaks
across the chromosome.
28. (iii) Acentric rings are the result of two transverse breaks and an exchange within the chromosome.
The linear separation between the two breaks is greaterthan in the case of minutes so that the e-xcised paired
fragments are larger and are ring-shaped. The distinction between minutes and rings is often arbitrary.
since it is based purely on the size of the interstitialregion of the chromosome that is deleted.
29. (iv) Centric rings are ring-shaped chromosomes resulting from an exchange between two breaks
occurring on either side of the centromere. The centricring aberration is clearly distinguished from its acentriccounterpart and is accompanied by one (rarely two)acentric fragments.
30. (v) Pericentric inversions result from two
breaks. one on each side of the centromere, followedby the inversion of the centromeric segment and its
reincorporation into the chromosome. If the two breaks(or points of exchange) are not equidistant from thecentromere, then the pericentric inversion is clearly
characterized by the altered location of the centromere
within the chromosome. However. if, as is probably
most often the case, the exchange points are approximately equidistant from the centromere, then the in-101and of the restrictions on chromosome movements ininterphase. 17 • 31 there must be a much higher probability of exchange between points equidistant fromthe centromere than between points at different distances. Similarly, in the case of symmetrical interchange aberrations, exchanges between points equidistant from the centromeres of the two chromosomes
involved will be very frequent. Thus, if the chromosome
complement contains a number of chromosomes havingarms of equal or similar lengths, as is the case in thehuman complement. then a symmetrical exchange often
will not result in an altered morphology of the exchanged chromosome ( s), and the aberration will passundetected.

version cannot be detected in mitotic cells but could
be detected in meiotic cells following chromosome
pairing.
31. Paracentric inversions, where both points ofe.xchange lie on the same side of the centromere, cannot be cytologically detected in mitotic cells but could
be identified at meiosis.
32. In types (ii) to (v), the exchange eYent may,in a small proportion of cases. be incomplete, only twoof the four free ends involved in the exchange actually
undergoing rejoining. Thus, an incomplete paracentric
inversion will be scored as a terminal deletion, and aring chromosome may be accompanied by two, rather
than one, acentric fragments.
33. (vi) S·ymmetrical i11te1·cha.tiges (recipi-ocal tra11slocations) are exchange aberrations resulting from a
breakage in each of two chromosomes followed byaberrant rejoining such that the distal regions of thetwo chromosomes are transferred (translocated) from
one to the other. The aberrations are described as
being symmetrical since they do not result in theformation of a dicentric structure (paragraph 34).
Occasionally, if the exchange is incomplete, an acentric
fragment may result. If the e.xchange is equal. that is.
if an equal length of chromatin is translocated fromone chromosome to the other, then the exchange could
be detected at meiosis. Such an exchange could not be
detected in somatic cells, however, except when incomplete, in which case it would appear as a simple
terminal deletion. Symmetrical interchanges between
acrocentric chromosomes are sometimes referred to as
centric fusions. They result from the translocationof entire chromosome arms, the exchange occurringin the region of the centromeres of the chromosomes
involved.

37. The inefficiency of scoring symmetrical andequal inter- and intrachange chromosome-type aberratfoms is not encountered in the comparable chromatidtype aberrations. At the chromatid-type level, becauseof the close pairing between sister chromaticls. asymmetrical and symmetrical aberrations can all be scoredwith equal efficiency. Studies on chromatid-type aberrations in plants and animals have shown that the symmetrical and asymmetrical variants of any given aberration type occur at approximately equal frequencies, 17 • 21
and it has generally been assumed that this approximate
equality must also obtain at the chromosome-type level.
This assumption has recently been confirmed in studies
on the morphologically well-marked large chromosomesof the plant View faba. where the frequency of involvement of these chromosomes in symmetrical (reciprocaltranslocation) and asymmetrical ( clicentric) chromosome interchange was shown to be equal.3~
38. Since it is to be expected that asymmetricaland symmetrical interchange events occur with equal
frequency in irradiated human cells. it is possible toestimate from published data 16 • 33 •35 that the efficiency
of scoring symmetrical events in man·s chromosomes
is not more than 20 per cent. This follows from thefact that the frequency of dicentric- plus centric-ringaberrations should equal the frequency of reciprocaltranslocations plus pericentric inversions, whereas, inthe data available, dicentric and ring ch:romosomes
are approximately five times as frequent as abnormal
monocentric chromosomes. Similarly, the frequencyof acentric-ring plus minute aberrations should correspond to the frequency of paracentric inversions. As
noted earlier, paracentric inversions cannot be detectedin mitotic cells.

34. (vii) Asymmetrical interchanges( dicentricaberrations or more complex polycentrics) are exchange aberrations due to a breakage in each of two
or more chromosomes followed by aberrant rejoining
such that the proximal regions of the chromosomes
become united, thus forming a dicentric or polycentric
structure and an associated acentric fragment (rarely
two fragments, when the exchange is incomplete andthe two distal regions do not unite).
35. As mentioned above, incomplete rejoining ine..xchange results in an increased frequency of freefragments associated with an aberration. It should be
stressed, however, that the fragment associated withan exchange aberration, such as a dicentric or centricring, is part of the exchange aberration and is notscored as a separate fragment. that is. as a terminal
deletion. The presence of a dicentric- or centric-ring
structure with no accompanying fragment is an almost
certain indication that the aberrant cell has proceeded
through at least one mitotic division after irradiationand prior to observation.

39. In addition to the general difficulties in detecting symmetrical aberrations, it should be notedthat the efficiency of their detection varies between
different obsen,ers. However, these problems do notarise in the case of the asymmetrical aberrations. It is
for these reasons that it has long been the practice ofradiation cytogeneticists working on plant and animalcells to classify chromosome-type aberrations into
terminal deletions. minutes. acentric rings, centric ringsand dicentrics (polycentrics) and to use data on theseaberrations, but not data on pericentric inversions andsymmetrical interchanges. for quantitath·e studies.

36. :Mention has been made of the difficulties indetecting certain forms of pericentric inversions (interarm intrachanges) and of symmetrical interchanges insomatic cells so that not all seven chromosome-type
aberrations considered can be scored with equal efficiency. It is to be expected that pericentric inversions
that do not result in a change in the relative arm
lengths of a chromosome must comprise a significant
proportion of the total of such aberration types. Thisls so because. as a result of the V-shaped arrangementof the chromosomes following anaphase separation

40. It should be stressed that the aberrations described under the above five headings form the bulkof the structural alterations that can be observed.
They can be scored efficiently and there is little variation due to subjective differences between different
observers. In the case of the possible use of chromosome
aberration yields as indicators of absorbed dose inman, there is little doubt that the classification of the102seded by the descriptive clas:sification. In more recent
years, an additional and more general classification
has been introduced. 44 • 45 This classification places emphasis on the cells carrying the aberrations, cell types
being defined, as indi'cated in the subsequent paragraphs. on the basis of the kinds of aberrations thatthey contain.
46. Type A Cells have no apparent evidence of astructural chromosome abnormality. They may be
divided into "modal A cells" that are apparently
normal diploid cells and "non-modal A cells" that are
aneuploid, that is. that contain fewer or more chromosomes than the normal diploid number.
47. Type B cells. as originally defined. 44 included
two sorts of cells, those containing chromatid gaps
or isochromatid gaps (non-staining regions of thechromosome either affecting one or both chromatids).
which are not discontinuities and do not result in theformation of acentric fragments. and cells containing
simple chromatid breaks ( chromatid terminal deletions) but not isochromatid aberrations nor presumably
chromatid minutes. In other words, type B cells. as
originally defined, contain either non-staining gaps or
one of the many possible types of chromatid-type aberrations. Fortunately, however. the phrase. "type B
cells", has come to be used to describe cells containing
chromatid-type aberrations of any kind (see reference33) as opposed to cells carrymg chromosome-type
aberrations (type C cells).
48. Type C cells contain chromosome-type aberrations. Type C cells were originally put into three categories, C1. c~. Ca. 44 but later 45 they were reclassified
into two categories. Cu and C•. C., cells contain asymmetrical aberrations or incomplete symmetrical aberrations. that is, dicentrics (polycentrics), ring chromosomes or fragments. The suffix, u. in Cu denotes thefact that the cell contains an ''unstable" aberration
which will either result in mechanical difficulties atmitosis or result in a loss of chromosome material
in the form of an acentric fragment. C. cells contain"stable" aberrations. that is, complete symmetricalaberrations (symmetrical interchanges or pericentric
inversions). which can only be detected if the e..xchange
events result in a change in the position of the centromere or in the lengths of the chromosomes involvedin a rearrangement (paragraph 36).
49. It has already been stressed that the Cs cells
can only be detected with very low efficiency. It should
also be pointed out that certain types of Cu cells (forexample, those containing small terminal or intercalary deletions) may be unambiguously scored as clear
Cu cells at the first mitosis after irradiation, but that,
if viable, their descendants could well be scored at
the second or subsequent mitosis following irradiation
as either normal A cells or as C. cells.
50. The use of the C. and C,, form of classification
may be useful as a short-hand system, particularly
when considering the fate of cells carrying aberrations
over very long periods of time after irradiation ; thisis precisely where this general scoring system has been
largely employed. Nevertheless, there can be noquestion that the maximum information can only beobtained when aberrations are categorized on the basisof their detailed structure. The Committee, therefore.
strongly recommends that the detailed system of scoring be employed, particularly in those cases where
attempts are made to obtain information on doseresponse relationships.

aberrations into these categories is essential. However,
it is important to note that the aberrations that are
simplest and least ambiguous to score include manyof those that may result in cell death.
41. For instance. at the anaphase stage of mitosis,
a proportion of the dicentric aberrations form chromatin bridges linking the two anaphase groups andinterfering with the mechanical separation of the ty,rodaughter cells. Such an interference frequently resultsin the death of the cells. In addition, chromosome
fragments lacking centromercs may be excluded fromthe daughter nuclei produced as a result of mitosis.
and, depending upon the gene content and amount ofmaterial lost. such genetically deficient nuclei may beindable. In general. therefore. a considerable proportion of the asymmetrical aberrations constitute a shortterm hazard in the sense that the cells that carry them
have a Yery much reduced potential for survh-al. However. those cells that can survive will still be mutant
and constitute a long-term hazard both to the individual
and. if present in the germ line, to his offspring.
42. On the other hand, symmetrical aberrations.
which are simply a consequence of the rearrangementof chromosome material either within or between thechromosomes in the complement. will not rrsult inany chromatin deficiency when induced in somatic
cells. Cells carrying such aberrations will encounterno mechanical difficulties in proceeding through themitotic process and may be perfectly viable. However,
if such changes are present in the germ line, then, asa consequence of chromosome pairing and segregation
at meiosis, they may result in sterility and in the production of unbalanced gametes. For example, a numberof spontaneous translocations are kno\vn to exist in
man 3 and these are, in certain cases, responsible fora reduced fertility and. in others. for the productionof viable offspring with harmful traits, for example,
Down's syndrome.
43. It should, therefore, be re-emphasized here thatthe criteria for scorin~ aberrations are not based on
their particular biological importance. Thus. although
the symmetrical aberrations which result in little or nochange in chromosome morphology are difficult toscore objectively, they nevertheless may constitute avery important long-term hazard, since they result intranspositions and rearrangements of chromatin between non-homologous chromosomes and in duplications and inversions of the genetic materials. :Moreover. these aberrations are equally as frequent as theirasymmetrical counterparts. At present. the scoring ofsuch symmetrical aberrations in somatic cells is bothtedious and extremely inefficient, but their detection
might well be improved with the advent of mechanization and computer techniques in cytogenetics. 36 •4 :?
44. The method of classification of the aberrations
that has been outlined is that generally in use by radiation cytogeneticists and is based on the structure of
the aberrations. This method of scoring does not involve any assumptions as to the precise mechanismof formation of any particular aberration and does notgroup together aberrations that are structurally different but which may have similar mechanical consequences at mitosis.
45. An alternative system, which does involve
assumptions as to the mechanism of formation ofaberrations. was originally proposed by Darlington andUpcott 43 in their early work on chromosome aberrations in plant cells. This system, however, was super-103

C. CHROMATID-TYPE

ABERRATIONS

51. Chromatid-type aberrations occur if damage issustained either at the time of, or following, chromosome splitting and replication in late G1 and S of thecell cycle17 • 29 • 3o (paragraphs 19-21). In the case of
unsplit G1 chromosomes, any radiation damage sustained is itself replicated when the cell proceeds into S
so that the whole chromosome (both chromatids) is
involved in a chromosome-type aberration, both sister
chromatids being affected in exactly the same way andat identical loci. Chromatid-type aberrations are thus
distinguished by the fact that the unit of breakage or
exchange is the single chromatid.
52. Chromatid-type aberrations are. therefore. induced in cells irradiated while in the DNA synthesis(S) and post-DNA synthesis (G2) stages of interphase. Such aberrations also arise spontaneously, presumably as a consequence of replicating errors. They
can be readily induced by e..xposure of cells i11 'l'ivo
and in i,·itro to a wide variety of chemical agents andhave been found in peripheral blood leucocytes takenfrom individuals suffering from certain virus infections
and in cultured fibroblasts exposed to viral and otherinfectious agents.
53. A large proportion of the chemical agents that
produce mutations in micro-organisms. insects andplants induce chromatid-type aberrations in plant 46 " 48and mammalian 47 • 49 cells exposed in vivo or while incontinuous culture in vitro. A number of these agents.
particularly those known to interact with DNA or tointerfere ·with DNA synthesis (including carcinogenichydrocarbons such as dibenzanthracene) .~ 0 have now
been tested in human cells and shown to produce
chromatid-type aberrations in peripheral blood leucocytes and in fibroblasts exposed in vitro. 5 1-7° Similar
aberrations are also to be found in leucocytes taken
i rom patients treated for certain clinical conditions withpotent mutagens, for example, nitrogen mustard. 71-7 5
54. Considerable interest has recently been arousedin the possibility that the hallucinatory drug, lysergicacid diethylamide (LSD-25). might act as a mutagenic agent in man. Cohen et al. 76 originally presented
evidence indicating that e..xposure of peripheral blood
leucocytes in culture to LSD resulted in the production of chromosome aberrations; in addition. a low.
but significantly increased. aberration yield was foundin cultured peripheral blood cells taken from a patient
extensively treated with this drug over a period offour years.
55. More recent studies on LSD users (which frequently include individuals taking other drugs inaddition to LSD) have yielded conflicting results.
Some authors 77·i 9 have reported small. but significantly
increased over normal, aberration yields in cultured
leucocytes of LSD users. whereas others have foundno evidence of a change in aberration yield in LSD
users 80• 81 or in patients treated with LSD. 81 • 82 Studies
on the mouse 83 have yielded suggestive evidence of aslight effect on high doses of LSD on meiotic chromosomes. whereas mutation studies in Drosophila
han either shown no effects 84 at does levels comparable to those used in the mouse work or significant
increases in the yield of recessiYe lethals when massive,highly toxic, doses were used. s;;
56. The demonstration by a number of workersss-1os
that viral infections may produce chromatid-type aberrations in human and in other mammalian cells and104

the possible implications of virus infection in relation
to carcinogenesis (paragraphs 286-288) has prompteda variety of studies on this aspect of aberration production.' A wide varietv of both DNA and RNA
Yiruses has been reported as being responsible for the
production of chromatid aberrations in human peripheral blood leucocytes and in human and other mammalian fibroblast cells maintained in continuous culture. The viruses that have been claimed to induce
aberrations include Sendai virus, 86 chicken pox Yirus,s7
measles ,-irus, 90 • 100 • 102 yellow fever. 93 vaccinia. 109 poliomyelitis.ss the Schmidt-Ruppin strain of the Rous sarcoma virus.~ 4 • 99 • 103 herpes simplex. 104 • 107 cytomegalovirus,165 infectious hepatitis virus 87 • 88• 95 • 96 and various human and simian adenoviruses.106
57. Studies on human embryo cells e..xposed toavian pseudo-plague virus have shown that infection
with viable virus results in the formation of chromosome structural changes but that no such changes
are produced following e..xposure of the cells to heatinactivated virus. 110 In addition. chromatid aberrations have been reported in cultured human fibroblasts
infected with M ycoplasma 111 • 112 and similar aberrations reported 113 in Drosophila. e..xposed to Rous sarcoma virus and in other arthropods infected with a"Rickettsia-like ., organism. 114
58. Although there are conflicting reports on thepresence or absence of aberrations in cultures of peripheral blood leucocytes from patients suffering fromvarious virus infections. 91• 93 • 108 there is no doubt that,
under certain conditions, viral and other infectious
agents can induce chromatid-type aberrations in humancells. The general conclusion arrived at by many
workers in this field is that the effects of these agentson chromosomes are very similar to the effects ofchemical mutagens that interfere with DNA synthesis.
This conclusion is supported by the observations of
Nichols et al. 68 of a synergistic action of the SchmidtRuppin strain of the Rous sarcoma virus and of cytidine triphosphate (a nucleoside triphosphate that induces chromatid-type aberrations in human cells) in
producing chromatid aberrations in human leucocytes
treated in ·vitro. Moreover, the aberrations induced by
nucleosides and by viral agents are both localized toparticular chromosomes and chromosome regions 67 • 68
and thus differ from radiation-indnced aberrations
which are more randomly distributed.
59. It is important to note that Stich and Yohn 106
have recently obtained evidence that, at least in thecase of certain types of adenovirus. aberrations are only
produced by Yiruses which initiate but do not complete a full replication cycle. Moreover. it should bepointed out here that chromatid aberrations observedin peripheral blood leucocytes of patients sufferingfrom virus infection are actually produced when thecells are in culture. ( Chromatid-type aberrations produced in vivo would appear. if the cells were viable.
in the first mitosis observed in cnlture as "derived"
chromosome-type changes.) Indeed. it has been suggested115 that the conflicting reports from different
individuals and laboratories may be simply due topositive results being obtained more often when cells
are allowed to proceed through more than one cell
cycle in culture prior to obserYation.
60. It should be strongly emphasized that, in thecase of the viruses. of the alkylating agents and of the
majority of the other chemical mutagens that havebeen studied. onl'J' chromatid-type structural changes

have been seen at the first mitosis follO\ving treatment.
The aberrations produced by these agents presumably
arise as a consequence of misreplication. 116 • 117 so thataberrations are not directly produced in cells exposed
while in the G1 phase of interphase. On the other
hand, ex:posure of such G1 cells to ionizing radiations
results in the formation of typical chromosome-type
aberrations.
61. The kinds of chromatid-type aberrations produced by ionizing radiations in S and G2 cells and by
a variety of mutagens. including ultra-violet and thechemical and infectious agents referred to above, areall basically similar and are similar to those that arise
spontaneously in culture. However. because the unitof aberration formation is the chromatid. and because
sister chromatids remain closely paired at mitosis.
these aberrations exhibit a greater variety and are
more efficiently detected at mitosis than their chromosome-type counterparts.
62. The variety of possible chromati<l-type aberrations has been discussed in detail by a number ofauthors. 17• 118 and a detailed description and illustration of these aberrations will not be given here. In
brief, the aberrations include terminal deletions. intercalary deletions ( chromatid minutes). acentricrings, isochromatid deletions, duplications, inversions,interarm asymmetrical intrachanges (centric rings) ,interarm symmetrical intrachanges (equivalent to pericentric inversions) and symmetrical (reciprocal translocations) and asymmetrical ( chromatid dicentrics) interchanges.
63. In addition to these chromatid-type structuralchanges, cells irradiated in the S or G2 phases ofinterphase. or subjected to infectious agents or to certain chemical mutagens, may also contain achromaticlesions that are usually referred to as gaps or erosion
zones. These gaps do not represent transverse breaks
across the chromatid thread but are simply unstained
regions similar in appearance to normal secondary
constrictions or nucleolar organizing regions. 24• 119
There is evidence from studies on plant chromosomesthat these gaps are reparable lesions that do not resultin a permanent structural change in the chromosome.120, 121.
64. Information on the spontaneous yield ofchromatid-type aberrations in human cells has beenobtained from studies on peripheral blood 1eucocytes.
Although a number of reports (paragraphs 65-66)
have indicated that the frequency of spontaneouschromatid aberrations (more particularly chromatid
deletions) in these cells may be somewhat variable(averaging around 0.05 per cell), there is no doubtthat much of this variability is due to cells being
allowed to proceed through more than one cell cycle
in culture before sampling. The aberrations are, therefore, produced in culture (probably as a consequenceof misreplication). Culture conditions are extremely
important in this connexion, particularly since the cells
and cell products themselves contribute to changing
conditions.
65. 1fouriquand et al. 122 in a study of 1.000 leucocytes taken from ninety individuals and cultured forseventy-two hours prior to observation, reported achromatid-deletion frequency of 0.057 per cell and achromatid-gap frequency of 0.077 per cell. In a somewhat larger study carried out by Court Brown atal.,123 the frequency of chromatid aberrations. m105

eluding gaps, in 12,000 leucocytes cultured for seYentytwo hours was around 1 per cent. More recently. ina survey 124 in which care was taken to sample cells
at their first mitosis in culture. 1,200 leucocytes from
400 individuals were examined. and it was found thatthe frequency of chromatid-type aberrations in thesecells was very low (0.033 aberrations per cell) anddid not vary with the age of the donor. Moreover,in this latter survev. it was clearlv shown that thefrequency of these aberrations increased with incre:lsing
duration of the period of leucocyte culture.
66. A number of workers have noted the presenceof chromatid-type aberrations in peripheral blood leucocytes irradiated in vitro during the G1 phase and incells obtained from individuals exposed to ionizing
radiations. The frequency of these aberration types in
irradiated individuals is very low (for example.
around 0.02 aberrations per cell) 125 and is usuallysimilar to the frequency of these aberrations in blood
cells of unirradiated personnel. Indeed, studies onpatients following radio-therapy treatment, 16•126.127
accidental or occupational radiation exposure1 25,i2 7 .ia2
and exposure to radiation follow-ing a nuclear explosion133·135 have revealed no significant differences inyields of chromatid-type aberrations between irradiatedand control personnel.

67. On the other hand. there have been some suggestions of a slightly increased chromatid-type aberration yield in irradiated personnel. 136 and, in two instances, reports of much higher yields in cells irradiated in vitro and sampled seventy-two to ninety-six
hours after exposure.13 7,138
68. Since chromatid-type aberrations cannot bedirectly induced by the irradiation of unstimulated
leucocytes. their presence in such cells is generally
agreed to be almost certainly due in part to a possible
secondary effect giving rise to these aberrations in
culture (and this is particularly true in in vitro radiation experiments) and largely to effects occurring in
culture that may have no connexion whatsoever witha radiation exposure. It should also be pointed out
that. when relatively high yields of "spontaneous''
chromatid aberrations are observed. very few e..xchangeaberrations are noted, virtually all the aberrations
being simple deletions (see reference 139). This very
low frequency of exchange suggests that some of thechromatid breaks that are observed may well be consequences of the mechanical forces operating when
the cells are being dried during cytological processing.
69. Chromatid-type aberrations can. of course. be
induced bv radiation in vitro if the radiation is delivered du,ring late interphase.11 • 140 •146 Moreover, such
aberrations are induced if the cells are exposed toradio-activity labelled DNA precursors. 147 • 148 Similarly. these aberration tvpes will be produced in viva
in those cells that are actively engaged in proliferation
at the time of radiation exposure. However. thesechromatid-type aberrations will be lost before cells
develop into circulating lymphocytes. or. if the aberrations are symmetrical and therefore do not result inmechanical difficulties in the separation of chromatids
at anaphase, they will pass into the daughter cells,
proceed through a replication phase and reappear as"derived" chromosome-type changes at the following
mitosis (paragraph 74 and figure 3).
70. Since chromatid-type aberrations can only be
induced in cells irradiated while in theor c~ phasessof the cell cycle, chey are clearly quite useless as indicators of dose in cells, such as the peripheral blood
leucocytes of man, exposed to radiation while in theG1 phase. However. these aberrations can be used asdose indicators in normally proliferating cell populations, although it should be noted that the yield ofchromatid-type aberrations at any given dose level isvery much dependent upon the exact stage of development of the cell at the time of exposure (see reierence
149).
71. Detailed studies on cells of the plant Vicia
fa.ba 121 have shown that the chromatid-type aberration
yield induced by x rays in mid-G2 cells may be threeor four times higher than the yield induced in earlyG2 cells. Moreover. G2 cells are more sensitive thanS cells, and variations also occur within the S phase.
Similar variations have also been observed in -mammalian cells irradiated either in vivo or in vitro. For
instance, the yield of chromatid-type aberrations in
Chinese hamster fibroblasts receiving doses of 250 radsfrom cobalt-60 gamma rays in vitro 'vas found to be
three times higher in cells exposed while in G!! thanin cells exposed while in S. 150 Similarly, data on bonemarrow cells taken from Chinese hamsters that hadreceived 100 rads from x rays 1 " 1 (240 kV, lSmA. HVL= 2mm of Cu) or 100 rads from cobalt-60 gamma
rays 1 r> 1 in. vivo also show that cells in G2 are much
more sensitive than cells irradiated in earlier phasesof the cycle.
72. The limited number of studies that have been
carried out with human cells on change in aberration
yield with change in cell phase all accord with theearlier observations made on plant and other animalcells in showing that changes in response occur withchanges in development phase. :Most of the studies onradiation-induced chromatid-type aberrations in humancells have either been made on samples observed atonly one fixation time after x irradiation141 • 14 a. H 4 ormany hours after exposure, 8 or on samples fi.xed at
unspecified times after irradiation. The four studies
on peripheral blood leucocytes 142 • 144 • 146 and on "fibroblast-type"' cells in culture, 11 where samples were fi.'Ced
NORMALCHROMOSOMESINQ
2EXAMPLES OFONECHROMATID-TYPE OF THE POSSIBLE
ABERRATIONS AT
ANAPHASE"IRST METAPHAS! CONFIGURATIONS(X 1)
AT ANAPHASE (X 1)

at various times after irradiation, indicate that. for agiven x-ray exposure. the yield of chromatid-type aberrations is higher in G2 than in S cells. This change in
response both within and between cell phases at thechromatid-type level underlines one of the difficulties
inherent in the use of these particular aberration types
as indicators of dose.
73. The intraphase variation in sensitivity chat has
been observed for chromatid-type aberrations is notapparent with the chromosome-type aberrations. Studies
on plant cells18 • 22 • 103 • IM and human peripheral blood
leucocytes34 • 115 • 155 •157 (paragraphs 124-137) indicatethat the yield of chromosome-type aberrations is constant throughout the G1 phase. In the plant and animalstudies referred to above. it has generally been foundthat. at a given dose level, the yield of chromosometype aberrations is less than the yield of chromatidtype aberrations. The limited comparisons that havebeen made between the frequencies of those twokinds of aberrations in hwnan peripheral blood leucocytes suggest that a similar pattern exists, the maximum sensitivity occurring in cells irradiated while inG2. l4l, H2, 144
74. It should be emphasized here that symmetricalchromatid intrachanges (including duplication, deficiencies and pericentric inversions) and symmetricalinterchanges will all result in an abnormal monocentricchromosome in one (or in both in the case of interchange) of the daughter cells produced as a result of
mitosis. The replication of these abnormal chromosomes
will result in the appearance of "derived" symmetricalchromosome-type aberrations at the second mitosis
following their induction. Similarly, if asymmetricalchromatid interchanges and chromatid fragments areincluded in the daughter nuclei, then these also will
result in "derived" chromosome-type aberrations appearing at the second mitosis. Such asymmetricalchromatid aberrations (for example, dicentric chromatids) have a finite probability (up to P0.5) ofbeing transferred intact to one of the daughter nuclei
so that up to one-half of them will inevitably result in''derived" chromosome-type aberrations (figure 3).

=

REPLICATIONIN SUCCEEDING
INTER PHASE"DERIVED"CHROMOSOME-TYPE
ABERRATION AT SECO"DMETAPHASE ex,)I.~NORMAL(, )~ ·~ r ii<\ )7: ~ f I'~ ~NORMALNORMAL

If

~

u

u

0~

Figure 3. Examples of "derived'' chromosome-type aberrations at the second (X:::'i mitosis after
irradiationa.i. These aberrations ba,·e be".:"n derived from aberrations that were of the chromatid·type at the first mitosis after
irradiation. Note that only a Em:ted number of the possible anaphasc configurations arc shown and that. in man::·instances an acentric fragment ·.\-ill be losr and may not, ior example. be found in association with the chroma..
somc(s) from which it js derh:ed.

106

D. SuBCHR01rATID-TYPE ABERRATIONS

tain two or more cell lines differing in chromosome
number as a result of chromosome loss or gain occurring in the early stages oi development of the individual.
These particular instances of chromosome mosaicism are
not entirely relevant here, since the majority of such cases
are to be found in individuals having a cell line
possessing one or more chromosome additional to thenormal diploid chromosome number or, in abnormal
females. containing a proportion of cells lacking an X
chromosome.
81. A number of workers have reported an increased
incidence of aneuploidy (hypodiploidy) in peripheral
blood leucocyte cells obtained from individuals e..xposed to radiation, 44 • 128• 129 • 16"·16 8 and similar increases
have been reported in in vitro studies.1as, 138, 169-112
Thus, two groups of workers 136·138 suggest, on thebasis of their in vitro data, that the incidence of aneuploidy increases linearly with increasing x-ray exposure,at least up to a certain dose. It should be pointed out,
however. that in all these studies, the cells sampled
had been ailowed to grow in culture for seventy-two
hours or more so that in many cases cells in theirsecond and third division in culture were being sampled(paragraphs 124-137).
82. In some of the earlier 12· m, 174 and in most of themore recent studies, 16· 127· 132· 1 ~3 • 135 no differences havebeen found in the incidence of aneuploidy in blood
leucocvtes from irradiated individuals or in cells
irradi~ted in vitro as compared with controls. Buckton
et al. 16 quote a mean value of 3.8 per cent aneuploidy
for fifty-three patients treated with x rays for ankylosing spondylitis and a value of 3.7 per cent in unirradiated control patients. Similar values of 3 percent were noted by Ishihara and Kumatori175 in theirThorotrast patients and in controls, although rather
higher values of around 10 per cent in both control
and irradiated personnel were found by Visfeldt. 127
83. There seems little question that many of thehypodiploid cells that have been noted in radiation
studies were either cells that were carrying chromosome structural changes. such as dicentrics, or were moreprobably cells that had contained aberrations but hadproceeded through more than one mitosis in culture
and had lost aberrant chromosomes (for example,
dicentrics or centric rings) at their first division. The
incidence of aneuploidy cannot. therefore, be simply
correlated with a previous radiation history and cannot be used in a quantitative manner as an indicatorof radiation absorbed dose.
84. In genetic terms, the loss of a normally genetically active chromosome from the complement isserious, and, if it does not result in the death of thecell, clearly constitutes a mutation. The presence ofaneuploid cells in the germ line would lead to theformation of inviable zygotes, except in the case of
the loss of one sex chromosome (as discussed in the
1966 report3 of the Committee) or in very rare instances of the loss of a chromosome in group G. 17 a.i 75

75. Subchromatid-type aberrations are exchanges
within or between chromosomes which appear to involve a subunit of the chromatid.1' These aberrations
arise spontaneously in meiotic prophase cells of a wide
variety of plant and animal speciesH but have not so
far been recorded in man. They can be induced bychemical agents, ultra-Yiolet light and ionizing radiations, 17 • 46 • 48 but they are produced only in cells e......:posed while in early prophase of mitosis or meiosis.
76. Because subchromatid-type aberrations are relatively infrequent and because they cannot be inducedby irradiation of unstimulated peripheral blood leucocytes (these cells being in early G1). no informationon their frequency in irradiated human cells is available. They will, therefore. not be considered further inthis review.

E. ANEUPLOIDY
11. In cytological preparations of cells from normal
diploid individuals, a small percentage of the cells
appears to be deficient for one or more chromosomes(i.e.. are hypodiploid) and an even smaller percentage
may contain one or possibly more extra chromosomes(i.e., are hyperdiploid). The frequency of such aneuploid cells may vary bet\veen individuals as well as
between samples taken concurrently from the same
individual. A certain proportion of the aneuploid cells
is certainly an artefact resulting from cell breakage
during cytological processing. However, there is reason
to believe that aneuploidy is a natural phenomenon in
peripheral blood leucocytes of standard diploid individuals and that its frequency may be related to age
and se..x of the individual.
78. Cytogenetic surveys on human populations 122·
124· ISB-1 64 have shown that aneuploidy is slightly more
frequent in peripheral blood leucocytes of females thanof males. In females, this aneuploidy is largely a consequence of the loss of a chromosome in groups 6 to
12 (possibly an X chromosome, presumably the inactive X), whereas, in males, it is largely a consequence of the loss of the Y chromosome. Aneuploidy inblood leucocytes of the new-born may be less than
3 per cent but may reach a value as high as about 13
per cent in adult females and 7 per cent in males. In
males, this increase with age is not clearly apparent
until around age si..xty-five, but, in females, it is apparent a decade earlier. Kerk-is et al. 162 have suggested tl1at this increase of aneuploidy with age may
be a consequence of differences in the response of cells
to hypotonic treatment in culture.
79. It has been shown 122 that the incidence of aneuploidy in any given cell culture increases with increasing
culture time as a consequence of cells undergoing morethan one mitotic di.,.;sion in culture. Reference has alreadybeen made to the fact that the presence of chromosome
structural changes in cells will frequently lead tochromosome loss. and this is to be expected particularly with aberrations that may result in bridges
at anaphase. Thus, the presence of even a low frequency of spontaneous or radiation-induced aberrations
at the first division of the cells in culture will leadto an increase in hypodiploidy in the daughter cells
observed at the second and subsequent mitoses.

F.

POLYPLOIDY

85. Manv workers have noted that irradiation invivo1:., .u, 126, 179, 180 or in ·vitro136, Ho, 173 may result in
an increased incidence of polyploidy in peripheral blood
leucocytes. Thus. Kelly and Brown136 reported that,with x-ray doses of up to 400 rads in vitro, the incidenceof polyploidy increased in proportion to the square of
the x-ray dose: these authors used culture times of

80. In addition to the increasing frequency of hypodiploidy with increasing age, it should be mentioned
that, in rare instances, certain individuals may con107

seventy-two to ninety-six hours. Other authors, forexample. Fischer et al. 179 in their work with Thorotrast patients. although noting an increase in polyploidy
in blood cells of irradiated patients, have found noquantitative relationship between the frequency ofpolyploidy and radiation dose. On the other hand, insome of the ot.her early work, both in viva12 8 • 129 andin ·vitro. 125 there appeared to be no association whatsoever between radiation exposure and an increase inpolyploidy.
86. More recent work has shown that few or, quite
often, no polyploid cells are obsen•ed following invi.,,·o 45• l8l, 1 82 or i11 'l!itro 115, 1 83 irradiation, provided theleucocytes are cultured for no more than forty-eight
to fifty-four hours. However. if cells are cultured for
longer periods of time. then polyploid cells appear(even in unirradiated samples) , and their incidence
increases with increasing culture time up to sixtyeight hours.UG
87. It has been noted45 • 182 • 183 that a very high proportion of polyploid cells contain aberrations, particularly chromosome dicentrics and centric rings, inpairs (i.e., the aberrations have been duplicated). In
the studies of Ishihara and Kumatori182 on the incidence of polyploidy in leucocyte cultures irradiated invitro and sampled seventy-two hours or ninety-six
hours later (that is. cells in their second. third or fourth
mitosis after irradiation-paragraphs 124-137). it was
found that the tetraploid and octoploid cells consistently contained a higher frequency of chromosome
aberrations than the diploid cells in the cultures: thatis, the number of pairs of identical aberrations in thepolyploid cells was greater than the number of single
aberrations found in an equivalent number of diploidcells.
88. In leucocyte cultures sampled ninety-six hours
after an exposure to 350 roentgens of gamma rays, thesame authors found that all the polyploid cells contained pairs of aberrations (mainly dicentrics. tricentrics, and rings), whereas only one-third of the
diploid cells contained aberrations of any sort. Thiskind of observation suggests that polyploidy in thesecases is largely a consequence of the presence of chromosome aberrations. Asymmetrical aberrations willinterfere with the separation of sister chromatids atmitosis, and interlocked chromosomes and chromosome bridges will pre\•ent a clean separation of theanaphase groups. Thus. the nucleus may not be allowed
to complete its division but passes into interphase inits doubled state and re-emerges as a polyploid nucleus
at the next division.
89. It is clear from these considerations that thebulk of the observed polyploidy is probably a consequence of the mechanical difficulties arising from thepresence of asymmetrical aberrations in cells that areallowed to proceed through a number of divisions in
culture. Polyploidy is. therefore. a secondary phenomenon, and. since its incidence will Yary not only withdose but also with the number of mitotic cycles completed in vitro, it cannot be used as a reliable indicatorof absorbed radiation dose. It is probable that polyploidy in somatic cells may be of little significancein terms of somatic hazards (for instance. polyploidy
exists as a natural phenomenon in a proportion of
normal adult liver cells), although there is no direct
information on this point. Polyploidy in primiti\·egerm cells (meiocytes) will, however, result in theformation of unbalanced gametes.
108

G. EKDOREDUPLICATION
90. The presence of cells containing endoreduplicated chromosomes has, on occasion, been noted inperipheral blood leucocyte cultures exposed to x irradiation.rn>, 146 • 184 • 185 As in the case of polyploidy, cells
shO\\;ng endoreduplication are sometimes found in
untreated leucocyte cultures, and the process of endoreduplication can be facilitated by exposure of the cells
to certain spindle inhibitors, such as colcemid.
91. No relationship has been found between radiation dose and the frequency of endoreduplication. Ho.
146 164 185••
Moreover, it is clear that endoreduplicationnecessitates two or more cycles of chromosome replication in culture and is. therefore. rarely observed inleucocytes cultured for forty-eight hours. whereas itincreases in frequency with increasing culture time.Ho.
186
The incidence of endoreduplication has no real
merit as an indicator of absorbed radiation dose.
H.

CoNcLus10~s

92. The aberrations that have been described must
now be considered from the two viewpoints of theirimportance in relation to genetic hazards, particularly
in somatic cells, and of their use in providing a measure of absorbed dose. These aspects will, of course.
be considered in detail in the later sections. but there
are a number of both general and specific points thatcan best be made at this time.
93. It is clear from the description of the aberrations that all result in some kind of genetic change andthat the majority result in genetic deficiencies. If the
deficiencies are small. they may be tolerated. The
amount of loss that can be tolerated will depend bothupon the nature of the genetic information lost andupon the normal destined role of the cell in the body.
Genetic deficiencies in stem cells will. in general. be of
far greater importance than similar deficiencies in cells
that were undergoing differentiation.
94. ~Iany of the asymmetrical aberrations will becell-lethal so that their consequences are more or lessimmediate. Cell death may follow, either as a directconsequence of the loss of genetic information in theform of acentric fragments or even whole chromosomes, etc., or as a result of the mechanical difficulties
that occur at mitosis. Aberrations in resting cells may
play no role until the cells are stimulated to undergo
mitosis. Only in those tissues that normally containproliferating cells, therefore, will chromosome aberrations be a significant contributory cause of the cell
depletion that occurs very shortly after radiation e::-..--posure. However. only a proportion of the aberrations
will result in early cell death, since the presence of
certain structural changes, such as the symmetricalaberrations described, does not result in a rapid lmvering of cell viability. If symmetrical aberrations areproduced in germ-cell precursors. they may result ingenetic imbalance in the gametes. leading either todominant lethality in the embryo or to drastic effects
in the resultant offspring. It is clear, therefore, thatthe aberrations that will contribute to long-term
hazards in both somatic cells and germ cells are essentially the small deficiencies and symmetrical changes(duplications. inversions and reciprocal translocations)that are difficult to score efficiently and are not celllethal.

95. In considering the various kinds of chromosome aberrations produced by ionizing radiations (andby other agents), it has already been indicated thatcertain types of aberrations may offer a more usefulindex than others in the context of their possible application as biological indicators of absorbed dose. Aneuploid and polyploid cells produced following radiation
exposure have been shown to arise largely as a secondary consequence of the presence of chromosome structural changes. Moreover, the presence of these abnormal cell types depends upon the fact that cell divisionmust intervene between the time of radiation e..xposure and the time of observation. These particularanomalies are. therefore, not only Jess frequent thanthe chromosome structural changes that give rise tothem, but also highly variable in their frequency, andmust be considered as very inferior biological endpoints relative to the chromosome structural changes.
96. A vast amount of information e..xists on therelationship between radiation dose and the yield of
the two kinds of chromosome structural changes (i.e.,
chromosome-type and chromatid-type aberrations) ina variety of plant and animal cells. This has shown 17 • !!:?
that, for a given quality of radiation, there exists a
strict relationship between aberration yield and absorbed dose and that, for certain aberration types, theyield is markedly dependent upon the dose rate andthe stage in development of the cell at the time ofirradiation. Detailed studies, particularly with plantmaterials. have shown that. for a given cell type andknown sampling time after irradiation. the variation
between individuals is small and that different observers
score similar aberration yields when materials are
exposed under similar conditions. Indeed, trained
observers using materials such as Tradescantia microspores and Vicia faba root-tip cells can. through determining aberration yield. estimate doses to within a
few per cent.
97. The experience and information obtained on
chromosome damage in species other than man suggestthat a similar strict relationship between radiation doseand chromosome aberration yield should also apply tohuman cells. and in vitro radiation studies strongly
indicate that. such a relationship, in fact, e..xists. Moreover, there is every prospect that. in man, the variation in response at the chromosome level between
different individuals will be of the same order as thesmall variation observed between individual plants. andpreliminary studies indicate that this is. in fact, thecase.
98. It has been stressed earlier that chromatid-type
aberrations are only induced by radiation when cells
are e..xposed while in the S or G:! phases of interphase.
These aberrations are, therefore, of little use as dose
indicators in the case of peripheral blood Jeucocyte cells
existina in a normal G1 state. but they can be used innormally proliferating cells. However, i~ sh?uld . beemphasized that great care must be exercised 111 usmg
such aberrations, since. even within the confines of the
G:? (or S) period, the yield of chromatid-type changes
is very markedly influenced by the degree of development of a cell within a given interphase stage. No suchdependence has been noted in relation to chromosometype aberrations induced in G1 cells.
99. In addition to the constant sensitivity of G1
cells to chromosome-type aberration induction byradiation, these particular aberration types have an

109

added advantage as possible indicators of radiationabsorbed dose since their spontaneous · frequency is
extremely low. A number of cytogenetic surveys onhuman populations, in which chromosome analysis has
been carried out on peripheral blood leucocytes takenfrom hundreds of individuals, have shown that thepresence of an asymmetrical chromosome-type aberration is an extremely rare event. For instance. an analysis of some of the available data (table I) indicatesthat a dicentric aberration occurs. at most, about oncein a sample of about 2,000 cells. and possibly its frequency may be even less than one in 8.000 cells.
100. It is evident from the foregoing considerations
that, on general cytological grounds. chromosome-type
aberrations are superior to other forms of chromosome
aberrations in terms assessing absorbed close. It should
also be noted that the simplest and most convenientsource of human material for studying aberration yieldfollowing in "'ivo exposure are the peripheral blood
leucocytes and that the aberrations that are induced inthese cells by ionizing radiations are chromosome-type
aberrations.
101. It has already been concluded that the frequency of chromatid-type aberrations in cultured peripheral blood leucocytes may bear no relationship to
the radiation e......posure of an individual, and it has
been pointed out that these aberrations may arise spontaneously in culture or be produced in culture following virus attack or exposure to certain chemical agents.
It should be added here that the aberrations that may
be induced by virus attack will not influence the yieldof chromosome-type aberrations, provided that only
cells in their first division in culture are sampled. One
can feel fairly sure on this point. since the frequencyof chromosome-type aberrations in blood cells of individuals previously exposed to a virus infection is nohigher than in unexposed individuals.
102. The fact that (paragraph i 4) chromatid-type
aberrations seen at the first mitosis following irradiation can result in "derived" chromosome-type changes
at the second division will be of importance if cells areallowed to proceed through more than one division in
culture. The relative importance of such ''derived"changes will depend markedly on dose and will besmall when the yield of true chromosome-type aberrations is high but will be very important when the yieldof true chromosome-type changes is low.
103. Although the possible complications introducedby chromatid-type aberrations and their "derived''
chromosome-type counterparts is obviated if only cells
in their first mitosis after irradiation are sampled(paragraphs 124-137), we should note that the exposure of individuals (as opposed to cells in <Jitro) tocertain chemical agents 187 may well result in increased
frequency of chromosome-type aberrations in their
peripheral blood leucocytes. It is important, however.
to recall that leucocytes (small lymphocytes) carrying
chromosome-type aberrations may survive in the bodyfor long periods (up to many years). This complication of in vivo effects of chemical mutagens may only
be important in relation to individuals that have beentreated for certain clinical conditions with potent chemical mutagenic agents, 71• 75 for example. nitrogen mustard.
104. Finally, it should be stressed that. whenattempts are made to obtain information on absorbed
dose through scoring chromosome-type aberration
yields. it would be valuable if the aberrations wereclassified in detail in the form described in paragraphs
25 to 34.

III. Materials and methods of studyA.

IKTRODUCTIO!\

105. Any tissues containing cells that are normally
involved in proliferation or cells that can be made to
proliferate by various means can be utilized for chromosome analysis. Chromosome studies on mammaliancells have been carried out on a variety of proliferating
tissues, including skin, intestinal epithelium. corneal
epithelium, bone marrow, various lymph nodes, spleen,
thymus and gonads (particularly testis). In addition,certain cells from tissues that do not normally proliferate in the adult animal can be made to undergo
mitosis; this has been done mainly with liver tissue
and blood. Mitotic divisions occur in liver cells when
the liver regenerates following partial hepatectomy, andblood leucocytes can be stimulated by various means
to proceed into a mitotic phase in short-term i11 vitro
cultures.
106. All the above cells or tissues have been usedby various workers in studies on radiation-induced
chromosome aberrations. In laboratory mammals, bonemarrow, lymphatic tissues, corneal epithelium, liver
cells, peripheral blood leucocytes and gonads have been
the principal tissues used. In man, the great majorityof the work has been carried out with peripheral blood
leucocytes, and some information has been obtainedusing bone marrow and skin.
107. Chromosome preparations can be made from
cells that are proliferating in viva without the necessity for in vitro culture, and direct preparations madein this way exclude the possibility of adverse effects
arising during in vitrn culture. However, in the caseof man, the only tissue from which a sufficient numberof dividing cells can be directly obtained without
recourse to surgery is the bone marrow. 1\Iitotic cells
can, of course, be obtained from skin, but to obtainsufficient cells of good cytological quality from small
samples of skin requires in vitro culture.
108. Excellent techniques188• 180 exist for obtaining
direct chromosome preparations for human bonemarrow cells, but. to obtain good quality spreads, thecells must in all cases be exposed to certain prefixation treatments, including treatment with colchicine
and hypotonic saline so that some handling of theliving cells in vil1'o is required. In the case of humanskin, the standard technique (see reference 190) involves setting up primary cultures and making cytological preparations from outgrowing fibroblasts somedays after culture initiation.
109. One major disadvantage of the use of bonemarrow and skin cells in terms of their possible use
in "aberration dosimetry" is the fact that these tissues
consist of populations of asynchronously developing
cells. Thus. when an individual is exposed to radiation.
bone marrow and sldn cells in all stages of development in the mitotic ncle will be irradiated. This meansthat both the type and yield of chromosome aberrations
will change quite rapidly with time even in the first
few hours after exposure.
110. Information on the sensitivity of man's proliferating cells (both somatic and gonadal) to radiationinduced chromosome damage is of the utmost importance in relation to assessing hazards. However, it
110

is clear that. except under very exceptional circumstances, for example, where serial-marrow aspirates can be taken shortly after irradiation and when
the time of exposure is accurately known. the idea ofttSing chromosome damage in these particular cellsystems as a general means of estimating absorbed dosecannot be entertained.
111. Because of the change in radiation responsewith cell-development phase, cells that are exposed toradiation while in a resting phase and are thenstimulated under controlled conditions to proceed into
mitosis offer the best possible system for radiation
dosimetry from the cytological viewpoint. Althoughregenerating liver cells fall into this category, clearly
their use in man must be ruled out. However, thedevelopment of a technique (paragraphs 113-123) for
culturing peripheral blood leucocytes and stimulatingthem to undergo mitosis in short-term culture has
provided a most suitable cytological system. The
peripheral blood Jeucocyte culture technique is simple
and reliable. By using this technique, large quantities
of mitotically active cells can be obtained quickly andpainlessly and, if required, large numbers of samples
can be taken from any one individual without causing
bodily injury or suffering.
l 12. Peripheral blood leucocytes from normal
healthy individuals do not usually undergo mitosis inperipheral blood vessels, and exposure of these cells
to tritium-labelled thymidine191-1 93 reveals that Jessthan one cell in 1.000 undergoes DNA synthesis. It has
been established 193 •105 that these leucocytes rest in an
early interphase or G1 state so that, following radiation
exposure, they contain chromosome-type aberrations
if and when they appear at mitosis. Moreover. sincethese cells are all in the same stage of development. thevariation in response between cells should be minimal.
Since most of the work on radiation damage in man's
chromosomes has been carried out with these cells, andbecause some differences between results obtained indifferent laboratories have emerged, some of the details
and various modifications of the techniques used will
now be briefly considered.

B.

THE PERIPHERAL BLOOD CULTURE TECHNIQUES

113. A number of recent articles190• 197 have detailed
the basic principles of the leucocyte culture techniqueand have described the earlier developments by Osgood
and his colleagues198 • 199 and by Nowell 200 • 201 that
culminated in the successful use of this system forhuman cytogenetics. 14 • 202 In this brief account. therefore, we shall be concerned only with the general principles and with certain specific variations in methodology that have been used in the studies on radiationinduced chromosome damage to be considered in the
succeeding sections.
114. The culture of peripheral blood leucocytes inYolves the introduction of leucocytes, either following
their separation from the other blood elements or
simply in whole heparinized blood. into a tissue-culture
medium containing a mitotic stimulant. The tissueculture media used contain a standard. defined, synthetic medium (such as TC medium 199) with antibiotics plus serum (or plasma), the serum mak'"ing up
from 10 to 40 per cent of the total volume (usually
about 6-10 ml). The serum ttSed may be autologous
or homologous (usually AB) human serum. or fcetal
or adult bovine serum.

115. The mitotic stimulating agent (or mitogen)normally employed is the plant mucoprotein phytohemaggiucinin (PHA). and it is clear that the cells
that are stimulated to transform into blast cell types
under the action of PHA. and then to proceed into
mitosis are the small lymphocytes. 19 ~· 203 · 20 ~ Although
PHA is the mitogen that has been used in almost allradiation 'vork, another plant extract from the poke\veed ( Plz:ytolacca americana) is also effectiYe. 205
116. The fact that PHA stimulation appeared tobe restricted to the immunologically competent cell waspartly responsible for the work that led to the findingthat small lymphocytes from donors sensitive to aparticular antigen may be stimulated to transform
into blast cells and divide in the presence of this antigen.:?06·208 In fact, this conversion to blast cells also
occurs when lymphocytes from different donors are
mixed in culture in the absence of any plant
mi togen.:?os, ~ou

in acetic alcohol are dispensed as drops onto clean
microscope slides and allowed to dry,' the cells flattening out during the drying process. ~Iost laboratories
have their own variations on the gener<:l cytological
technique outlined here, but the end result is the production of a number of slides containing scores ofwell spread metaphase cells from each culture. Thesecells are usually stained with acetic orcein or Giemsa,
etc., according to individual preferences.
121. Although first class preparations are required
for accurate scoring. the variations between laboratories
in their detailed cytological procedure after fixation
may not be of grea<: importance in the context ofinfluencing the frequency of the c~romosome aberrations that are eventually scored in metaphase cells.
However. it has been suggested :m. :? 12 that variationsin the techniques of culture (for example, the question of type of serum used and the use of whole bloodor leucocytes separated by sedimentation or by centrifugation) rnay be important, since different aberration yields have been obtained in in •lJitro studies carriedout by laboratories using slightly different techniques(paragraphs 154-170).
122. In most of the work on chromosome aberrations induced following in vivo radiation exposures,leucocytes have been separated from blood samples
either in huffy coat, namely. following fairly high speed
centrifugation (,...,,,, 3,000 rpm in a clinical bench centrifuge, or around 1,800 G), or by low-speed centrifugation ( <500 rpm or about 25 G) and/or gravity
sedimentation with or without the presence of an agglutinating agent. Separated leucocytes have also beenused in many of the in vitro studies, whereas in otherin vitro work small samples ( ,.._ 0.3 ml) of whole blood
have been used to set up microcultures.:? 13123. It is not yet known whether there is any selective loss of cells suffering from radiation damage whenhigh speed centrifugation is used. At equivalent doselevels, however. aberration yields are significantly
higher in laboratories using the microculture techniquefollowing in vitro irradiation of whole blood than inlaboratories using cells obtained follO\ving huffy coatseparation (paragraphs 154-183). Recent comparisons2H, :?ls have indicated that there is no differencein aberration yield between leucocytes separated bygravity sedimentation and leucocytes cultured as partof a whole blood inoculum. Direct comparisons between
each of these techniques carried out within the different
laboratories are certainly required.

117. The amount of heparinized blood used for asingle culture may be as little as a fraction of amillilitre ( - ;4 ml) if whole blood is inoculated into a
culture vessel containing around 5 millilitres of culture
medium, giving a so-called microculture; or, leucocytes
may be separated from samples of around 5 to 10
millilitres of whole blood and approximately 107 cells
inoculated into a culture bottle containing 5 to 10
millilitres of culture medium.
118. In cultures containing PHA and incubated at
37°C. the small lymphocytes are stimulated to undergo
RN A and protein synthesis, enlarge in size and proceed
through a DNA-synthesis phase and thence pass into
mitosis. The first cells to reach mitosis do so after
thirty-six to forty hours in culture. At forty-eight
hours, a considerable number of cells are in their firstdivision in culture (paragraphs 124-137). We shouldnote, however, that temperature is, of course, a very
important factor in influencing the rate of cell development and that fluctuations of as little as 1°C havemarked effects. ~Ioreover, there may be other. as yet
undefined. factors that may influence the cell development rate.
119. The cells that are stimulated to pass into
mitosis may go through a series of cell cycles so thatthe maximum number of cells in division in the culture
are usually to be found approximately seventy-two
hours after culture initiation. The cultures, however,are strictly short-term and cannot be maintained indefinitely, since there is a continued decline in the numberof viable mitotic cells after the first few days. In alarge proportion of the laboratories that carry out
human cytogenetic studies. peripheral blood leucocytes
are cultured for seventy-two hours so as to obtain the
maximum number of divisions. This practice has also,
unfortunately, been the custom in much of the work inradiation cytogenetics.
120. Up to four hours (or more in certain cases)
prior to termination of a culture, a small amount ofcolchicine or diacetylmethylcolchicine ("Colcemid"-around 0.05 µ.g ml-1 of medium) is added to accumulate cells at the metaphase stage of mitosis. After anappropriate time, the culture medium is removed,
following centrifugation ( 5 or 10 minutes) at about
25 to 100 G. and the cells resuspended in a hypotonic
solution such as 1 per cent sodium citrate3 or a 0.75
molar solution of KC! for a few minutes before they
are fixed in acetic alcohol. 210 Suspensions of the cells

c. CULTURE Skl.IPLING TI:.\lE
124. It has already been indicated that chromosometype aberrations in peripheral blood leucocyte cells
offer the best cytological combination. if aberrations
are to be used for biological dosimetry. Consideration
should. therefore. now be given to the influence of
culture sampling time on the yield of these aberrations.
since in recent years it has been clearly shown thatthe y;eJd of chromosome-type aberrations declines. as
is, of course, to be expected, with increasing leucocyte
culture time.
125. Most of the published data on chromosometvpe aberration frequencies in human leucocytes cultured in vitro have been obtained from cells that wereallowed to grow in culture for seventy-two hours before fixation and slide preparation. As has alreadybeen mentioned. however, it has been !mown for some111

time that a number of cells enter into mitosis as earlv
as thirtv-six h-0urs after culture initiation and thathigh pr0portion of the cells can be seen in division at
forty-eight hours. The presence of mitotic cells in
culture at forty-eight hours after culture initiation wasnoted in some of the early studies by N owel1 201 andalso bv Bender and Prescott. 195 More recent work has
now made it quite clear that, at seventy-two hours after
initiation. a majority of the mitotic cells may be in
their second or third division in culture. It is, in fact,
now evident that the cell-cycle time between successive
mitotic divisions in culture is about twenty to twentyfour hours. 62, us, 210, zn

a126. It was established by Buckton and Pike 45, 181
from their studies on patients exposed to x rays fortreatment for ankylosing spondylitis that the frequencyof chromosome-type aberrations in the blood cells ofthese patients varied according to the duration forwhich the cells were allowed to grow in culture. Cells
observed after seventy-two hours were found to contain fewer aberrations than cells from the same blood
sample that were allowed to grow in culture for only
forty-eight hours. In cultures fixed after seventy-two
hours, tetraploid ce!ls were present. and these cells
contained duplicated aberrations. Such duplicated aberrations were not observed at the earlier times, tetraploid cells being rare or absent in culh1res that wereallowed to grow for only forty-eight hours.

127. Observations similar to those of Buckton andPike were reported by Ishihara and Kumatori175 fromtheir sh1dies on blood cells obtained from Thorotrast
patients and from in •vitro studies on x-irradiatedblood samples obtained from normal individuals. Theselatter authors observed that the yields of aberrations
in both £11 vivo and in vitro studies were twice as highin cells harvested after forty-eight hours as they wereafter seventy-two hours in culture and roughly fourtimes as high as the aberration yields in the same cell
populations after ninety-six hours in culture. :r-.Ioreover, their later studies 182 on the incidence of polyploidy are also in accord with the observations of
Buckton and Pike. Similar observations of a decline inaberration yield with increasing culture time weremade by Nowell. 218 although in his studies the longer
culture times of seventy-two and 120 hours were used.
128. In summary, the work at Edinburgh in the
United Kingdom and at Chiba in Japan showed (a) areduction in the yield of chromosome-type aberrations
with increasing period of culture from forty-eight toseventy-two hours: ( b) the virtual absence of polyploid cells at forty-eight hours but their presence inhigh frequency at seventy-two and ninety-si.x hours;and ( c) the presence of duplicated aberrations in manyof the polyploid cells. These observations Jed inescapably to the conclusion that a considerable proportionof the cells observed in samples cultured for seventytwo hours and ninety-si.x hours were cells in theirsecond (X~) or later (X 3 , X.1, etc.) divisions in culture. This conclusion 'vas later confirmed by workers
in the Soviet Union!! 17 who showed that many of thecells observed at metaphase after seventy-two hours inculture at 37°C were in their third mitosis.
129. The reduction in the frequency of chromosometype aberrations with increasing culture time follows
from the fact that the aberrant chromosome structure
may be lost at anaphase of mitosis and that a proportion of the cells carrying aberrations will. therefore,
be unable to participate in any further mitotic acth·ity.

112

For instance. acentric fragments tend to be excluded
from anaphase groups at mitosis so that both daughter
cells. if viable, may be difficult to distinguish fromnormal cells when they divide at the second divisionin culture, particularly if the fragments are yery small.

130. In the case of dicentric- and centric-ring aberrations, a proportion of these structural changes will
result in anaphase bridges so that the aberrations and,in most instances, the cells carrying them. will be Jost
from the dividing population. Recent studies by Norman and his colleagues34 • 218 on aberration yields in cells
cultured for fifty hours and seventy-two hours indicate that the probability of Joss of a dicentric is 0.5 perdivision. Although aberrations and cells carrying aberrations may be Jost, undamaged cells will proliferate
normally and will, therefore. comprise an everincreasing proportion of the cell population as culture
time increases.
131. Attention was earlier drawn to the fact thatby far the majority of workers have used seventy-twohour culture periods in their in viz'o and in vitro studies and that most of these quote the autoradiographic
data of Bender and Prescott194 as demonstrating thatcells observed at mitosis after seventv-two-hours inculture are cells in their first mitosis· in culture. In
fact, Bender and Prescott stated that "the cells are in
their first post-labelling division at seventy-two hours''
when the cells were exposed to tritium-labelled thymidine for thirty minutes after forty-eight hours in culture. In their experiment, it was clearly pointed outthat "numerous mitoses accumulated (by colchicine)
between forty-two and forty-eight hours in culture''.
and the authors refer to ''the first wave of mitoses"
occurring at this time. From the recent extensive dataof Sasaki and Norman 216 and of Heddle. Evans andScott, 115 it would appear that the first post-labelled
mitoses seen at seventy-two hours may well havebeen cells in their second mitosis in culture, the majority of the cells being exposed to label in the interphase period following the first mitosis in culture.
132. In studies of Sasaki and Norman. 216 culturesof separated leucocytes were exposed to tritium-labelledthymidine after various times during culture and thensampled at various fixation times to determine the frequency of labelled mitotic figures and the patterns oflabel over the chromosomes. In addition. cells werealso x irradiated, and the frequencies of polyploid cells
and of cells containing doubled sets of acentric fragments were studied after culturing for either fifty hours
or seventy-two hours. The results obtained with thesefour different parameters showed that. at seventy-two
hours. 70-80 per cent of the cells were in their second
mitosis in culture. whereas there were no indicationsof second division cells being present after fifty hours
in culture.
133. In the 'vork of Heddle et al ..115 the mitotic
index, the incidence of polyploidy and the yield ofchromosome-type aberrations were studied using a"·hole blood microculture technique. These three parameters were scored in a series of cultures grown :ior
periods of from thirty-six to 100 hours. The cultures
were tenninated at successive four-hour intervals
throughout this period. and the cells 'rvere subjected
to a four-hour colchicine treatment prior to fixation.
This technique made possible an effectively continuous
sampling of all cells from the time of first appearanceof mitosis in culture up to sixty-four hours later. The
results showed that first division cells were observed

up to fifty-two hours and that a small proportion of
second diYisions appeared at around sixty hours. A;
approximately sixty-four hours, a significant proportion of the cells were in their second division. and. atseventy-two to seventy-six hours, by far the majorityof cells were in their second or even third divisions
with only a few first division cells present.
134. The data of Heddle et al. 115 show that, in cultures irradiated in 'Vitro with x rays (150 rad) andsampled at seventy-two hours, the yield of dicentric
and ring aberrations was approximately half the yielc
found in similar cultures grown for up to fifty-six
hours. Furthermore. it was shown that this culture
time of seventy-two hours was at a transition point
between a peak of mitotic activity (due to second divisions) occurring at sixty to sixty-four hours and alater peak (due to third divisions) occurring at seventy-six: hours.
135. One of the reasons contributing to the use of
seventy-two hours as a standard culture time was thepossibility that irradiated cells were delayed in theirprogression through the cell cycle. It is well known
that irradiation can result in mitotic delay in proliferating cells but that the amount of delay depends.
amongst other things. on the stage of development of
the cells at the time of irradiation. For instance, it has
been reported2 1" that x irradiation of human fibroblasttype cells in tissue culture results in virtually no delay
at the first post-irradiation mitosis of cells irradiated inearly G1 but in a considerable delay in the development of cells irradiated while in late G1 • S or G2.
The recent data of Sasaki and N orman210 on blood
cells given a dose in 'Vitro of 500 rads from x rays
and of Heddle et al. 115 on microcultures given a close
in 'llitro of 150 or 300 rads from x rays have indicated that little or no mitotic delay occurs at thesedose levels.
136. The data of Evans 1 " 5 • 156 show that the response of the peripheral blood leucocytes to x-rayinduced chromosome damage does not change with
development of the cells through the G1 phase in culture. But these data. obtained from cells sampled at
one fi..xation time, do not preclude the possibility that
more than one cell population with differing radiosensitivities may be present in culture. There is not a
great deal of information available on this point. but
the data of Norman 3 ·1 and of Heddle et al.11 5 show that
there is no difference in aberration yield between cells
that undergo an early. as opposed to a late. transformation to blast cell types. The available observations,
therefore, suggest that. if there is a variation in the
average rate of development of blood cells in culture
between different individuals. and if there is some indication that certain blood donors may be "slow growers" ,115 then this may be of little consequence provided
that only first division cells are sampled for aberration yield.
137. This point of cell cycle times has been considered at some length, since. for comparing quantitative data on aberration yield. it is clearly of the utmost
importance to ensure that aberration frequencies are
determined using only first division cells. On the information that is at present available. this would necessitate the use of cultures grown for around forty-eight
hours at 37° C. "; e should note, however. that at leastone Iaboratory4 " has reported the presence of a proportion of cells in their second mitosis in cultures exposed
to colcemid in their final three hours and terminatedat forty-four to fifty-two and a half hours. This has
113

led other work~rs 216 • 219 • 221 to expose fifty-hour cult?res. to colcem1d for t\venty-four hours prior to fixation 111 order to prevent cells from proceedina into asecond mitosis in culture. The duration of col~emid orcolchicine prefixation treatment may, therefore. be an
additional factor to consider in conjunction ·with duration and other conditions of culture.
D.

CONCLUSIONS

138. Chromosome analysis in man can be carriedout quite readily on cells from three sources, namely,
skin, bone marrow and peripheral blood. For qualitative work. cells from each of these three sources canbe used. and a number of studies· have, in fact, been
made on the persistence and proliferation of cells containing symmetrical aberrations in bone marrow and inblood. In these studies, the presence of clones of cells
derived from an original single cell containing a radiation-induced symmetrical aberration have been noted inindividuals studied many years after exposure to ionizing radiations. 13 "1• 222 -22 -i The possible importance ofsuch persistent symmetrical aberrations as long-term
somatic hazards is considered later.
139. At the quantitative level. ·where the questionof using the aberrations produced in proliferating cells
to estimate absorbed dose is concerned, various physical and general biological problems arise. These problems will be considered later, whereas this section has
been confined to the relative cytological merits of thevarious proliferating cell systems. From this discussion, it is clear that, from the cytological viewpoint,
as well as because of the ease and simplicity of obtaining single or repeated cell samples. the peripheral blood
lymphocytes are far superior to bone-marrow or skincells for quantitative work.
140. These peripheral blood lymphocytes exist ina uniform stage of development in G1 so that only
chromosome-type aberrations are produced in themfollowing radiation exposure. The more comple..x cbromatid and subchromatid-type aberrations that will be
induced in a proportion of the skin and bone-marrowcells are, therefore. normally absent. However, somechromatid-type aberrations are observed in irradiatedlymphocytes. but these have been shown to arise during
cell development in culture. and their relatively low
frequency does not raise complications in the scoring of
the chromosome-type changes if only cells in their firstpost-irradiation mitosis are scored.
141. Despite the apparent simplicity of the peripheral blood leucocyte system, certain differences inin <Jitro response have been observed between different
laboratories (paragraphs 154-183). Some of these differences are a consequence of the use of radiations ofdiffering qualities (paragraphs 154-177) but. from thediscussion of the methods of leucocyte culture used invarious laboratories. it is evident that there are at least
two other factors of importance.
142. First. it is clear that different aberration frequencies are observed if cultures are allowed to growfor nrious periods of time in e.xcess of fifty-four hours
and that this is almost certainly a consequence of theappearance in culture of second and subsequent mitoses
which result in increasing the proportion of undamaged cells. The usual standard fixation time of seventytwo hours used in many laboratories is not only toolate to find many first division cells but may also be .on
the border-line between waves of second and third
mitoses in culture so that small differences in timingand in culture conditions can be expected to have exagge:-ated effects on the aberration frequency. Information on the rate of decline in aberration yield withincreasing culture time suggests that estimates ofaberration yield made at this late time of seventytwo hours may be too low by as much as a factor oftwo.
143. Second, it has been suggested that differencesin the methods used to handle the cells ( rtnd. in general, differences in culture techniques) might conceivably contribute to variation in aberration yield. Thissuggestion needs to be explored. and the various techniques must be compared within laboratories. Moreover,because of the present Jack of knowledge concerning
possible subtle effects of minor variations in technique,
it is of the utmost importance for workers in this field
to define clearly the conditions of culture being used,
including temperature. centrifugation methods and cytological techniques.
144. Finally, it cannot be over-emphasized that laboratories should endeavour to standardize scoring methods and presentation of data, giving, where possible.
the maximttm amount of information on the frequencies
of all the various aberration types as outlined inparagraphs 24 to 34.
IV. The relationship between aberration )ielcl
and doseA.

INTRODUCTION

145. In considering the relationship between radiation dose and aberration yield and the significance of
the aberrations in terms of their potential hazard, it
is important to note that a statistically significant increase in chromosome-type aberrations is observed in
the peripheral blood leucocytes of individuals exposed
to low doses of diagnostic radiation. The humanchromosome complement is, therefore, sensitive toaberration induction.
146. Reference was made earlier (paragraph 96) to
the relationship that exists between absorbed dose andaberration yield in a wide variety of organisms andcell types. A similar relationship between aberration
yield and dose must also exist in the case of man'scells, and it is this relationship, coupled with the highsensitivity of the human chromosome complement, that
forms the basis of the possible use of aberration yieldin dosimetry.
147. Interest in the potential application of aberration yields to estimate dose was largely stimulated byearly observations that the peripheral blood leucocytesof persons exposed to radiations, either accidentally or
for therapeutic purposes. contained chromosome aberrations.a It was generally suggested that, in the caseof accidental e..xposure. determining chromosome aberration yields in an exposed individual might provide
not only a simple but also a much more valid alternath·e to dose estimates based on physical measurements
and would make possible some sort of direct estimateof the degree of biological damage incurred.
148. The merits and possible disadvantages of"chromosome aberration dosimetry" are considered inparagraphs 326 to 340. The present section is moreconcerned with the kinds of data that have been obtained from experiments, from radio-therapy treatments
and from incidents where aberration frequencies were
determined under conditions where some kind of physi114

cal estimate of dose was available. The available data
can be separated into two categories, namely, thosethat have been obtained in in ·vitro experiments whereaccurate physical estimates of dose were available andthose obt:dned following in vivo (whole-body or partialbody) exposure ·where physical dose estimates were,in general. rather less accurate.
149. There are three principal reasons why studies
on the effects of irradiation in vitro are very importantin the context of any biological dosimetry techniquethat involves using aberrations produced in -.:fr:o. They
are as follows :(a) Chromosome damage sustained by leucocytes
in vivo can only be readily observed following shortterm in vitro culturing of the cells. As has atready been
seen, studies on aberrations induced in vitro areproviding a means for determining the optimum
conditions for sampling and for defining the typesof observations that are required and the conditions
under which they should be made.
( b) Since many of the fundamental aspects ofaberration induction, such as the kinetics of response
and the influence of radiation quality and of exposure
time. etc., are similar in different species and in in·vitro and in ·vi·vo exposures in organisms other thanman. the response of human cells exposed in vitro
is not expected to be different from the response obtained i11 v1'.vo.
( c) Various authors13°· 223 have concluded thatthe x-ray-induced aberration rates in mammaliancells in vitro are very similar to those seen invivo. There is. thus, a strong possibility that thesensitivity of the peripheral blood leucocytes in vitro
may be similar to that in vivo.
150. Bender's conclusion was arrived at following comparisons between the response of chromosomes
in Chinese hamster bone-marrow cells151 and corneal
epithelium ce1Js!?2 6 and. more particularly, between
in vivo exposure of bone-marrow cells of the spider
monkey (Ateles spp.) and in vitro exposure of continuously cultured kidney cells of the same species.12
151. It might be expected that the in vitro irradiation of freshly drawn human whole blood. as opposed
to irradiation studies on established human cell culture lines, might closely approximate the irradiation ofthese cells while they are circulating in the peripheral
blood system. As yet there is only a small amount of
direct information on this point. 2 :! 7
152. In line with this suggestion are the recent
preliminary and unpublished studies of Cleminger228
on rabbits. In this work. blood was taken from animals
and given a total gamma-ray dose of 300 or 500 radsfrom cobalt-60 following which the animals themselves
were given either of these doses. \Vithin each doselevel, the aberration yields in blood cells exposed in
1;itm and in cells from blood sampled ten minutes after
whole-body irradiation were found t-0 be closely similar.
153. In the in vfr..·o radiation studies made on man.
a considerable amount of data has been accumulating
on aberration yields in peripheral blood lymphocytesof individuals exposed to radiations. although a great
deal of these data. as is the case in much of the i11;..•itro work. comes from cultures grown for seventytwo hours or more. Since the conditions of in <:h·oexposure are so diverse and the information obtained isso varied. the ill ;:i<.,•o studies will be considered separately according ro type of e..'posure.

B. !11 -.:itro STUDIES

these e.."'periments. and no firm conclusion on the relationship between dose and yield can be drawn fromthese data.

1. X rays and ga111111a rays
154. Dose-response data from i11 i:itro x-irradiation
studies on peripheral blood leucocytes haye been obtained by fourteen groups of workers. These data appear in twenty-three separate publications,3·1• 3 ;;, 12;;, 137.
us, uo, 106• 211 • ::~o. ~ss only thirteen of which report
data on cells cultured for less than fifty-four hours. In
assessing these data. there are at least three importantdifferences in experimental conditions that are of importance. These are (a) the use of different culture
times; ( b) the irradiation of whole blood prior toculture as opposed to the irradiation oi blood in culture: and ( c) the use of different qualities of x rays.
155. In the original data of Bell and Baker, 140 terminal deletions and exchange aberrations both increased approximately linearly with increasing x-raye.."'(posnre, and the yield of exchange aberrations wasdose-rate dependent. For instance, at 200 roentgens.
2.1 exchanges per cell were recorded when the exposure
rate was 160 roentgens per minute but only 1.0 e..'Cchange per cell when the exposure rate was 1.6 roentgens per minute. In these e..xperiments, however.. thecells were cultured for 100 hours, and, in addition, insome of the experiments, radiation was given at various
times after culture initiation. No firm conclusions on
response with dose can, therefore, be drawn from thesedata.
156. The data of Bender and his collcagues 125• 229
were obtained by irradiating whole blood with up to
200 roentgens (250 kV x rays. HVL 2 mm Cu) andculturing separated leucocytes by using the huffy-coat
technique. In Bender's laboratory, cells were sampled
at seventy-two hours so that the aberration yields reported may be under-estimates. Coefficients of aberration production were given as yield per cell-roentgenin the case of deletions and yield per cell-roentgen
squared in the case of dicentrics and rings. In two
C..'..lJeriments, values of 0.9 and 1.1 lo-3 deletions percell-roentgen and 5.2 and 6.0 10-6 dicentrics
rings
per cell-roentgen squared were obtained.
157. It has been claimedlll6 , 211 that Bender's dicentricring data give a best fit to the relationshipy = kD 1 •4 rather than to y = kD 2 , where y =yield,
k = a constant and D = dose. A very relevant point ofinterest in these data is the fact that, in the first experiment of Bender and Gooch. a culture was gro"l'm forfifty-four hours in addition to parallel cultures grown
for seventy-two hours. In the fifty-four-hour culture.
the aberration yield was 50 per cent higher than insimilarly irradiated cells grown for seventy-two hours.
This is in accord with the expectation that the coefficient
for aberration production obtained from seventy-twohour samples may be too low.
158. Kelly and Brown 13 i irradiated whole blood(200 kV x rays, HVL 1.5 mm Cu) and cultured separated leucocytes for seventy to ninety-six hours after
exposures of from 100 to 1.600 roentgens. The data wereanalysed according to the equation y = kD 2 , but they
were not uniform. Over the full dose range the coefficient for the yield of dicentrics was 0.9 10-6 per cellroentgen squared and was considerably lower than thatobtained by Bender and his colleagues. If the data obtained at exposure above 200 roentgens were omitted.
a coefficient of 5.6 10-6 per cell-roentgen squared (i.e.,
similar to that obtained by Bender) was obtained.
Oearly, culture time played a very important part in

+

+

115

159. The original data of Norman and his col-~eagu.es~30 .were obtained from lymphocytes that were

1rrad1ated m whole blood (100 kV to 1.9 i\IeV x rays)
and then cultured. after separation by centrifugation.
for seyenty-two to ninety hours. Doses of up to 1,200
rads were used, and dose rates ranged from 10 to 200
rads per minute. No effect of dose rate was observed,
an~ a coefficient for the production of dicentrics oi
2.1 10-6 per cell-roentgen squared was obtained. These
data for dicentrics gave an excellent fit to the equation y = kD 2 , and the coefficients obtained in thesedata are approximately one-half of those obtained bY
Bender's group. No significant differences were observed between the effects of these two radiations ofdifferent qualities.
160. In the very recent publications from Norman's
group,34 · 240 cells have been cultured for fifty hours
as well as seventy-two hours, and doses of up to 5,000
rads have been used. In these experiments, higher aberration yields were observed at fifty hours, and thecoefficient for dicentrics and rings in these shorter
term cultures was 5.7 104' per cell-roentgen squared.
almost identical with that obtained by Bender and hiscolleagues. These data were obtained with x rays froma linear accelerator giving a mean photon energy of
1.9 MeV, although it should be noted that Bender'sdata were obtained with the more efficient 240 kVp
x radiation.2 -H
161. Visfeldt 242 in his in vitro work has used only
three dose levels of up to 200 rads of cobalt-60 gamma
radiation and has reported higher aberration yields
than Norman et al. and Bender et al, In Visfeldt' s
work. leucocytes were separated without resort tocentrifugation, and cultures rather than whole blood
samples were irradiated. Cells were sampled after
forty-eight hours in culture, and it is of interest tonote that these gamma-ray data from irradiatedcultures approach the high yields obtained with
x-irradiated whole blood microcultures. It is important to note here also that the RBE for chromosomeaberration production is about 0.8 for cobalt-60 gamma
rays relative to 250 kV x rays.243, 2«
162. ~Iouriquand et al. 138 have exposed separated
leucocytes (gravity sedimentation) in autologous serum
to x irradiation (160 kV, 7.5 mA, 100 R per min) prior
to culture and have sampled cells seventy-two hours
later. The yields of dicentric aberrations obtained bythese workers were higher (coefficient of 8.2 1~ percell R 2 ) than those obtained by other authors
who irradiated whole blood. The data of Mouriquand
et al. 138 were very probably obtained from a
mixture of first and second division cells. and theiryields closely approach those obtained in x-irradiatedmicrocultures sampled at seventy-two hours115 ;md areabout 25 per cent lower than the yields obtained in
microcultures sampled at fifty-four hours.
163. Evans 156 • 211 • 212 has reported data from fivee.'Cperiments on whole blood x-irradiated (250 kV HVL
1.2 mm Cu) prior to or during culture, using dosesof up to 460 rads and dose rates of from 17.5 to 230.5
rads per minute. These experiments differ from theothers in that whole blood microcultures were used.
The cells were sampled at fifty-four hours, and no differences were observed between cultures exposed at
different dose rates. In these experiments. the dicentricand ring aberrations did not increase in proportion to
the square of the dose but give a best fit to the equation y = kD 1 •2 • The pooled data from all experiments
analysed as a quadratic, i.e., y = k
aD f3D 2 , givea = 3.42 10-3 and f3
3.5 10-6 •164. Bajerska and Liniecki 231 have recently reported experiments on x-irracliated cultures ( 180 kVwith 1.05 or 1.8 mm Cu filtration) and have obtained
results somewhat similar to Evans. In these e..xperiments, using a dose range of up to 415 rads given atdose rates of around 100 rads per minute, the yields ofdicentric aberrations best fit the equation y = kD1 ·s,
and the total yields are similar to those reported in theother published data on x-irradiated cultures (figure 4).

A (CO) Linear accelerator x rcys
with mean photon energies
of 1.9MeVand l.5MeV+ +=O Bajerska and Lin1ecki• Evans (1967)

211

x Heddle !! ~;

115

231

(250 kV x rays)

J(250 kV x rays)

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C (•) 200kVpx ray~(!
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B (+)

+ 0.1
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RADIATION DOSE (RAD)SOO

Figure 5. Dose-response relationship for dicentrics plus rings
for different qualities of radiationa

0.1
0' Cells irradiated prior to culture and cultured for fifty houn.=

0.05

•

0.03 -~---._........._._........___ _..__...__.__._.._
20
50
100
200
500
RADIATION DOSE (RAD)Figure 4. Dose-effect relationship for dicentric aberrations(irradiation with 180 and 250 kV x rays after PHA
stimulation)"•Whole blood x-irradiated in n<lture. Cultures grown at 37° C for

fifty to fift:y·f(JUr ,,,,,.rs. Regression line fitted to data obtained fromblood cells irradiated with 180 to 300 kV x rays prior to culture(figure 8).

165. Up until very recently the x-ray in -z:itro data
appeared to be very confusing. ·within indiYidual laboratories. consistent and repeatable results were obtained, but little uniformity in the form of the relationship between aberration yield and dose existed
between laboratories. Recently acquired experimental
data have markedly improved the picture, however.
particularly if separate consideration is given to dataobtained using different techniques and radiations ofdiffering quality.
166. Studies by a number of workers have now
clearlv demonstrated that 2 Me V x rays are less efficient than 180 to 300 kVp x rays. tf1e RBE being
0.8 232 • 234 •236 when comparisons are made between samples handled in the same way with regard to irradiation technique and culture sampling time. The data of
Sasaki23·1 on the dose-response relationship of dicentrics
rings for radiations of differing qualities areshown in figure 5. These data were obtained using irradiat~d whole blood cultured for fifty hours prior to
sampling. and the constants and dose exponents for the+116

fitted lines are as follows : 1.9 Me V and 1.5 Me V
x ray, y = 8.50 10-6 D 1•0 ·1 ; cobalt-60 gamma rays,
y=25.5 10-6 D 1•78 ; 200 kV x rays (HVL 1 mm Cu),y = 81.14 10--0 D 1 •60 : 14.1 MeV T(d,n) fast neutrons,y = 1.039 10-G D 1 •24 • The D 2 relationship for the
1.9 MeV x rays confirms the earlier studies34 • 240 using
radiation of this quality.
167. Sasaki's data on the cobalt-60 gamma rays
are very similar. both in terms of absolute yield ofdicentrics and of dose kinetics, to recent data obtainedwith this radiation, under similar culture conditions.
by Sevankayev and Bochkov237 (figure 6). Both thesesets of data differ. however, from Visfeldt's 242 results
and from some recent data of Scott et a/. 234 In theselatter studies, cells were sampled after forty-eight tofiftv-four hours in culture. but the cultures themselves(stimulated cells) rather than the freshly drawn whole
blood (unstimulated cells) were irradiated. In thiscontext. it is of interest to note that all the data on
x- or gamma-irradiated cultures give lower dose exponents than do the data on irradiated whole blood,the difference being largely a consequence of higher
yields at low doses in the irradiated cultures (figures
4 and 7).
168. Recent data from five different laboratories on
the vields of dicentric aberrations in cells irradiated inwhoie blood prior to culture for up to fifty-four hours~with x rays of peak kilovoltage ranging from 180 k\
to 300 kV. all giYe consistent results. These data areshown in figure 8, and the slope of the fitted line gi,resa dose exponent of 1.53. The aberration yields in these
five sets of data are higher than the yields reported by02
0 Sascki 234
Sevankoyev end Bochkov

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237

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232-c Scoll('.!00 kV x rcys)

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50
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500

200

0.01 .___

RADIATION DOSE (RAD)10

Figure 6. Dose-effect relationship for dicentric aberrations(irradiation with 60(o gamma rays before PHA stimulation) 8_.I_.__ _.__.__._.._,__...._.___..___...._.__...................._
SOO

200

100

20

RADIATION DOSE; (RAD)Figure 8. Dose-response relationship for dicentric aberrations(irradiation with 180 to 300 kV x rays before PHA stimulation)a• Whole blood irradiated prior to culture and cultured for fifty tofifty·fo•r hours.

• Whole blood x·irradiated Prior to culture and cultured for fifty tofifty·folfr hours.

3

••
•

•

-'
-'
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Q'.

Scott et cl. (1969)

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242

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RADIATION DOSE (RAD)Figure 7. Dose-effect relationship for dicentric aberrations(irradiation with 60(o gamma rays after PHA stimulation) 3*Bender and Gooeh 125(250 kVp x rays)• Mouriquand ~ £1. 138(160 kVp ll ·rays)
34
0 Norman(1.9 MeV Linoc x rays)
230• Vonder E lst et al.
(220 kVpr-;;"ys)0x-• Whole blood irradiated ;,. C11lture. Cultures grown at 37° C for
forty-e1'qlil to fifty hours. Regression line fitted to data obtained fromblood cells irradiated with 00 Co gamma rays prior to culture (figure 6),

I II

0.01
20

other authors using longer culture periods; for the
purpose of comparison, some of these latter data areplotted in figure 9.
169. It would appear, therefore. that, in the caseof irradiated whole blood sampled after fifty-four hours

50

I 00

I I I I II

200

500

RADIATION DOSE (RAD)Fig11rc 9. Dose-response relationship for dicentrics plus ringsa.
• Cel'5 irradiated with various qualities of x rays prior to culture,and cultured at 37° C ior stvnu:;-trro hour•.

117

in culture or less. the interchange aberrations inducedby high-energy x rays are predominantly a consequenceof the interaction between two lesions produced byindependent tracks. However, over the same aberration yield range (up to about two dicentrics per cell)a significant fraction of the exchange aberrations induced by conventional x rays (150-300 kVp) are theresult of the interaction between two lesions produced
by a single track. The importance oi this one-trackcontribution will. of course, decrease with increasing
dose level.
170. Despite the excellent agreement between setsof data recently obtained quite independently in different
laboratories, the difference in response, particularly at
low doses. between irradiated cultures and irradiatedwhole blood requires further investigation.

2. Fast neutrons
171. Dose-response data from peripheral blood Ieucocytes exposed to fast neutrons in ·uitro have been reported by three groups of workers, namely, in the
United States, the United Kingdom and Japan.
172. Gooch et al. 229 in the United States irradiatedwhole blood with 14.1 MeV DT and 2.5 MeV DD
fast neutrons with doses of up to 200 rads, and theseparated leucocytes were cultured for seventy-two
hours before determining the induced aberration yields.
\Vith the 14.1 il1eV neutron dose delivered at 6 rads
per minute, it was found that "chromosome breaks"(terminal deletions+ intercalary deletions?) increasedslightly more than with the first power of the dose
and that dicentrics plus rings increased as approximately the square of the dose (apparently the best
fit to the equation y = kD", for dicentrics + rings
1.42). The coefficient of abergives a value245 of nration production for deletions >vas 2.0 lo-3 deletions
per cell-rad and for dicentric and rings 12.1 10-s aberrations per cell-rad squared.

=

173. It has been argued 1 M, 211 that the curvilinearityof these dicentric and ring data with 14.1 MeV neutrons might be due to the sampling of predominantly
second and third division cells at low doses and. asa result of mitotic delay, to sampling of an increasing
proportion of first division cells with increasing dose.
There is no direct information on this possibility, but
previous experiments with this quality of radiation on
chromosome-aberration induction in plant cells haveshown that all the aberration types increase approximately linearly with increasing dose. 246·2·i 9 However, it
is important to note that, in the more recent studies
of Sasaki234 in Japan, using 14.1 MeV neutrons butwith the cells sampled after fifty hours in culture, adose exponent for dicentric aberrations of 1.24 was obtained.
174. Gooch et al.~ 29 compared their 14 ).1eV fastneutron data ·with those obtained with 250 kV x rays
and obtained an RBE for these neutrons of approximatelv two. Preliminarv data with 2.5 MeV DD neutrons -yielded a linear dose response for all aberration
types and an REE of approximately four to five fordeletions. It is of interest to note here that theseauthors der-ived an estimated REE from in viva exposure of three men to fission spectrum neutrons during
a criticalitv accident. These estimates were arrived atafter maki:;Jg certain assumptions. and an RBE value.of the order of five was obtained (figure 10).
118

175. Scott et al. 245 in the United Kingdom exposed
whole blood to fast neutrons of 0.7 Me V mean energy
using doses of up to 150 rads. The cells were e.xposed
prior to or following their introduction into whole
blood microcultures. Continuous irradiations were given
over periods of up to twenty-four hours and involved
dose rates of 6.7 5 rads per hour and 3.41 rads perhour. and short-term irradiations were given at theapproximately thousandfold higher dose rate of SO rads
per minute. No differences in efficiency between chronicand short-term exposures were found. and in bothcases all aberration types increased linearly with increasing neutron dose. Comparisons with 250 kV x-ray
data (doses of up to 500 rad) gave an RBE value forthese 0.7 Me V fission neutrons of around three(figure 10).
176. In t\vo of the five sets of 0.7 MeV neutrondose-response data, some indication of a saturation in
aberration yield was indicated at doses of above 100
rads in the chronic low dose-rate e.xperiments. In these
two experiments, the data also indicated a higher yield(of up to 10-20 per cent) in those cells irradiated whilein the G1 phase in culture (PHA-stimulated blood
cells), as opposed to those cells irradiated prior toculture (i.e., unstimulated blood). The possibility ofdifferences between the response to irradiation of stimulated and unstimulated cells has alreadv been com·mented on (paragraph 171).
177. It has been suggested that the saturation effectresults from a preferential loss of damaged cells due
to ·'interphase death'' under conditions of continuous
irradiation in culture. This possibility again raises thequestion of preferential cell loss both prior to and during
culture. It is of interest to note here that, in parallelto the higher yields of x-ray-induced aberrations observed by Scott et al. relative to those observed by
Gooch et al .. similar, but rather more pronounced. increases in vield have been observed with the neutron
data. This· difference must, of course. partly be theconsequence of differences in culture times and partlyof differences in radiation quality. The difference issuch that. at equivalent dose levels. the yields of dicentrics and rings with the 0.7 MeV neutrons is someten times higher than the yields reported with the 14.1
MeV neutrons (figure 10). Further work is mostcertainly necessary here.
3. The variation in response bet-,;1een blood sain plesobtained from different in.dh•iduals
178. Perhaps the only consistency among laboratories which has emerged from the in vitro studies is
the indication that the variation between the in vitro
response of blood cell5 obtained £rom different adultdonors of both sexes is very smalJ.l'.?5, 131, 143, 229, 250
Five donors haYe been used by the Oak Ridge group.
and data have been presented in two publications. 125· 229
In no case did the responses to the same dose level ofcells obtained from different donors differ significantly.
179. In the Harwell studies. 250 seven donors wereused. and all yielded closely similar aberration yields
when exposed to similar doses. Moreover. cells fromone donor were used for much of the x-ray doseresponse work. and cells were, therefore, sampled atvarious times throughout a three-year period. Throughout this period, there \Yas no significant change in theaberration yield at any given dose. 211

2.5

D 50 rod/min (slimulcted)

o 6.75 rod/h (stimulated)• 6.75 rod/h (unstimubted} 0,7 MeV z;;" 3.41 rod/h (stimulated)
i. 3.41 -ad/h (unstimulated)
2.0• 6 rad/min 14.1 MeV (unstimulot.d) 22 ~

Cl

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NEUTRON DOSE (RAD)Figure 10. Dose-response data for in ·vitro fast neutron

180. Recent ·work in the Sovi~t Union 251 • ~;; 2 has
indicated that the yield of x-ray-induced chromosome
aberrations in peripheral blood lymphocytes irradiatedin vitro may be slightly higher in cells taken from
infants and from elderly people than in cells takenfrom other healthy adults. However, Migeon and
Merz 2;; 3 had previously reported no significant differences between the responses of lymphocytes from infants and adults. Bochkov et al ..2 ;; 4 in a study of fiftynine individuals, reported that there was an influenceof age on the frequency of spontaneous structural rearrangements in these individuals. The possibility of
an influence of age of donor on radiation response clearly
merits further study.
181. Studies on plant and animal cells (see references 17 and 255) have shown that, in a given species,
the presence of an extra chromosome or chromosomes
over and above the normal diploid complement results
in an increased aberration frequency in these cells.
The induced aberration frequency at any given doselevel is, therefore, closelv correlated with chromosome
number as well as with 'chromosome size and chromosome morphology.~ 55
182. Because of the disparity in size between the
X and Y chromosomes in roan. 250 it might be expected
that, at a given dose, the frequency of chromosome
aberrations in normal females might be very slightly
greater than the aberration frequency in normal males.
This expected very small difference between se..xes has
not yet been demonstrated. but, in line with expectation. a small increase in aberration yield in in vitro
x-irradia1ed leucocvtes from individuals trisomic forchromo.;;ome 21 (Down's syndrome). as compared withnormal individuals, has been reported by three groups
of workers. 143 • 145 • ~ 57
183. The constancy between individuals, and within
individuals onr a period of a few years, in the responseof their blood cells to in vitro radiation exposure isheartening. :N" evertheless, it must be strongly emphasized that the number of donors that have been
compared to date is extremely small. Bearing in mind
the influence of age and of sex on aneuploidy (paragraph 78), it is clear that more information is required
from a la:·ger number of donors of both sexes encompassing a wide age range. Such studies are currently
in progress in a number of laboratories.
119

irradiation~~5c. In vivo STUDIES
184. Quantitative studies on aberrations induced invivo are beset with a number of difficulties additional
to those considered in the ·in vitro work. These difficulties will be considered in some detail in this section
as well as in paragraphs 341 to 343. Nevertheless, a
discussion of the in vivo data cannot be initiated without emphasizing that difficulties with regard to physicaldosimetry and to biological sampling are inherent inall the £11 vivo work. For instance, many studies have
involved partial-body irradiation. and a variety ofqualities of radiation has been used. Estimates of absorbed dose in such cases may not be very meaningful,and, in fact, difficulties with regard to the non-uniformity of absorbed dose e..'<ist even in the case of
so-called uniform whole-body irradiation. Sampling
problems are present because of the distribution. life
span and mobility of the small lymphocyte within the
body. These difficulties should. therefore, be borne inmind throughout the following discussion.
1. Clinical exposure185. A number of instances have been recorded
where an increased frequency of chromosome-type aberrations has been observed in peripheral blood leucocytesof individuals following the exposure of these individuals to diagnostic x rays. Although, to date. all theobservations have been made on cultures grown forseventy-two hours or more (and, hence, some of thechromosome-type aberrations observed could have been
derived following duplication of chromatid-type aberrations produced in culture-paragraphs 67 and 93),
control data, where they exist, were also obtained from
cultures grown for a similar period. The increases,
therefore, must be a real consequence of radiation e..'{posure.
186. The first observation of a possible effect of
diagnostic x rays in inducing aberrations was madeby Stewart and Sande.-son~" 8 who reported the presenceof two cells containing a dicentric out of a total of
thirty-one cells scored in a patient with Klinefelter's
syndrome. This patient was subjected to a skeletal
survey involving a skin dose of less than 2 rads from
60 kV x rays, and blood samples were taken eight
hours after exposure. Unpublished evidence 259 • 260 onthe yield of spontaneous aberrations in Klinefelter patients who have not been exposed to diagnostic x rays
shortly prior to study has indicated that the spontaneous yield in these individuals is no higher than innormal individuals (table I).
187. Observations similar to those of Stewart weremade by Corren et al.~ 61 • !?o:! who found two dicentricaberrations in 121 blood cells of an infant examined
one week after exposure to a series of diagnostic x rays
giving :i total dose of 0.8 rad. Bloom and Tjio 263 did
not detect anv dicentric aberrations in blood cells fromsix patients 'given diagnostic chest x rays involving
exposures of from 20 to SO milliroentgens, but four
dicentric aberrations were observed in 300 cells of fivepatients :-ubjected to gastro-intestinal examination using
fluoroscopy. The exposures of these five patients ranged
from 12 to 35 roentgens. and blood samples were in
all cases taken thirty minutes after irradiation.

high aberration frequency in damaged cells is typicalof damage induced by high LET radiation, such as
the alpha particles of thorium.
192. The high aberration yields obtained by Buckton et al. 224 in their Thorotrast patients are considerably greater than the yields obtafried by theseauthors 1G in patients receiving a whole-bodv dose of
50 rads of x rays or a partial-body dose o( 300 rads.
Howe\'er. in the Thorotrast work, although the volumeof Thorotrast administered and the time interval between
treatment and observation were known, no relationshipbetween these parameters and aberration yield could
be demonstrated.
193. Fischer et a/. m. 268 examined blood cells fromtwenty individuals \vho had received Thorotrast fromnineteen to twenty-seven years prior to sampling, andin these patients estimates of residual body burden
were made through whole-body counting. Nineteen ofthese cases showed a significant increase in aberration
frequencies above the background level: the only one
that did not show such an increase had undergone a
retrograde pyelography and had. thus, retained little
radio-activity as was confirmed by the extremelv low
burden r<-gistered by the whole-body counter.

188. Further indications of what appears to be asignificant elevation in aberration yield are seen in thedata of Court Brmvn:!0-1 on ankylosing spondylitis patients subjected to diagnostic x rays (columns b and c
in table I). Moreover. Sasaki et aJ.:! 65 have reported
that, in a scan of over 7.000 cells taken from a totalof eleven individuals. dicentrics were only found in one
man who had received a number of lumbar spinal x-ray
examinations some five years previously.

194. In this work it was shown that, with an increasing amount of Thorotrast (estimated from wholebody gamma-ray counting), there was an increasing
amount of chromosome damage and a significant linear
correlation between these parameters when the data
were weighted by the time interval elapsinu between
administration and observation. However. ;e shouldnote that there are several difficulties in assessing these
dose-response relationships, since considerable variation
exists in the distribution of Thorotrast within thereticulo-enclothelial system, 224 and estimates of dosefrom thorium and its decay products, based on gammaray measurements, require that allowance be made forthe self-absorption of alpha rays; this is of particularimportance if the thorium is not uniformly distributed.
Because of these difficulties in physical dosimetry andsince, in the work of Fischer et al., 179 the leucocytes
were cultured for seventy-two hours. it is difficult toarrive at any meaningful coefficients for aberration production.

189. These observations suggest that very low-dosepartial-body x irradiation at low (diagnostic) kilovoltage is capable of inducing a detectable frequencyof chromosome-type aberration. The fact that we candetect the effects of such small doses of x ravs is aconsequence, first. of the relatively high sensitivity ofhuman peripheral blood leucocytes to the inductionof chromosome damage by radiation. and. second, ofthe extremely low frequency with which cbromosometype aberrations are found in individuals not exposed
to ionizing radiations.
190. More recently. information has become available from patients treated with Thorotrast, a stabilized
colloidal suspension of the dioxide of thorium-232.
Thorotrast is taken up by the reticulo-endothelial cells
and deposited in liver and spleen, and. to a lesser
extent, in bone marrow and lymph nodes. Only minutequantities are excreted so that these tissues are subjected to continuous irradiation. much of which is due
to densely ionizing alpha particles.

195. Since the original paper of Tough et al. 1 :; whoreported gross chromosome damage in cells from blood
cultures of two patients after x-ray therapy for ankylosing spondylitis. a number of publications dealing
with aberrations induced in patients following radiotherapy have appeared. Most of this work has been
concerned with aspects of aberration induction (suchas the qtiestion of the longevity of the small lymphocyte - paragraphs 252-263) other than the quantitative
correlation between induced chromosome damage andabsorbed dose. However. a limited amount of data on
close response in ·viz:o ·has been obtained. and moreshould he aYailable in the near future.

191. Ishihara and Kumatori 182 • !?GG reported that asignificant yield of aberrations was to be found in blood
leucocytes of persons given Thorotrast injections sometwenty-five years prior to observation. The residual
body burdens of these persons were estimated by wholebody counting. but no definite correlation was foundbetween body burden and aberration yield. 207 Similarly,
Buckton et al.. 2'.H. in a cytogenetic study of thirty-six
patients who received intra-arterial injections of Thorotrast some eJe,·en to thirty-one years prior to study.
reported a marked increase in aberration yields in the
Jeucocytes of these patients as compared with those ofcontrol indh·iduals. The cells in this latter studv were
cultured from forty-eight to fifty-two hours. a";1d 9.2
per cent of the cells were found to contain unstable(asymmetrical) aberrations and 5.7 per cent stable(symmetrical) aberrations. It is of interest to note
that, in this work. a very high frequency of tricentricaberrations was found (3.8 per 100 cells) and thatmany cells contained more than one aberration. This

1%. Norman et al.~ obtained a limited amount ofdata on two patients receiving doses of 300 rads from250 kY x rays, using blood samples that were collected
immediatelv after irradiation. No details of the methodof exposu;e \Yere given. and the geometric mean of
the aberration vields from the two rather different
samples was nearly equal to the yield obtained fromnormal blood irradiated in vitro and receiving a doseof 300 rack In these cases, all the cultures "·ere grown
for se,·enty-two hours.
39120

19i. A more recent paper 3·1 reports aberration yielddata from six patients treated with (partial-body?)
radio-therapy for malignant disease. Cultures weregrown for fifty hours and se\"enty-two hours, but noestimate of dose is gi,·en. In considering such partialbody exposures. the question, of course, arises as towhat significance can be attached to a partial-bodydose if such a dose is reported.
198. The aberration yield observed in peripheral
blood leucocytes sampled after a partial-body irradiation will depend upon a number of variables including
the following: the physical characteristics of the radiation : the region of the body and the volume of tissue
exposed; the absorbed dose in this volume and theduration of the radiation exposure: the proportion of
the total body lymphocytes that were resident in thisvolume during irradiation; the proportion of blood
lymphocytes that traversed this region during irradiation : the amount of exchange of lymphocytes between
the lymphatic tissues and peripheral blood and the timeof sampling after irradiation.
199. The question of dosimetry in cases of partialbody exposure is, therefore, comple.x and will be considered in some detail later (paragraphs 264-273).
However, it is pertinent to note here the recent studies
bv \;yinkelstein et al.227 on chromosome aberrations inl~ucocytes of three patients whose blood was exposed
to extracorporeal irradiation (ECI) prior to renaltransplantation.
200. In this work. blood was passed through ateflon loop outside the body. using a standard QuintonScribner shunt, and was subjected to ECI by exposure
to beta-emiting ll 0 Sr-00 Y sources. Exposure times of up
to four to eight hours were used. and the frequency
of dicentric aberrations in Ieucocytes cultured with
PI-IA immediately after the termination of the ECI
period was determined. In addition, the frequency ofdicentric aberrations in fo vitro studies on blood put
through the radiation applicator in a single passage
was also determined.
201. In these ECI studies, physical dose estimatesto the blood cells were made on the basis of patient
blood volume, flow rate through the applicator andduration of irradiation exposure .•AJthough no detailed
data on aberration yields were given. the relationshipbetween calculated physical doses (integrated over thewhole blood volume) and the doses estimated on
the basis of dicentric aberration yields in sampled blood
leucocytes were compared. The dose was estimatedfrom the aberration yield through the use of the proportionality constant (paragraph 160) of dicentric yield
being equal to 5.i ± 0.5 10· 0 per cell-rad squared asdetermined from previous in 'Vitro studies 34 • 240 of thisgroup. A very close correspondence between physicallyand biological estimated dose was found (table II) ,provided that samples were taken after no more thana four· to eight-hour ECI exposure so that blood
leucocytes were not replaced by populations of leucocytes from the unirradiated lymphoid tissues.
202. In an ECI study carried out by Sharpe et al.~ 43
on a patient wiTh reticulum cell sarcoma. it was foundthat the relationship between estimates of dose basedon blood flow rate and on total blood volume of apatient and of those based on the yield of dicentricaberrations differed b,· a factor of 2.7. The data obtained inc~icated that. 'in a treatment lasting three anda half hours, several cells made many transits through121

the irradiator and that there was a fairly rapid e.xchange
between leucocytes of peripheral blood and those inmuch larger pools in e.xtranscular sites.
203. Sharpe et al. 269 have recently reported on somefurther ECI studies made on a patient v>ith Hodgkins
disease. This work has confirmed and verv much extended their earlier findings, and the results are somewhat at variance with the conclusions of \Vinkelstein
et aF 2 ' In this recent stucly. 269 it was found, as previously shown by others. 22 ' that the yield of dicentricaberrations (0.83 per cell), in a sample of the patient's
blood taken prior to ECI treatment and receiving an
in ·vitro dose of 300 rads from 2 ~IeV x rays, wasclosely similar to the dicentric yield (0.87 per cell)obtained from blood allowed to proceed through one
transit of the radiation coil ( 320 rad from a caesium137 source) over an e.xposure period of four seconds.
However, in blood samples taken from the patient
after one and a half, three and tv:enty-four hours. continuous ECI-treatment. dicentric yields of less than
0.09 per cell were obtained.
204. From these data and from studies on the
distribution of aberrations between cells, Sharpe etal. 269 concluded that there is a rapid exchange between
lymphocytes in blood and lymphocytes in the extravascular pool. It was estimated that the peripheral
blood contained 3 grammes of lymphocytes. whereas theextravascular pool contained between 800 and 1,070
gramm~s. Two independent estimates of the mean
residence time of lymphocytes in blood gave values of
4.7 and 7.5 minutes.
205. The results of the ECI studies bv these twogroups of workers, although somewhat conflicting. aremost interesting, and further work in this field will be
particularly rewarding both from the point of viewof yielding information on the population structure
and movements of the leucocytes and in providing
information applicable to the possible use of chromosome aberrations in dosimetry.
206. Following up on its original studies 15 on aberrations induced by x irradiation of spondylitis patients.
the Edinburgh group has recently reported 16 data on
the relationship between aberration yield and radiationdose in these patients and in patients suffering fromneoplastic disease. With the spondylitis patients. single
partial-body doses of 100 to 700 rads (250 kV x rays.
HVL = 2.7 mm of Cu) were given and the cells cultured for forty-two to fifty hours. At all doses, blood
samples were taken twenty-four hours after exposure.
but, in some instances, samples were also taken atearlier and later times. The data indicate a slightly
lower aberration yield in cells cultured immediately
after exposure than in cells sampled for culture twentyfour hours later. A summary of the data obtained isgiven in table III.

20i. Although we cannot define the absorbed dosesin these cases for whom data are given in table III,
there is evidently a clear relationship between skindose and aberration yield. The data are somewhat
variable. and for dicentric and ring aberrations theaberrations appear to increase in proportion to the
1.5 to 2.4 power of dose, at least for doses up to
300 rads. If the data from one patient given a partialbody dose of 700 rads is included in the kinetic analysis.
then this high dose yield reduces the dose-squared
component considerably. Extrapolation from the data
shown in table III shows that these yields are higherthan those obtained by Millard 126 from patients exposed
to partial-body (lower abdomen) radiation following
orchidectomy. In Millard's data (2 MeV Van der Graff
x rays), 20 per cent of the cells showed aberrations
after doses between 925 and 1.550 rads and 32 per centafter doses between 3JOO and 4.330 rads. These latterdata. however. were obtained from peripheral blood
cells that were allowed to grow for seventy-two hours,
and, moreover, the radiation treatment was spread overa period of thirty-six to seventy-four days.

208. Similar observations to those of l\!Iillard havebeen made by Dubrova!!iO on two myeloma patients
receiving radiation therapy. In this work, patients weretreated \vith an accumulated partial-body exposure of
9,000 roentgens. and up to 42 per cent of the leucocytes cultured for forty-eight hours \vere found to contain chromosome aberrations. High aberration yields\Vere also observed in blood cells of a similar patient
sampled thirty-two months after the completion of asimilar course of treatment.
209. Spondylitis patients were also given ten partialbody x-ray dose fractions over a period of twelve tofourteen days ;16 blood samples were collected immediately after each treatment. Again, there was a clear
relationship between radiation dose and aberration
yield, the data giving a very good fit to a linear relationship (figure 11). In these data. each fraction

0.4

.

•
Yield of frcgments• Dicentrics + rings
..J
..J

0.3

~

,..::

g 0.07£-<
5 0.06

I-

~

UJ

"'

0.10

..l

0.2

<

<(

213. Analysis of the data obtained from these wholebody exposures revealed that the yield of dicentric and:ing .ab.erra~ons in blood sampled immediately after
irradiation mcreased as the 1.13 power of dose(n = 1.13 with 95 per cent confidence limits of 0.52
to 1.74), whereas, in blood sampled twenty-four hours
later. the yield increased as the 1.88 power of dose(n = 1.88 with 95 per cent confidence limit of 1.24 to2.50). These two sets of data are shown in figures 12
and 13.

<.:

UJ

"'"'

212. In these data, there is a significant increase inaberration yield in bloods sampled twenty-four hours
P?St-treat:nent as opposed to bloods sampled immediately atter exposure in patients receiving doses of
50 rads. This obsenration is entirely in line with theearlier observations made by this group of workers on~nkyl.os~ng spondylitis patients exposed to partial-body
irrad1at10n (paragraphs 206 and 210).

> o.on"'a.
VI

211. The Edinburgh group has recently extended its
studies on whole-body irradiation of patients withbron~hial carcinoma and has now reported2 71 dataobtamed from a further nine patients exposed within
the dose range 17 to 50 rads. The dicentric and ringaberration frequencies in fifty-three-hour cultures ofblood cells sampled immediately after irradiation andtwenty-four hours later are summarized in table V foreach of the si.xteen patients.

~

UJ

u

z
2

yield obtained following a whole-body dose of 50 rads
to the cancer patients was equivalent to the yieldobtained with a partial-body dose of 250 rads to theankylosing spondylitis patients.

~,., 0.05

O.J

00

0 0.04
0

..l
..l

300

600

[; 0.03

900

i::
p.

DOSE (RAD)~i'3

Fig111"e 11. Yield of fragments and dicentrics plus rings in~ 0.0~

ankylosing spondylitis patients exposed to doses of up to
1,500 rads (250 kV x rays) given as a series of fractionsof 150 radsl6+of 150 rads (partial-body dose) gave an average yieldof 3.6 dicentrics and rings per 100 cells analysed.

u210. Buckton et al. 16 also reported preliminary data,summarized in table IV. on aberration vield in sevenmen suffering from bro1~chial carcinoma' who received
low doses (25 or 50 rad) from whole-body x irradiation (2 MeV Van der Graff). In contrast to the darno~t~ined from patients exposed to partial-body irradiation (table III), no differences in aberration vields
were found between bloods sam;:Jl.:=d immecliatel·/ after
exposure and twenty-fou:- ho:1;-s later (howe\'er, seep~ragraph ?11). :'>.foreo,·er. the dose response for dicenrncs and nngs after whole-bodv irradiation was linear( 1'! in the equation y = caD" being equal to 0.92
with 90 per cent confidence limits of 0.5 to 1.4). From
the data presenced, i• would ~eem that the aberration

+

i22

u
"'

z~

"'i5

10

~tl

~o

;,r,

•;u ;o so soDOSE (RAD)Figure 12. relationship between yield of dec~ntriC' plus dng;;il<::r ce!I and uniform whole-body dose in patients exposed to

2 =-.re V x ray3~71 •c Date. from blood sar.1.p}es t:iken immediately a:ter radiation expoS!..:re. Po:nts are from dn.rn given Ln iable V, ~nd c·.:-:\'ed Enc~ rtpre£e-;it

the 93p~r ce:1t

co:.:fidence Em1ts of there-~re!sio:ilb::.

214. It is of interest to compare the 25-n~d and 50rad whole-bodv irradiation data with data obtainedusing "imilar -doses (but with various quaiities ofradiation) by other authors in in 7!ifro st:.idies. These~ 0.10l:i 0.00~ 0.08g
0.07....
S O.OtlB:::? 0.05"'
00

0 0.04I,.J,.J

i'.l 0.03
r::

bl

"'
"'§0 0.02
+

"'
2

u

....z
bl

u

'5

0.01
·10

20

30

40

50

60

70 80 90

DOSE (RAD)Figure 13. Relationship between yield of dicentrics plus rings
per cell and uniform whole-body dose in patients exposed to
2 Me V x rays271 a• Data from blood samples taken twenty.four hours after radiation
exposure. Points are from data given in table V, and curved lines
represent the 95 per cent confidence limits oi the regression line.

comparisons are set out in table VI where it may beseen that scores obtained in the in vivo work fall inbetween the lowest and highest yields reported for thesame absorbed doses in the in vitro studies. It is disappointing to note that. in the only in vitro workwhere a radiation quality similar to that used in thein vivo studies was used and where the cells were
cultured for fifty hours, the aberration yield at a doseof 50 rads was four times lower than that found in
the in ·vivo studies. However. the observations weremade in different laboratories where techniques were
not entirely comparable.
215. In addition to the data on therapeutic exposure to external radiation sources. there is some
information on aberration yields in patients to whomradio-active materials had been administered internallv
for therapeutic purposes. Boyd et al. 272 initially reported
that types of chromosome damage similar to those reported by Tough et al.1 5 were to be found in blood
cells of patients treated with radio-active iodine. It wassuggested that, in quantitative terms, the effects of 100
millicuries of radio-iodine were similar to the effects
of a partial-body dose of 250 rads of x rays and thata 10-millicurie dose of radio-iodine was probably sufficient to produce recognizable chromosome damage.
Similar findings with radio-active iodine were also reported by other authors. 172• 273• 275 and. in one report, 274
a significant yield of aberrations was observed fourteen
years after completion of treatment.
2. Occupational exposure216. A number of workers have reported the presenceof chromosome-type aberrations in individuals receiving
chronic low doses from external sources.127 • 1s2 • 13 s, 1 ss,
168, 239, 2sa, 2as. 2ss, 2 7 s-2 so Norman and his colleagues 265
observed seven dicentrics in 5, 138 cells from ten hospital
123

radiation workers who had received dose equivalents oi
up to 88 rems accumulated at an average rate of from
1 to 3 rems pe:- year, whereas in ten control individuals
no dicentrics were observed in 4.219 cells. Later
studies 16 :; were carried out on thirtv-si.'\: radiation
workers who had received cumulative - doses of from10 to 98 rads with a median annual dose rate of 1.45
rads per year. In this later work. fourteen dicentrics
were observed in 14.839 cells. whereas in t\ventv-threecontrol individuals no dicentrics were observed i; 5.784
cells. Ob.;ervations similar to these have recentlv been
made by Lisco and Lisco27 ' and by GorizontoV'a. 27 S
217. Studies somewhat similar to those of Norman
and his colleagues have been reported by Court Brown
et aF 76 and Buckton et al. 13 ~ These authors studied sixtyseven adult males working in atomic energy establishments
and divided their sample into five groups: (a) a control
group that had received a cumulative dose of less than
1 rad : ( b) a group with an average accumulated doseof 3.8 rads, with a range from 1 to 10 rads ; ( c) a
group with an average accumulated dose of 27 rads.
with a range from 23 to 34 rads ; ( d) a group withan average accumulated dose of 24 rads, with a range
from 15 to 37 rads: (e) a group with an averageaccumulated dose of 84 rads, with a range from 75
to 98 racls. The irradiated groups differed not only inthe doses received but also with respect to the timeover which the exposures occurred. The control group
did not differ cytologically from a control population
drawn from outside atomic energy establishments, but
all the irradiated groups showed a highly significant
increase in aberration yield. Although yields of dicentric and ring aberrations of as high as eight per 1.000
cells were observed, no correlation between dose andyield could be discerned.
218. Visfeldt127 studied aberration yields in peripheral blood cells of ten members of the staff of the
Copenhagen Radium Institute who had received
cumulative doses ranging from 1 to 116 rads over a
period of ten years. Again. a clear increase in aberration yield was observed in irradiated (thirteen dicentrics plus rings in 950 cells) personnel as opposed tocontrol (zero dicentrics plus rings in 300 cells) personnel, but the data are once more too meagre toshow any correlation \Vith the dose received.
219. EI-Alfi et al.2 w analysed blood cells fromtwelve radiation workers. exposed over periods as longas four years. who received cumulative dose equivalents
of up to 1.110 millirems of x, gamma or beta rays or up
to 9,722 millirems of neutrons. No details on the qualityof the radiation nor on the kinds of exposures were
given, but significant increases in aberration yields wereobserved in the si.x individuals exposed to neutrons
when compared with nine control individuals.
220. Data somewhat similar to those of Visfeldt's
have recently been reported by Wald et al. 136 Theseauthors studied aberration yields in six nuclear industry\Yorkers who had received external body dose equivalents ranging from 25 to 55 rems at an averageaccumulation rate of 4.3 rems per year. A significant
increase in the frequency of stable and unstable aberrations in irradiated as opposed to control personnel wasnoted, but no relationship with the various dose levels
could be discerned, and no detailed cytogenetic data are
given.
221. A number of studies have been carried out on
persons who have worked· in the luminizing industry

and who have, as a consequence, high body contentsof radium-226. These studies also have shown a significant increase in aberration yields in exposed versus
unexposed individuals, even in individuals having bodyburdens well belov.r the maximum permissible level.1 67 • 281
Vlith these internal emitters. there is some evidenceof a consistent gradient of increasing aberration yieldwith increasing radium body burden. 282 In the dataof Boyd et al.'2 82 on individuals who accumulated bodyburdens between 0.10 and 0.56 microcuries of radium226 eighteen years prior to study, some 3.2 per centof the cells were classed as "unstable" and found tocontain asymmetrical aberrations. In this work, thetotal occupational dose equivalent from external gamma
rays averaged about 90 rems. There was no association between these low-level external exposures andaberration yield.
222. These data on occupationally exposed individuals all show significant increases in aberration yieldin persons exposed to very low dose levels. This is, ofcourse. in line with the earlier observations on theeffects of low doses of diagnostic radiation. Moreover,it should be emphasized that, in those cases whereaccurate physical dosimetry has been carried out. it ispossible to state that significant aberration yields havebeen observed in individuals receiving doses below thepermissible levels.

3. Accidental e:cposttre
223. Bender and Gooch128 • 129 studied aberration
yields in peripheral blood cells of eight men exposed
accidentally to mixed gamma and fast neutron radiation. The doses were estimated to range from 23 rads
to 365 rads. with the neutrons comprising some 26
per cent of the total dose. No chromosome-type aberrations were found in five control individuals (total of458 cells) . but dicentrics and rings "·ere present in all
five individuals exposed to doses calculated to be over
200 rads. Blood samples were first collected twentynine months after the original exposure and then ayear and a half later. Aberrations were present in all
individuals except the person exposed at the lo\vestdose level. In one individual who received an estimated
339 rads. the frequency of dicentric and ring aberrations was 0.166 per cell (table VII). All the culturesin these cases were grown for seventy-two hours. Goh 283
followed up these observations and examined cells fromblood samples, cultured for seventy-hvo hours. takenfrom six of these men seven years after the original
accident. Cells from bone marrow were also sampled.
Aberrations were observed both in cells from the marrow and in cells from peripheral blood, but dicentricaberrations were absent in marrow cells. Although theover-all frequencies of aberrations in peripheral blood
cells had declined with time after exposure. significant
yields of dicentric and ring aberrations were observedon each of the three occasions when samples weretaken. The published data on dicentric and ri1ig aberrations in these studies are summarized in table VII.
224. In a later criticality accident.131 • 2 ~ 9 three men
received estimated doses cif 12, 22.5 and 47 rads of
mixed radiation ( !!amma and fission neutrons), theneutrons contributing, in the different individuals.
25-50 per cent of the total dose. fo these cases, blood
was sampled from four hours up to two years after
exposure. and the cultures were grown for seventytwo hours. Dicentric aberrations were observed in all
124

three individuals, their frequency showing a clear increase ·with increasing dose. A reasonable correlation
between estimated physical dose and aberration yield
was observed with some 3 per cent of the cells being
affected at the highest dose level. Using these data.
previous in vit1·0 information and certain assumptions,
it was suggested that the RBE for fission-spectrum
neutrons versus gamma rays was of the order of five
to one.
225. Biola and Le Go 284 have described studies onblood samples taken from an individual four davs after
a highly non-uniform exposure to mixed gam~a andneutron radiation in an incident at Mol. Belgium.
Physical estimates of dose suggested that the individual
had received a mid-line exposure of around 500 rads.
In parallel with the studies on the blood sample fromthe irradiated individual, studies were also made on
blood cells taken from a normal individual and thenexposed to cobalt-60 gamma rays with doses of 400
and 600 rads. For comparison with the data of Goochand Bender.229 all cultures were harvested at seventytwo hours or ninety-si.x hours, although it was evident
that at seventy-two hours at least 10 per cent of thecells were in their second mitosis in culture. The aberration yields observed, therefore. were dearly anunder-estimate of the true vield. but. since cultures ofthe blood of the irradiated· individual and of the cells
irradiated in vitro were handled in exactly the sameway. valid comparisons could be made. The actual
yields observed in vitro were similar to those obtained
by Gooch and Bender229 at the lower dose levels and to
those obtained by Kelly and Brown137 at the higher
doses. The in vitro yield at a dose of 450 rads was
equivalent to the yield obtained in the cultures fromthe irradiated individual so that a good correlation
existed between physically estimated and biologicallyestimated dose.
226. More recently, Buckton et a/. 132 analysed cells
from two men who accidentally received whole-bodydoses of 17 and 18 rads. the men having additionallyaccumulated 10 and 9 rads. respectively, as an occupational exposure over several years of routine employment. Dicentric and ring aberrations were present in
the blood cells of both men at levels up to a ma.ximumof 3 per cent. depending on whether blood was taken
at forty-eight hours or at one or three months following
exposure. One of the two controls \vho had received
occupational exposures of approximately 2 to 3 roentgens had a dicentric and ring frequency of approximately 1 per cent on two out of the three occasions
on which his blood was sampled. The data here are too
scanty to draw any conclusions on dose relationship.
227. Sugahara et al. 285 have reported data obtained
from two men exposed to external irradiation from250 kV x rays and cobalt-60 gamma rays, with estimated e..xposures of. respectively. 66 and 40 roentgens.
and studied ten and twelve months after exposure.
In addition, data were obtained from a further three
men ,...-ho inhaled urany! fluoride and from whom blood
cells "·ere taken forty days after the accident. The
amounts of uranyl fluoride (estimated from urine excretion) taken up ranged from 2.2 milligrammes to
3.9 milligrammes. representing an inhalation of between
2.6 and 4.6 10-3 microcuries.

228. In blood cells obtained from all fiye men inthis study, 285 significant increases in aberration :yields.
as compared with those in cells obtained from control
individuals, were noted. and dicentric and ring aberra-tions were present in cultures from all but one of theindividuals. The authors point out that the frequency
of aberrations in the x- and gamma-irradiated individuals were in the range expected from earlier
observations of other workers 128 • 131 but that almost
equivalent yields were obtained in two of the three
men who had inhaled uranium ( 3.2 per cent enricheduranimn). The cumulative. external, occupational dose
equivalents of these men were small, ranging from 128
to 936 millirems. The observations made on thesethree men were comparable with those reported by
Boyd et al. 282 on luminous dial painters (paragraph219).
229. Vhld et al. 136 have carried out cytogenetic
studies on a group of seven workers who accidentally
inhaled iodine-125 and whose body burdens were
measured by direct counting methods. Body burdens
ranging from 1.2 to 111 microcuries were determined.
These workers had also been exposed to e.."<ternalsources and had accumulated dose equivalents ranging
from 1 to 18.8 rems at an average rate of 1.4 rems
per year. No details of the qualities of the externalradiations were given. The data clearly show a significant increase in the frequency of cells carrying unstable aberrations over controls, but no breakdown of
the aberration data is given.
230. Observations similar to those reported above
have been made by Lejeune and his colleagues 286 on
four individuals, one of whom received an estimatedmaximum dose of 33 rads of neutron and gamma rays
(following an accidental e..'Cposure to a proton beam),and the others were exposed to unknown quantities ofgamma rays, although in one individual the estimateddose was between 35 and 50 rads. Significantly increased aberration yields relative to controls wereobserved in samples taken at various intervals up to
one year after irradiation. The aberrations included
dicentrics and rings as well as a number of symmetricalchanges. and the cells were cultured for seventy-two
hours prior to preparation. In the individuals in whom
physical estimates of dose were available. it was shownthat the aberration yields observed were reasonably
consistent with those predicted on the basis of the aberration-yield coefficient quoted by Bender and Gooch 229(paragraph 156). The authors were careful to pointout. however. that the data were insufficient to draw
any firm conclusions on the relation between aberration yield and dose in these individuals.
231. Lisco and Lisco 287 have recently examined
peripheral blood leucocytes (forty-eight-hour and seventy-two-hour cultures) of two radiation workers who
e..xposed their right hands to mixed gamma-beta radiation from an iridium-192 source. The e..'CpOSure was fora ten-minute period. and the physically estimated doseto the hands was 3,000 rads . 10 per cent of which wasfrom gamma radiation. Eleven days after exposure.
the yield of dicentric and ring aberrations in cells ofboth individuals was around 0.05 per cell (equivalent
to that observed with a 50-rad whole-bodv dose fromx rays). and high aberration yields were noted in eachof the follow-up studies carried out at intervals up to
three years after the accident. No aberrations wereobserved in bone-marrow cells.
232. The rather sparse data obtained from, fortunately rare, accidents underline the earlier statementthat one of the complications in accidental exposure is
the difficulty of obtaining good physical estimates ofdose, particularly in those cases of mixed radiation
125

exposure. In general, the cytological data that havebeen obtained are not inconsistent, but they are still
too scanty to draw any firm conclusions on the usefulness of aberration yield for biological dosimetry inthese particular cases.
4. Nuclear explosion
233. A number of studiesl33·135, 1ss, 175, is2. 221 havebeen made on survivors at Hiroshima and Nagasaki
who were exposed to radiation from nuclear e..xplosion
in 1_9.+5. The data of Ishihara and Kumatori,135, 17 s
obtamed from blood cells of persons who were between
500 and 2,000 metres from the hypocentre and who
were studied nineteen years later, show a significant
increase over control individuals in the yield of asymmetrical and symmetrical aberrations.
234. Bloom et al. 133 • 134 • 288 • 289 carried out surveys
on survivors in different age groups. In the firststudy133 on ninety-four exposed individuals and ninetyfour matched controls. all the individuals sampled were
under the age of thirty years at the time of the bombings in 1945. Chromosome aberrations were found in
0.6 per cent of the peripheral blood leucocyte cells in
the exposed individuals. whereas only 0.01 per centof the cells contained aberrations in control individuals.
In the control individuals, no dicentric aberrations wereobserved in the 8,847 cells scored, but nine such aberrations were found in 8,283 cells from the irradiatedpopulation sampled twenty years after exposure.
235. In the second survey by Bloom et al. 134 observations were made on seventy-seven heavily e.."<posed(estimated dose greater than 200 rad of mixed gammaneutron radiation) survivors and eighty control individuals, all of whom were over the age of thirty
years at the time of the bombings. Sixty-one per centof the heavily e.."<posed survivors and 16 per cent of
the controls were found to contain aberrations at afrequency of 1.5 per cent in cells of exposed individuals
as opposed to 0.3 per cent in cells of control individuals.
One dicentric aberration was detected in the 7,188 cells
scored from controls, and this was observed in a cellfrom an eighty-year-old male. In the irradiated individuals, eight dicentrics were found in the 6,778 cells
studied.
236. The relative frequencies of the asymmetricaldicentric. ring and fragment aberrations in the survivorsof the two different age groups were very similar.
However, the symmetrical translocations and inversions were found to be more frequent in the older
exposed survivors than in the younger ones. In parallelwith this latter observation, it was noted that symmetrical aberrations were also more frequent in theolder of the two control groups. These observations
once more raised the question of whether the sensitivity to aberration induction by radiation might berelated to age. a problem that requires urgent attention.
237. The most frequent aberrations observed by ·Bloom et al. 13.,i in the exposed older survivors weretranslocations which were present in seventy-two of
the 6.778 cells scored. It was noted. however. thattwenty of these cells containing a translocation \Veredetected in four individuals, and these represented fivedifferent types of translocation or five possible cellclones.
238. Estimates of the dose sustained by the exposed
older survivors ranged from 204 to 991 rads of mixed
gamma and neutron radiation. A preliminary attempt

to correlate aberration frequency with physically estimated dose suggested that the aberrations increased
approximately linearly over the dose range studied,the over-all aberration frequency being about 1 per centat 200 rads and increasing by approximately 0.5 percent per 100-rad increment. It should be noted. however, that, in this work. the leucocytes were cultured
for a period ranging from 66 to 70 hours, and. ofcourse. samples were studied twenty years after an
original radiation exposure.
239. Workers at the Atomic Bomb Casualty Commission 289 have analysed the karyotypes of 128 individuals who were born after at least one of theirparents had received a minimum exposure of 100
rads as a result of the atomic bombings at Hiroshima
and Nagasaki. Control studies were carried out on
fifty-seven sibs of these individuals who were born
before the time of the bombings. Particular attention
was devoted to the 103 individuals who were born inthe first five years after parental e.xposure, but nosignificant increase in chromosomally abnormal individuals could be detected. However, a detailed studyof thirty-eight in utero e.xposed survivors, 288 whose
mothers had been exposed to more than 100 rads
(estimated range 104-477 rad) at the time of thebombings, revealed a small but significant increase in
the frequency of lymphocytes with complex chromosomal rearrangements (0.52 per cent) as comparedwith matched control individuals (0.04 per cent).
240. Sasaki and Miyata221 re-opened the questionof the relationship between aberration yield and estimated physical dose in the atomic bomb survivors
and presented a considerable amount of detailed dataobtained from the scoring of over 80.000 cells fromexposed and control individuals. Chromosome analysis
was carried out on fifty-one Hiroshima sun'ivors andeleven controls twenty-two years after the original
exposure. Care was taken to score only cells dividing
for the first time in culture. and the cultures were
terminated after fifty hours. The aberrations wereclassified as outlined in paragraphs 24 to 34, and it
was found that the mean number of dicentrics andrings in the exposed individuals (201 in 73.996 cells)
was 0.0024 per cell as compared with 0.0002 per cell
(2 in 9.510 cells) in the controls. It was also shownthat the frequency of cells carrying stable symmetrical
rearrangements (largely reciprocal translocations) was
0.40 per cent in exposed individuals as compared with
0.07 per cent in controls.
241. A significant yield of aberrations was observed
among nineteen sun,ivors who were more than 2.4
kilometres from the hypocentre and who. as wasestimated on the basis of physical considerations, hadreceived a dose of the order of 1 rad. Eleven of these
individuals entered the bombed area within three days
after the bombing, and the frequency of dicentrics andrings in these individuals was 0.0013 per cell as compared with a frequency of 0.0006 per cell in the eightindividuals who did not enter the bombed zone.
242. The exposed individuals were divided into fourgroups based on distance from the hypocentre at thetime of the bombing and on whether they were directly
exposed or shielded by wood or by concrete. Proportionally more cells with aberrations appeared in survivors exposed at the shortest distance from the hypocentre, and, at a given distance, the aberration yield~\·as highe~t in those directly exposed. intermediate
111 those shielded bv wood and lowest in those shieldedby concrete.
126

243. Since the observations were made twenty-two
years after the original exposure, Sasaki and Miyata221
used two different methods in an attempt to obtaindose estimates. Studies by other workers33 , 44 • 2 00 haveshown that the proportion of peripheral blood lymphocytes carrying stable chromosome rearrangements (Cacells) obsen•ed many years after an irradiation exposure
remains unchanged from the proportion observed
shortly after exposure. The ratio of Cs cells to normal
cells could, therefore, be used as an end-point in these
Hiroshima sun·ivors. and the relationship between thisend-point and distance from the hvpocentre is shownin figure 14.
·
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rl 2.0

l

ri.

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r.J

~ 1.5

...z
r.i

u

~ 1.0
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DISTANCE FROM HYPOCENTRE (km)Figure 14. Frequency of C. cells as a portion of all (except C.)
cells plotted against distance from hypoccntre221

244. Asymmetrical aberrations would, of course.
have been largely eliminated over the twenty-two years
since exposure so that the total frequencies of dicentric
and ring aberrations in individuals at different distancesfrom the hypocentre would not offer a good methodof relating aberration yield to absorbed dose. However, the distribution of chromosome aberrations within
X 1 Cu cells which had not divided since exposure shouldbe the same as that existing immediately after exposure.
In figure 15 are shown the number of dicentric andring aberrations in C11 cells in relation to distance fromthe hypocentre.
245. Using the proportion of C. cells to normal
cells and the number of dicentric-plus-ring aberrations
in Cu cells, Sasaki and Miyata 221 obtained absorbeddose estimates simply by relating the values given in
figures 14 and 15 to equivalent yields obtained withmeasured doses of 2 MeV x rays in in vitro studies.
Figure 16 shows the dose estimates obtained in thisway (without attempting to make corrections forshielding or for quality of radiation. etc.) plottedagainst distance from the hypocentre. The dashed linein this figure shows a recent 21>1 indirect physical estimateof air dose for comparison with the dose arriYed atby using the biological methods.
246. The estimates based on chromosome aberration
yields compared with the physical estimates (figure 16)
are low in the survivors exposed close to the hypocentre and high in the remotely e.'-""J>OSed people. The
authors 221 suggest that these differences may reflect a
selectiYe mortality over the twenty-two-year period in
the population exposed near the hypocentre and that

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DISTANCE FRC.M HYPOCENTRI!: (km)Figure 15. Kumber of dicentrics plus rings per X1Cu cell versus distance from hypocentre221

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DISTA."CE FROM HYPOCENTRE (km)Figure 16. Average total-body absorbed dose estimated from the frequency of chromosome aberrations in Hiroshima survivors:?21 n•Vertical lines through points represent 50 per cent confidence limits, and dashed line T66D, gives estimateof air dose derived from physical considcrltions. Estimates of absorbed dose based on the C, cell data (fiirure 14)
are those given as open symbols, and estimates based on dicentric and rinir aberrations in X 1C. cells (figure 15)
are given as closed symbols. Tri'1llgle•, circle• and squares represent directly exposed sui·vivors and survivors shieldedby wooden or concrete houses, respectively. Subscript e indicates those survivors who had epilation.

posited radio-active material. Samples were first taken
ten years after e.">Cposure and repeated sampling has
continued since that time. In the original study 135 it
was found that the aberration yield in the irradiatedindividuals was significantly above controls and that
dicentric aberrations were present. but it was notpossible to correlate aberration yields with physicallyestimated closes. However. when the individuals weredivided into three groups according to the degree ofdamage indicated by the lowest neutrophil levels
reached shortly after exposure, it was found that themean frequencies of cells containing aberrations in thesethree groups were correlated with the e.">Ctent oi damage
indicated by the original haematological findings. Follow-

individuals who were more than 2.4 kilometres awaymight have received radiations from sources other thanthe primary rays.
247. Three papers describe obsen·ations on peripheral blood chromosomes of individuals accidentally
exposed to radia!ion from radio-active fall-out due tothe explosion of a thermonuclear device at Bikini in
1954.
248. Ishihara and Kumatori 135 • '.!e'.!, 2u3 studied cells
obtained from eighteen of the twenty-two fishermen
whose e::-.."1:emal exposure resulted in doses estimatedto range from 220 to 660 rads and who had also received an unknown contribution from internally de127

up studies showed that three individuals possessedclones of cells with chromosome abnormalities in theirbone marrow, and it was noted that these three persons were, in fact, in the group who had the lowest
neutrophil counts shortly after exposure.
249. Lisco and Conard2114 have recently studied
blood cells obtained from fifty-one Marshallese, ofwhom thirty had received an estimated whole-body
gamma-ray dose of 175 rads, thirteen had received
approximately 70 rads, and eight had not been exposedand served as controls. The results are curious in that
more acentric fragments were found in controls thanin irradiated individuals. However. if we consider only
the dicentric and ring aberrations. a difference between
exposed and unexposed individuals is observed. In thecontrols. no asymmetrical exchange aberrations were
found in the 400 cells analvsed. but three dicentrics
and rings were found in 6SO cells from the 70-radgroup and six in 1,500 cells in the 175-rad group. A
similar difference between control and exposed individuals was found for symmetrical e_-x:change aberrations.
250. These data from individuals exposed to radiation following nuclear explosions all show significant
aberration yields in the survivors studied. l\foreover.
the obsen•ations confirm earlier studies which showedthat radiation-induced chromosome aberrations can bedetected in leucocytes of individuals exposed to radiation for clinical reasons up to twenty-two years prior
to observation. Most of these earlier studies did not
permit quantitative conclusions relating aberration yield
to absorbed dose. but the recent eA-tensive data andanalyses on Hiroshima survivors suggest that a fairmeasure of agreement exists between dose estimates
based on the yields of chromosome aberrations and indirect estimates of air dose arrived at through the useof physical methods.
251. From the data discussed thus far it is evident
that the aberration yield must decline with increasingtime interval between irradiation and blood sampling.
and the influence of this factor on aberration vieldshould now be considered.
'

5. Time of sampling after radiation exposure252. Since the first publicationsH· 128 demonstratingthat a significant yield of aberrations could be observed
in hlood cells of individuals exposed to radiation many
years earlier, virtually all of the publications on in vivo
exposure present data confirming these observations.
253. In the original work of Buckton et a/.. H it
was shown that the frequency of cells carrying asymmetrical aberrations ("unstable" Cu cells) showed an
approximately exponential decline \\'ith increasing timeafter exposure. whereas the frequency of cells withsymmetrical changes (''stable" C. cells) stayed roughly
constant with time (paragraph 243). Cytological eYidence indicated that manv of these aberrant cells withasymmetrical changes were in their first post-irradiation mitosis when sampled so that these data offereda means of determining- the average in ~·i'Z:o life spanof the dormant non-didding leucocytes (small lymphocytes). Further studies by Euckton et a/. 33 • ~ 9 ;; and by
N" Orman et al. ~Do, ;n have extended and refined theanalysis and led to the conclusion that the mean life
span oi this cell lies somewhere behveen 500 and 1,500
days in accordance with the tritium-labelled tlwmidine
data on these cells obtain'=d by Little et al. 103 '

128

254. Goh:! 83 has suggested that not all the aberrations observed in peripheral blood leucocytes many
years after exposure are aberrations induced at thetime of irradiation. The evidence in support of thissuggestion is meagre and is open to an alternativeinterpretation. It is known, however,296 that chromatidtype aberrations can be induced in human fibroblasts
by exposure to extracts of allogenic lymphocytes, andGoh presents evidence297 for a small, but significant,effect of irradiated human plasma in inducing aberrations in peripheral blood Ieucocytes. an observation
supported by the independent work of Hollowell andLittlefield. 298 In Goh's experiments, the cultures wereincubated for seventy-two hours and. from the figuresin the published account. it is evident that most of theaberrations observed were chromatid-type changes withsome "derived" (paragraph 74) chromosome-type fragments: no chromosome-type dicentrics or rings werenoted. Hollowell and Littlefield298 used plasma frompatients given doses up to 4.500 rads from 2 MeV
x rays and observed chromatid-type and chromosometype changes in blood cells from normal individuals
cultured in the presence of such plasma. Five dicentrics and rings were observed in 476 cells, butthese aberrations were probably <>f the "derived" type.
Unpublished works of other observers 299.soi have showneither no discernible effect or a small increase inchromatid aberration yields in first divisions of cultured
cells exposed to irradiated plasma.
255. It is of interest to note here reports 30!!, :ioa
indicating that the long persistence of chromosomallydamaged lymphocytes so clearly demonstrated in man
and Rhesus monkey may not occur in certain othermammals. Studies on PHA-stimulated blood cells
from monkey, rat, guinea pig and pig have shownthat unstable chromosome aberrations are not obsen·ed
for more than a few hours after irradiation except in
the monkey where Cu cells were observed for as longas the animals ·were obsen·ed (seven months). This
rapid loss of C.. cells in these animals is of particularinterest, since, at least in the rat. iii vivo studies withtritium-labelled thymidine304• :JOj have clearly demonstrated the presence of long-lived lymphocytes in unirradiated animals.
256. From the point of view of biological dosimetry,
there is normally little interest in samples of blood
cells taken from irradiated individuals many years
after radiation exposure. However. follow-up studies
will. of course. be of importance in connexion withthe possible somatic risks that may be associated withthe presence of aberrations. Chromosome studies onbone-marrow cells in particular will be of paramountin1portance with regard to leukremia risks. In thecase of the peripheral blood leucocyte (small lymphocyte). the composition of this cell population in peripheral blood (as well as in e.xtravascular areas) willchange with time simply because of the role playedby this cell in immunological response. 306 The workof NowelP 18 • 30 ~ suggests that the aberration yields
obser•ed in leucocytes many weeks. months or years
after exposure will be dependent upon the number andtype of antigenic stimuli received by the individual
between the time of exposure and the time of sampling.
257. Although these small lymphocytes are invo!Yedin immune responses. they are. neyertheless. normallynon-dh·iding so that. in man. there should be no preferential Joss of cells carrying chromosome damage insamples taken many hours. days or possibly ''"eeksafter e.xposure, provided the cells are not i1wolved in

1

an immune response. Support for this concept has comefrom the in 1:itro studies of Scott et al.~ 45 and of
Kozlov 307 who have compared aberration yields inperipheral blood leucocytes placed in a culture medium
containing PH:\ immediately after irradiation withcells held for twenty-four hours post-irradiation or lessin a culture medium devoid of PHA. In both thesestudies. using fast neutrons~ 4 ;; (paragraphs 176-177)
and cobalt-60 gamma rays, 30 ' the general finding wasthat an increased time between irradiation and mitotic
stimulation had little or no influence on the aberration
yield.
258. There is little information, however, on thevariation in life span of the small lymphocyte andon the multiplicity of populations present throughout
the body. Nevertheless, on the basis of information
available. it would seem that. within any particulardefined subpopulation, the aberration yield should remain constant until that population becomes involvedin an immunological response. The difficulty is. of
course. that of defining a subpopulation or compartment in a mixed population of morphologically identical but functionally diverse cells which migrate
throughout the body.
259. In terms of biological dosimetry. the location,
distribution and population density of lymphocytes
throughout the body is a problem of no importance if
an individual receives a uniform whole-body e.'Cposure
to a radiation of high penetrating power. This problem
could, however. be of great importance in the case of
an individual receiving partial-body exposure or whenthe exposure is to radiation of low penetrating power
or of mixed quality. The problem may be minimized if
a sufficient cell mixing within the population occurs between the time of irradiation and sampling.
260. "When blood is sampled immediately after anacute e."'<posure to radiation. then leucocytes that werein the peripheral blood vessels at the time of exposure
will presumably be the only population that is sampled.
However, if sampling is delayed for a sufficient but
unknown time, then leucocytes that were in various
lymphopoietic centres at the time of irradiation may
have been mobilized into the blood stream. The dataobtained in the extracorporeal studies227• 243 • 244 suggest that this time interval is not greater than eight
hours and may, in fact, be of the order of a few minutes(paragraphs 199-204). At later sampling times.
some of the leucocytes observed in peripheral blood
will not have been directly exposed to radiation but
will have been derived from irradiated stem cells.
261. To date. there is not a great deal of information on the change in aberration yield in blood samples
obtained at frequent intervals throughout the first twoor three days following exposure. In the data of
Bender and Gooch 131 obtained from three men receiving
up to 47 rads of mixed gamma and neutron radiations
in a criticality accident. there appeared to be a little
difference in aberration y;eld in samples taken fourhours, two weeks and four weeks after e.xposure.
262. In some more recent work of Buckton et al. 16
which involved the partial-body exposure of ankylosing
spondylitis patient~ to various closes of x rays, it wasobserved that higher aberration yields were obtained
from blood samples taken twenty-four hours after exposure than from samples obtained immediately after
exposure (table III). l\fore detailed studies were thencarried out by these authors on patients who had received a partial-body exposure of 300 roentgens. Sam-pies were taken from these patients at zero. three.
six. twelve, twenty-iour and forty-eight hours after exposure. The aberration yield increased with samplingtime up to twenty-four hours and then declined at fortyeight hours. At zero hours and forty-eight hours, thefrequency of "unstable"' cells was approximately onehalf the maximum vield that was observed in thetwenty-four-hour saniples. There was no significant
difference behveen vields obsen,ecl in the six-. twelveand twenty-four-hour samples.
·
263. The very limited data on whole-bodv exposure in the criticality accidents, often involving uneven exposure. are insufficient to make any pronouncements on change in aberration yield in the first few
hours following exposure. vVhat few data exist do notcontradict the expectation of no change in yield withtime of sampling with this kind of exposure. However,
the recent whole~body x-ray studies 271 support theobservations made in the earlier partial-body x-ray
work (paragraph 212 and table V). In the partial-bodyand in the extracorporeal blood e..xposures. there isclear evidence that the aberration yield changes withtime even in the first twenty-four hours folJO\ving e.'Cposure. There is but little information on the amountand rate of change and no information on whether theseparameters are influenced by dose level. site of exposure, age and health of the exposed individual.

D.

CONCLUSIONS

264. It is evident from the studies carried out onpatients receiving low doses from diagnostic x rays
and on individuals receiving chronic low doses as aconsequence of their occupation that doses of the orderof a few rads from x or gamma rays result in a significant increase in the yield of aberrations in blood
leucocyte cells. This increase is particularly impressive
when dicentric and ring aberrations. that are extremelyrare occurrences in the blood cells of une.."'<posed individuals, are considered. Thus. in terms of its possible
use in biological dosimetry, it is clear that this particular system is a very sensitive one.
265. A knowledge of the form of the relationshipbetween aberration yield and radiation dose is essential
in any attempt to e.'Ctrapolate from one to the other.
Until very recently, the in .._>itro data that were available seemed rather disappointing, since considerable
differences were evident when data obtained in different
laboratories were compared. Nevertheless, within any
one laboratory, when the same techniques were employed, the results were consistent and highly repeatable (paragraphs 178-179). The more recently available data on the response to radiations of differing
quality reveal a high degree of consistency between
laboratories when similar conditions of irradiation andof duration of culture are used.
266. The main factors contributing to differencesin results between laboratories are (a) the use of different culture times ; ( b) the irradiation of whole blood
prior to culture as opposed to the irradiation oi bloodin culture; and ( c) the use of different qualities ofradiation. It is evident that the aberration yields obtained in cultures maintained from forty-eight to fiftyfottr hours are generally higher than in cells irradiatedand cultured under the same conditions but sampled atseventy-two hours. Higher aberration yields are also
reported, particularly at dose levels below approximately 150 rads of x or gamma rays. if blood cells
are irradiated after culturing rather than as whole129

blood prior to culture. As a consequence of this. thedose exponents from e."\:periments with irradiated cultures are somewhat lower than from experiments using
cells irradiated prior to culture. The reason for thesedifferences is not clear, and further work is certainly
necessary in this area. Large differences in response
are observed between radiations of differing quality. andthe RBE for 2 :Me V x rays as compared with 250
kVp x rays is 0.8.
267. Sh1dies on the yields of dicentric aberrations inwhole blood irradiated with 2 Me V x rays, cobalt-60
gamma rays or 150 to 300 kVp x rays, and cultured
from forty-eight to fifty-four hours. show that the dose
e.'\.--ponent. n. in the equation y = kDn, is 1.9 to 2. 1.7
to 1.8 and 1.5 to 1.6, respectively. For blood cells irradiated in culture with 180 to 250 kV p x rays andcobalt-60 gamma rays, the dose exponents are reduced
to around 1.2 to 1.3. These data indicate that exchangeaberrations induced in vitro by 2 MeV x rays are predominantly a consequence of the effects of two separate
tracks. \Vith conventional x rays ( 150-300 kV p), however. up to dose levels yielding less than two d.icentrics
per cell. a considerable proportion of the exchange aberrations is the consequence of single track events. Studies with 2 to 5 MeV DD neutrons and 0.7 1feV(mean energy) fission neutrons indicate that with theseradiations there is a linear relationship between doseand aberration vield over a wide range of doses. In thecase of 14.1 ~feV neutrons, the relationship is not
linear. the dose exponent lying between 1.2 and 1.4.
Further information on the effects of fast neutrons,particularly in relation to the influence of irradiationand culture conditions and technique, is required.
268. In the in vivo studies on the relationship between aberration yield and dose. much of the data
have been obtained from individuals exposed to partialbody doses. Here. numerous complications arise, andmost of the data indicate that the aberration yieldchanges with time within the first day or so after exposure. These changes reflect alterations in the number,
distribution and mobility of the different leucocytes
within and between the exposed and unexposed regionsof the body at the time of exposure and the degree of
mixing that occurs between the time of irradiation andthe time of sampling from peripheral blood. It is clearlydifficult to derive a measure of absorbed dose underthese circumstances.
269. In view of the complications referred to above,
there can, of course. be no simple standard doseresponse relation for aberration yield in cases of partialbody irradiation. In those cases where defined areas
oi the bodv have been exposed to radiation for therapy
purposes and blood samples taken at short defined intervals after exposure, an increase in aberration yieldwith increasing skin dose has been noted. In thesecases. the relation between the vield of dicentric andring aberrations and ''skin dose"' may be almost linear(ykD 1· 2 ) or approximately follow a dose-squared(ykD 2 ) relationship.
270. In the whole-body studies in which patients
receh·ed from 17 to 50 rads from 2 MeV x rays. theyield of dicentric and ring aberrations increased approximately linearly with dose in blood samples taken
immediately after exposure (y = kD 1• 13 ) but increased
approximately in proportion to the square of the dosein samples taken twenty-four hours later (y = kDLSB).
This increased dose exponent in the later samples wasa consequence of significant increases in the yields obtained in two of the patients given a dose of 50 rads.

=
=

130

271. A number or laboratories are now studying
aberration yields in patients exposed to unifonn wholebody irradiation so that more data on the relationshipbetween aberration yield and radiation dose andquality should become available in the near future.
272. In the criticality accidents in which individuals
were non-uniformly e.xposed to mixed gamma-neutronradiation (paragraph 225), the data are naturally difficult to evaluate because of the complications of dose
distribution. radiation composition and RBE. However,in the individual that received a physically estimateddose of 47 rads ( 47 per cent of which was estimatedto be due to gamma rays) the yield of dicentrics andrings was 0.033 per cell in cells sampled between fourhours and four weeks after e_xposure and cultured forseventy-tw-o hours. This yield is somewhat lower thanthat (0.056 dicentrics and rings per cell) observed following forty-eight-hour cultures of blood from individuals exposed to whole-body irradiation of 2 MeV
x rays with a dose of 50 rads (paragraphs 210-213.
and table V.)
273. The limited data on whole-body exposure indicate that the aberration yield could serve a useful
purpose in dosimetry in such cases, but more information is certainly required. In this respect. we havepointed out that results from samples taken shortly
after ( 'vithin the first few hours or days) uniform
whole-body irradiation may not be influenced by the
distribution, etc. of the leucocytes at the time of irradiation or sampling. On the other hand. in the case offollow-up studies or where the first samples are taken
many weeks or months after an original exposure, theaberrations will be studied in a selected population of
long-lived cells. The primary aberration yields in surviving long-lived damaged cells will, however, be areflection of the close received in the original exposure(see the Hiroshima data referred to in paragraphs
240-246), and it may still be possible to make estimates of absorbed close from such cells. However, much
more work in this area is necessary, and a great deal
more information is required on the rate of decline
in aberration frequency with time and possibly, undervarious conditions. in the months or years following
exposure.
V. Possible hiological significance of theaberrationsA.

INTRODUCTIO~

274. The possible biological significance of chromosome aberrations, particularly with reference to theirpresence in germ cells, has been the subject of continued review bv the Committee. 2 •3 There are no direct
data on the genetic consequences of radiation-induced
chromosome aberrations in the germ cells of man,although information on the genetic consequences ofradiation-induced chromosome anomalies in laboratory
mammals and on constitutional chromosome anomalies
in man is available. Most of this information was
reviewed in detail in the 1966 report 3 so that only avery brief consideration of the little new material will
be attempted here.
275. The significance of chromosome aberrations insomatic cells has been the subject of much speculation.
The idea that aberrations might be causal factors incertain somatic disease states in man has been considered since the early suggestion by Boveri 308 that
abnormalities of chromosome constitution might besignificant causal factors in neoplasia. The fact thatneoplasms can be induced by ultra-violet ionizing radiations and chemical carcinogens. that these agents induce
chromosome aberrations and that such aberrations areto be observed in many tumours has naturally provided
a good deal of incentive to examine the possible relationships involYed. This has been further stimulated by
the more recent studies on virus-induced neoplasia inmammals and on virus-induced chromosome aberrations and also by the discm·ery of a specific chromosome change in bone-marrow cells of humans sufferingfrom a particular form of leuka:mia.
276. This section will, therefore, examine the mutational importance of chromosome aberrations in relation
to metabolic effects, cell killing, life-shortening. neoplasia and immunological deficiency, effects which haveall been suggested to be a consequence, at least inpart, of genetic imbalance.

B.

mosome constitution. since they ha\'e been reported
to occur with relatively high frequency in patient,; with''dangerous, violent and criminal propensities'' in statehospitalsz 22 and in prison populations. 3 ~ 3 • s24 Jacobs
et al. 32 ~ reported that 3 per cent of 314 patients in astate hospital in Scotland for the mentally sub-nonual
had an XYY chromosome constitution and that themean height of such patients was significantly in excessof the mean height of men in the hospital with an XY
chromosome constitution. It had previously been reported32;; that XYY individuals were fertile.
280. The possible significance of an e:-.1:ra Y chromosome with regard to criminal behaviour has not yet
been clarified. as a number of conflicting reports 326• 327
have appeared since the original suggestion of thisassociation. 322 Good estimates of the incidence of XYY
males in "normal" populations are only just becoming
available. It is reported3 ~ 8 that only one XYY male
was discovered in a sur\'ey of over 2,000 Edinburgh
males examined prior to the state hospital and prison
surveys. Recent studies in Canada329 indicate a frequency of XYY males among new-born of two in
1.000, and a frequency between one and two in 1.000
in the general population is suggested by a survey
carried out in France.aao

ABERRATIOXS IN GERM CELLS

277. The varieties and incidence of constitutional
aneuploidy in man were considered in detail in the
1966 report3 of the Committee, and there is only little
new information of relevance to be considered here.
In that report, reference was made to two well characterized syndromes causally related to a loss of autosomal chromosome material, the "cri du chat" syndrome(a deficiency in the short arm of chromosome 5) 309
and a congenital anomaly resulting from a deficiency ofthe long arm of chromosome 18 ( 18q - ) .310• 311
Another deletion syndrome, first reported by Lejeune
et al. in 1964, 312 was shown to be associated with adeletion of chromosome 21 (giving a partial monosomy), the syndrome showing many signs that appeared
to be the reverse of those characteristic of Down's
syndrome. Since that time, a number of cases of a
syndrome31 3 have been reported in \Vhich the affectedindividuals are completely monosomic for a group G
chromosome 11 11-17 s (generally considered to be chromosome 21). Further evidence is. therefore, accumulatingthat indicates that certain deficiencies in certain chromosomes and even the loss of a group G chromosome (presumably chromosome 21) may not be incompatible
with life. although all appear to be associated withgross physical and mental abnormality.
278. Consideration was also given in the 1966 report3
to the incidence of chromosome anomalies in spontaneous abortion and still-born, and reference
was specifically made to the work of Carr.314 • 315 In thiswork, it had been shown that the incidence of 45X
zygotes in man occurs at a frequency of about 8.3 per
1,000 conceptions; the majority of such XO conceptions abort, 316 one estimate indicating that only one
in fortv reach full term. 317 In the mouse, XO individuals ·are viable fertile females. and the average
incidence of this condition in the mouse is around sevenper 1,000 births318• 319 (annex C, table VI, of the 1966
report3 ). The probable incidence of the X 0 condition at
the time of conception in man and mouse may, therefore, be very similar. and it is of interest to note that,in the mouse, depending on the stage of developmentof the germ cells at the time of irradiation, it has now
been shown that the incidence of XO offspring may be
increased up to three or four times following an exposure of 100 roentgens of x rays. 318 · 321
279. In the last three years. considerable interest
has been aroused in individuals having an XYY chro-

281. Five sexually abnormal patients have been reported331-334 to contain chromosome complements with
an apparent dicentric Y chromosome, and it has beensuggested335 that some isochromosomes for the longarm of the X chromosome are dicentrics. These arereferred to here simply to point out that. in those rare
instances when the two centromeres of a dicentricchromosome are very clearly ju.'<taposed. the dicentric
may behave in a manner functionally similar to anormal monocentric chromosome at anaphase of mitosis
and, hence. be transmitted to all the descendant cells.
282. It was concluded in the 1966 report3 thatchromosome anomalies are to be observed in the somatic
cells of about ten per 1,000 live new-born infants andthat half of these abnormalities are accounted for bytranslocations. This estimate of five individuals withtranslocation per 1,000 live new-born is certainly aminimum estimate, and better estimates will not be
available until considerably more work on meiotic cells
is carried out. For this reason. it is reievant to consider here some new data 331 obtained from a highly
selected population comprising fifty males attending
a subfertility clinic.
283. These individuals were selected on the basis ofbeing chromatin-negatiye and having a sperm countof less than 20 106 per millilitre ; a proportion wereazoospermic. The obser\'ation of interest here is thatfour of the men were found to be translocation heterozvgotes on the basis of meiotic studies, but in only two
o-f these men could the translocations be discerned atmitosis in somatic cells. This result serves to underline the inefficiency of translocation detection in somatic
cells (see paragraph 38 where it is concluded that only
approximately 20 per cent of the radiation-induced
translocations can be detected in somatic cells) andpoints to a very high incidence of translocation heterozygosity in subfertile men. This observation is in
itself significant, since it is usually assumed that thesemi-sterility that is to be found in insects or laboratory
mammals that are translocation heterozygotes will beof little consequence in man because family-size falls
short of the fecundity of the species.
131with alterations in tryptophane metabolism,3 ·19 butthese may reflect a general alteration in control mechanisms for RNA or protein synthesis rather than involvement of specific structural loci.350 Abnormalities inhremoglobin and haptoglobin synthesis have been reported in constitutional anomalies invoh·ing group D
chromosomes (chromosomes 13-15), but again it is notclear whether structural genes or regulatory mechanisms
are involved. 351 • 353 In abnormalities of the human X
chromosome. specific metabolic alteration in affected
cells has not been recognized; apparently the abnormal X is consistently inactive genetically in such cells. 354
The metabolic effects of specific chromosome abnormalities in human cells thus remain almost totally

284. A study has recently been reported 338 on theincidence of constitutional chromosome anomalies inoffspring of mothers exposed to abdominal diagnostic
radiation (estimated maximum gonadal dose up to 7rad) prior to conception. and the authors interpret
their results as indicating an increased risk in suchoffspring. However. in this study. the frequency of
trisomic offspring in matched-control unirradiatedmothers was unexpectedly low. and, moreover, theincidence of still births in this control group was significantly higher than in the irradiated group. The Committee is continually reassessing this problem, butthese new data do not alter its opinions stated in the
1962 and 1966 reports. 2 • 3 that is, that exposure toionizing radiations might result in an increase in theprevalence of developmental congenital malformations
but that no quantitatiYe estimates can be made at this
time.

C.

Un1."IlOWn.

ABERRATIOXS IN SOMATIC CELLS

285. There has been much speculation concerning
possible relationships between radiation-induced chromosome aberrations in somatic cells and various diseases
in man. To date. however. almost no data havebeen obtained which permit precise statements relatinga particular chromosome aberration to a particular lesion
or a given yield of radiation-induced aberrations to a
predictable incidence of a specific disease in man orother mammals. Nonetheless, the postulated relationships between chromosome changes and such disorders
as neoplasia. auto-immune disease, and non-specificaging are worth revie\ving if only to stimulate furtherwork.

1. Somatic mutation and metabolic effects
286. One general hypothesis has suggested thatradiation-induced chromosome aberrations may be an
important mutational mechanism by which cellular alteration or depletion takes place in mammalian organs
with increasing age leading to disease or death, but
few corroborative data are available. 339 • 340 Deleterious
metabolic changes in mammalian parenchymal cells
surviving radiation injury have not been directly related
to induced chromosome changes. :Material for suchstudies is possibly available in the clones of cells withchromosome aberrations which repopulate the peripheral
blood and the hrematopoietic tissues of humans androdents surviving large doses of radiation,1 31 · 13 5 , 222, 3 41344 To date, however. the alterations of specific functional capacity of the enzyme activity of these chromosomally abnormal cells have not been compared tothose of non-irradiated populations. although superficially, at least, these ceils appear to be functioning
normally.
28i. In human chronic granulocytic leukremia. including those cases belieYed to be radiation-induced,
alkaline phosphatase levels are consistently reduced inleukremic leucocytes which also carry an abnormal
chromosome 21 (the Philadelphia chromosome) .345
V1Thether this enzymatic deficiency has any deleterious
effect on the cells involYed is not known.
288. Only very sketchy data are available on enzyme
changes associated with constitutional chromosome
abnormalities in man (these conditions were reviewed
by the. Committee in its 1966 report). 3 Trisomy 21(Downs syndrome) is associated \vith increased levelsof a number of leucocyte enzymes3 -! 6 • 3 ~ 8 as well as132

2. Somatic mutation and cell killing
289. With respect to the relationship between radiation-induced chromosome aberrations and cell killing,
more extensive information is available. These data
largely involve reproductive cell death in vitro (forexample. inability to complete mitosis or to proliferatesufficiently to produce a viable clone) as opposed tointerphase death (for example. death of non-proliferating cells), and this may be of importance in considering
the relevance of such studies to the effects of radiationin the human body. In general, the x-ray dose-response
curves have been similar for both cell reproductive
survival and production of chromosome abnormalities
in a number of cell systems. 149 • 355 and there is goodevidence that certain types of chromosome aberrations
may result in cell killing (paragraphs 24-50).
290. However, discrepancies have been observed insome experiments. both in the shape of the cunres, withconsequent considerations of recovery and repair mechanisms, and in the effect of modifying factors. Thus,in some instances, mechanisms not involving chromosome aberrations can apparently cause reproductive cell
death following radiation injury.14 9• 197 Both somatic
mutation and cell killing may underlie various somaticeffects of ionizing radiation in the mammalian organism,and chromosomal damage may be visible eddence of the
primary site of injury, but any quantitative statements,
based on presently available data. attempting to relatethese phenomena may subsequently prove to be oversimplified.
3. Somatic mittation and life-shoi-te11ing
291. There has been considerable speculation about
the role of chromosome aberrations as mediators of theaging process. The fact that radiation results in lifeshortening has long been well documented in experimental animals with extensive consideration of a variety
of radiation parameters. 356 but whether reduced life
span results simply from radiation-induced increases incertain specific diseases or from acceleration of some
generalized non-specific aging process has been debated.
·where radiation-induced life-shortening is clearly related to increased tumour incidence. the same conceptsof the role of chromosome aberrations would apply.
as will be discussed in the next section. If some nonspecific aging phenomenon is to be postulated, thepossible significance of chromosome aberratio'.1s depends
on the particular senescence theory being proposed.
If radiation-induced aging of tissues and organs is considered to result from mutational eYents leading todeath or diminished function of non-replaceable cells
or to reproductive death of cells needed for renewal,

1'chromosome aberrations might obviously play a central
role. Similarly. aging attributed to auto-immune mechanisms mediated by "forbidden clones" arising throughsomatic mutation (paragraph 317) could also have achromosomal basis.357· 359 On the other hand. theories
of aging based on e.."\:tracellular, degenerative changes.
such as increased cross-linkage of collagen, 360 would
seem unrelated to chromosome alterations.

295. It is certainly well documented that many
agents and conditions which produce chromosome aberrations also cause tumours. Not only ionizing radiation,
but also ultra-violet light, a number of oncogenic viruses
and several carcinogenic chemicals have been sho\'rn
to have this capacity (paragraphs 53 and 56).
296. Both DNA and RNA tumour viruses, includingRousvirus.
adeno-viruses.
SV40
andpolyoma, 92 • 95 • 101 • 3134 have produced chromatid aberrations in human and animal cell cultures (paragraph56). It is of interest that. while the Schmidt-Ruppin
strain of the Rous virus causes chromatid abnormalities
in human leucocyte cultures and also tumours in experimental animals, the Bryan strain of the Rous virus,
under similar circumstances. produces neither chromosome changes nor neoplasia. 103
297. Benzene, perhaps the best documented leukremogenic chemical in man. also appears capable ofcausing chromosome changes in human cells.3 65 • 367
Carbon tetrachloride and other hepatic carcinogens
have been shown to produce both tumours and chromosome aberrations in the rodent liver.368• 370 Although anumber of other mutagenic chemicals have recently
been shown to yield chromatid alterations in humancells (paragraph 53). their carcinogenicity remains to
be demonstrated.
298. It is also now clear that there is an increased
incidence of leukremia and lymphoma in several rarehuman diseases (Bloom's syndrome. Fanconi's syndrome, and perhaps ata.xia-telangiectasia and xeroderma
pigmentosum) in which there is a constitutional propensity for increased spontaneous chromosome aberrations observed as chromatid aberrations in leucocvte
cultures • 34 o, 371• 372 Such data I involvina
a varietv ·ofhagents and conditions. have suggested that genetic
damage, as indicated by chromosome aberrations,might be a common mechanism by which most, if notall, carcinogens act.
299. The frequency of chromosome abnormalities
in tumours has been used to support this argument.
Certainly most neoplasms, radiation-induced or not, doshow chromosome aberrations. Numerous studies inrecent years3 ' 3 • 381 have demonstrated that. e..xcept forsome human acute leukremias and some virus-induced
rodent leukremias, nearly every mammalian tumour. by
the time it reaches macroscopic size, is characterized bychromosome changes.

292. Attempts to approach these questions experimentally have been relatively few. Curtis. in a seriesof studies in mice. 340 extending concepts and techniquesof earlier workers. 339• 361 • 362 has demonstrated a direct
correlation between life-shortening and frequency ofchromosome aberrations in liver cells. This relationshipwas observed after various types of radiation exposureand also in strains of mice differing nahirally in life
span. Curtis has cited these data to support the somatic
mutation theory of aging. postulating radiation-induced
life-shortening as the direct result of radiation-induced
chromosome aberrations. This conclusion has been challenged on various grounds. including the lack of correlation between liver chromosome change and hepatic
dysfunction and the failure of chemicals which produce
liver chromosome aberrations in mice to induce lifeshortening.303 Kohn:!oo h;is suggested that the observed
aberrations might be the consequence, rather than thecause. of altered metabolic states or of diseases associated with life-shortening.
293. Comparable data are not available in man, although an increased incidence of aneuploidy with age
has been observed in peripheral blood lymphocyte cultures from a large human population (paragraph 78).
The incidence of such alterations is always low. however. and no increase in structural aberrations has beenreported. In the absence of a generally accepted definition of biological aging. the relationship between radiation-induced life-shortening in e..xperimental animals
and age-related processes in the human population isdifficult to assess. Even if such a relationship is accepted as valid. additional experimental evidence is required that specifically relates radiation-induced aging
to chromosome aberrations as opposed to other possible
mechanisms.

~

4. Somatic mutation and ncoplasia
294. The most e..-..;:tensive evidence relating radiationinduced chromosome changes in somatic cells to significant biological effects in man would appear to bein the area of neoplasia. It has been recognized formanv years that ionizing radiation produces chromosome aberrations and also tumours in both man andanimals. Since chromosome changes have been
demonstrated in nearly all tumours studied bymodern cytogenetic techniques. older concepts concerning the causal role of chromosome aberrations in
neoplasia 30 s have been revived. and it has been tempting to speculate that radiation-induced tumours result
directlv from radiation-induced chromosome aberrations. -Much evidence has accumulated which at leastindirectly supports this hypothesis. but. at the sametime. it has proved extremely difficult to demonstrateprecise quantitative relationships. following radiation,
between aberration frequencies and subsequent tumour
incidence. In the following paragraphs, the evidence
relating chromosome aberrations and tumours is briefly
summarized, and some limitations of available data andconcepts are indicated.

300. Furthermore, the neoplastic cells bearing theseabnormalities frequently appear as stemlines or clones.
particularly in the leukremias. but in many solid
tumours as well. The entire neoplasm often consists ofa single clone with all cells showing the same alteration in karyotype. or of a small number of clones,
usually with related chromosome changes. This clonal
phenomenon has suggested that a tumour may consist
entirely of the progeny of a single aberrant ce!L 344 • 382
a cell having a proliferative advantage as a result of its
altered karyotype. Subsequently. additional clones may
appear and even come to predominate as further karyotypic changes confer additional selective advantages.
Sequential studies in human neoplasms have supported
this concept of the important role which chromosome
alterations play in the progressive development oftumours.382, ass301. However. it is still debatable whether thechromosome changes observed in mammalian neoplasms
are primary or secondary phenomena and whether they133are involved in the initiation of the tumour or only inits subsequent progression. The alteration obseffed m
one tumour is usually different from that observed in
the next even when the two neoplasms are clinicallyand histologically identicaJ.3i 3 • 381 In general, each typeof mammalian neoplasm has not been characterized bya specific chromosome abnormality. This. plus theoccurrence of some leukaemias and e,·en a few solid
malignancies 373 · aso. 38 i
without any demonstrable
chromosome changes. has led many investigators, but
not all .a'"· 3 i 9 to conclude that the chromosome changes
seen in most tumours are secondary phenomena superimposed on an already neoplastic process. This conclusion does not rule out genetic alteration. or alterations,388 as the initiating event in neoplasia; it simply
suggests that it may be submicroscopic.
302. The first example of a neoplasm it.•ith a specificchromosome change is human chronic granulocytic
leukremia. where, in 90-95 per cent of the typical cases,
the karyotype is characterized by the same abnormality,
namely, a chromosome 21 lacking approximately halfof its long ann, the so-called Philadelphia chromosome( Ph1 ). This is not an inbom change but rather anacquired abnormality. 405 ordinarily limited to the neoplastic hremopoietic cells (myeloid. magakaryocyticand erythroid) and not present in lymphocytes or othertissues of the body. 374 (In irradiated individuals without leukcemia. the Ph 1 chromosome has occasionallybeen observed in extrarnedullary tissues (paragraph309), and in two incompletely documented instances it
may have occurred as a familiar abnormality) .389, 390
303. ·when present in neoplastic cells, the Philadelphia chromosome is associated with chronic granulocytic leukremia. although it has been. on rare occasions,observed in related myeloproliferative disorders such aspolycythaernia vera. 389 It persists throughout thecourse of the disease with additional karyotypic changes
frequently superimposed in the late stages. During
remission, when immature myeloid cells disappear fromthe peripheral blood. the Ph 1 may not be demonstrable
in peripheral leucocyte cultures, but it is still observablein dividing marrow cells.an
304. The constancy oi the association behveen the
Philadelphia chromosome and chronic granulocytic
leukcemia has suggested that, in this instance. a chromosome change is a primary phenomenon and that theoccurrence of this aberration in a marrow stem cell isdirectly involved in the initiation of the neoplasm.
305. A sin1ilar suggestion might also be made forthe relatively few other human and aninml tumours inwhich some of the cases have apparently demonstrated
a characteristic chromosome change.39;;· 397 These include such neoplasms as \Valdenstrom's macroglobulinemia, Burkitt lymphoma. multiple myeloma, certain human ovarian and testicular tumours and several
leukcemias in rats and mice. In the first three instances,for example, a number of cases have shown a large
abnormal ·'marker'' chromosome, similar to a group A
chromosome (chromosome 1-3) in the Burkitt and\Valdenstron1s tumours. and similar to a large D chromosome (chromosome 13-15) in multiple myeloma.
306. In none of these various human and animal
tumours, however. has the constancy of the particularchromosome change noted in each instance approachedthat of the Ph1 in chronic granulocytic leukremia. For
most of these tumours, the abnormality characteristic
for the particular neoplasm has been found in lessthan half of the cases examined. In these ciisorders,
therefore. it is much more difficult to postulate apri.n1ary role for the chromosome alteration observed.
307. In truth, of course, one cannot currently makean absolute statement about the primary nature of any
chromosome change in any tumour. Only when specificchromosome changes can be related to specific metabolic abnormalities (and we also know what metabolicchanges are critical in the initiation of neoplasia) willsuch a statement be made with assurance. In ourpresent state of knowledge, one may perhaps only
suggest that the changes observed in most neoplasms.
because of their apparent inconstancy from case tocase, seem likely to be evolutionary phenomena important in progression, while the Philadelphia chromosome.. because of its constant and specific association
with chronic granulocytic leukaemia, is probably involved in the initiation of that disease.
308. Radiation-induced tumours have proved to be
no exception to these general concepts of the significance of chromosome alte:-ations in neoplastic cells.
Apparently all, or nearly all, radiation-induced tumours
show chromosome changes. but, as with most othertumours. these vary from case to case.aH. 345, aaG, am.
398 399•
In addition, not all cell clones with radiationinduced chromosome aberrations are necessarily neoplastic. Heavily irradiated humans and animals havebeen found to have clones of cells marked by radiation-induced chromosome aberrations which are apparently non-neoplastic and functionally normal and persist
in their kematopoietic tissues for Jong periods of timeafter recovery. 131· 135 • 2 ~ 2 • 341·3 H The frequency of such
non-leukaemic clones has made it impossible to use chromosome studies to predict which irradiated individuals
will eventually develop leukaemia. although in nonirradiated individuals a clone of marrow cells with achromosome abnormality appears to be a good indication that a preleukremic disorder is in transition to afrank leukcemia.::.u309. The Philadelphia chromosome is present inthose cases of chronic granulocytic leukaemia whichappear to be radiation-induced. 345 • 4 oo 4 o4 as well as inthose with no radiation history. In irradiated individuals without leukremia, the Ph1 chromosome has
been observed (both in single cells and in clones) inperipheral lymphocytes and in skin cells, 13 5, 4os but,with one possible exception. 223 it has not been reported
in the bone marrow. Since only in a marrow stem
cell does the Ph1 appear to have a role in initiating amalignancy. those indidduals in which it is found insuch a cell will be of particular interest to foilow todetermine if leukremia subsequently develops.

310. In addition to these considerations. lack ofquantitative conclusions on the role of chromosomechanges in radiation carcinogenesis has also resulted
from the difficulty of obtaining precise dose-response
curYes for either aberrations or tumours, although. ingeneral. both show an increasing incidence with increasing dose. The induction of tumours by ionizing
radiation is a complex problem which was extensh·ely
reviewed by the Committee in its 1964 report.3 92 In
addition to the important modifying effects on tumour
incidence of such variables as radiation quality. dose
rate. and non-uniform dose distribution over the bodv,
one mnst also consider possible indirect mechanisms
i1n-oh·ed in radiation carcinogenesis. 40 ~ -.no Activationof oncogenic Yirus. depre;:sion of the immune response.

1

alteration of hormone levels und non-specific cell killing
and regeneratiYe stimulation may all be radiationinduced effects in t.he body which '.llay an importantrole in carcinogenesis.
311. At the level of the indi\·idual cell, one must,of course, consider not only direct genetic damage byradiation but also the possibility of the cell pre\'iously
damaged by radiation being more susceptible to attackby an oncogenic Yirus. 408 Such a radiation-damaged
cell may also have a greater propensity ior subsequent·'spontaneous" chromosomal rearrangements duringmitosis or other "spontaneous" genetic alterations 286
and may as well be more liable to reproductive cell
death:109 The evidence incriminating viruses in many
radiation-induced tumours has recently become particularly strong, 410 but all of these Yarious factors havecontributed to make it e.."\:t:-emely difficult to predict
with confidence the tumour incidence subsequent to agiven radiat'.on exposure.
312. Similarly, radiation quality and dose rate areimportant modifying factors, although some in 'j,•ivo
data and many iu vit1<0 data now make it possible torelate quantitatiYely radiation doses to chromosomeaberration yields in human, as well as in animal. cells.
In addition, the quantitative results of in vivo studies
may also be affected by the frequency of cell division in
the irradiated host of the particular cells studied.!?18• 302
313. Taking these variables together, it is perhaps
not surprising that no data, either animal or human,are yet available on which to establish precise quantitative relationships involving a given radiation e.."<posure. the resultant aberration yield and the number
or kind of tumours to be subsequently expected. It isof interest that the RBE for neutrons versus x rays
appears to be comparable for tumours·107 and chromosome
aberrations. at least in some circumstances. Also, several
studies have indicated that Thorotrast, through its
primary localization in the reticulo-endothelial system,
produces both leukremias and hemangioendotheliomas
as well as demonstrable chromosome alterations inlymphocytes, whereas radium, localizing in bone, produces osteogenic sarcomas more frequently than leukremias and yields fewer chromosome changes in lymphocytes.179, 224, 2as, 411, u2314. However, very few experiments have approacheddirectly the quantitative relationships between chromosome aberrations and tumour incidence. In one study
that compared the effects of high and low dose radiation on the mouse liver, 413 the chromosome aberration
yield was higher, but the subsequent incidence of hepatomas lower. after exposure to a high dose rate thanit was after exposure to a low dose rate. The authors
postulated a cell-killing effect of the high-dose-rateradiation. associated with visible chromosome aberrations, which removed potentially neoplastic cells fromthe surviving population.

5. S 0111atic mutation and immwiological deficiency
316. Both mt;tational events and cell depletion havebeen cited as possible mechanisms for immunologicaldisorders and deficiencies, and both could result fromradiation-induced chromosome aberrations.
317. Although radiation can produce acute immunological deficiency through its cell-killing effects on thelymphoid system. 4 1-! ho•v much of this effect is mediated through chromosome aberrations is not knmYn.
~or is it known if the tendency towards increasing
immunological deficiency with a<lYancing age is due toeither cell depletion or genetic alterations in the
immune system. In a system, hO\vever, in which cell
division is apparently required for the initiation of its
specific functions. deleterious effects of chromosome
aberrations can be readily visualized.
318. It has been postulated that human auto-immune
disorders might stem from somatic mutations in thelymphoid system. For instance. statistical study of theage and sex distribution of rheumatoid arthritis. lupus
erythematosus, multiple sclerosis and other possible
auto-immune diseases has led Burch. 3ii 7 extending theconcepts of Burnet415 and others, to suggest somatic
mutation as a source of "forbidden clones" of lymphoidc~ll~ capc:ble of reacting against '"self" and producing
clmtcal disease. Although such theories do not necessarily require either radiation as the mutagen or visible
chromosome aberration as the form of genetic change,such possibilities obviously exist.
319. Among the group of rare human diseases whichhave recently been shown to be characterized by increased chromosome fragility, 366• 371 it is of interest
that at least one of them, ataxia-telangiectasia, is also
associated with immunological deficiency.H 6 Immune
defects have not been prominent in either Fanconi's
anremia or Bloom's syndrome. the other two disorders
showing excessive spontaneous chromosome breakage
in lymphocyte cultures. but neither disease has been
extensively studied from this standpoint. 417
320. It must be stated, in summary, that at present
there is neither epidemiological nor experimental evidence directly relating immunological disorders or deficiencies in man or animals to chromosome aberrations known to be induced by radiation.

D. CONCLUSIONS
321. In relation to aberrations in germs cells. there
is little to add to the conclusions arrived at by theCommittee in its 1966 report. 3 The only point worthyof extra emphasis here is the need for more information on human meiotic cells so that better estimatesof the spontaneous level of translocations in man anda better understanding of their genetic consequences
can be obtained.
322. At the somatic cell level, although attempts at
relating certain constitutional chromosome abnormalities in man with specific metabolic deviations fromthe normal are continually being made, as yet there
is little direct information relating alteration or lossof gene function with alteration or loss of a particularchromosome or chromosome segment. The demonstrations of the existence of clones of cells containing
abnormal karyotypes in the peripheral blood leucocytes of individuals previously exposed to radiation
now offer an opportunitv for detailed metabolic studies
on a wide variety of chromosomally aberrant cells. No

315. Smnmarv. In both man and animals. radiationinduced chromosome aberrations and radiation-induced
neoplasms regularly appear together. The chromosomechanges may represent visible evidence of intracellularalterations involved in the neoplastic process. However,
the mechanism of radiation carcinogenesis is still far
from clear, and the number or type of radiation-induced
chromosome aberrations observed in an irradiated individual cannot at present be confidently used to predictthe risk of his later developing a neoplasm, except
perhaps in the case of chronic granulocytic leukremia.
135such studies have yet been reported. but work in thisarea will not only clarify the variety of detrimental
effects that are to be expected to result from thepresence of aberrations in somatic cells but will also
provide information for the genetic mapping of thehuman chromosome complement.
323. It is to be expected on a priori grounds thatcertain kinds of chromosome aberrations will be celllethal and so will contribute to cell depletion. However,
it is not possible to state in quantitative terms the relative importance of the variety of chromosome aberrations in contributing to cell killing in human somatic
cells.
324. If one attempts to relate specific somatic effects, such as immunological deficiency or life-shortening, to chromosome changes, the problem becomeseven more complex:. Hypotheses have been advanced
relating these disorders to radiation-induced somatic
mutations. but strong, supportive, experimental evidenceis lack-ing. and non-mutational mechanisms have also
been advocated.
325. The significance of the role played by chromosome aberrations in the aetiology of neoplastic diseaseis also far from clear. In the case of chronic myeloid
leulaemia, the evidence strongly implicates a specificchromosome aberration (the Ph1 chromosome) as playing a significant role in the initiation of this disease.
Although the possibility remains open that other specific chromosome abnormalities may be involved withother types of neoplastic change, the presence of awide variety of chromosome aberrations in most tumours, and their complete absence in a few, militatesagainst the notion of a simple causal relationship. The
inconsistencies that have been observed may well bea consequence of there being many different pathways
that lead to a common biological end-point. If neoplasia is a multi-step process, the possibility exists thata radiation-induced alteration of the genome may provide a more favourable environment for the development of additional essential alterations. through increased susceptibility to an oncogenic virus, or greater
liability to •·spontaneous'' mitotic errors, or throughsome other mechanism. Until the basic mechanismsof radiation carcinogenesis are better understood, it
will be difficult to define more clearly the role ofchromosome aberrations in this process.

VI. ConclusionsA.

APPLICATION OF ABERRATION YIELDS FORBIOLOGICAL DOSIMETRY

326. The possibilities of estimating the absorbed
dose received by individuals through biological rather
than (or where possible, in addition to) physicalmethods have an obvious interest, particularly in thosecases where accidental e..xposure has occurred. Attempts to make rapid estimates of dose based on film
badges worn by radiation workers are liable to error,
especially if assumptions have to be made regarding
shielding by objects intervening behveen the exposed
individual and the radiation source. Moreover. thedegree of radiation exposure is frequently not uniform
over the whole bodv. and this leads to complications
if the dosimetric dedce is in essence a point receiver.
327. In cases of criticality accidents, physical measurements made at a later time usuallv involve an experimentally contrived incident in an attempt to obtaina reasonable approximation of the dose received during
136

the accident. Alternative biological techniques that havebeen studied from the point of view of their use as
dosimeters have so far proved disappointing. A number of possibilities have been investigated.27 6. us including studies of metabolic products excreted in the urine
and of the frequency of lymphocytes with bilobed or
double nuclei or of neutrophils carrying particles withspecific staining properties. However. none of thesebiological parameters has been shown to vary consistently ·with radiation dose.
328. Considered in qualitative terms, there is no
doubt that the presence of a significant number ofchromosome aberrations, more particularly of the dicentric- and ring-type, may be indicative of a previous
radiation exposure. This follows, since it has beenshown that the spontaneous in vri.Jo occurrence of adicentric or ring aberration is an extremely rare eventin the blood cells of the many hundreds of individuals
that have been e..xamined from this aspect. Such aberrations occur, at most. once in every 2.000 cells takenfrom unirradiated subjects. whereas 20 ± 5 such aberrations would be expected in 2.000 cells from an individual, or individuals. that had received a whole-body
x-ray dose of 10 rads or its equivalent. The frequencyof these aberrations would appear to be uninfluenced
by previous exposure to infectious agents or (except
in certain specific cases) to chemical agents. The frequency of dicentric and ring aberrations in a population
or in an individual is. therefore. a good qualitative
screening test for a previous radiation history. Moreover. because of the long life span of at least a proportion of the lymphocyte population. aberrations ~n. beobserved in these cells many years after the ongmal
radiation exposure.
329. A good example of the use of chromosomeaberration vield in this qualitative context is afforded
by a recent" report16 of an examination of an individual
who. on the basis of film badge measurements. wasassumed to have received a dose of 300 rads. It waspossible to state after chromosome analysis that a significant dose of radiation had not, in fact. been received
by this individual. since no chromosome abnormalities
were seen in cultured blood or even in direct preparations of bone-marrow cells. The badge was. therefore,
assumed to be faulty or to have been e>..-posed independently of the individual. In circumstances such as
this, where there is no way of reconstructing the conditions under which the exposure occurred and therebyof checking the validity of the film badge information.
information from chromosome analysis can be of greatvalue.
330. The main advantages oi a biological method as
opposed to physical methods of measuring absorbed
dose in man would follow from the directness andpermanent availability of the biological method. fromthe fact that information on and extrapolation from therelative biological efficiencies of a spectrum of radiations received in a mixed exposure is not required andbecause a biological dosimeter is in itself one stepnearer to the problem of assessing immediate damage
and future risk. In the particular case of a chromosome-aberration dosimeter, it may be added that chromosome aberrations are believed to be responsible for acertain proportion of the ceil killing following radiation exposure: they are mutational events and they are
generally believed to play some part at least in thedevelopment of late somatic effects. Howe.-er. it shouldbe emphasized here that. in the current state of our1knowledge. aberration vields cannot be related to abiological effect and th~t the presence of a low frequency of aberrations in the peripheral blood cells of
an individual can. therefore. in no way be regarded as
constituting a medical risk.
331. The starting points for considering the possible
use of chromosome aberrations as opposed to otherpossible biological systems as indicators of dose are(a) the high sensitivity of the human chromosome complement and the high resolution of the method. since
the effects of doses of around 5 rads or less can bedetected. and ( b) the fact that the yield of chromosome aberrations induced is closely correlated in a
specific manner with the dose of radiation received.
332. It was concluded (paragraphs 138-144) that,from the cytological viewpoint. by far the best system
for use in chromosome-aberration dosimetry was theperipheral blood leucocyte system, provided chromosome-type aberrations were scored, preferably at thefirst mitosis following radiation exposure. Moreover,there is little doubt that a measure of the radiation
damage incurred by leucocytes (more particularly thesmall lymphocytes) that are widely distributed throughout most tissues and areas of the body should be a good
indicator of the effect of radiation on the individual asa whole. There are. however, a number of disadvantages to the system, although it should be noted thatmany of these disadvantages also apply to physicaldosimetric systems.
333. Dosimeters require calibration, and the calibration with the peripheral blood leucocyte system willalmost certainly require an accurate correlation between
the response of these cells in vitro and their responsewhen exposed in viva. This cross reference to thein vitro system seems necessary, since opportunities foranalysing cells from individuals exposed to various
levels of whole-body radiation are fortunately rare.
Thus. good in ,,.jvo dose-response curves, particularly
from healthy individuals accidentally exposed to radiation. will be difficult to obtain. However. recent studies(paragraphs 199-205), including the use of the technique of extracorporeal irradiation, indicate that the
response in ·vitro is equivalent to that obtained in viva.
although further 'vork here is clearly necessary.
334. In the in 7'-ilro work, despite the fact that accurate physical estimates of dose are available and that,within most laboratories, a repeatable quantitative relationship between aberration yield and dose is always
found, differences in results are, nevertheless, to befound between laboratories in both the absolute aberration yields at given dose levels and in the shape of thedose-response curves. Some of the explanations forthese differences have now become clear. vVhen factors
such as radiation quality, methods of irradiation andduration of culture are taken into account, good interlaboratory agreement is obtained. Further studies onthese factors are, however, necessary before the prospect of obtaining standard sets of coefficients relating
aberration yield to radiation dose can be realized.
335. In vitro studies on the influence of dose rate
on aberration yields induced by x rays have been
carried out bv a number of laboratories. and it can beconcluded from the results obtained that there is little
dependence of aberration yield on exposure time overtimes ranging from one to thirty minutes. This is. of
course, a feature of some importance in attempting torelate aberration yields with dose. Similar studies with137

~ission neutrons show that the yield of aberrations
mduced by these particles is. as expected, independentof dose rate and exposure time. and that fission neutrons (mean energy 0.7 ~IeV) and fast neutrons (2.2
:MeV and 14 l\Ie\i ) are two to five times as effectiYeper rad as 250 kV x rays in inducing aberration;;.

. ~3<'.i. In the i11 vh•o work. in only one study have
mdrv1duals been exposed to accurately measured uniform whole-body radiation (2 l\IeV x rays). In thisstu~y. the yield of dicentric and ring aberrations inperipheral blood cells was shown to increase approximately linearly with exposure over the dose ranaestudied (0-50 rad) if samples were taken immediately
after radiation exposure. the coefficient of yield forthese aberrations being 0.001 per cell-rad. Vniform
whole-body exposure to a penetrating radiation, so that
all cells receive a similar dose, is the ideal state for the
purpose of biological dosimetry. Such a state is . ofcourse, rarely encountered. and most of the in vi110
work that has been done relates to partial-bodv or nonuniform exposures, and it is here that the grea'test dosimetric problems arise.
337. In the case of acute partial-body exposure. theaberration yield in peripheral blood leucocytes will
depend upon a variety of factors. These include (a) theamount of radiation energy deposited in that area of the
body that is exposed and the duration of the exposure;( b) the area and volume of the body that is irradiated:( c) the proportion of peripheral blood leucocytes thatare exposed to radiation and the time that they spend
in the irradiated area during exposure: ( d) the proportion of leucocytes (lymphocytes) in extravascular areas
within the exposed region : ( e) the amount of exchangeof lymphocytes between peripheral blood and the extravascular pools : and (f) the time at which blood issampled after the radiation exposure. Since there isevidence indicating that there is an appreciable e.xchangeof lymphocytes between the blood vessels and e~-tra­vascular sites in the first few hours following irradiation (paragraph 204). it is not yet possible to make~ny reasonable estimates of physical dose from ananalysis of chromosome-aberration yields in partialbody exposures.

338. The question of what is meant by dose. interms of biological effect or biological consequence, isitself not very meaningful in the case of partial-body
irradiation. This follows. not only because aberration
yields may be very much dependent upon the regionsof the body that are exposed. but also because aberration frequency cannot be related to any given somaticeffect. It should be noted, however, that the difficulties
encountered with the type of biological dosimeter underdiscussion may be far less than the difficulties encountered with a point receiver measuring physical dose.
Studies on individuals who received partial-body exposures at varying dose levels over similar regions (andareas) of the body have. in all cases. revealed a strict
proportionality between physically measured sh.in doseand aberration yield. However, it is not possible simply
to relate the aberration yield to skin dose and area orregion exposed. since, because of cell mixing. themeasured aberration yield varies with the time at whichblood is sampled in the first twenty-four hours afterradiation e.."<posure.
339. It can onlv be concluded from this discussion
that, in the case of partial-body exposure, a great deal
more knowledge is required about the structure of thepopulations (and subpopulations) of small lymphocytes
and about the distribution. mobility and longevity of
the cells before it is possible to equate an aberration
yield ohsen,ed at any given time to the dose absorbed
in the lymphocytes. It should be noted that, in any
case, within the limitations of eJ\.-isting knowledge. any
given aberration yield in these cells can only be related
in physical terms to an "equivalent whole-body dose''.
340. It should be re-emphasized here that the difficulties that confront us in the case of partial-body
exposure do not exist in the case of uniform wholebody exposure. With uniform whole-body exposure,there is no doubt that the yield of chromosome-type
aberrations in peripheral blood Jeucocytes can be used
as an accurate measure of dose received. In the caseof accidental. non-uniform. whole-body exposure. theaberration frequency in peripheral h!ood cells can yield
but little information on the degree of non-uniformityof the exposure but may more readily provide anestimate of an "equivalent whole-body dose". 423 Careshould be taken to note, however. that dose estimates
require the use of dose-yield kinetics obtained inin vit1·0 studies and possibly also information on theform of the distribution of aberrations ·between cells.

B.

ASSESSMENT OF RISKS

341. There is no new information about the estimatedfrequency of aberrations induced in germ cells byradiations and the consequent risk to individuals andto offspring. These risks have been fully discussed
in the 1966 report, 3 and the only new point to add isthe preliminary observation of a possible association
between translocation and subfertility in the hwnan
male. In this connexion, however, further data are
required before any real assessment can be made.
342. In somatic cells, infom1ation on the yields andtypes of chromosome aberration does not as yet provide either a new approach to or a better estimate ofrisk, except in one specific case. \Vith existing information, knowledge of chromosome-aberration yield in
peripheral blood leucocytes does not enable us to makeany quantitative statement regarding the risk of developing neoplastic disease. immunological defects andshortening of life span. etc. As a consequence. no information of clinical significance can be obtained from thepresence of aberrations. Little can. therefore, be added
to the statements on assessments of risk of somatic
disease that have already been made by the Committee
in its earlier reports. From the point of view of assessing risks, the only direct use of the aberration yieldis when this is the only parameter from which a"physical" estimate of the dose can be obtained. Clearly.
where physical dose estimates can be made. the aberration yield may serve as a valid supplement to thephysical data.
343. The exception mentioned in the aboYe paragraphrelates to the association between chronic granulocytic
leukremia and the presence of the Ph 1 chromosome incells of the bone marrow. The presence of such anabnormal chromosome in bone-marrow cells is, apartfrom one possible exception.~~s always associated witha blood dvscrasia-in almost all the cases with chronic
granulocytic leukremia and. in the others. with adisorder such as polycythaemia yera. It has not yet
been shown whether a Ph 1 chromosome can be observed in bone-marrow cells prior to the developmentof overt h.::ematological disease. Ho\\·eyer. its presence138

in a bone-marrow cell of any irradiated individual must,on present knowledge, be taken as indicating an extremely. high risk for the indiyidual of developing
leuka:m1a. On the other hand, it should be stressedthat Ph 1 -like chromosomes have been observed insingle cells and in clones in peripheral blood lymphocytes and skin cells of irradiated individuals. but theirpresence in such cells has not. to date, been associatedwith any kind of neoplastic change.

C.

RECOMMENDATIO:N"S FOR F'URTIIER STLllY

344. Human cytogenetics is still a relatively new andvigorously de,·eloping field. and it is evident that there
are large gaps in our lmowledge both of the responseof human chromosomes to radiation and of the consequences of radiation-induced chromosome aberrations
in human germ cells and somatic cells. Some of themore immediate requirements and questions that needanswering and some of the more general longer-term
problems that require further attention are outlined in
the follchving paragraphs.
345. Further studies should be undertaken on humanmeiotic cells, particularly from the point of getting
better information on the frequency and possible genetic
consequences of symmetrical spontaneous aberrations
that cannot be detected in somatic cells.
346. A better understanding of the effects of culture
conditions on aberration yield in peripheral blood
leucocytes should be achieved. the aim being to develop
a standardized technique for interlaboratory use so thatstandard coefficients for the yields of the various aberration types can be obtained.
347. Further in vitro studies on the effects of dose
rate and exposure time with radiations of low LET
and on the relative efficiencies 'vith which aberrations
are produced by radiations of different quality should
be made.
348. Very much more information is required todefine to what extent there exists a range of sensitivity
to aberration induction in a human population andregarding the influence of age on response. Thesestudies can be carried out by measuring the responseof blood cells to in vitro exposures.
349. Further work should be undertaken on therelation between in vivo and in vitro responses. including studies on laboratory mammals and. where possible. on humans exposed to e.....:tracorporeal radiation
treatments.
350. Further information is needed on the influenceof Yarious patterns of non-uniform radiation exposureand a ~etter understanding required of the lymphocyte
populat10ns in the body, their distribution. age structure and turn-over.
351. Further data on the rate of decline of aberration yield with time following exposure under a variety
of radiation regimes should be acquired. Attentioi'1
should be deYoted, in this respect. not merely to longterm changes occurring OYer periods of weeks. months
or years after exposure. but to changes that occur inthe first few hours and days. Consideration should also
be giYen to the importance of immune response andother possible factors in such \YOrk.
352. \:Vhere possible, the utmost effort should bemade to obtain data from individuals undergoing uniform whole-body irradiation.

353. Further work on the qualitative and quantitative differences between aberrations induced by chemical and infectious agents in relation to ionizing radiations and studies on the response of peripheral blood
leucocytes (and bone-marrO\v cells) to radiation whilst
in the S and G2 phases of the cell cycle should also be
undertaken, since current information on chromatidtype aberrations induced in these cells is minimal.
354. Future advances in all aspects of population
cytogenetics will be greatly facilitated by the introduction of automation into the processes involved in cytogenetic analysis. In the case of sun'eys of populations
exposed to mutagens that induce chromosome abnormalities, although a considerable amount of valuable
information will continue to be obtained through conventional methods. automatic systems should provide
powerful tools for the cytologist, and their cb·elopment
should be fostered.
355. A continued effort should be directed at
attempts to relate both yields and types of chromosome
aberrations to specific somatic diseases. In this connexion, follow-up studies on individuals exposed toradiations are desirable, and continued studies of irradiated individuals possessing Pb 1 -like chromosomes andother clonal changes in their proliferating cells are essential.
356. Further studies on the possible relationshipbetween the incidence of aberrations on the one handand neoplasia on the other. both in man and in experimental animals. are required. In this connexion. it

should be pointed out that there is no direct information whatsoever on a possible synergistic interaction
between oncogenic viruses and radiation-induced
chromosome damage. Experimental attack on this problem is now possible.
357. There is an absolute dearth of information
relating radiation-induced aberrations to biological endpoints. It should, therefore. be emphasized that theexistence of clones of cells containing abnormal karyotypes in the skin. bone marrow and peripheral blood
leucocytes of individuals previously exposed to radiation now offers an opportunity for detailed metabolic
study on a wide variety of chromosomally aberrant
cells. In this context. human-animal hybrid cells 41 9, 4 20
may provide a useful tool. Such studies will provide
valuable genetic information as well as information on
the detrimental effects to be expected from certain kindsof aberrations.
358. A follow-up of the studies indicating a possible
difference in the in vivo response of lymphocytes tochromosome-aberration induction in different laboratory
mammals should be made.
359. Further studies on the radiation response indifferent tissues and the radio-sensitivitv of different
cell types, together with the possibilitY of utilizing
materials other than blood cells to measure aberration
yields, should be carried out.
360. Studies on the frequency, types and consequences of constitutional chromosome anomalies in man
should be continued.

139

T AllLE J.

FRF.QUENCY OF D!CENTllIC ABERRATIONS IN "NORll!AI." SUBJECTS NOT lcXl'OSEDTO RADIATION OTIIF.R THAN ROUTINE IJIAGNOSTIC EXPOSURE

The forty-seven individuals in col1111111 c were patients with anlcylosing spomlylitis, and the samples were taken prior to any
therapy, hut shortly t1fl1!r the individuals had rcccivecl diagnostic radiation exposures. The thirty-eight individuals in colum11 b
were individuals from a general population and serve<l as controls for 11opulalion c
Nonnauet a1.10."

Autlim·s.......

.f>.
0

Isl1i/1ara
mid

Kumator·i11 5

Eva usand
Speed"'"

,,

a

23

20

200

?

94

438

38

47

11

Number of cells scored .....................

5,784

2.875

2,400

2,295

8,847

12,420

1,060

2,269

9,510

Numhcr of 11icc11trics observed ...............

0

()

0

0

0

7

0

3

2

f'rcqucm:y nf dicentrics per cell ..............

<tin
5.8 x JO:!

<I in
8.8x103<I in
1.8 x JO:l

<1 in
1x103

TADLE I I.

<I in
2.8 x 10:1

<Jin
2.4 x 103

<I in
2.3x10 3

PHYSICAi. DOSES AND ESTJM ATED OOSES AASF.ll ON vmr.n OF' DlCENTIHC AmmRATJONSIN PERIPHERAJ, LEUCOCYTES IN BLOOD CELLS 01' I'ATJENTS227

ll-After several passages through the irradiator

Total calcu!tlttd
f>/1ysical dostr (rad)A fo vitro studies: blood sampled after asingle passage at a flow rate of 3.0 to
15.7 1111-min ......................... .

4500
56Sa
29Sn
14811

B Ju vivo studies: blood sampled immediately after a four- to eight-hour extracoqiorcal irradiation ................. .

12Qb
12Qb
240\J

Total dose estimatedfro111 dr'ccutric aberTntion

J.80

frrq11c11cy/>er cc11I
co11/i e11ce)

440--490
510-565245-310115-175
145e
!8()c

230e

11 Physical dose calculated on the basis of now ralc and volume of blood passed through irradiator.
" Physical dose estimated on the basis of blood volume, flow rate and duration of irradiation.
e Estimated dose based on frequency of dicentric aberrations in 200-300 metaphase figures inblood leucocytes immediately after completion of irradiation.

!'°

Afij•atan•cNumber of individuals in sample ............

A-Exposed to one passage through a nosr-ooy extracorporeal irradiator

L

Sasakiaud

Court Browt1''"Bloom
et a1.1ss

Nor·mau"<I in
0.76 x 1()3

<l in
4.7x10aTABLE III.

ABERRATION YIELDS FOLLO\\'IXG A SDIGLE PARTIAL-BODY EXPOSURE OF
AXKYLOSIXG SPOXDYLITIS P.\TIEXTS 70 X R.-\YS (250 KV)IGA cells-Undamaged cells
B cells-CeJls containing chromatid-type aberrations
Cu cells-Cells with unstable chromosome-type aberrations (rings, dicentrics, fragments)
C5 cells-Cells with stable chromosome-type aberrations
A cells

Time of.$G11Jf>/i11g

Si:iiidosein rads

Total
postc:posure cells
in hours analysedMada!

Na11-modal

No.

No.

Per
cent

No.

Per
cent

No.

Per
cent

Ring Plus
dicentrics

c, cells

C. cells

B cells

No.

Per
cent

No.

Fragments

Per
cent

No.

Per
cen:

100

0
24

300
300

256
266

13
8

4.3
2.7

22
14

7.3
4.7

6
5

2.0
1.7

3
7

1.0
2.3

3

1.7
1.0

2
2

0.7
0.7

150

0
24

400
500

349
408

14
18

3.5
3.6

16
30

4.0
6.0

13
32

3.3
6.4

8
12

2.0
2.4

8
20

2.0
4.0

7
13

1.8
2.6

200

0
24

250
300

205
240

12
7

4.8
2.3

11
21

4.4
7.0

17
27

6.8
9.0

5
5

2.0
1.7

15

16

6.0
5.3

5
13

2.0
4.3

250

0
24

300
300

218
221

8
7

2.3

7-·'

33
30

11.0
10.0

24
32

8.0
10.7

17
10

5.7
3.3

20

3.7
6.7

14
20

4.7
6.7

300

0
24

400
350

319
240

14
23

3.5
6.6

14
13

3.5
3.7

41

67

10.3
19.1

12
7

3.0
2.0

28
55

7.0
15.7

19
27

4.8
7.7

700

24

100

'29

29.0

30

30.0

18

18.0

T.-\BLE IV.

5

11

ABERR.o\TIOX YIELDS FROM SEVEN PATIENTS EXPOSED TO SINGLE WHOLE-BODY
DOSES OF X RAYS (2 11'fEV) 16

A cells-Undamaged cells
B cells-Cells containing chromatid-type aberrations
Cu cells-Cells with unstable chromosome-type aberrations (rings, dicentrics, fragments)c. cells-Cells with stable chromosome-type aberrations

Dose inrads

Time ofsampling
pastexposurein hours"0"
control

Total
cells
ana/;y,.d700

c. cells

B cells

A cells
Mada/

Rings plus
diccntrics

C, cells

Fragments

Non-modal

No.

No.

Per
cent

No.

Per
cent

Na.

Per
cent

No.

Per
cent

Ko.

Per
cent

No.

Per
cent

618

45

6.4

27

3.9

5

0.7

5

0.7

2

0.3

3

0.4

16
16

2.7

2.8

2.7

18
11

3.0
1.8

3.1
3.1

3i
53

4.6
6.6

27

'29

3.4
3.6

-7
:>.-

25

0
24

600
600

489
499

38
31

6.3
5.1

27
30

4.5
5.0

31
23

3.8

15
17

50

0
24

800
800

632
637

56
34

7.0
4.3

26
28

3.3
3.5

61
76

7.6
9.5

25

141

r
_;,

?__ ;,-TABLEv. FREQUENCIES OF DICENTRICS PLUS RIXGS Di PERIPHERAL BLOOD CULTURES TAKEN
U.C\IEDIATELY, OR TWEXTY-FOUR HOURS AFTER, WHOLE-BODY EXPOSt.:RE OF PAT.IBN"TSTO 2 ME\i X RAYS AT THE DOSES INDICATEI>2'1

Dose

Case

tiumbcr

3
8
5
7

1
0
0
0
0
0
0
0
3•

6
8
5
4
14
10

l

9

0
1
0
0
0
0

11

,-

50
50
50
50
50
50
17
28
36
40

12
13
14
15
16

5

25
25
25
25
25
_;:,

II

u lum~strtc.tmetst(200 cells}

Co11trol(1?0 cells}

(rad)

2
3
4
5
6
7
8
9
10

I mm,diately
after trtatmnd(200 cells)

TCC~ved

2

9

4

3
15
14

15
9
15
7b

9
2

1

4
7
7

1

5
8

a One cell, containing a dicentric and a tricentric, has been scored as three dicentrics.
b Seven rings and dicentrics in seventy-five cells analysed.

TABLE VI.

DICExTRICS PLUS RINGS FOLLOWIXG ia ;nlro AXD in -.·ivo (WHOLE-BODY) IRRADIATIOXRadiation
quality

AutM1's

Evans212················· ·······

240 kV x rays

Gooch (!f a/.2W ...................

250 kV x rays

Langlands c:I a/.!!il ...............

2 1'.fe V x rays.... ······· ..

160 kV x rays

Norman and Sasaki!!·IO ...... ······

1.9 Me V x rays

Mouriquand et a/.138

Vander Elst et af.230 .............
Visfeldt2~2.......................

T ADLE VJI.

Case

220 kV x rays
GOCo ;>,.rays

Dose

in rads

lrradiatio11

Sa111plit111
timt
in hours

Culturetime
in hours

25
50
r
-:>
50
25

in "itro54

in ;.,•ilro

72

-:>

in :•i<•o

,-

50
50

r
-:>
50
50
?"
-:>

50
50

0
24
0
24

53

iu dtro72

in vitro50

in ;1ilro72

fo r:itro

48

FREQUENCIES OF DlCENTRICS PLUS RI:>GS PER CELL IN PERlPHER-~L BLOOD LEUCOCYTESAT VARIOUS INTERVALS AFTER EXPOSURE OF INDI\"lDUALS TO MIXED GAM!UA AXD
FAST NEUTRON RADIATION (CULTURES GROWN FOR SEYEXTi"·T\VO HOURS: OBSER\'ED
NU!>IDERS IN PARENTHESES)

E.rtimateddose (rad)Time after exposure
29 IUOHf/:sl!.?'8_.\
B

42 111011t/isl:O

7 years""

0.01 (!)
0.04 ( 4)0.02 (2)365
0.01 ( 1)0.02 (2)0.03 (3)270
0.02 (2)0.166 (24)0.018 (2)c339
0.04 ( 4)0.023 (2)D
0.07 (2)327
0.013 (1)E
0.03 (3)0.05 (5)
236
F
0
0
68.5
0
0
68.5
0
G
0.01 (1)H
22.8
0
X o dicentrics or rings were observed in 900 cells from control indh·idualsl'.!ll142

Diccntric

pl11s rinfis
per ct l

0.065
0.15
0.003
0.017
0.029
0.025
0.05
0.07
0.025
0.05
0.014
0.009
0.021
0.02

1

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