Estimations of Radiation Risk for the Population of Contaminated Territoryof Belarus
Vladimir A. KNATKO, Mikhail M. KOMOCHKOV* and Alla E. YANUSH
Institute of Radiobiology of the National Academy of Sciences of Belarus
Kuprevich str. 2, 220141 Minsk, Belarus: V_Knatko@radbio.bas-net.by*Joint Institute for Nuclear Research, 141980 Dubna, Moskow Region, Russia

Abstract
Accumulated doses for adult residents of contaminated regions of Belarus were calculated for
different periods after the Chernobyl accident. The average dose during the period of 1986-2001
was evaluated to be 50 mSv and 39 mSv in the southern and the eastern contaminated regions ofBelarus, respectively. Data obtained were used to evaluate the excess of the relative risk of cancer
mortality for adult residents of contaminated regions in terms of Two Defence Reactions (TDR)
model developed on the basis of modern results of radiobiological investigations. The results showthat the excess of the relative risk (ERR) of cancer mortality as a result of the Chernobyl accident isabout 5–6% during the whole life. The results were compared with risk values estimated in theframework of the 1990 Recommendation of the ICRP. The ERR values obtained in terms of TDR
model is about six times larger than values calculated on the basis of ICRP recommendations.

Introduction
The radionuclides released during the Chernobyl accident contaminated large areas in the southern
and the eastern parts of Belarus. At the present time more than 200 000 people live in regions where thelevel of 137Cs deposition density is higher than 185 kBq/m2. According to dose assessments [1, 2],radiation exposure of the population of the contaminated territory is characterized by low dose
distribution. Doses (without the thyroid doses) received by the majority of people in the first decade after
the accident were smaller than permissible dose limit for life — 70 mSv. Note that doses from external
and internal exposure in the first decade are equal to about 60% and 90% of doses for life,
correspondingly (see [3]).
To evaluate risks of damage at low doses an extrapolation of high dose results to low dose ones isimplemented. At present the non-threshold linear theory is most often, as well as officially, used to makeevaluation of radiation risks. However, experimental data accumulated during the last years show
evidence against the linear extrapolation of high dose effects to low dose range (see [4, 5] and references
therein). In this connection it is of interest to evaluate radiation risk for the population of the contaminatedregions of Belarus by using dose-effect relationship based on recent data on radiation-induced effects. The
present work gives radiation risk estimations obtained by using nonlinear dose-effect relationship derived
in terms of the Two Defence Reactions (TDR) model [6] developed on the basis of modern results ofradiobiological investigations. The calculation results are compared with the risk values estimated in theframework of the 1990 Recommendation of the ICRP [7].
Methods and Materials
Accumulated doses for adult residents of contaminated regions of Belarus
Estimation of radiation risk requires the knowledge of accumulated dose for residents ofcontaminated regions. An expression for evaluation of the average accumulated dose for adult residents ofn-th settlement is written152kBq/m 237-185
VITEBSK185-555555-1480
>1480
MINSK

MOGILEVGRODNO

GOMEL
BREST

Chernobyl

Figure 1: Belarus territory contamination with 137Cs.
tf

Dn (∆t = t f − ti ) = ∑ d (t ) S n (t ),t =t i(1)where ∆t is the time period after the accident (ti=1986), Sn(t) is the mean 137Cs deposition density in the nth settlement; d(t) is average annual dose of adult population exposure normalized to 137Cs deposition
density. Note that according to measurement results the concentration of 137Cs activity in organism ofadult inhabitants is in average about 1.5 times larger than that in organism of children of 10-15 years old[8]. As a consequence, radiation doses for adult population are higher than doses for children. Estimationsof Sn(t) were obtained by using of empirical data on 137Cs deposition density in settlements [2]. Values of
the average annual normalized dose d(t) for tf ≤ 1995 were taken from [9], and the sum of two exponents
was used for fitting and extrapolating these data for tf > 1995.
The distribution of 137Cs deposition density as of 01.01.1995 on the territory of Belarus is shown inFig. 1. One can see that the contaminated territory consists of the southern and the eastern regions (spots).
It is necessary to note that the regions mentioned above are characterized by different values ofnormalized dose d(t). The southern spot, closely situated to Chernobyl NPP, includes the Belorussian
Polesie area, where coefficients of 137Cs transfer from soil to agricultural plants are several times larger
than the coefficients obtained for other contaminated regions. Therefore the doses from 137Cs perroral
intake and respectively normalized average annual dose for residents of the southern contaminated region
are larger than doses in the eastern region (see [9]).
Taking into account the mentioned dose difference, calculations of accumulated dose Dn(∆t) werecarried out separately for adult residents of the southern and eastern contaminated regions. Values of theaccumulated dose Dn(∆t) estimated for the case of 137Cs deposition density Sn(t) = 370 kBq/m2 are shown
as example in Fig. 2. Dash line corresponds to the accumulated dose calculated by multiplying ∆t by
constant value of the permissible annual dose of exposure for population (1 mSv per year) (see [7]).
According to Fig. 2, doses accumulated by inhabitants of the eastern and the southern contaminatedregions during the first five years after the accident are about 5 and 10 times higher, correspondingly, than153

Dn, 70mSv
65
60
55
150
45
40
35
30
25

220
15
10
3

501986

1988

1990

1992

1994

1996

1998

2000

2002

2004

time after the accident, years

Figure 2: Accumulated doses Dn for residents of the southern (1) and the eastern (2) regionswith 137Cs deposition density Sn(t) = 370 kBq/m2. Dash line (3) corresponds to thepermissible annual dose: 1 mSv [7].
permissible level 5 mSv; doses for the 15 years period are about 2(4) times higher than permissible level
15 mSv.
The results show that about 80% of dose accumulated for the 15-years period were taken during thefirst five years after the accident. It means that the intensity of exposure in first years after the accident
was several times higher than permissible level.
Using assessments of Dn(∆t) and the population data of settlements, distributions of accumulated
doses for ∆t = 5, 10, 15 years were deduced for adult residents of the southern and the eastern regions
where the level 137Cs contamination is more than 37 kBq/m2 (see Fig. 3). Note that the percentage of adult
population in settlements was supposed to be constant for the period under consideration. The number ofthem are 103,680 and 153,380 in the southern and the eastern regions, respectively (see [2]). According tocalculation results, average values of the accumulated dose for residents of the southern region are about
30% higher than those for the eastern region. As it is shown in Fig. 3, the dose accumulated during tenyears and fifteen-years periods following the accident are about 20% and 25% larger than dose for the firstfive-years period, correspondingly. In the second five-years period the average annual increase of the dose
was about 1.3 mSv/y. During the third 5-years period the dose increase rate was about two times smaller:
0.6 mSv/y.
Dose-effect relationship and radiation risk estimation in terms of model of Two Defence Reactions
Dose-effect relationship (DER) is the basis of radiation risk evaluation. Presently the official form ofDER is the linear non-threshold (LNT) relationship, which was accepted by UNSCEAR [10] andICRP [7]. It is the foundation for all present-day norms, standards and rules of radiation safety. In theformulation of UNSCEAR, the main merits of LNT hypothesis are its simplicity and agreement with mostconcepts and numerical data. At the same time modern results of epidemiological observations andradiobiological researches show difference between a variety of DERs obtained for stochastic
radiobiological effects at low dose range (see, for example [11]). The epidemiological data of the surveyof Japanese survivors cohort prove the non-linear dependence of the excess of the relative risk on dose at
low range [12, 13]. It is very likely that DER depends on kind of radiation damage, type of biological154

1986-1991
Adult population% 353025
Av=40.0 mSv
20151050A

Adult population%B
Av=31.1 mSv

1986-1996
25

Av=36.5 mSv

Av=47.0 mSv

201510501986-2001
25

Av=38.8 mSv

Av=50.0 mSv

2015105c

0D ,mSv
10 30 50 70 90 >100
110

cD ,mSv
10 30 50 70 90 >100
110

Figure 3: Distributions of accumulated doses for adult residents of the southern (A) and theeastern (B) contaminated regions.
object and exposure conditions.
In this paper the Two Defence Reactions (TDR) model [6] was used for radiation risk assessment.
The basis of the choice are the results of the observations of low dose radiation damages in cells ofdifferent biological objects: HT29 human lines [14], V79 Chinese hamster lines [11] and root meristem ofbarley grains [15]. The results obtained in the above mentioned works show nonlinear increase of damage
with dose growth (see Fig. 4–7). Data presented in these figures were treated in the framework of TDR
model [16]. According to the results, TDR model satisfactorily describes DERs for considered biologicalobjects.
The TDR model is based on the assumption that the cell response to irradiation is regulated byconstitutive and adaptive mechanisms. On the level of organism the response of the immune system toirradiation also consists of two components: innate and adaptive. The defence reaction at the organism
level acts in addition to the cell level defence. The differential equations describing interaction of two
components of defense system are similar for both the cell and the organism levels. Curves in Fig. 4–7 are
obtained through the fitting procedure of observed DERs by using the function found by solving the TDR
model equations.
Taking into account the results of TDR model application for description of DERs, the excess of therelative risks (ERR) for the population of the contaminated regions of Belarus was estimated on the basisof this model and on the dose distributions given in Fig. 3. The ERR for the group of N residents isERRN =∑ ERR N = ∑ ERR N ,N
∑Niiiiiiii(2)155

14% inactivated cells HT291210864
20040

2060

80

100Dose, cGrFigure 4: Dependence of the yield for human tumor inactivated cells of HT29 line on X-ray dose [14].

Aberrant cells, %2520

151050010050150200Dose, cGy

Figure 5: Dependence of the frequency of Chinese hamster cells with chromosome
aberrations on γ-radiation dose from 60Co [11].
54F, %

32100204060

80

100Dose, cGy

Figure 6: Dose-effect relationship in root meristem cells of barley grains. F - output ofaberrant cells [15].
where ERRi is the excess of the relative risk for i-th subgroup of residents irradiated with the dose Hi.
According to the TDR model,

ERRi =(1 − Wc )(1 − f i ),Wc(3)
where Wc is the spontaneous risk of mortality from cancer without irradiation (background), accepted tobe 0.28 [12], and function fi is determined as:
156

0.10
0.08

ERR

0.06
0.04
0.02
0.00
0510

1520

Dose, cGy
Figure 7: Excess of the relative risk (ERR) of mortality from all cancer other than leukemia forJapanese A-bomb survivors (men) who were 20-40 years old at the time of bombing [6].

f i = exp(− µ n H i ) +νµ n{exp[− µ a ( H i − H t )] − exp[− µ n ( H i − H t )]},
µn − µa(4)with (Hi-Ht) ≥ 0 (see [6]).
The model parameters µn, µa, ν, Ht are found from the best description of empirical risk data by theTDR model. Parameters µn and µa characterize two types of defence (response) reaction of biologicalobjects: constituent (or innate) and adaptive (or inducible) radiosensitivities, correspondingly. Theparameter ν characterizes capabilities of the adaptive protective mechanism. The dose Ht is the thresholdof inducible reparations action. Various ranges for the mentioned characteristics are considered in [16].
Epidemiological data for adult survivors of Hiroshima and Nagasaki, who were 20–40 years old at
the time of atomic bombing [4], were treated in the framework of TDR model for evaluating the model
parameters. The fitting results of ERR of mortality from all types of cancer except leukaemia for Japanesesurvivors are presented in Fig. 7. One can see that the magnitude ERR is characterized by non-linear
growth at low dose range. The behavior of ERR is similar to the dose-effect relationships observed in cells(see Fig. 4–6).
Results and Discussion
Obtained model parameters were used for the assessments of ERR of cancer mortality for the adultresidents of the contaminated regions of Belarus on the basis of accumulated doses Dn(∆t). The results ofcalculation of the excess of radiation risk for residents of the southern and the eastern regions arepresented in the Table 1 in comparison with the values obtained in the framework of ICRP
recommendations.
The results show that excess of the relative risk of cancer mortality for the adult residents ofcontaminated regions of Belarus as a result of Chernobyl accident is about 5–6% during the whole life.
The main contribution to ERR was provided by the doses accumulated during the first five years after theaccident; further increase of ERR is 10–15%. The difference between ERR values predicted for thesouthern and the eastern contaminated regions is smaller than 10%. ERR values obtained in terms of TDR
model is about six times larger than values calculated on the basis of ICRP recommendations [7].
On the basis of the ERR assessments obtained in terms of TDR model the average annual excess ofcancer mortality for adults can be estimated as 6% / 50 years ≅ 0.1 % in a year. As the excess of cancer
mortality is proportional to excess of cancer morbidity the average annual excess of cancer morbidity isexpected about 0.1%. Statistical data on cancer incidence in Belarus for the period 1991-2000 (see [17])157Table 1: The excess of the radiation risk (ERR) for the residents of the southern (A) and theeastern (B) regions.
Period after the accident, years

Model5101520TDR

0.05540.05900.06100.0620ICRP 1990

0.00930.01060.01120.0113TDR

0.05040.05530.05780.0582ICRP 1990

0.00770.00870.00920.0093

ABshow that the average annual excess of cancer morbidity among adult residents of contaminated regions isabout 1.2%. Note that the same excess of cancer morbidity was observed for uncontaminated regions. So
according to assessments evaluated by using of ERR estimations and data from [17] one might expect thatthe contribution of the Chernobyl accident to annual excess of cancer morbidity would be about 10%. It
should be stressed that the latent period for solid cancers is about 10 years and the annual excess of cancer
morbidity increases with age. On this reason the mentioned value of the Chernobyl contribution to cancer
morbidity for the period 1991–2000 would be considered as overestimated assessment.
It is necessary to note that the quantity ERR in question is general index including all types of solidcancer. An excess of some types of cancer, which are characterized by the small weight coefficient, may
be considerably larger than the above-mentioned assessments. For example, in the period 1991–2000 thethyroid cancer morbidity of the population of the easten contaminated region increased about 7 times(see [17]). For this reason it is of great interest to perform long–term epidemiological investigations inBelarus to deduce dose-risk relationship for some types of cancer.
Conclusion
Accumulated doses for adult residents of contaminated regions of Belarus were calculated for
different periods after the Chernobyl accident. The results show that about 80% of dose accumulated forthe 15-years period were taken during the first five years after the accident. Distributions of accumulated
doses were deduced for adult population of the southern and the eastern contaminated regions. Averagevalues of the accumulated dose for residents of the southern region are about 30% higher than those forthe eastern region.
Modern data of epidemiological observations of Japanese survivors cohort and radiobiologicalresearches of low dose damages in cells of different biological objects were analysed in terms of Two
Defence Reactions (TDR) model. The results show that this model satisfactorily describes non-linear
dose-effect relationships for considered biological objects. The TDR model parameters obtained bytreating of data on mortality of adult survivors of Hiroshima and Nagasaki were used for calculation of theexcess of the relative risk for adult residents of contaminated regions of Belarus on the basis of theaccumulated dose distributions. The results show that the excess of the relative risk of cancer mortality asa result of Chernobyl accident is about 5–6% during the whole life. ERR values obtained in terms of TDR
model is about six times larger than values calculated on the basis of ICRP recommendations.
References[1] Catalog of Exposure Doses fot Citizens of Settlements of Republic of Belarus, Minsk (1992), (in russian).
[2] Catalog of Annual Exposure Doses for the Population of Belarus. Report of the Institute of Radiation Medcineand Endocrinology, Minsk (1995), (in russian).
[3] Levels of Irradiation and Effects as a Result of Chernobyl Accident. Report of ICEAR-2000, Application 7,Moskow, 41 (2001), (in russian).

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[4] M. M. Komochkov, On Similarity of Dependences of the Dose-Stochastic Radiobiological Effect at Cell andOrganism Level. Preprint of JINR, P19-99-78, JINR, Dubna (1999), (in russian).
[5] E. A. Krasavin, R. D. Govorun, Radiobilogical Research in JINR. Preprint of JINR, P19-2000-245, JINR, Dubna(2000), (in russian).
[6] M. M. Komochkov in Fundamentals for Assessment of Risks from Environment Radiation. "A Model of Two
Reactions as Dose-Stochastic Radiological Effect Relationship and Estimation of Radiation Risks" edited by C.
Baumstark–Khan et al. (Kluwer Academic Publishers, Netherlands, 1999), pp. 25–34.
[7] International Commission on Radiological Protection. 1990 Recommendation of the ICRP, Pub. 60, Vol. 21, No.
1–3 of Annals of the ICRP (Pergamon Press, Oxford, 1990).
[8] Health Consequences of the Chernobyl Accident. Scientific Report. International Programme on the Health
Effects of the Chernobyl Accident (IPHECA) edited by G. N. Souchkevich and A. F. Tsyb (World HealthOrganisation, Geneva, 1996), p. 219.
[9] V. F. Minenko, V. V. Drozdovich, S. S. Tretyakevich, A. V. Ulanovskiy in Analytical and informational bulletin"Medical and biological aspects of ChNP accident". “Irradiation of Population of Belarus after the Chernobyl
Accident: Collective Doses and Prognosis of Stochastic Effects”, 50 (1996), (in russian).
[10] Report of United Nations Scientific Committee on the Effects of Atomic Radiation, Medical Radiology andRadiation Safety, 46, 28 (2001), (in russian).
[11] N. L. Shmakova, E. A. Fadeeva, E. A. Krasavin, M. M. Komochkov, O. A. Abouzeid, Cytogenetic Effects ofLow Dose Radiation in Chinese Hamster Cells, Nucleonika, 44, 539 (1999).
[12] D. A. Pierce, Y. Shimuzu, D. L. Preston, M. Vaeth, K. Mabuchi, Studies of the Mortality of Atomic BombSurvivors. Report 12, Part I. Cancer: 1950--1990. Radiation Research, 146, 1 (1996).
[13] D. L. Preston in Proc. of an International Conference on Low Doses of Radiation: Biological Effects andRegulatory Control. Invited papers and discussions. "Low Dose Radiation and Human Health. Risk Estimates",(IAEA, Vienna, 1998), p. 217.
[14] M. C. Joiner, P. Lambin, E. P. Malaise et al, Mutation Research, 538, 171 (1996).
[15] S. A. Geraskin, V. G. Dikarev, A. A. Udalova, N. S. Dikareva, Regularities of Induction by Low Doses ofIonizing Radiation of Cytogenic Damages in the Cells of Barley Root Meristem. Radiation Biology.
Radioecology, 39, 373 (1999), (in russian).
[16] M. M. Komochkov, Dose Dependence on Stochastic Radiobiological Effects in Radiation Risk Estimation.
Preprint of JINR, E19-99-295, JINR, Dubna (1999).
[17] Belarussian Cancer Registry. Malignant Neoplasms in Belarus: 1991–2000, Minsk (2001).

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