A radioecological model for thyroid dose reconstruction of the Belarus population following the Chernobyl accident

A radioecological model was developed to estimate thyroid exposures of the Belarus population following the Chernobyl accident. The input of the model includes an extensive data set of the ¹³⁷Cs activity per unit area deposited during the Chernobyl accident, the rainfall data for different regions of Belarus, the ¹³¹I/¹³⁷Cs ratio in the deposit and the start of the grazing period in Belarus in April/May 1986. The output of the model is the age-dependent thyroid exposure due to the intake of ¹³¹I with fresh milk. Age-dependent average thyroid doses were assessed for selected regions of Belarus. The maximum thyroid doses were estimated for the inhabitants of Gomel oblast where the highest deposition was observed among the regions considered here. The lowest doses were estimated for Vitebsk oblast with the lowest level of depositions. The mean exposures for the oblasts of Grodno, Minsk, Mogilev and Brest were very similar. The results were compared with estimations of thyroid exposure that were based on ¹³¹I measurements in human thyroids, and they are in good agreement. The model may be used for the assessment of thyroid doses in Belarus for areas where no ¹³¹I measurements are available.     

The effect of Chernobyl accident on the development of malignant diseases--situation after 20 years

The accident that occurred at the Chernobyl Nuclear Power Plant in 1986, released large quantities of radionuclides--among them radioiodine--into the atmosphere, thereby raising public concerns about its influence on thyroid structure and function, especially the development of malignancy. There were even reports about 700 deaths due to thyroid carcinoma in Russian Federation, Ukraine and Belarus, resulting from the accident. In this review we discussed the incidence of thyroid cancer in different parts of the world, especially in heavily contaminated countries, as Ukraine and Belarus, and the possible link between radioisotope activity in the thyroid and the development of malignancy. The study carried out in Minsk showed 40-fold increase of the incidence of thyroid cancer in the years 1986-1994, in comparison to the period 1977-1985. An increase of the incidence of thyroid cancer has generally been observed in many countries after the Chernobyl accident. We focused on the factors that may have an influence on this phenomenon, especially diagnostic tests, health care, social and environmental factors, like iodine level in water and soil. The results of molecular biology studies, e.g. RET translocation in carcinoma type RET/PTC1 in elderly and RET/PTC3 in children, and expression Ax1 and Gas6 in children were reviewed as well. We also mentioned other thyroid diseases, like nodular goitre, cysts, the disturbance of thyroid function and autoimmunity, possibly linked to the radiation after Chernobyl accident. Data obtained from the regions near Chernobyl showed no increased risk of other types of malignancy (leukaemia, Hodgkin's and non Hodgkin's lymphoma) in 1986-1996. In this article the epidemiology of thyroid diseases in Poland was also reviewed.     

Monte-Carlo prediction of changes in areas of west Cumbria requiring restrictions on sheep following the Chernobyl accident

Following the 1986 Chernobyl accident radiocaesium levels in sheep meat in some upland areas of the United Kingdom were above the national intervention limit. West Cumbria was one of these areas and restrictions are currently still in place. In addition to deposition from the Chernobyl accident, Cumbria has been subject to radiocaesium deposition from atmospheric nuclear weapons tests, the 1957 Windscale accident and routine releases from the Sellafield nuclear reprocessing plant. A Monte-Carlo approach has been used to try to predict areas in west Cumbria where radiocaesium activity concentrations in lamb meat would require the imposition of restrictions at different times after the Chernobyl accident. The approach models the transfer of radiocaesium from soil to vegetation, based upon soil organic matter, and from vegetation to lamb meat. Spatial inputs are soil organic matter and total post-Chernobyl (137)Cs and (134)Cs deposition; a ratio of Chernobyl (137)Cs to (134)Cs deposition has been used to differentiate Chernobyl and pre-Chernobyl (137)Cs deposition. Comparisons of predicted radiocaesium transfer from soil-vegetation and the spatial variation in lamb (137)Cs activity concentrations are good and predicted restricted areas with time after Chernobyl compare well to the restricted areas set by UK government. We predict that restrictions may be required until 2024 and that in some areas the contribution of pre-Chernobyl (137)Cs to predicted lamb radiocaesium activity concentrations is significant, such that restrictions may only have been required until 1994 as a consequence of Chernobyl radiocaesium deposition alone. This work represents a novel implementation of a spatial radioecological model using a Monte-Carlo approach.     

Micronucleus frequency in Gomel (Belarus) children affected and not affected by thyroid cancer.

Cytogenetic monitoring was carried out on a group of children from Gomel (Belarus), one of the areas most severely affected by radioactive contamination following the accident at the Chernobyl nuclear power plant, in 1986. We performed the micronucleus test (MN) in binucleated lymphocytes of 42 children (mean age: 11+/-2.34 years), 16 of whom were affected by thyroid gland tumor. Thirty healthy children living in Pisa (mean age: 14.96+/-2.17 years) were enrolled in the study as controls. Thyroid tumor affected children living in Gomel showed a statistically significant increase in the frequency of micronucleated cells as compared with the healthy children from Pisa. Moreover, a significant correlation was found between MN frequency and both the presence of tumor and higher 137Cs contamination. In addition, higher 137Cs contamination was more frequently observed in tumor affected children. These results suggest that the increased MN frequency is attributable more to 137Cs contamination rather than to the presence of the tumor itself.     

The radiological situation before and after Chernobyl disaster

The nuclear reactor accident, which occurred on 26 April 1986 at Chernobyl, has been one of the greatest ecological disasters in human history. In our study we discussed the most recent data on the accident, and the natural and synthetic sources of radiation. According to the recent data, the air at Chernobyl had been contaminated with about 5300 PBq radionuclide activity (excluding rare gases), including 1760 PBq (131)I and 85 PBq (137)Cs. The highest radiation received by the liquidators (0.8-16 Gy), lower doses were received by the population which was evacuated or inhabited the contaminated areas (in which the level of (137)Cs activity deposited in the earth was 37 kBq/m(2)). In the European countries the highest mean radiation dose per year for the whole body in the first year after the accident was in Bulgaria (760 microSv), Austria (670 microSv) and Greece (590 microSv), while the lowest radiation dose was observed in Portugal (1.8 microSv) and Spain (4.2 microSv). In Poland the mean effective equivalent dose resulting from Chernobyl accident was 932 microSv and is close to the limited dose permitted in Poland, equalling 1 mSv/year. The highest radiation dose to thyroid was received by inhabitants of the states previously known as Bielskopodlaskie, Nowosadeckie and the north-east region of Poland. Lowest dose was received by inhabitants of the areas previously known as Slupski and Rzeszowski.     

Estimation of 131I thyroid doses for the evacuees from Pripjat

  The thyroid doses of 49 360 inhabitants of Pripjat evacuated after the accident at Chernobyl were reconstructed. During their evacuation most of the evacuees passed through highly contaminated territories. The evaluation of a large-scale public survey showed that only about 50% of the evacuees had left the contaminated areas within 5 days and that 30% of them stayed there for more than 30 days. As a first step, the model of dose estimations was improved, and thyroid doses were assessed for the group of evacuees for whom the ¹³¹I activity in the thyroids was measured. The ¹³¹I incorporation during the first 5 days after the accident was described by a single-intake model (inhalation); later incorporations were assumed to be proportional to the radioiodine activity in milk. As a second step, the correlation between the calculated doses and individual parameters (place of residence in Pripjat, intake of stable iodine, and age at the time of the accident) was described by an empirical equation. This equation was applied to all evacuees who completed the questionnaires of the public survey. Previous dose assessments were found to overestimate the thyroid doses especially for the younger evacuees. On the basis of these estimations, collective doses and the resulting radiation risks for thyroid cancer were assessed for different age groups. Received: 28 November 1995 / Accepted in revised form: 6 March 1996     

Clinical immunological disorders in children from various observation cohorts exposed to radiation factor during various stages of oncogenesis

The immune status disorders and features depending on the radiation impact type in various cohorts of radiation observations long after the Chernobyl (CNPP) disaster and the possible role of these disorders in development of chronic somatic pathology in children are shown. Lymphocyte depletion, T-cell immunity component disorders in the form of cell contraction with CD3, CD4, CD8 markers and the B-cell immunity component disorders in the form of reducing the quantity of CD10, CD23 marker cells were observed in children subject to combined chronic irradiation by 131I, 137Cs, 90Sr radionuclides. The descendants of irradiated parents (the 1st generation; children of the Chernobyl accident consequences liquidators, children of the citizens of radiation contaminated territories with various 137Cs levels) had immunity disorders of different type. A change in the total amount of NK-cells (CD16(+)-lymphocytes) is the general sign for all radiation risk groups; however, people subject to direct radiation impact demonstrated reduction of the antitumor protection potency, whereas descendants of irradiated ones demonstrated its activation with typically increasing number of CD16(+)-lymphocytes. In all radiation risk groups, a tendency to reduction of a number of cells involved in the leukocytal activation with the "pluripotential activation" marker (CD38 marker cells), proliferating cells (CD71 marker cells) and the increase of relative amount of cells with apoptosis marker (CD95(+)-lymphocytes). Immune disorder markers under the radiation impact in various cohorts of children's observation are suggested: antigens: CD4, CD8, CD10, CD23, CD16, CD38, CB71, CD95.     

Thyroid dose assessment for the Chernigov region (Ukraine): Estimation based on131I thyroid measurements and extrapolation of the results to districts without monitoring

Based on the results of¹³¹I thyroid activity measurements in three districts of the Chernigov region (Ukraine), individual doses were calculated and an approach of the age dependence of the average thyroid exposure was derived. Using the relationships between the thyroid doses and the¹³⁷Cs deposition as well as the location relative to the Chernobyl Nuclear Power Plant (NPP), age-dependent average thyroid doses were extrapolated also for those settlements of this region where no monitoring measurements have been carried out. The highest doses were found in the west of the region with the lowest distance to the Chernobyl NPP. In this part, the highest mean of the thyroid dose in a settlement was 3.3 Gy for infants and 0.5 Gy for adults. The collective thyroid dose was 31000 and 27 000 person-Gy for children and adults, respectively. Based on this assessment, 140 and 21 excess thyroid cancer cases are predicted for children and adults, respectively. In the years 1989 to 1991, in the whole contaminated territory of the Ukraine 0.4–1.2 cases per 100000 children were observed. Although the absolute numbers are very small, this indicates the possibility of an increase in thyroid cancer morbidity among children. The same trend also seems to be indicated in the Chernigov region. A careful epidemiological study in the future is necessary to enable a final evaluation of radioinduced cancers in this region.     

Biological effects in natural populations of small rodents in radiation contaminated areas. The frequency of bone marrow polyploid cells of bank voles in different years following Chernobyl accident

On the basis of the metaphase analysis results, the peculiarities of dynamics of the genome mutation frequency (polyploid cells) were studied in bone marrow of bank voles inhabiting the areas with different radiocontamination level due to the Chernobyl accident (8-1526 kBq/m2 for 137Cs) in 1986-1991. Unexpectedly high frequencies of polypoid cells exceeding the pre-accidental level by a factor of 10(1)-10(3) were recorded in all populations studied. Relationship between the frequency of the parameter studied and the concentration of radionuclides incorporated in animal carcasses was proved. A statistically significant rise in the frequency of genome mutations with the time was revealed up to 1991, i.e. approximately to 12th post-accidental animal generation.     

Estimation of I131 doses absorbed by the thyroid in the period from May to June 1986

In a randomly selected group of subjects the uptake of iodine radioisotopes was measured over the thyroid in the period May 6-23, 1986. From the obtained results the doses were estimated which were absorbed from April 28, 1986 until the time of complete disappearance of I131 from the thyroid. The highest dose of radiation absorbed from I131 by the thyroid was found in the youngest group (children aged 5 to 12 years) and it was 227.8 mGy (about 23 rads).     

Post-Chornobyl thyroid cancers in Ukraine. Report 1: estimation of thyroid doses

About 1.8 EBq of 131I was released into the atmosphere during the Chornobyl accident that occurred in Ukraine on April 26, 1986. More than 10% of this activity was deposited on the territory of Ukraine. Beginning 4-5 years after the accident, an increase in the incidence of thyroid cancer among children, believed to be caused in part by exposure to 131I, has been observed in different regions of Ukraine. A three-level system of thyroid dose estimation was developed for the reconstruction of thyroid doses from 131I for the entire population of Ukrainian children aged 1 to 18 at the time of accident: (1) At the first level, individual doses were estimated for the approximately 99,000 children and adolescents with direct measurements of radioactivity in the thyroid (so-called direct thyroid measurements) performed in May-June of 1986; (2) at the second level, group doses by year of age and by gender were estimated for the population of 748 localities (with 208,400 children aged 1-18 in 1986) where direct thyroid measurements of good quality were performed on some of the residents; and (3) at the third level, group doses by age and by gender were estimated for the population of the localities where no thyroid measurements were made in 1986. The third-level doses were then aggregated over the population of each oblast. Data, models and procedures required for each level of thyroid dose estimation are described in the paper. At the first level, individual doses were found to range up to 27,000 mGy, with geometric and arithmetic means of 100 and 300 mGy, respectively. At the second level, group doses were found to be highest for the younger children (aged 1 to 4 years); doses for the older children (aged 16 to 18 years) were 3.5 times smaller. At the third level, average population-weighted doses were found to exceed 35 mGy in the five northern oblasts closer to the Chornobyl reactor site; to be in the 14- to 34-mGy range in seven other oblasts, Kyiv city and Crimea; and to be less than 13 mGy in all other oblasts.     

Somatic mutagenesis at T-cell receptor locus in inhabitants of radiation polluted regions as a result of the Chernobyl disaster

In the period of 2001-2004, frequency of cells bearing mutations at T-cell receptor (TCR) locus was assessed in 553 inhabitants of radiation polluted regions of the Russian Federation and 154 unexposed control persons. The inhabitants were divided into three groups according to age at the moment of the Chernobyl disaster and 137Cs pollution density: 1) in utero, 37-555 kBq/m2; 2) 0-14 years old, 20-555 kBq/m2; 3) 18 and more years old, highest 137Cs density (185 more than 555 kBq/m2). The most intense changes of the TCR-mutant cell frequency were observed in the group of persons exposed to ionizing radiation in utero. The mean frequency of the mutant cells was higher in the first group than in age-matched control group by about 1.5-fold: 4.0 x 10(-4) vs 2.7 x 10(-4) accordingly (p < 0.0001). Elevation in the mean TCR-mutant cell frequency was less expressed in group of inhabitants aged 0-14 years at the moment of irradiation start: 1.3-fold increase in comparison to age-matched control (3.8 x 10(-4) vs 2.9 x 10(-4), p = 0.0002). It was not found significant differences in mutant cell frequencies between control group and adults consisting in the third group (18 and more years old at the moment of the Chernobyl accident). The changes of the TCR-mutant cell frequency in persons exposed in pre- and postnatal periods differ not only quantitatively, but qualitatively. In the fist case all persons react to irradiation by increasing number of the TCR-mutant cells in some degree. In the second case - only a part of population. Proportion of reacting persons depends on age at the start of irradiation and, perhaps, on dose absorbed. The TCR-mutant frequency was significantly higher in persons with benign tumors of different localizations and nodules in thyroid gland than in persons without this pathology.     

The pale grass blue butterfly in ex-evacuation zones 5.5 years after the Fukushima nuclear accident: Contributions of initial high-dose exposure to transgenerational effects

Biological impacts of the Fukushima nuclear accident have been reported in various organisms but have not been evaluated sufficiently. Here, we collected the pale grass blue butterfly, Zizeeria maha, in September 2016, 5.5 years after the accident, from ex-evacuation zones in Fukushima Prefecture to evaluate the remaining biological effects of the accident. Although the morphological abnormality rate was not high, it was significantly positively correlated with the initial ¹³¹I and ¹³⁷Cs deposition levels (as of March 2011). The capture rate was negatively correlated with the initial ¹³⁷Cs deposition level at the borderline of significance. Neither the abnormality rate nor the capture rate was correlated with the ground radiation dose (as of September 2016). Generalized linear model (GLM) analysis also indicated contributions of the initial ¹³¹I and ¹³⁷Cs deposition levels but not the ground radiation dose. These results suggest that the butterflies present in the ex-evacuation zones in 2016 were still affected, although not severely, mainly because of the initial high-dose exposure to radioiodine, radiocesium, and other radionuclides. We conclude that transgenerational effects of radioactive contamination have successively manifested in the 5.5 years since the accident, possibly through genetic mutations, although potential contributions of other modes of damage cannot be ignored.     

The concentration and distribution of 137Cs in soils of forest and agricultural ecosystems of Tula Region

The paper deals with a comparative study of 137Cs contamination in forest, old arable and cultivated soils of Tula Region. Initial interception of Chernobyl derived 137Cs is higher in forest ecosystems: oak-forest > birch-forest > pine-forest > agricultural ecosystems. Vertical migration of 137Cs in deeper layers of soils was intensive in agricultural ecosystems: cultivated soils > old arable soils > birch-forest soils > oak-forest soils > pine-forest soils. In study have been evaluated spatial variability of 137Cs in soil and asymmetrical distribution, that is a skew to the right. Spatial heterogeneity of 137Cs in agricultural soils is much lower than in forest soils. For cultivated soil are determined the rate of resuspension, which equal to 6.1 x 10(-4) day(-1). For forest soils are described the 137Cs concentration in litter of different ecosystems. The role of main accumulation and barrier of 137Cs retain higher layers of soils (horizon A1(A1E) in forest, horizon Ap in agricultural ecosystems) in long-term forecast after Chernobyl accident.     

A preliminary assessment of the consequences for inhabitants of the UK of the Chernobyl accident

The accident with the nuclear power reactor at Chernobyl in the USSR resulted in the release of substantial quantities of radioactive material and subsequent increases in radioactivity in the environment in many countries. In this paper the situation in the UK is considered and, from the preliminary monitoring measurements, the major routes of exposure of the population are identified and quantified. For the most part exposures in the UK are within variations in levels of natural background radiation to be found in Europe. An exception is the dose likely to have been received by the thyroids of young people in the north of the UK. From reported measurements of I-131 in milk it is predicted that thyroid doses up to 10-20 times the annual doses received from 'normal' natural background radiation might have affected young children drinking fresh cows' milk. The ways in which this component of exposure could have been reduced and the criteria that govern decisions as to whether or not to implement counter-measures are discussed. The importance of I-131 in milk as a route of exposure of the population to radioactivity is a feature that the Chernobyl accident has in common with the Windscale accident in the UK in 1957, and underlines the importance of milk-producing regions in relation to reactor-sitting policy.     

Cancer incidence in northern Sweden before and after the Chernobyl nuclear power plant accident

Sweden received about 5 % of the total release of ¹³⁷Cs from the Chernobyl nuclear power plant accident in 1986. The distribution of the fallout mainly affected northern Sweden, where some parts of the population could have received an estimated annual effective dose of 1–2 mSv per year. It is disputed whether an increased incidence of cancer can be detected in epidemiological studies after the Chernobyl nuclear power plant accident outside the former Union of Soviet Socialist Republics. In the present paper, a possible exposure–response pattern between deposition of ¹³⁷Cs and cancer incidence after the Chernobyl nuclear power plant accident was investigated in the nine northernmost counties of Sweden (2.2 million inhabitants in 1986). The activity of ¹³⁷Cs from the fallout maps at 1986 was used as a proxy for the received dose of ionizing radiation. Diagnoses of cancer (ICD-7 code 140-209) from 1980 to 2009 were received from the Swedish Cancer Registry (273,222 cases). Age-adjusted incidence rate ratios, stratified by gender, were calculated with Poisson regression in two closed cohorts of the population in the nine counties 1980 and 1986, respectively. The follow-up periods were 1980–1985 and 1986–2009, respectively. The average surface-weighted deposition of ¹³⁷Cs at three geographical levels; county (n = 9), municipality (n = 95) and parish level (n = 612) was applied for the two cohorts to study the pre- and the post-Chernobyl periods separately. To analyze time trends, the age-standardized total cancer incidence was calculated for the general Swedish population and the population in the nine counties. Joinpoint regression was used to compare the average annual percent change in the general population and the study population within each gender. No obvious exposure–response pattern was seen in the age-adjusted total cancer incidence rate ratios. A spurious association between fallout and cancer incidence was present, where areas with the lowest incidence of cancer before the accident coincidentally had the lowest fallout of ¹³⁷Cs. Increasing the geographical resolution of exposure from nine county averages to 612 parish averages resulted in a two to three times higher value of variance in the regression model. There was a secular trend with an increase in age-standardized incidence of cancer in both genders from 1980 to 2009, but significant only in females. This trend was stronger and statistically significant for both genders in the general Swedish population compared to the nine counties. In conclusion, using both high quality cancer registry data and high resolution exposure maps of ¹³⁷Cs deposition, it was not possible to distinguish an effect of ¹³⁷Cs on cancer incidence after the Chernobyl nuclear power plant accident in Sweden.     

Microscopic distribution of iodine radioisotopes in the thyroid of the iodine deficient new-born rat: insight concerning the Chernobyl accident.

Thyroid cancer markedly increased in children exposed to iodine radioisotopes following the Chernobyl accident. This increase exceeded predictions based on dose estimates to the whole organ. We sought to investigate whether iodine deficiency may have influenced the pattern of microscopic distribution of radioiodines, which may be important to interpretation of the observed effects. Iodine-deficient new-born rats were injected with iodine-129 (129I) and the microscopic distribution in the thyroid tissue was studied at 24 hr and at one week after administration, using secondary ion mass spectrometry (SIMS). Twenty-four hr after administration, SIMS images showed large differences in 129I uptake among thyroid follicles, with more than a factor ten variation in the local concentration. In addition, the distribution of 129I inside follicles varied with time. At 24 hr, the highest concentration was found at the periphery of the colloid, close to the thyroid cells. There also was enhanced concentration of 129I at one pole of follicles. Distribution inside follicles was homogeneous at 7 days. Conclusions: 1/Dosimetric models, which assume uniform iodine uptake by thyroid follicles, give an oversimplified picture of radiation dosimetry in cases involving iodine deficiency, which induces patchy tissue irradiation. 2/The dynamic pattern of iodine distribution within thyroid follicles suggests that decay events from short-lived iodines will occur closer to thyroid cells than events resulting from iodine-131.     

Major Factors Affecting Incidence of Childhood Thyroid Cancer in Belarus after the Chernobyl Accident: Do Nitrates in Drinking Water Play a Role?

One of the major health consequences of the Chernobyl Nuclear Power Plant accident in 1986 was a dramatic increase in incidence of thyroid cancer among those who were aged less than 18 years at the time of the accident. This increase has been directly linked in several analytic epidemiological studies to iodine-131 (¹³¹I) thyroid doses received from the accident. However, there remains limited understanding of factors that modify the ¹³¹I-related risk. Focusing on post-Chernobyl pediatric thyroid cancer in Belarus, we reviewed evidence of the effects of radiation, thyroid screening, and iodine deficiency on regional differences in incidence rates of thyroid cancer. We also reviewed current evidence on content of nitrate in groundwater and thyroid cancer risk drawing attention to high levels of nitrates in open well water in several contaminated regions of Belarus, i.e. Gomel and Brest, related to the usage of nitrogen fertilizers. In this hypothesis generating study, based on ecological data and biological plausibility, we suggest that nitrate pollution may modify the radiation-related risk of thyroid cancer contributing to regional differences in rates of pediatric thyroid cancer in Belarus. Analytic epidemiological studies designed to evaluate joint effect of nitrate content in groundwater and radiation present a promising avenue of research and may provide useful insights into etiology of thyroid cancer.     

Post-Chernobyl thyroid cancers in Ukraine. Report 2: risk analysis

On April 26, 1986, the worst nuclear reactor accident to date occurred at the Chornobyl (Chernobyl) power plant in Ukraine. Millions of people in Ukraine, Belarus and Russia were exposed to radioactive nuclides, especially (131)I. Since then, research has been conducted on various subgroups of the exposed population, and it has been demonstrated that the large increase in thyroid cancer is related to the (131)I exposure. However, because of study limitations, quantified risk estimates are limited, and there remains a need for additional information. We conducted an ecological study to investigate the relationship between (131)I thyroid dose and the diagnosis of thyroid cancer in three highly contaminated oblasts in Northern Ukraine. The study population is comprised of 301,907 persons who were between the ages of 1 and 18 at the time of the Chornobyl accident and were living in 1,293 rural settlements in the three study oblasts. Twenty-four percent of the study population had individual thyroid dose estimates and the other 76% had "individualized" estimates of thyroid dose based on direct thyroid measurements taken from a person of the same age and gender living in the same or nearby settlement. Cases include 232 thyroid cancers diagnosed from January 1990 through December 2001, and all were confirmed histologically. Dose-response analyses took into account differences in the rate of ultrasound examinations conducted in the three study oblasts. The estimated excess relative risk per gray was 8.0 (95% CI = 4.6-15) and the excess absolute risk per 10,000 person-year gray was estimated to be 1.5 (95% CI = 1.2-1.9). In broad terms, these estimates are compatible with results of other studies from the contaminated areas, as well as studies of external radiation exposure.