The problem of post-Chernobyl dosimetry is unique in its complexity in the history of radiation medicine and radiation protection. This is because the early experience of mass exposure of people (bombing of Hiroshima and Nagasaki, Windscale and South-Ural accidents, exposure of inhabitants of Nevada in the United States of America, the Semipalatinsk area in the former USSR, the Marshall Islands, and the Goiånia accident in Brazil, and others) differed both in the much simpler structure of the irradiation source and in the number and characteristics of exposed persons. It is obvious that post-Chernobyl dosimetry, both as an independent problem, and as a tool for epidemiological studies, requires significant expertise and economic and technical expenditures. Extensive and deep research has been carried out in Ukraine for the past 10 years. This article reviews the main results of these studies.
The estimation of the effective dose equivalents of exposure to accidental and non-accidental sources was made on the basis of the studying of radiation situation variations in Kiev after the Chernobyl accident in 1986-1989. The total annual effective dose equivalents to the Kiev population from all sources of exposure for 1986-1989 were 9.19, 7.29, 5.34 and 5.03 mSv, respectively. The necessity of radiation health measures aimed at limiting the intake of long-acting radionuclides of Chernobyl nature and reducing the exposure doses by optimizing the x-ray procedure and by regulating the natural radioactivity of building materials are under discussion.
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.
After the Chernobyl accident in 1986, about 150,000 monitoring measurements were performed in Ukraine. From this data base, 40,000 measurements were selected for which the results of the reference-source measurements could be analyzed by statistical means. The majority of these measurements are of high quality. In this paper, the uncertainties introduced due to the variabilities of anatomic parameters and the measurement geometry are quantified by measurements using a thyroid-neck phantom. Parameters considered are the thyroid mass, the thickness of the tissue overlying the thyroid as well as the detector-neck distance, the orientation, and the horizontal and vertical position of the detector. The uncertainty introduced due to the variability of these factors corresponds to a coefficient of variation in the range of 25-40% for the measured activity.
Comment In: Health Phys. 1996 Feb;70(2):269-708567297
The paper considers the radiation doses of the thyroid gland in the inhabitants from the Ukrainian SSR areas (Kiev, Zhitomir, Chernigov, and Vinnitsa Regions), which have been obtained by instrumental studies of thyroidal radioactivity in May-June, 1986, and calculated by the most conservative single-dose administration model. A hygienic evaluation has been made of the findings, taking into account the age and residence. The cumulative irradiation doses of the thyroid have been estimated for children and adults. Possible late sequelae for the areas in question may account for 1060 and 300 thyroid carcinoma cases during the whole life for children and adults, respectively.
In the Zhytomyr region, about 52,000 measurements of the 131I activity in thyroids were performed. On the basis of these measurements, individual doses have been assessed for the people monitored and age-dependent average doses have been estimated for those settlements with more than 11 direct measurements. In order to estimate the pattern of thyroid exposure in the Zhytomyr region, these doses have been interpolated or extrapolated to population groups who were not monitored during May-June 1986. For this purpose, a model has been developed based on a correlation between thyroid dose estimates with the 137Cs deposition and the co-ordinates of the settlements relative to Chernobyl. Collective doses of people who were born in the years 1968 to 1986 were calculated. The radiation-induced thyroid cancer incidence in the period 1991 to 1995 was assessed by subtracting the spontaneous incidence from the observed incidence. The result is considerably lower than that observed in longer periods after external exposures. Possible reasons for this difference are discussed.
Based on the results of 131I 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 the 137Cs 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 31,000 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 100,000 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.
The paper is devoted to one of the most important aspects of the accident at the Chernobyl Nuclear Power Station: thyroid irradiation in the residents of 8 areas in the Ukrainian SSR and the town of Pripyat, affected with radioactive iodine. Irradiation doses, prognosis of medical effects of the accident, methods and practical measures for the organization of endocrinological monitoring of the affected population were thoroughly considered. The authors proposed a scheme of health care of the population in these areas. They also discussed the structure of data bases for analysis of medico-dosimetric information on "iodine" effects of the accident. The main trends in medico-dosimetric problems of the effects of thyroid irradiation were outlined.
To estimate thyroid radioactivity in the Ukrainian population from May-June 1986, more than 150,000 individual examinations were carried out by special dosimetric teams. The results of these total measurements were approved to be a basis for assessing individual absorbed doses of infant and adult thyroid irradiation associated with the 131I exposure. The dosimetric radioiodine data bank of thyroid irradiation of the Ukrainian population was created to analyze these measurements. The analysis was performed using the data for eight Ukrainian districts and the town of Pripjat, which were all heavily contaminated due to radioiodine exposure. Results of the dose assessments are given using two models: the more conservative model of "single radioiodine intake" and a more realistic model that considers the individual duration of radioiodine intake. In accordance with the more realistic model, the predictions of late effects have shown that a collective thyro-oncogenic dose is equal to 64,000 person-Gy, stimulating the possibility of the emergence of 300 cases (30 incurable) of thyrocancers. Considering this information for the next 35 y (1991-2026), it is possible to predict a 1.4-fold increase over spontaneous thyroid cancer morbidity for children who lived in the heavily contaminated regions of the Ukraine in 1986 (spontaneous and radiogenic to spontaneous).