Age- and sex-specific relative thyroid radiation exposure to 131I in Ukraine after the Chernobyl accident.

The age- and sex-dependence of the 131I induced count rates is determined for the measurements performed in Ukraine after the Chernobyl accident on the thyroids of over 60,000 persons. For this, the individual measurements are scaled in such a way that the mean values over age and sex on one side and the mean values over measurement series on the other side are normalized to one. The resulting distribution of all scaled measurements is roughly log-normal. Half of them lie within a factor 1.6 of the median. 131I induced count rates have a minimum at birth year 1986, about half the value of adults. The maximum count rates with about 30% above adults are reached for males around age 16 y. The count rates are up to about 40% (at age 14-17 y) higher for males than for females. The results are within statistical uncertainties independent of the geographical area and the urban or rural nature of the settlements. Starting from the relative count rates, the age- and sex-dependence is calculated for the thyroid activities 1 mo after the accident for the integrated activities and for the doses. The dose of young children is a factor of about 6.5 higher than that of adults. Uncertainties are estimated throughout.

Thyroid dose and thyroid cancer incidence after the Chernobyl accident: assessments for the Zhytomyr region (Ukraine).

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.

Thyroid dose reconstruction for the population of Belarus after the Chernobyl accident.

An 131I environment transfer model--adapted for Belorussian conditions--was applied to estimate thyroid doses for different population groups. For this purpose the available data were analysed and the important radioecological parameters assessed i.e. (a) the elimination rate of 131I from grass due to weathering and growth dilution, (b) the initial interception of 131I by vegetation, (c) the transfer coefficient for 131I from grass to cow's milk, (d) the yield to pasture grass and (e) the milk consumption rate. Additionally, the influence of applied countermeasures has been taken into account, such as the interruption of locally produced milk consumption, and the appropriate correction factors have been estimated. As a result, the average age-dependent thyroid doses were assessed for the Belorussian population. The highest average doses in children (> 1 Gy) have been estimated for the Bragin, Khoiniki, Narovlia and Vetka raions of the Gomel oblast. The thyroid exposure tends to decrease from the southeastern (closest to the Chernobyl nuclear power plant areas) to the northwestern part of the republic. When comparing the assessed thyroid doses with estimates derived from direct 131I activity measurements in thyroids (for the locations with more than 15 direct measurements), the results agree fairly well. The model calculation may perhaps overestimate thyroid doses of the population residing in the settlements of the central and northern parts of Belarus, distant from the areas with direct measurements of 131I activities in soil, grass and milk. These thyroid dose estimates may serve as a basis for further epidemiological studies and risk analyses.

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.