Thyroid Dose Estimates for a Cohort of Belarusian Children Exposed to Radiation from the Chernobyl Accident

Abstract

The U.S. National Cancer Institute, in collaboration with the Belarusian Ministry of Health, is conducting a study of thyroid cancer and other thyroid diseases in a cohort of about 12,000 persons who were exposed to fallout from the Chernobyl accident in April 1986. The study subjects were 18 years old or younger at the time of exposure and resided in Belarus in the most contaminated areas of the Gomel and Mogilev Oblasts, as well as in the city of Minsk. All cohort members had at least one direct thyroid measurement made in April–June 1986. Individual data on residential history, consumption of milk, milk products and leafy vegetables as well as administration of stable iodine were collected for all cohort members by means of personal interviews conducted between 1996 and 2007. Based on the estimated 131I activities in the thyroids, which were derived from the direct thyroid measurements, and on the responses to the questionnaires, individual thyroid doses from intakes of 131I were reconstructed for all cohort members. In addition, radiation doses to the thyroid were estimated for the following minor exposure pathways: (a) intake of short-lived 132I, 133I and 132Te by inhalation and ingestion; (b) external irradiation from radionuclides deposited on the ground; and (c) ingestion intake of 134Cs and 137Cs. Intake of 131I was the major pathway for thyroid exposure; its mean contribution to the thyroid dose was 92%. The thyroid doses from 131I intakes varied from 0.5 mGy to almost 33 Gy; the mean was estimated to be 0.58 Gy, while the median was 0.23 Gy. The reconstructed doses are being used to evaluate the risk of thyroid cancer and other thyroid diseases in the cohort.

Thyroid Dose Estimates for a Cohort of Belarusian Children Exposed to131I from the Chernobyl Accident: Assessment of Uncertainties

“…We compared the arithmetic means of stochastic thyroid doses calculated in this study with the deterministic doses estimated previously (13) for the same cohort (Fig. 5).…”

“…For some study subjects, the scaling factor was found to be very high (i.e., greater than 100) or very low (i.e., less than 0.01), and biases in the parameter values of the ecological model are not sufficient to explain these extreme values. Case examples of such study subjects were presented and analyzed in previous work (13). The results indicate that biases in parameter values cannot fully explain the discrepancies between the instrumental doses and in the ecological doses when the scaling factors are either very high or very low.…”

“…A more detailed description of the methodology used to calculate the “instrumental” thyroid doses for all members of the cohort of Belarusian children can be found elsewhere (13). …”

“…In those instances, the method used to estimate P bg ( t m ) depended on the availability of outdoor exposure rates (13). …”

“…To evaluate the radiation risk of thyroid diseases, deterministic thyroid doses were reconstructed for the study subjects based on estimated 131 I activity in the thyroid derived from direct thyroid measurements of each cohort member and responses to interviews of all subjects on residential history, consumption of milk, milk products and leafy vegetables and administration of stable iodine (13). It is clear that there are uncertainties associated with these dose estimates since it is impossible to obtain precise and complete data for exposure assessment.…”