Selection of the cohort for the long-term clinical follow-up and assessment of radiation risks for thyroid diseases under the joint medical research project conducted by Sasakava memorial health foundation and MRRC of RAMS

The description of the cohort of the children of Bryansk and Kaluga regions of Russia exposed to radioiodine in 1986 as a result of the Chernobyl accident is given in the paper. The cohort has been selected under the Joint Medical Research Project on Thyroid disease conducted by Sasa-kava Memorial Health Foundation and MRRC of RAMS for the long-term clinical follow-up of the thyroid. It comprises 3299 persons, 1187 of them are residents of Bryansk region and 2112 - residents of Kaluga region. All subjects were under 10 years old at the time of exposure. All members of the cohort have individual radiometric data for thyroid obtained in May-June of 1986. The main part of the subjects lived in the most contaminated areas of Bryansk and Kaluga regions in 1986. The preliminary evaluation of absorbed radiation doses to thyroid has been performed before the verification of incorporated activity of 131I in thyroid, dynamics of radioiodine deposits in the territory of Russia as well as specialised cross-examination of the cohort members. Analysis of the results has shown that statistical distribution of the absorbed doses of internal exposure of thyroid is close log normal distribution with mean value of 240 mGy and standard deviation of 750 mGy for Bryansk part of the cohort, and 360 mGy - for Kaluga one. In 5% of the members the absorbed radiation dose in thyroid is under 1 mGy, the maximal one is 12 Gy for the Bryansk part of the cohort and 6 Gy for the Kaluga part. One of those who were followed up within the decade after the Chernobyl developed thyroid cancer. For the next period of the follow-up by the cohort study technology radiation risks of developing thyroid cancer and non-cancer thyroid diseases are planned to be assessed.