S. Shinkarev and Y. Gavrilin
All-Russian Public Organization of Invalids “Chernobylets”, Scientific Center “FENIX”, Moscow, Russia Moscow
February 2004
OBJECTIVES The main purpose for FENIX in the current project was to provide the realistic age-dependent thyroid dose estimates for the Belarusian children exposed as a result of the Chernobyl accident occurred on April 26, 1986. The main tasks for FENIX were as follows: • Critical examination and consistency checks of individual thyroid dose estimates. Revision of individual thyroid doses where necessary; • Estimation of average age-dependent thyroid doses and associated uncertainties in thyroid doses for Belarusian settlements with more than 10 measurements of 131I-activities in human thyroids after the Chernobyl accident, that are considered to be representative to characterize thyroid exposure to the residents; • Application of the generalized model (semiempirical model) developed by FENIX to provide age-dependent thyroid dose estimates and associated uncertainties in Belarusian settlements, including those for which no direct thyroid measurements are available; • Estimation of average age-dependent thyroid doses for the children population in each of the Belarusian oblasts; • Comparative analysis together with the Belarusian Research and Clinical Institute of Radiation Medicine and Endocrinology and German GSF – Institute of Radiation Protection of age-dependent thyroid dose estimates obtained by using different approaches for the considered Belarusian settlements and oblasts.
MATERIALS AND METHODS Input data used in the project were as follows: – database with individual measurements of the 131I thyroidal content carried out within a few weeks following the Chernobyl accident and subsequent estimates of individual thyroid doses based on those thyroid measurements using corresponding functions of the 131I intake for about 130,000 Belarusian inhabitants (description of the database is presented in ANNEX 1); – database of 137Cs ground deposition densities for all Belarusian settlements prepared by BelHydromet; – database of the results of spectrometric measurements of various radionuclides, including 131I, in environmental samples and foodstuffs carried out in May through July 1986, prepared by the Institute of Biophysics (Moscow); – demographic data on Belarus. The main methods developed outside the framework of the project but used in the project were as follows: – method to assess individual thyroid doses for the Belarusian people on the basis of the results of individual 131I thyroidal content (description of the method is presented in ANNEX 2); – semi-empirical method to assess age-dependent average thyroid dose estimates in Belarusian settlements where no direct thyroid measurements were conducted within a few weeks following the accident (description of the method is presented in ANNEX 3).
WORK PERFORMED IN THE PROJECT AND RESULTS Critical examination of individual thyroid dose estimates was done in order to check whether the estimates of thyroid doses available in data bank are consistent. Such analysis revealed inconsistency in the data related to the residents of Gomel city. Special work was conducted to revise individual thyroid doses of the residents of Gomel city. Detail description of the work and the results of revision of individual and average thyroid doses are presented in the report “Re-evaluation of the estimates of age-dependent average doses for the residents of Gomel city” (ANNEX 4). Also, additional analysis of the direct thyroid measurements for children from Minsk city was done. On the basis of the revision of individual and average thyroid doses for inhabitants of Gomel and Minsk cities the draft of the paper “Thyroid dose assessment for the population of Minsk and Gomel cities in Belarus as a result of the Chernobyl accident: Part 1 – Minsk city” was prepared (ANNEX 5). Analysis of the individual thyroid doses available in the data bank showed that for 331 persons thyroid doses exceeded 10 Gy. Taking into account that high doses have significant influence on average thyroid dose estimates in the settlement, it was decided to investigate whether the “measured” doses exceeding 10 Gy were realistic. This work had been done before the following step in the project to calculate average age-dependent doses for the settlements was conducted. Detail description of this investigation was written in the report “The problem of doses exceeding 10 Gy (Analysis of the Belarusian databank)”. Based on the materials of this report, the draft of the paper “Reliability of assessment of the individual thyroid doses greater than 10 Gy for the Belarusian population based on in-vivo monitoring of iodine-131 activity in human thyroids following the Chernobyl accident” was prepared (ANNEX 6). The main conclusion of this investigation was as follows. Number of doses exceeding 10 Gy looks realistic. The high thyroid dose estimates should be used in the calculation procedure to assess average age-dependent thyroid doses for the settlements with direct measurements. In order to realize one of the key tasks in the project to assess average age-dependent thyroid doses and associated uncertainties in thyroid doses for Belarusian settlements with more than 10 measurements of 131I-activities in human thyroids a special methodology was developed. Detail description of this methodology is presented in the report “Methodology of assessment of age-dependent average thyroid doses for the settlements in Belarus” (ANNEX 7). The results of estimates of average age-dependent thyroid doses and associated uncertainties in thyroid doses for Belarusian settlements with more than 10 measurements of 131I-activities in human thyroids are presented in file “Belarus-settlements-doses.xls”. The form of presentation of those results is available in ANNEX 8. Estimation of average age-dependent thyroid doses for the children population in each of the Belarusian oblasts was done calculating the average age-dependent thyroid dose estimates for the settlements using (1) direct thyroid measurements if a given settlement had more than 10 measurements of 131I-activities in human thyroids and (2) semi-empirical method if no or less than 10 measurements of 131I-activities in human thyroids were available for a given settlement. The results of estimation of average age-dependent thyroid doses for the children population in each of the Belarusian oblasts are presented in ANNEX 9.
CONCLUSION The main results of the work performed by FENIX in the current project are the estimates of average age-dependent doses for the children populations living 1) in the settlements with more than 10 measurements of 131I-activities in human thyroids and 2) in each of the six Belarusian Oblasts. Those estimates were an important input to derive the estimates of thyroid cancer incidence rates for Belarus in the project. Important intermediate results were 1) revision of individual and average thyroid dose estimates for the populations of the cities of Gomel and Minsk; 2) justification of necessity to account for high thyroid dose estimates under calculation of average age-dependent thyroid doses for the settlements with direct thyroid measurements. During implementation of the project four research reports considering various issues discussed in the framework of the project and the drafts of two papers have been prepared.
