"Radiation & Risk", 1996, issue 7 Scientific Articles Thyroid doses for the population of Russia as a result of the Chernobyl accident (retrospective analysis) Stepanenko V.F., Tsyb A.F., Gavrilin Yu.I.*, Khrushch V.T.*, Shinkarev S.M.*, Skvortsov V.G., Kondrashov A.E., Yas’kova E.K., Ivannikov A.I., Parshkov E.M., Shakhtarin V.V., Moskovko L.I., Petin D.V., Chebotareva I.V., Proshin A.D.**, Rozhko Yu.N.**, Dorokhov V.V.**, Rivkind N.B.**, Kvitko B.I.**, Kuz’min P.S.**, Leshakov S.Yu.***, Omel’chenko V.N.*** Medical Radiological Research Centre of RAMS, Obninsk, Russia; * Institute of Biophysics, Russia; ** Department of Health, Administration of Bryansk region; *** Department of Health, Administration of Kaluga region. The paper discusses a methodological approach developed for re- construction of thyroid doses from internal exposure to 131I and prospects of future work in this field. Estimated levels of thyroid irradiation for the population of Russia after the ac- cident are presented. These results have been obtained based on the retrospective analysis using the developed approach. Also, long-term health consequences of thyroid irradiation in the population of Russia have been predicted. Retrospective esti- mates have been made for individual thyroid doses for children and adolescents with the diagnosis of thyroid cancer living on the contaminated areas. Introduction 1. Key characteristics of available ar- rays of dosimetric data The recent data are indicative of an increase in the frequency of detection In essence, three classes can be of malignant neoplasms of thyroid on the identified in individual internal doses territories contaminated after the Cher- of thyroid exposure to radioactive io- nobyl accident [1, 2]. This enhances the dine: importance of the studies to comprehen- Class 1 includes individual doses sively assess the factors of thyroid ex- calculated from measurements of 131I con- posure to radiation. In this respect, tent in thyroid (people examined in May- based on available results of measure- June 1986). With all reservations con- ments of radioiodine in thyroid and cerning errors in measuring radioiodine various characteristics of the environ- in thyroid and uncertainties in calcula- ment and using additional data which can tion models used for estimation of indi- be obtained in the near future, it is vidual doses, these data should be re- necessary to bring the accuracy of dosi- ferred to as most reliable and objec- metric evaluations of thyroid exposure tive. to the level required by practical medi- Class 2 consists of estimates of in- cine and epidemiological studies, ide- dividual doses for those population ally, to the accuracy which would allow points for which sufficient amount of adjustment of dose coefficients of risk data of Class 1 are available. Based on of thyroid diseases in irradiated per- results of Class 1, parameters of dis- sons. This problem should be first ap- tributions of individual internal doses proached from the standpoint of total for corresponding age groups (mean value absorbed dose, namely internal radiation and deviation from it) are determined dose resulted from intake of radioactive and then for a specific individual for isotope 131I. whom data of Class 1 are not available, an individualised dose is estimated af- ter additional information on life style, milk intake and other information 62 "Radiation & Risk", 1996, issue 7 Scientific Articles is provided. While this additional in- In May-June 1986 in Belarus, Russia formation is missing, as a personal and Ukraine an unprecedented dosimetric characteristic of thyroid internal irra- survey was undertaken which produced re- diation mean values for a corresponding sults of direct measurements of the ra- age group are taken. The objectivity of dioiodine content in the thyroid in more such characteristics is quite high, as than 400 thousand people. The survey was they were derived on the basis of direct performed by numerous groups of special- measurements of radioiodine in the thy- ists of differing training, working in roid. different conditions (from specialised Class 3 includes estimates of indi- establishments to field conditions in vidual internal thyroid doses for resi- the contaminated areas) and having at dents of the rest populated points their disposal different types of meas- (which are the majority) for which the urement equipment (from energy-selective required amount of Class 1 data are not specialised devices to -radiation dose available and it is not possible to de- rate detectors of DF-5 type). As a re- rive estimates of Class 2. In this case sult, a variety of factors influenced the group characteristics of internal the reliability of results of direct exposure of thyroid can be estimated measurements. Some of these factors have from data on characteristics of the ra- been identified and corresponding ad- diation situation and its formation in justments have been made, as, for exam- each populated point of interest using ple, was the case with measurements in one of the calculation models. Belarus which were distorted because of The problem is that the existing data the wrong measurement procedure used and on the radiation situation are rather conversion of measurement units without limited and they are not sufficient for indication of the type of device used application of the known well-developed [6]. multi-parameter models. The most realis- One of the issues which are still de- tic way to solve the problem is to use a batable is a possible distortion of re- large volume of data of Class 1 avail- sults of determination of 131I in thyroid able for the contaminated areas for de- due to “ glow” of detectors because of veloping a semiempirical model of ex- external radioactive contamination of trapolation-interpolation type which body and clothes and as a result of in- would be rather general, but reflect the take of other radionuclides and, first knowledge of the basic processes respon- of all, radioisotopes of cesium (134Cs, sible for thyroid exposure and be based 137Cs). It should be emphasised that the on available empirical material. At the contribution of -radiation of incorpo- same time, it would be desirable that rated radiocesium in the detector read- such a model be quite simple, i.e. would ings (not -spectrometric), when measur- require only available information on ing 131I content in thyroid, changes de- formation of thyroid doses or informa- pending on the date of measurement: from tion that can be obtained by the exist- insignificant in early-middle May 1986 ing methods. to predominant in middle June and later. The features of large-scale measure- The Class 1 data bank on residents of ments of radioiodine in thyroid in May - Kaluga region [3, 13] contains informa- early June 1986 on the contaminated ar- tion of May 1986 only. The Class 1 data eas of Belarus, Russia and Ukraine and bank for the population of Belarus has main aspects of the following verifica- only individual doses determined based tion of measurements, models used and on measurements made not later than 6 individual internal thyroid doses have June 1986. The data on residents of Bry- been described in detail in national and ansk, Tula and Orel regions based on the international publications and discussed difference in detectors readings with at scientific conferences [3-13]. In the measurement geometry of “ thyroid” general, it can be said that separate and “ thigh surface” [8] can signifi- subsets of Class 1 data formed by now by cantly underestimate the true values of different investigation groups differ individual internal thyroid doses for either by the method of obtaining pri- those for whom external radioactive con- mary data of radioiodine measurements in tamination of thigh surface was not thyroid or by schemes for calculating eliminated. individual internal thyroid doses. By far, factors influencing the reli- ability of the determination of 131I con- 63 "Radiation & Risk", 1996, issue 7 Scientific Articles tent in thyroid can not all be quanti- “ per os” intake of 131I with local milk, fied. That is why, at this stage we use which is primarily characteristic of ru- an approach based on conventional clas- ral residents and a model of inhalation sification of results of 131I content intake with consideration of documented measurements in thyroid into several protection measures (such as evacuation certainty groups. Each of these groups or relocation to relatively “ clean” are assigned a relative error by ex- areas). The minor differences in the perts. The data on more than 200 thou- used values of calculation parameters sand residents of Belarus were first di- (constants of thyroid cleaning from 131I vided into three groups of certainty of for separate age groups, cleaning of 131I determinations in thyroid with al- pasture grass and leaf vegetables) lowance for conditions of measurements should be taken into account at the next and devices used with relative errors of stage. The assumption of a short one- 50, 300 and 500% [6]. As a result of time contamination of the environment verification of available data and re- was based on data on daily depositions lated work, the Class 1 data bank on of 131I registered by Goskomhydromet net- residents of Belarus contains about 130 work which, regrettably, is not as dense thousand individual doses, whose cer- as one would need for dose reconstruc- tainty groups, in terms of 131I measure- tion. The general scheme of applying ad- ment in thyroid, can be characterized by justments for the long-term character of relative errors: to 60%, within 60 to fallout is described in detail in Guide- 200% and 200 to 400% respectively. As- lines [14] and will be used in future as suming that the indicated intervals cor- much in detail as kinetics of local respond to 95% confidence probability depositions of 131I is refined, which was with lognormal distribution, the intro- promised, in particular, by the authors duced certainty groups can be character- of work [16].