P-11-267
Thyroid cancer among children and adolescents in Russia following the Chernobyl Accident: retrospective estimation of thyroid doses and case – control study. V K Ivanov, V A Pitkevich, S Yu Chekin, M A Maksyutov, O K Vlasov, A F Tsyb, E Cardis*, A Kesminiene*, Y Shibata**, S Yamashita** Medical Radiological Research Centre of Russian Academy of Medical Sciences, 4 Korolev Street, Obninsk 249020, Russia, e-mail:
[email protected]; * International Agency for Research on Cancer, Lyon, France, e-mail:
[email protected]; ** Atomic Bomb Disease Institute, Nagasaki University School of Medicine, Nagasaki, Japan, e-mail:
[email protected]
Introduction Today the dramatic growth of thyroid incidence among residents of the areas of Belorus, Russia and Ukraine most contaminated after the Chernobyl is an established fact [1-4].At the same time, epidemiologically some issues of principal importance for interpretation of the accumulated data remain unresolved. In particular, it has to be definitely proved that these thyroid cancers are induced by exposure of thyroid to radiation in the form of incorporated 131I. The territories affected most by the radioactive contamination after the Chernobyl disaster included the four regions of Russian Federation: Bryansk, Kaluga, Orel and Tula regions. In the western areas of the Bryansk region the highest 137Cs soil contamination density was 4 MBq/m2 and the highest 137Cs soil contamination density in three other regions was 0.6 MBq/m2. The precise pattern of 131I depositions in the territory of Russia is unfortunately not known to date. The available data indicate that the highest 131I depositions in the Bryansk region was 15MBq/m2 as of 10 May 1986. After the Chernobyl accident the Government of the former USSR took a decision to set up All-Union Distributed Registry of persons affected by radiation. In 1992 (time of USSR disintegration) the database of the Registry included individual medical and dosimetric information for 659292 persons, among them 284919 were the Chernobyl recovery workers. Today the Russian National Medical Dosimetric Registry (RNMDR), which was the core of the All-Union Registry before 1992, contains data about 530965 persons exposed to radiation. The problem of thyroid cancer remains to be central in the RNMDR system and in the long- term radiation and epidemiological studies. This problem required dealing with three major goals: - estimation of dynamics of thyroid cancer in terms of SIR before and after the Chernobyl accident; - estimation of age dependence of thyroid cancer among residents of the contaminated regions of Russia; - estimation of dose dependence of thyroid cancer incidence on incorporated 131I. The first large - scale study of thyroid cancer among young people in Russia following the Chernobyl accident was conducted. The study population consists of all children and adolescents (17 years and less) who were resided in Kaluga, Orel and Tula oblasts of Russia at the time of Chernobyl accident. Kaluga, Orel and Tula oblasts are approximately equally contaminated oblasts of Russia (up to 15 Ci/km2 for cesium – 137). All prevalent and new carcinoma cases from 1 January 1992 to 31 December 1998 were collected in 1997 – 1998 years as a result of joint project on thyroid disease conducted by the Medical Radiological Research Center (Russia), International Agency for Research on Cancer (France) and Sasakawa Memorial Health Foundation (Japan).
Registration and verification of thyroid cancer cases
In accordance with the Decree of Government of RF residents of the territories with 137Cs soil contamination density of more than 185kBq/m2 are subject to registration in RNMDR. By this criterion, residents of only three of 28 districts in the Kaluga region are to be entered to RNMDR. Within this project, it was decided to study children and adolescents at the age 0-17 years at the time of the Chernobyl accident living in all districts of the above three regions who were diagnosed thyroid cancer in 1992-1998. To this end, RNMDR requested individual information from cancer dispensaries of the Kaluga, Orel and Tula regions about people having diagnosis of thyroid cancer and meeting the above criteria. An important aspect of the studies under discussion was morphological verification of all thyroid cancer cases by the international panel of morphologists. All cases included in the study were verified in Russia (Prof. E.Lushnikov) and UK (Prof Williams). Finally, 60 verified thyroid cancer cases were included in the study: 11 cases from the Kaluga region, 26 cases from the Orel region and 23 cases from the Tula region. Table 1 shows sex and age structure of the cases (age at the time of the accident). The pappilar form of cancer was diagnosed in 6 persons (10%).
Selection of controls and interviewing Controls of each thyroid cancer case were selected by the procedure of matching by age and sex. To this end, assistance was asked from the entities responsible for marriage and death registration (ZAGS) and 1
P-11-267 address registration (Passport office). For each case were randomly selected two and four controls: two controls were matched on age, sex and on settlement of residence at the time of accident; four controls were matched on age, sex and on oblast of residence at the time of accident. Each patient and corresponding controls were examinated by the high – resolution ultrasonographic instrument and were interviewed by special trained medical staff. Consideration was given to risk factors of radiation nature (incorporated 131I and short-lived isotopes) and genetic predisposition. With allowance for possible migration processes and change in residence place for reasons not related to the accident (army service, university study), the number of selected controls initially was three times more than necessary (18 instead of 6).Practice shows that such excess is justified. For interviews the stuff of regional centers of RNMDS in the Klauga, Orel and Tula regions were involved who were specially trained for doing this. Since almost all districts in the region were studied, interviews were conducted in 3-5 points in each region. By volume and complexity of organizational work this project has no analogues among a large number of post-Chernobyl international projects. The support number of cases and controls in this work was 427 persons. Summary for the information, obtained through the interviews, is shown in the Table 2. The collected individual information was effectively used at the next stage, namely reconstruction of the individual thyroid doses. The new technique for reconstruction of the individual dose of internal irradiation of thyroid with incorporated 131I was developed. The method allows to take into account major factors influencing formation thyroid doses of the population: (1) three sources of receipt 131I in organism of the man: inhalation, consumption of milk and green vegetables; (2) types of food production: public and private (individual) sectors of its (her) manufacture; (3) weather conditions of spring of 1986 in regions of residing of the population for determining the dynamic contamination of milk and greens; (4) multichamber model of the dairy for the "Russian" cow adjusted on direct experimental data; (5) standard age dependent multichamber ICRP model of the man; (6) the data from the special questionnaire about changes of his (her) diet, origination of the meal and his (her) migration during May, - July 1986; (7) results of reconstruction on the basis of direct thyroid radiometry for 28229 persons in Kaluga oblast and for 2967 persons in Bryansk oblast.
The results obtained In analysis of the dynamics of SIR for thyroid cancer incidence in the above indicated four regions (control- Russia) in 1982-1998 it was found that SIR
