A substantial and early increase in the number of thyroid cancers has been reported in CIS countries, particularly Belarus, following the Chernobyl accident. Although there is strong circumstancial evidence that this increase is due to radioactive fall-out following the accident, a number of questions remain to be answered. In particular, there is a need to clarify the role of factors which may modify the risk of radiation induced thyroid cancer.
A case-control study of thyroid cancer in young people was therefore conducted in contaminated regions of Belarus (Gomel and Mogilev regions) and the Russian Federation (Bryansk, Kaluga, Orel and Tula regions). The primary objective of this study is to identify and quantify the role of modifying factors in radiation induced thyroid cancer in young people in Belarus and the Russian Federation, particularly genetic predisposition and iodine status. An additional objective is to study the role of exposure to short lived isotopes of iodine. Other modifying factors to be considered include age at exposure, reproductive factors (in girls and women), dietary factors, history of benign thyroid disease and family history of thyroid diseases. This study is a collaborative effort between epidemiologists, clinicians, pathologists, molecular biologists, geneticists and dosimetrists.
The study period was from 1 January 1990 until the 31st of December 1999. All prospective cases and a sample of retrospective cases were included. Two sets of controls were selected and interviewed; controls were; matched on age, sex and on geographical area of residence - one set very closely (on village or settlement), the other more loosely (on oblast, i.e. administrative region) in order to study the role of different factors -.
Overall, interviews and examination of about 250 cases and 1500 controls in Belarus and 80 cases and 480 controls in the Russian Federation have been carried out. An international panel of pathologists has verified the diagnoses.
Methods for estimating individual thyroid doses from 1-31I (and associated uncertainties), from questionnaire information on behaviour of the subjects at and around the time of the accident, have been developed and validated. I-131 doses have been estimated for all study subjects, as well as doses from external exposures and long-lived nuclides. An index of exposure to short lived isotopes has also been developed.
Information on iodine deficiency at the time of the accident and subsequently has been collected and reviewed critically for analysis of a possible modifying role of iodine deficiency on the risk of radiation-induced cancer. Maps of iodine deficiency by district and time period are being prepared, and an iodine deficiency index is being developed. Information on iodine prophylaxis and supplementation has also been collected.
Information on a possible genetic predisposition has been obtained by questionnaire. If a subject reports a history of the diseases of interest among first or second degree relatives, an investigation is carried out and the pedigree drawn; genetic linkage will be performed. In a second phase, once candidate genes have been identified in the ongoing studies, blood samples obtained from study subjects will be analysed to screen for mutations of these genes and thus evaluate the risk of radiation induced thyroid cancer associated with the genetic predisposition.
The first results of this study have been published in the JNCI in May 2005 –
http://jncicancerspectrum.oupjournals.org/cgi/reprint/jnci;97/10/724?ijkey=78i7V4IS1XnJ5v0&keytype=ref
(see also IARC Press release 162)
A strong dose-response relationship between radiation dose to the thyroid received during childhood and the risk of thyroid cancer. The risk was mainly attributable to I-131 and appeared to be similar in magnitude to that seen following external radiation exposures to X and gamma rays.
This risk was three times higher in iodine-deficient areas than in other areas. Use of potassium iodide as a dietary supplement reduced that risk by a factor of three compared with no supplementation. Both iodine deficiency and iodine supplementation appear to be important and independent modifiers of the risk of thyroid cancer after exposure to I-131 in childhood. This result has important public health implications in the case of exposure to radioactive iodines in childhood that may occur after radiation accidents or during medical diagnostic and therapeutic procedures. ”Indeed, stable iodine supplementation in iodine-deficient populations may reduce the subsequent risk of radiation-related thyroid cancer in these situations".
Analyses of other risk factors and modifying factors (including age at exposure, reproductive factors (in girls and women), dietary factors, history of benign thyroid disease and family history of thyroid diseases) are underway.
Work under this study was supported through grants from the Nuclear Fission Safety, INCO-Copernicus and Quality of Life Programmes of the European Commission. The field work was carried out jointly with that of a study funded by the Sasakawa Memorial Health Foundation of Japan.