Thyroid cancer: lessons of Chernobyl and projections for Fukushima

Estimating radiation risks of thyroid cancer in the range of middle an low doses and prediction of the incidence with account of screening effect is a complex problem. The present article considers the problem of radiation risks estimation and screening effect on detection of thyroid cancer. Development of methodology and risk assessment were based on radiation-epidemiological information accumulated for 30 years at the National Radiation Epidemiological Registry (NRER). In order to research into screening effect and relationship between radiation dose and thyroid cancer the cohort method was used. National statistical data on thyroid cancer incidence among Russian population were used as referent. In the present study residents of the most contaminated territories of Bryansk, Orel, Tula and Kaluga oblasts exposed to radiation as a consequence of the Chernobyl accident in 1986 composed the cohort. Taking into account the 5-year latent period the study was carried out from 1.01.1991 to 31.12.2013. Such information as place of residence in 1986, and the age available for all members of the cohort was used for estimation of thyroid dose from incorporated radioiodine. The total number of the cohort members was 327 thousand. Previously published data demonstrated dependence of thyroid dose on age of children and adolescents, younger than 18 years at exposure, the dose exponentially reduced with age. At the same time thyroid dose of adults, older than 18 years at exposure, did not depend on age. Average thyroid dose to children and adolescents was 174 mGy, and it was 36 mGy to adults, the highest doses were 4.48 Gy and 0.66 Gy respectively. All cohort members underwent yearly comprehensive medical examination. It is possible to suggest that recorded increase in thyroid cancer incidence, as compared with national statistical data, is a result of the examinations. This increase can be defined as screening effect. During the follow-up period (1991 through 2013) screening effect was 6.74 in the group younger than 18 years at exposure and 1.5 in the group older than 18 years at exposure. From 1996 the effect reduced twofold as compared with the first 5-year period (1991-1995). Obtained methodology was used for estimation of expected pediatric thyroid cancer incidence in Japan after the accident at the Fukushima-Daichi NPP. It is necessary to stress that actual data on thyroid cancer incidence are in good agreement with predicted data.