Thyroid cancer risk in Belarus after the Chernobyl accident: comparison with external exposures.

Within the time period 1990-1993, childhood thyroid cancer incidence due to the Chernobyl accident increased dramatically in Belarus, especially with regard to the birth cohort January 1, 1971, to May 31, 1986. This rise subsequently slowed down, i.e. during the period 1994-1996. The respective data were analysed and compared with the results of an analysis on the time dependence of thyroid cancer incidence in a pooled cohort of persons who had been exposed during childhood to external radiation with high dose rates. Concerning the period of 5-10 years following exposure, the excess absolute cancer risk per unit thyroid dose in the latter (external) exposure group was found to exceed the one in the Belarus group by a factor of two. This difference, however, is not statistically significant. The age-adjusted average excess absolute risk per unit thyroid dose for the period of 5-50 years following external childhood exposure was found to be 8 female and 14 male cases per 10(4) person-year Gy, which is a factor about 2.5 times higher than for the non-adjusted risk in the pooled cohort, as reported by Ron et al. in 1995. Assessments of future excess thyroid cancer cases due to the Chernobyl accident were done on the basis of the time dependence of thyroid cancer risk following external exposure. The thyroid cancer incidence among the birth cohort considered in Belarus and for a period starting from the cessation of the available observation data (1 January 1997) and extending to 50 years after the Chernobyl accident has been estimated to be about 15,000 cases, with an uncertainty range of 5,000-45,000 cases. According to our calculations, 80% of these cases exceed the baseline risk under enhanced thyroid surveillance.

Childhood exposure due to the Chernobyl accident and thyroid cancer risk in contaminated areas of Belarus and Russia.

The thyroid dose due to 131I releases during the Chernobyl accident was reconstructed for children and adolescents in two cities and 2122 settlements in Belarus, and in one city and 607 settlements in the Bryansk district of the Russian Federation. In this area, which covers the two high contamination spots in the two countries following the accident, data on thyroid cancer incidence during the period 1991-1995 were analysed in the light of possible increased thyroid surveillance. Two methods of risk analysis were applied: Poisson regression with results for the single settlements and Monte Carlo (MC) calculations for results in larger areas or sub-populations. Best estimates of both methods agreed well. Poisson regression estimates of 95% confidence intervals (CIs) were considerably smaller than the MC results, which allow for extra-Poisson uncertainties due to reconstructed doses and the background thyroid cancer incidence. The excess absolute risk per unit thyroid dose (EARPD) for the birth cohort 1971-1985 by the MC analysis was 2.1 (95% CI 1.0-4.5) cases per 10(4) person-year Gy. The point estimate is lower by a factor of two than that observed in a pooled study of thyroid cancer risk after external exposures. The excess relative risk per unit thyroid dose was 23 (95% CI 8.6-82) Gy(-1). No significant differences between countries or cities and rural areas were found. In the lowest dose group of the settlements with an average thyroid dose of 0.05 Gy the risk was statistically significantly elevated. Dependencies of risks on age-at-exposure and on gender are consistent with findings after external exposures.

Time trends of thyroid cancer incidence in Belarus after the Chernobyl accident.

The rates of childhood thyroid cancer incidence observed in Belarus during the period 1986 to 1995 are described as a function of time after exposure, age at exposure, and sex. Conclusions are drawn for the excess absolute risk function. After a minimum latent period of about 3 years after exposure, this risk function has a linear increase with time for at least 6 years. After correction for the dependence of average doses on age, the radiation-induced absolute thyroid risk in Gomel is about a factor of 3 higher for children up to age 10 at exposure compared to older ones; this may be due in part to different case-collection quality. In addition, in the group up to 10 years at exposure, the thyroid of girls is more sensitive to radiation by a factor of about 1.5 than the thyroid of boys on an absolute scale. Risk estimates from external exposure are consistent with risk estimates from Gomel assuming that the increase in excess cases reaches a plateau soon.