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 subs equently 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 dep endence of thyroid cancer incidence in a pooled cohort of persons who had b een exposed during childhood to external radiation with high dose rates. Co ncerning the period of 5-10 years following exposure, the excess absolute c ancer risk per unit thyroid dose in the latter (external) exposure group wa s found to exceed the one in the Belarus group by a factor of two. This dif ference, however, is not statistically significant. The age-adjusted averag e excess absolute risk per unit thyroid dose for the period of 5-50 years f ollowing external childhood exposure was found to be 8 female and 14 male c ases per 10(4) person-year Gy, which is a factor about 2.5 times high er th an 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 Che rnobyl accident were done on the basis of the time dependence of thyroid ca ncer risk following external exposure. The thyroid cancer incidence among t he birth cohort considered in Belarus and for a period starting from the ce ssation of the available observation data (1 January 1997) and extending to 50 years after the Chernobyl accident has been estimated to be about 15,00 0 cases, with an uncertainty range of 5000-45,000 cases. According to our c alculations, 80% of these cases exceed the baseline risk under enhanced thy roid surveillance.