Radiation risks of thyroid cancer among the Chernobyl emergency workers

The paper presents results of analysis of thyroid cancer incidence among males participated in clean-up work within the 30-km Chernobyl exclusion zone from the 26 th April through the 31 st December 1986. The considered follow up period of the cohort made up of 27749 emergency workers (EWs), residents of six regions of the European part of Russia, is 1992-2010. The analysis was based on data of the National Radiation Epidemiology Registry (NRER). All cohort members have documented radiation dose from external exposure. The cohort consisted of two groups of the accident workers, the group 1 consists of 5650 so called “exposed” EWs worked from April 26 to May 31 and exposed both to external radiation and 131I; the group 2 of so called “unexposed” EWs consists of 22099 males worked within the zone from June through December 1986, and exposed to external radiation only. For the whole period of follow-up 49 thyroid cancer cases were registered. For estimating risk of thyroid cancer associated with exposure to 131I and exact confidential limits the data of “exposed” vs. “unexposed” contingency tables were used. For calculating risk of thyroid cancer associated with external exposure only (the group 2) the maximum likelihood estimation for relative risk linear model was used. The odds ratio is 1.4 (0.71, 2.70 90% CI). Excess relative risk per 1 Gy in the group 2 (“unexposed”) is 1.87 (-2.16, 26.38 90% CI). The calculated mean radiation dose to the thyroid from 131I is 0.22 (0.05, 0.70 90% CI) Gy. According to obtained estimates, contribution of 131I to radiation associated thyroid cancer among EWs worked in the 30-km Chernobyl exclusion zone in May 1986 is 53%.