Dose dependence of pediatric thyroid cancer prevalence in the 6 years after the Fukushima nuclear power plant accident

Background: After the Fukushima nuclear accident in March 2011, Fukushima Prefecture initiated thyroid ultrasound screening as part of the Fukushima Health Management Survey (FHMS). Regional differences in external radiation dose were often reported not associated with thyroid cancer prevalence in the firstround screening E-I (2011-2013). The association between childhood thyroid cancer and radiation exposure in the 6 years after the accident is studied by analyzing the results of the first and second-round examinations E-I+II (2011-2015). Methods: Dose dependence of thyroid cancer proportion in E-I and E-II for all residents aged ≤18 years at the accident was analyzed for FHMS external dose and UNSCEAR effective dose by regression analysis using Microsoft Excel. Two divisions of Fukushima prefecture, O-model in the order of decreasing external dose and S-model according to the initial screening schedule, were adopted. Results: In O-model, thyroid cancer proportion per 100,000 in E-II and in E-I+II were found to increase linearly to FHMS external dose in 0.2–1.4 mSv range and UNSCEAR effective dose in 1.6–5 mSv range. Thyroid cancer proportion in E-II and E-I+II was observed to increase linearly to effective dose in Smodel. Conclusion: The observed linear prevalence–dose relation after 6 years from the accident and incidencedose relation during 4-6 years after exposure suggest a possible association between pediatric thyroid cancer and radiation exposure. Regional differences were not obvious in E-I presumably because of the short interval from exposure to screening in high-dose areas. High prevalence of thyroid cancer cannot be attributed only to mass screening effect that does not depend on radiation dose and elapsed time from exposure.