Detection of thyroid cancer among children and adolescents in Fukushima, Japan: a population-based cohort study of the Fukushima Health Management Survey

Background Thyroid ultrasound examinations using a cohort study design (from the Fukushima Health Management Survey [FHMS]) were conducted after the nuclear power plant accident caused by the Great East Japan Earthquake in 2011. This study investigated the association between radiation exposure and the detection of thyroid cancer in children and adolescents. Methods The cohort study has been conducted in Fukushima prefecture in Japan since 2011. The primary outcome was the external dose. We enrolled 253346 examinees who lived in Fukushima at the time of the accident (Dataset A), including 113120 examinees who had data on external radiation exposure (ERE) (Dataset B). The median dose in the examinee's district was used for missing dose. The association between ERE and detection of thyroid cancer or suspected thyroid cancer was analyzed using Poisson regressions with two types of explanatory variables: sex, age, overweight status, and district (Model 1), and past medical history, family history of thyroid cancer, frequency of seafood consumption, and frequency of seaweed consumption in addition to Model 1 (Model 2). Findings During the second and third rounds of examinations, a total of 97 thyroid patients were detected, for a detection rate of 10.328 [10(5)year(-1)] (95% confidence interval: 8.464-12.602 [10(5)year(-1)]). Multivariate Poisson regression showed that the detection rate ratio of the ERE (1+ mSv) to <1 (mSv) was 1.577 (0.715-3.394) in Model 1 and 1.596 (0.726-3.512) in Model 2, for Dataset A; and 1.677 (0.746-3.773) in Model 1 and 1.669 (0.743-3.748) in Model 2, for Dataset B. Interpretation Our study showed no association between radiation exposure with extremely low dose which were more than 99.9% of all the exposure was less than 5 mSv, and thyroid cancer detection, when the follow-up period was an average of 3.7 years at the present, using the cohort study design.