LAUSR

BACKGROUND The 2011 Great East Japan Earthquake led to a nuclear accident at Fukushima Daiichi Nuclear Power Plant. This study examines the associations of radiation dose and lifestyle factors with incidence of thyroid cancer in Fukushima. METHODS We designed a prospective study with 300,473 participants aged 18 years or younger who underwent thyroid examinations from October 2011. Follow-up surveys were conducted through June 2017, and 245,530 participants (123,480 men and 122,050 women, 82% follow-up) received follow-up examinations. Fukushima Prefecture was divided into five areas based on individual external radiation dose. We calculated relative risks and 95% confidence intervals (CIs) for thyroid cancer in each area, with area of lowest dose as reference, using age-adjusted Poisson regression models. We also calculated risks associated with overweight and obesity. RESULTS The incidence per 100,000 for Groups A (highest dose), B, C, D, and E (lowest dose) were 13.5, 19.2, 17.3, 9.0, and 8.3, respectively. Compared with Group E, the age-adjusted risks (95% CIs) were 1.62 (0.59-4.47) for group A, 2.32 (0.86-6.24) for group B, 2.21 (0.82-5.94) for group C, and 1.02 (0.36-2.86) for group D. Obesity was positively associated with thyroid cancer incidence; the age- and sex-adjusted risk of thyroid cancer was 2.26 (1.03-4.95) for obese individuals compared non-obese. CONCLUSIONS Regional differences in radiation dose were not associated with increased risk of thyroid cancer among children in Fukushima within 4-6 years after the nuclear power plant accident. Obesity may be an important factor for further follow-up in Fukushima.