Thyroid cancer under 19 in Fukushima: The fourth report from the second round screening

Abstract

Introduction: After the nuclear power plant accident in March 2011, Fukushima Prefecture planned a 3-year thyroid screening (1st round) for all children ages ≤18 years at the accident. Then, the 2nd round is being conducted from April 2014 to March 2016. We report the latest results based on the data until Dec 31, 2015. Methods: We divided Fukushima Prefecture into 9 districts: (a) Nearest area (i.e., the most contaminated); (b) North middle; (c) Central middle; (d) Koriyama city; (e) South middle; (f) Iwaki city; (g) Southeastern; (h) Western; and (i) Northeastern. The districts (f), (g), and (h) correspond to the “less affected location” by the World Health Organization. As an external comparison, we estimated incidence rate ratios (IRRs) for thyroid cancer in the districts, compared with its annual incidence estimate among 5–24 ages from the Japanese National Cancer Center (i.e., 5 per 1,000,000). The time elapsed between the 1st and the 2nd round screening was 2.5 years in district (a) and 2 years in the rest. Results: IRRs (95% confidence interval: Number of cases) were (a) 38 (22–62: 16); (b) 20 (9–38: 9); (c) 25 (7–64: 4); (d) 33 (18–54: 15); (e) 6.9 (0.2–38: 1); (f) 6.1 (0.7–22: 2); (g) 7.5 (0.9–27: 2); (h) 6.3 (0.2–35: 1); and (i) 18 (0.5–102: 1) under an assumption that no cancer cases exist in the rest of examinees. Proportions of those who had a detailed examination among those with positive findings in the screening were 84.0% (279/332) in the Nearest area, 78.6% (990/1,260) in the middle, and 32.6% (182/559) in the least contaminated. Conclusions: We observed 20–38 folds excess of thyroid cancer incidence in the 4 districts (a)–(d) where more than 70% had detailed examinations. Although diagnostic decision of the 2nd round screening and its detailed examinations are still ongoing, the excess is unlikely explained by “screening effect” because the “harvesting” would have occurred mostly in the 1st round screening.