Abstract
Following the Fukushima Daiichi Nuclear Power Plant (FNPP) accident on 11 March 2011, there have been concerns regarding the health impacts of the ensuing radioactive environmental contamination, which was spatially heterogeneous. This study aimed to assess the geographical variability of thyroid cancer prevalence among children and adolescents in Fukushima Prefecture. We computed the sex- and age-standardised prevalence ratio using 115 diagnosed or suspected thyroid cancer cases among approximately 300,000 examinees at the first-round ultrasound examination during 2011–2015 from 59 municipalities in the prefecture, under the Fukushima Health Management Survey. We applied flexibly shaped spatial scan statistics and the maximised excess events test on the dataset to detect locally anomalous high-prevalence regions. We also conducted Poisson regression with selected regional indicators. Furthermore, approximately 200 examinees showed positive ultrasound examination results but did not undergo confirmatory testing; thus, we employed simulation-based sensitivity tests to evaluate the possible effect of such undiagnosed cases in the statistical analysis. In conclusion, this study found no significant spatial anomalies/clusters or geographic trends of thyroid cancer prevalence among the ultrasound examinees, indicating that the thyroid cancer cases detected are unlikely to be attributable to regional factors, including radiation exposure resulting from the FNPP accident.
Highlights
The accident on 11 March 2011 at the Fukushima Daiichi Nuclear Power Plant (FNPP), triggered by the tsunami waves following the Great East Japan Earthquake, raised serious concerns about the health impact of radioactive materials released into the atmosphere from the FNPP
Studies conducted in Japan just after the FNPP accident to estimate thyroid exposure from radioactive iodine released from the FNPP2–4 reported much lower thyroid doses among the evacuees, compared to the Chernobyl Nuclear Power Plant (CNPP) accident
This study showed no substantial geographical clustering in thyroid cancer prevalence among children and adolescent examinees in the first-round of TUE in Fukushima Prefecture
Summary
The accident on 11 March 2011 at the Fukushima Daiichi Nuclear Power Plant (FNPP), triggered by the tsunami waves following the Great East Japan Earthquake, raised serious concerns about the health impact of radioactive materials released into the atmosphere from the FNPP. In the first round of TUE from 2011 to 2015 (including one extended year), 116 thyroid cancer cases with malignancy or strong suspicion of malignancy based on fine needle aspiration cytology were detected from approximately 300,000 children and adolescents This larger-than-expected number of cases incited the suspicion of an ‘epidemic’ of early radiation-induced thyroid cancer[8]. Tsuda et al.[8] aggregated 59 municipalities in Fukushima Prefecture into 9 districts and compared the regional prevalence rates among them They reported excesses of thyroid cancer cases in the central middle district (odds ratio to the reference region, 2.6; 95% confidence interval [CI], 0.99–7.0) suggesting that the geographical excess was induced by exposure of the residents to radioactive contamination within the environment. Due to the high degree of uncertainty in the estimation of internal dose from radioactive iodine-131, which has a short half-life of 8 days[1,4], the distribution of internal exposure dose could differ from that of the external dose
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