2018 Volume 67 Issue 1 Pages 42-49
After the accident occurred at the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant, the Fukushima Health Management Survey (FHMS) was initiated. The FHMS consists of a basic survey and four detailed surveys: a thyroid ultrasound examination, a comprehensive health check, a mental health and lifestyle survey, and a pregnancy and birth survey. In this article, we briefly summarized whether an association exists between radiation exposure and the observation of thyroid cancer cases according to the results of the first-round thyroid examination in the FMHS. Regarding this issue, Tsuda and his colleagues showed an association using an internal comparison (odds ratio (OR)=2.6, 95% confidence interval (CI): 0.99-7.0) and an external comparison (incidence rate ratio =50, 95% CI : 25-90). However, for this internal comparison, Ohira and his colleagues used two ways of objective classifications of districts in Fukushima; (1) the group of municipalities of which proportion of the exposed external dose level of more than 5 mSv was higher than or equal to 1% (≧1% of 5 mSv), the group of municipalities of which proportion of the exposed external dose level less than 1 mSv was higher than or equal to 99.9% (≧99.9% of 1m Sv<99%), and others, and (2) the location groups applied by WHO. For the classification (1), they obtained OR=1.49 (95% CI : 0.36-6.23) from the highest group to the lowest, which was similar to the results of the classification (2). For the external comparison, Takahashi and his colleagues developed a cancer-progression model with several sensitivities under non-accident conditions, and showed 116 cases were possible to observe in Fukushima under non-accident conditions. Katanoda and his colleagues found an observed/expected ratio of 30.8 (95%CI: 26.2-35.9) of the prevalence of thyroid cancer among residents aged ≦ 20
years (160.1 observed of cases and 50.2 expected cases), and a cumulative number of thyroid cancer deaths in Fukushima Prefecture of 0.6 under age 40 with the same method. This large disparity implied the possibility of over-diagnosis in thyroid examinations.
A researcher reported the results were unlikely to be explained by a screening effect, which implied the association between thyroid cancer cases and external radiation exposure. However, subsequently, a possibility that it might be a result of over-diagnosis of the thyroid examinations was pointed. And, no significant associations were found by applying objective classification of districts and by raising comparability with the incidence data of whole Japan, respectively. In the Basic Survey of FHMS, only individual external doses in the first four months after the accident has been observed. So neither external dose after the four months nor internal dose was applied in these studies. Further studies are necessary to clarify the existence of the association by applying the estimation of individual overall thyroid dose.