The Authors Respond.

Abstract

To the Editor: We thank Tsuda et al.1 for indicating three issues to be addressed in the manuscript titled, “Re: Associations between childhood thyroid cancer and external radiation dose after the Fukushima Daiichi Nuclear Power Plant Accident.” First, although we only evaluated the external radiation dose,2 it was reported that the radiation doses to the thyroid gland were much lower in Fukushima than in Chernobyl even if the internal doses from both cesium and iodine were included.3,4 Furthermore, a recent study that analyzed a huge data set of the post-Fukushima food monitoring campaign reported that the internal exposure levels of iodine, even for infants and young children, did not reach 50 mSv in the worst-case scenario.5 Moreover, the areas, suggested to have high exposure to internal radiation,6 were not associated with an increased risk of thyroid cancer in our study.2 Therefore, in the first round of the survey, radiation dose, irrespective of whether it was external or internal, was not associated with thyroid cancer in Fukushima. Second, as pointed out by Tsuda et al.,1 we compared the prevalence ratio within the Fukushima Prefecture, and this may have led to an underestimation of the association between thyroid cancer and the radiation dose. However, in the actual situation, there are no other prefectures where thyroid examination among children and adolescents can be conducted in the same manner as in our study. In our study, the proportion of exposed external radiation of ≥1 mSv was <0.67% in the lowest exposed area,2 and exposed internal radiation in this area was much lower than those in areas near the nuclear power station.7 Because examination of the thyroid gland using ultrasonography could detect asymptomatic thyroid cancer, compared with that in other prefectures, the prevalence of thyroid cancer in the Fukushima Prefecture may be overestimated when different methods and criteria are used. Thus, we believe that the lowest exposed area in Fukushima is validated as a control area for the analysis of the regional differences in the associations between thyroid cancer and radiation exposure. Third, as indicated by Tsuda et al.,1 we analyzed the associations between radiation doses and thyroid cancer prevalence using the first-round survey of thyroid ultrasound examinations. Because the results of the first-round survey are insufficient to conclude the effects of radiation exposure on the risk of thyroid cancer, we shall publish the results of the follow-up surveys as early as possible. Tetsuya OhiraRadiation Medical Science Center for theFukushima Health Management SurveyFukushima Medical UniversityFukushima, JapanDepartment of EpidemiologyFukushima Medical University School of MedicineFukushima, Japan, [email protected] Hideto TakahashiRadiation Medical Science Center for theFukushima Health Management SurveyFukushima Medical UniversityFukushima, JapanNational Institute of Public HealthSaitama, Japan Seiji YasumuraRadiation Medical Science Center for theFukushima Health Management SurveyFukushima Medical UniversityFukushima, JapanDepartment of Public HealthFukushima Medical University School of MedicineFukushima, Japan