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Ohira, Tetsuya; Takahashi, Hideto; Yasumura, Seiji

doi: 10.1097/EDE.0000000000000897
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Radiation Medical Science Center for the Fukushima Health Management Survey, Fukushima Medical University, Fukushima, Japan, Department of Epidemiology, Fukushima Medical University School of Medicine, Fukushima, Japan, teoohira@fmu.ac.jp

Radiation Medical Science Center for the Fukushima Health Management Survey, Fukushima Medical University, Fukushima, Japan, National Institute of Public Health, Saitama, Japan

Radiation Medical Science Center for the Fukushima Health Management Survey, Fukushima Medical University, Fukushima, Japan, Department of Public Health, Fukushima Medical University School of Medicine, Fukushima, Japan

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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 Ohira

Radiation Medical Science Center for the

Fukushima Health Management Survey

Fukushima Medical University

Fukushima, Japan

Department of Epidemiology

Fukushima Medical University School of Medicine

Fukushima, Japan, teoohira@fmu.ac.jp

Hideto Takahashi

Radiation Medical Science Center for the

Fukushima Health Management Survey

Fukushima Medical University

Fukushima, Japan

National Institute of Public Health

Saitama, Japan

Seiji Yasumura

Radiation Medical Science Center for the

Fukushima Health Management Survey

Fukushima Medical University

Fukushima, Japan

Department of Public Health

Fukushima Medical University School of Medicine

Fukushima, Japan

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REFERENCES

1. Tsuda T, Tokinobu A, Yamamoto E, Suzuki E. Re: associations between childhood thyroid cancer and external radiation dose after the Fukushima Daiichi nuclear power plant accident. Epidemiology. 2018;29:e56–e57.
2. Ohira T, Takahashi H, Yasumura S, et al; Fukushima Health Management Survey Group. Associations between childhood thyroid cancer and external radiation dose after the Fukushima Daiichi nuclear power plant accident. Epidemiology. 2018;29:e32–e34.
3. Tokonami S, Hosoda M, Akiba S, Sorimachi A, Kashiwakura I, Balonov M. Thyroid doses for evacuees from the Fukushima nuclear accident. Sci Rep. 2012;2:507.
4. Tsubokura M, Kato S, Nomura S, et al. Absence of internal radiation contamination by radioactive cesium among children affected by the Fukushima Daiichi nuclear power plant disaster. Health Phys. 2015;108:39–43.
5. Steinhauser G, Chávez-Ortega M, Vahlbruch JW. Japanese food data challenge the claimed link between Fukushima’s releases and recently observed thyroid cancer increase in Japan. Sci Rep. 2017;7:10722.
6. Kim E, Kurihara O, Kunishima N, Momose T, Ishikawa T, Akashi M. Internal thyroid doses to Fukushima residents-estimation and issues remaining. J Radiat Res. 2016;57(suppl 1):i118–i126.
7. UNSCEAR. Annex A. Levels and effects of radiation exposure due to the nuclear accident after the 2011 Great East-Japan Earthquake and Tsunami. Sources and effects of ionizing radiation. 2014.New York, NY: UNSCEAR 2013 Report Vol. I. United Nations;
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