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The Authors Respond

Ohira, Tetsuya; Takahashi, Hideto; Yasumura, Seiji

doi: 10.1097/EDE.0000000000000941
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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

The authors report no conflicts of interest.

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To the Editor:

We thank Dr. Kato for indicating comments 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.”1

Kato pointed out that the high prevalence in high-dose groups has been counteracted by the short elapsed time from exposure to screening (1.4 years for A compared with 2.8 years for E) in the first-round examination.2 However, as shown in our previous study,3 the duration from the accident to the first-round thyroid examination was not associated with the prevalence of thyroid cancer, regardless of the location.3 Furthermore, there were no interactions between radiation dose and the duration from the nuclear accident to the thyroid examination.3 Because the thyroid cancers observed in the first-around examination are likely a mixture of prevalent and incident cases, we therefore think that the results of our study might not be affected by the short elapsed time in the high-dose group.

Kato also showed a linear prevalence–dose relation when the prevalence of thyroid cancer versus external dose is plotted using the data of both first- and second-round thyroid examinations. However, four major methodologic issues should be considered. First, we classified 59 municipalities in Fukushima into quintiles according to the proportion of residents who received an external radiation exposure of ≥1 mSv to avoid arbitrariness. The numbers in groups A–E varied because of the varying number of participants in each municipality and some municipalities having more than 40,000 participants. In contrast, the grouping by Kato appears to be arbitrarily assigned. Second, as mentioned earlier, the thyroid cancers observed in the first-round examination are likely a mixture of prevalent and incident cases, and intervals between the first- and second-round examinations are different among groups A–E. Therefore, analysis using only the combined data of the first- and second-round examinations is not appropriate. Third, because Kato analyzed the data published from the Fukushima Health Management Survey, it is impossible to analyze the association between thyroid cancer and radiation doses with adjustment for confounding factors. To avoid ecologic fallacy, individual data must be used to analyze the associations. Finally, the participation rate of the confirmatory examination was different depending on the municipality (from 50% to 100%),4 and the lower-exposed municipalities tended to have a lower participation rate in the confirmatory examination. Furthermore, the rate of the aspiration biopsy cytology in the confirmatory examination was also different depending on the municipality, and the lower-exposed municipalities tended to have a lower rate in aspiration biopsy cytology. Because the participation rate of the confirmatory examination and the rate of the aspiration biopsy cytology were strongly associated with the detection rate of thyroid cancer, the prevalence of thyroid cancer in lower-exposed municipalities must be underestimated, and this may lead to ecologic fallacy. Therefore, the results of the second-round thyroid examination must be interpreted with caution.

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. Kato TRe: Associations between childhood thyroid cancer and external radiation dose after the Fukushima Daiichi Nuclear Power Plant Accident. Epidemiology. 2019;30:e9e11.
2. Ohira T, Takahashi H, Yasumura S, et alAssociations between childhood thyroid cancer and external radiation dose after the Fukushima Daiichi Nuclear Power Plant Accident. Epidemiology. 2018;29:e32–e34.
3. Ohira T, Takahashi H, Yasumura S, et alComparison of childhood thyroid cancer prevalence among three areas based on external radiation dose after the Fukushima Daiichi Nuclear Power Plant Accident: The Fukushima Health Management Survey. Medicine. 2016;95:e4472.
4. Thyroid Ultrasound Examinations (First Full-Scale Thyroid Screening Program). The 28th Prefectural Oversight Committee Meeting for Fukushima Health Management Survey. Available at: http://fmu-global.jp/survey/the-28th-prefectural-oversight-committee-meeting-for-fukushima-health-management-survey/. Accessed December 10, 2018.
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