To the Editor:
Ohira et al.1 studied the associations between childhood thyroid cancer and external radiation dose in Fukushima from the results of the first-round thyroid ultrasound examination (E-I) conducted in FY2011-2013 and completed in FY2014 as part of the Fukushima Health Management Survey (FHMS).1 They concluded that regional differences in the external radiation dose were not associated with thyroid cancer prevalence in the first 4 years after the accident. However, the analysis should be extended to 6 years because the final results of the second-round examination conducted in FY2014-2015 (E-II) became available in 2016. In this letter, we analyzed data of E-I, E-II, and E-I+II (FY2011-2015) to study the relation between thyroid cancer and radiation dose during 6 years after exposure.
Ohira et al.1 analyzed the area dependence by classifying municipalities into five groups A–E, according to the proportion (P) of residents with external exposure dose of ≥1 mSv, with borders of P = 66%, 55.4%, 5.7%, and 0.67%. We could not reproduce their groups based on the publicly available data of the FHMS Basic Survey.2 Because the reconstructed groups B and E were too small, we combined groups A and B (A + B) and shifted the border of P from 0.67% to 0.80%. Map of four groups divided by the criterion P(A + B) ≥ 55.4% > P(C) ≥ 5.7% > P(D) ≥ 0.80% > P(E) is shown in Figure panel A. We estimated the external dose of each group from an average of doses for each municipality weighted by the number of primary examinees E-I, where the dose for each municipality was an average of doses weighted by the number of the Basic Survey respondents.2 Calculated external doses of groups decrease in the descending order of P. (Table).
Odds ratios (ORs) of thyroid cancer in E-I+II, using the least contaminated area E as reference, rare larger for higher external dose groups. Thyroid cancer cases per 100,000 versus external dose is plotted for E-I and E-I+II in Figure panel B. We observe a linear prevalence–dose relation in 6 years after exposure. These observations suggest a possible association between childhood thyroid cancer in Fukushima and radiation exposure.
Regional differences in external radiation dose were not associated with thyroid cancer prevalence for E-I, in agreement with Ohira et al.’s conclusion for the first 4 years after exposure. This is likely because the high prevalence in high-dose groups has been counteracted by the short elapsed time from exposure to screening (1.4 years for group A compared with 2.8 years for E).1 The high prevalence of childhood thyroid cancer cannot be attributed solely to mass screening3 which is independent of radiation dose and elapsed time from exposure.
The author is grateful to Professor Yutaka Hamaoka of Keio University for instructive discussions.
Kangoku-cho, Nara, Japan
1. Ohira T, Takahashi H, Yasumura S, et alFukushima 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. Suzuki S, Suzuki S, Fukushima T, et alComprehensive survey results of childhood thyroid ultrasound examinations in Fukushima in the first four years after the Fukushima Daiichi Nuclear Power Plant Accident. Thyroid. 2016;26:843–851.