Re: Associations Between Childhood Thyroid Cancer and External Radiation Dose After the Fukushima Daiichi Nuclear Power Plant Accident

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).

FIGURE.: Panel A, Map of four groups A + B, C, D, and E divided by the criterion P(A + B) ≥ 55.4% > P(C) ≥ 5.7% > P(D) ≥ 0.80% > P(E), where P is the proportion of residents with external exposure dose of ≥1 mSv. Panel B, Thyroid cancer cases per 100,000 residents aged ≤ 18 years on March 11, 2011 versus external dose for E-I (FY2011-2013) and E-I+II (FY2011-2015). An approximate linear dose relation is observed in E-I+II in 6 years after exposure, with t and p-values for slope parameters in the Figure.

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.

TABLE.: External Dose Based on the Fukushima Health Management Basic Survey, Number of Primary Examinees, Confirmed or Suspected Cancer Cases, Thyroid Cancer cases per 100,000 for Examinations I, II, and I + II

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.

ACKNOWLEDGMENT

The author is grateful to Professor Yutaka Hamaoka of Keio University for instructive discussions.

Toshiko Kato
Kangoku-cho, Nara, Japan
[email protected]