We appreciate an opportunity to respond to an attempt by Ohira et al.1 to "clarify" an inconsistency between conclusions of the international organizations and our study2 regarding the health impact of radiation exposure from the 2011 Fukushima Daiichi nuclear power plant (FDNPP) accident.
Ohira et al.1 cited four references3-6 in their criticism that our paper2 "suffered from fundamental limitations: unsuitable geographic classifications, disregard of the slow-growing nature of thyroid cancer and its relation with radiation dose, and inappropriate statistical methods."
The geographic classification of Fukushima Prefecture into nine districts was not arbitrary but attained by taking into consideration the schedule sequence of the thyroid screening program launched by the prefecture, wherein the whole prefecture was divided into three areas in the order of screening: the nearest area to the FDNPP in Fiscal Year (FY) 2011; the middle area in FY 2012; and the least contaminated area in FY 2013. The middle and the least contaminated areas were further divided into four districts each, by separating the three most populated cities—Fukushima City, Koriyama City, and Iwaki City—to minimize variance, creating nine districts.
Regarding "the slow-growing nature of thyroid cancer"1, 56 (81%) of 69 thyroid cancer cases detected by cytology in the second round screening did not have any nodules in the first round. These 56 cases had tumors that apparently grew from undetectable to more than 5.0 mm during 2.5 years in-between the two rounds. As for the relation with radiation dose, it is meaningless to solely depend on inference from a known dose-response relation when the dose estimates are widely distributed among various reports2. In addition, lack of individual dose estimates in Fukushima, as appropriately mentioned by Davis6, leads incidence rate ratio towards the null, resulting in underestimation in the sensitivity analysis.
The claim of "inappropriate statistical methods" lacked any specific reference to our statistical methods1. Takahashi et al.4, cited by the authors, only pointed out "mean latent duration" which we assigned 4 years to estimate incidence rate ratio. As we responded1, our conclusion would not change no matter what number of years—anywhere from one to several decades within a realistic human life—was assigned in the sensitivity analysis. Another reference cited, Takamura5, introduced prevalence in unexposed groups in Japan without any statistical inference, including the "distorted" prevalence at Okayama University7. Furthermore, Takamura5 compared Fukushima cancer cases within 5 years after the accident with the Chernobyl cancer cases observed more than 12 years after the accident in those under age 5 at exposure.
The authors' criticism that our paper "suffered from fundamental limitations" has no fundamental basis.
Next, our critiques concern two issues.
First, it is not true that their classification of highest dose area was "exactly the same" as the "Group 1" defined in the WHO report8. The WHO's preliminary dose estimation9, based on the 2013 report, excluded areas within 20 km from the FDNPP. The authors, however, included residents in the 20 km zone mostly as "Group 2" shown in Figure 1. Moreover, estimated doses for 10-year-old children and 1-year-old infants were the same in "Group 2" and "Group 3" in the WHO's 2012 report9 (Table 2 in pages 44-45; Table 4 in pages 46-47).
Second, they conducted only internal comparison within Fukushima Prefecture, when every data and reports indicate almost the entire prefecture, even "Group 3," was exposed as mentioned in our paper2. What is of critical interest here is not the variability of thyroid cancer within the prefecture but thyroid cancer as a potential health effect of the nuclear accident. This can be estimated by comparison with data outside the prefecture, or pre-accident data in the prefecture which is nonexistent. Thus our comparison was made with the Japanese national data before the accident as well as the Chernobyl data of the unexposed and the relatively low exposed, showing more than 20-fold excess of thyroid cancer2.
Toshihide Tsuda, MD, PhD (Graduate School of Environmental and Life Science, Okayama University)
3-1-1, Tsushima-naka, Okayama, 700-8530, JAPAN
1. Ohira T, Takahashi H, Yasumura S, Ohtsuru A, Midorikawa S, Suzuki S, Fukushima T, Shimura H, Ishikawa T, Sakai A, Yamashita S, MD, Tanigawa K, Ohto H, Abe M, Suzuki S, and for the Fukushima Health Management Survey Group: Comparison of childhood thyroid cancer prevalence among 3 areas based on external radiation dose after the Fukushima Daiichi nuclear power plant accident. The Fukushima health management survey. Medicine 2016; 95:35(e4472) http://dx.doi.org/10.1097/MD.0000000000004472.
2.Tsuda T (Tsuda 2016a), Tokinobu A, Suzuki E, Yamamoto E: Thyroid Cancer Detection by Ultrasound among Residents Aged 18 Years and Younger in Fukushima, Japan: 2011 to 2014. Epidemiology 2016; 28; 316-322.
3.Williams D. Thyroid growth and cancer. Eur Thyroid J 2015;4:164–73.
4.Takahashi H, Ohira T, Yasumura S, et al. Re: thyroid cancer among young people in Fukushima. Epidemiology 2016;27:e21.
5.Takamura N. Re: thyroid cancer among young people in Fukushima. Epidemiology 2016;27:e18.
6.Davis S. Screening for thyroid cancer after the Fukushima disaster. What do we learn from such an effort? Epidemiology 2016;27: 323–5.
7.Tsuda T (Tsuda 2016b), Tokinobu A, Suzuki E, Yamamoto E: The Authors Respond. Epidemiology 2016; 28; e21-e23.
8.World Health Organization. 1. Introduction, 2. Methodology, and 3. Results. Preliminary Dose Estimation from the Nuclear Accident after the 2011 Great East Japan Earthquake and Tsunami. Geneva: WHO Press; 2012:13–47.
9.World Health Organization. 5. Risk characterization. Health Risk Assessment from the Nuclear Accident after the 2011 Great East Japan Earthquake and Tsunami Based on a Preliminary Dose Estimation. Geneva: WHO Press, 2013;51–69.