We read with interest the paper by Ohira et al. of thyroid ultrasound examinations in Fukushima, which examines the relation between external radiation dose and thyroid cancer prevalence among Fukushima children.1 However, we point out that their classification of 59 municipalities in Fukushima prefecture into 3 areas is inappropriate. The "lowest dose area" was constituted of Aizu area with least thyroid dose and a distant Iwaki city with the highest thyroid dose, which led to a wrong conclusion that the external radiation dose was not associated with thyroid cancer prevalence among Fukushima children.
Ohira et al. of Fukushima Medical University examined the association between the prevalence of thyroid cancer and radiation dose among Fukushima residents.1 They used external radiation dose estimated by Fukushima Health Management Survey (FHMS) based on individual external doses from behavior data of 26.4% residents who responded the questionnaire.2 They classified municipalities based on fraction of respondents: "highest dose area" (≥1% received external radiation exposure of ≥5 mSv), "lowest dose area" (≤1% received ≥1 mSv), and the other "middle dose area". Mainly because the prevalence of thyroid cancer was found not to decrease in this order of decreasing external dose, they concluded that external dose due to nuclear accident is not associated with thyroid cancer prevalence. However, their classification of municipalities based on fraction of residents (1%) whose exposure exceeds 5mSv and 1mSv does not represent the average exposure dose in each municipality. Moreover, they seem to have made a serious mistake in their classification as follows.
Internal exposure to I-131 is known to be closely associated with the incidence of thyroid cancer among children. The UNSCEAR report on absorbed dose in thyroid 3a,b shows that the Ohira et al.'s "lowest external dose area" is composed of the Aizu area with least thyroid dose and a distant Iwaki city with the highest thyroid dose in Fikushima prefecture except evacuation zone. In a recent estimation of internal thyroid dose using a combination of thyroid measurement data, whole-body counter measurement data and atmospheric transport dispersion simulations, the residents of three municipalities including Iwaki city were shown to have the highest thyroid equivalent dose in Fukushima prefecture. 4 The "lowest external dose area" is found to be composed of the lowest dose Aizu area and the highest thyroid dose Iwaki city. Incidence rates of thyroid cancer for the highest, middle and lowest external dose areas in ref. 1 and those for Aizu and Iwaki districts constituting the "lowest dose area" are listed in Table 1. The average total effective dose to 10-year-old children estimated by UNSCEAR3b,c shows that the effective dose of Iwaki city is the 18th highest in 59 municipalities. If Iwaki city is classified as "middle external dose area" instead of "the lowest external dose area", dose response of thyroid cancer prevalence may be recovered.
The conclusion of ref. 1 that external radiation dose due to nuclear accident is not associated with thyroid cancer prevalence among Fukushima children is found to come from the wrong constitution of the "lowest dose area" as a sum of Aizu with lowest thyroid dose and Iwaki city with the highest thyroid dose. External radiation dose estimation may possibly reveal the dose dependence of thyroid cancer, if it is used carefully with referring to various estimations of external dose and absorbed dose in thyroid.
Toshiko Kato, Dr. Science (Kyoto University)
11-3-604 Kangoku-cho, Nara, 630-8242, Japan
1. Ohira, T, Takahashi H, Yasumura S, et al. 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): p e4472 doi: 10.1097/MD.0000000000004472
2. Fukushima Health Management Survey, Basic Survey Appendix: Estimated external radiation dose, Web site. http://fmu-global.jp/?wpdmdl=1872 Published Sep. 15, 2016, Accessed January 11, 2017.
3. UNSCEAR 2013 Report Vol. I Sources, Effects and Risks of Ionizing Radiation. Published 2014 March, Accessed January 11, 2017.
a. Absorbed Dose on Thyroid in Japan for the first year Web site.http://www.unscear.org/docs/reports/2013/UNSCEAR_2013A_C-16_Thyroid_doses_Japan_first_year_2014-08_corrected.pdf
b. Estimated doses to evacuees in Japan for the first year Web site.http://www.unscear.org/docs/reports/2013/UNSCEAR_2013A_C-18_Doses_evacuees_Japan_first_year_2014-08.pdf
c. Effective doses in Japan for the first year Web site. http://www.unscear.org/docs/publications/2013/UNSCEAR_2013_Annex-A_Attach_C-14.pdf
4. Internal thyroid doses to Fukushima residents—estimation and issues remaining. Kim E, Kurihara O, Kunishima N, Momose T, Ishikawa T, Akashi M. J Radiat Res. 2016 Aug; 57(Suppl 1): i118–i126.