To the Editor:
Tsuda et al. analyzed the incidence of thyroid ultrasound screening, referring to the interim results of Fukushima Health Management Survey, and reported that the highest incidence rate was observed in the central middle district of Fukushima Prefecture (prevalence, 605 per million), when compared with Japanese annual incidence. They concluded that an excess of thyroid cancer had been detected by ultrasound among children and adolescents in Fukushima after the accident at the Fukushima Dai-ichi Nuclear Power Plant.1
We recently conducted thyroid ultrasound screening, using the same procedures as the Fukushima Health Management Survey, in 4,365 children aged 3–18 years from three Japanese prefectures, and confirmed one patient with papillary thyroid cancer (prevalence, 230 per million).2 Furthermore, we recently reviewed findings of thyroid ultrasound screening conducted in Japan.3 In one survey, 9,988 students underwent thyroid screening and four students (including one foreign student) were subsequently diagnosed with thyroid cancer (prevalence, 300 per million). In another study at Okayama University that examined 2,307 students, three patients with thyroid cancer were found (prevalence, 1,300 per million), while at Keio High School, of 2,868 female students examined, one was found to have thyroid cancer (prevalence, 350 per million). These results show that the prevalence of thyroid cancer detected by advanced ultrasound techniques in other areas of Japan does not differ meaningfully from that in Fukushima Prefecture.
We should also note that it is important to evaluate carefully the relationship between the incidence of thyroid cancer and exposure doses. In fact, measured values of thyroid doses in more than 1,000 children in Fukushima just after the accident were less than 15 mSv in 99% of children 0–14 years old, with the maximum doses around 35 mSv.2 We must of course evaluate carefully the incidence of thyroid cancer and age at the time of the Fukushima accident. After the accident at Chernobyl in 1986, childhood thyroid cancer increased dramatically because of internal radiation exposure to I-131 through the intake of contaminated milk.4 In Chernobyl, many patients were younger than 10 years old, whereas in Fukushima, no children were under 5 years old and only a few were under 10. We should pay careful attention before making any definitive statements without accurate dose estimations.
Based on the experiences in Chernobyl and other nuclear disasters, we need to continue our efforts at appropriate, scientific evaluation of the health conditions in Fukushima.
Department of Global Health,
Medicine and Welfare
Atomic Bomb Disease Institute
1. Tsuda T, Tokinobu A, Yamamoto E, Suzuki E.. Thyroid cancer detection by ultrasound among residents age 18 year and younger. Epidemiology. 2016;27:316–322
2. Nagataki S, Takamura N.. A review of the Fukushima nuclear reactor accident: radiation effects on the thyroid and strategies for prevention. Curr Opin Endocrinol Diabetes Obes. 2014;21:384–393
3. Hayashida N, Imaizumi M, Shimura H, et al. Thyroid ultrasound findings in a follow-up survey of children from three Japanese prefectures: Aomori, Yamanashi, and Nagasaki. Sci Rep. 2015;5:9046