Many cases of thyroid cancer

Regular thyroid exams are being conducted for persons who were 18 or younger at the time of the Fukushima nuclear accident, and among them, 237 were diagnosed with or suspected of having malignant thyroid cancer, of which it was confirmed in 186 cases (based on published reports to February 13, 2020). In addition, at least 11 children underwent surgery or treatment for thyroid cancer at the Fukushima Medical University. This is because they were not counted in the totals of the Fukushima Health Management Survey if they were diagnosed as having thyroid cancer while under observation. In addition, persons who had thyroid exams in the Fukushima Health Management Survey, it was announced that 257 persons (as of December 2018) were in a support program for patients with thyroid lumps (nodular lesions), but the details are not provided.

[Table: Number of children with thyroid cancer (aged 18 and under living in Fukushima Prefecture at time of the accident)]

In any case, it is difficult to determine the actual numbers, and some patients are clearly missing from the statistics announced by the Fukushima Health Management Survey Committee. The results of surgeries are only being partially announced, but clearly many patients have lymph node metastasis and extrathyroidal invasion, and some have remote metastasis.

The male-to-female ratio ranged from 1:1 to 1:2, which is higher than with typical ratios for thyroid cancer (1:7 to 1:8).[1]

“Dozens of time higher”

The Thyroid Examination Evaluation Subcommittee of the Fukushima Health Management Survey Review Committee issued a report noting that during the first screening cycle, “the number of detected cases is dozens of times higher than the number of thyroid cancer cases estimated by the national government’s regionally registered cancer morbidity statistics.” Despite that, the report stated that “It was difficult to consider these to be impacts of the nuclear accident.” After the second screening, the committee reported the following:

  • The detection rate of thyroid cancer was slightly lower in the second compared to the first screening, but it was still tens of times higher than the morbidity rate estimated from patient statistics of thyroid cancer based on regionally registered cancer morbidity statistics.
  • Regarding the detection rate of malignant or suspected cases, when compared simply without considering gender or age, it is highest, in rising order, in 13 municipalities in evacuation zones, Nakadori, Hamadori, and the Aizu region.
  • No clear correlation was observed between radiation exposure and the rate of thyroid cancer detection as a result of using estimated thyroid absorption doses published by the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)

Data analysis expert Prof. Yutaka Hamaoka (Faculty of Business and Commerce, Keio University) responded, “In the summary, they claim that the detection rate of malignant or suspected malignant cases, when compared simply and without considering gender or age, is highest, in rising order, in 13 municipalities in evacuation zones, Nakadori, Hamadori, and the Aizu region. However in my opinion, for gender differences, there is no regional difference, so even if adjusted for gender, the result is the same. As for age, the exams were done first in 13 municipalities, including evacuation areas, so the age there at the time of examination will have been younger. Since the detection rate of the thyroid cancer increases with age, if adjusted for age, the regional differences will become larger.” Thus, he said that it is not correct to say that regional differences are due to gender and age differences. Also, he said that analytical methods using UNSCEAR dosages reduce detection capacity due to the way they break down analytical subjects, so there are problems with statistical methods.

Lymph node metastasis, extrathyroidal invasion, relapse

Initially, the high detection numbers for thyroid cancer were attributed to the so-called “screening effect”.[2] However, the detection at tens of times higher in the first round of screening cannot be explained by the “screening effect” alone, and it does not explain the 71 cases of thyroid cancer detected or suspected in the second round, just two years later.

Some experts claim that the high incidence of thyroid cancer detection can be explained by “over-diagnosis” (which means diagnosis of cancer that does not threaten one’s life or is asymptomatic).

However, Fukushima Medical University actually does conduct follow-up observation of minor and low-risk cancers. Prof. Shinichi Suzuki of Fukushima Medical University, who performed the surgeries, said that among 180 cases of thyroid cancer, 72% had lymph node metastasis and in 47% of cases it had spread to surrounding tissues, and surgery was necessary in all those cases.[3] He said that relapses occurred in 6% of the cases and required repeat surgery.

Advanced-stage cancer cases detected outside Fukushima Prefecture

Children with severe thyroid cancer have also been found outside of Fukushima Prefecture. Since December 2016, the 3.11 Fund for Children with Thyroid Cancer (President: Hisako Sakiyama) has helped to cover the medical expenses of patients who live in Tokyo and 15 prefectures, have thyroid cancer symptoms and were under the age of 18 at the time of the accident. Up to December 2019, it had supported 160 patients (104 from Fukushima Prefecture, 56 from elsewhere). The organization says that outside of Fukushima Prefecture there have been many cases of cancer being detected at an advanced stage because large-scale screening for thyroid cancer was not being done. Spokesperson Hisako Sakiyama commented, “Some committee members were saying that testing at schools should stop, but the people affected would actually like testing to be expanded and improved. For diagnosis, we should acknowledge that early detection and early treatment are succeeding,” she said.


[1] The male-to-female gender difference in ratio of juvenile thyroid cancer at Noguchi Hospital, Ito Hospital, and Kuma Hospital was 1:7.7. Source: Presentation materials by Akira Yoshida (Kanagawa Prefectural Medical Association) at International Symposium by Fukushima Medical University, March 2, 2020.

[2] The “screening effect” is the higher number of detections of a disease as a result of mass screening, because undiagnosed or pre-symptomatic disease is discovered, more than when people go for diagnosis after they notice symptoms.

[3] Second International Symposium of Radiation Medical Science Center for the Fukushima Health Management Survey, on February 3, 2020

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