Toshiko Kato

Abstract

After the Fukushima nuclear power plant accident in Japan on March 11, 2011, thyroid ultra-sound examinations (TUEs) were conducted as part of the Fukushima health management survey (FHMS) on all residents aged ≤18 years at the time of the accident. The result showed tens of times increase in thyroid cancer detection compared with the expected incidence from the cancer registry. Despite the extraordinary high incidence, the radiation origin of thyroid cancer has not been recognized and possible overdiagnoses are suggested, largely due to the low thyroid doses estimated by UNSCEAR 2020/2021.

The linear response of annual incidence rates per person-years (PY) to averaged thyroid dose of four areas in Fukushima based on UNSCEAR 2020/2021 in the first six years indicated dominant radiation origin, not due to overdiagnosis, of childhood and adolescent thyroid cancer. The excess absolute risk per Gray (EAR/Gy104PY) of 143 (95%CI: 122, 165) in the second TUE (p≤0.001) was approximately 60 times higher than the EAR/104PY Gy=2.3 observed after the Chernobyl accident, suggesting an underestimation of approximately 1/60 by UNSCEAR compared with the thyroid dose in Chernobyl. The increased childhood thyroid cancer in Fukushima was found to arise from radioactive iodine exposure comparable to that in Chernobyl. UNSCEAR’s conclusion that a large excess of thyroid cancer, as observed in the TUEs, would not be expected based on the low dose estimation should be retracted.

In addition to the epidemiological evidence, it was reported that “the majority of detected thyroid cancer patients in the second TUE (83%) and third TUE (64%) had no nodules in the preceding TUE”. This indicated that among young residents in Fukushima, thyroid cancer newly appears and grows in two-years between TUEs and the only possible explanation for thyroid cancers in FHMS —a small nodule growing rapidly—seems to be the effect of radiation exposure. Despite the high rate of recurrence (10%) and metastasis (including 5/220 cases of lung metastasis), the FHMS published “Merit and demerit of TUE” and requires parents of eligible people to understand the merits and demerits of TUE before making a decision to undergo TUE due to concerns about overdiagnosis.

Many patients are told at their initial diagnosis that “your thyroid cancer is unrelated to radiation exposure from the nuclear accident”. More than 400 thyroid cancer patients endure poor health, being unable to disclose that they had surgery of thyroid cancer given the official view of the government that the nuclear accident had no health effects. Discussions about thyroid cancer are criticized as “harmful rumors” that link Fukushima to radioactive contamination and hinder recovery efforts. Without any international recognition of radiation health effects in Fukushima, the situation experienced by patients in Fukushima may be experienced by people exposed in the next nuclear accident.