Individual Dose Response and Radiation Origin of Childhood and Adolescent Thyroid Cancer in Fukushima II: Possibility of High I-131 Exposure as in Chernobyl

Abstract

Background: Thyroid cancer incidence of individual dose groups in Fukushima residents exposed at ≤18 years of age demonstrated a linear response to thyroid dose estimated in the United Nations Scientific Committee on the effects of Atomic Radiation (UNSCEAR) 2020/2021. Increased childhood thyroid cancer in Fukushima was found to come dominantly from radiation exposure from the nuclear accident. The UNSCEAR 2020/2021 concluded that the apparent excess of thyroid cancers would not be expected at thyroid doses estimated by the UNSCEAR. The purpose of this paper is to solve the puzzle of the high childhood thyroid cancer incidence in Fukushima despite the estimated low thyroid dose. Methods: The conversion coefficient k connecting thyroid doses estimated in UNSCEAR 2020/2021 and doses based on direct thyroid dose measurements in Chernobyl: 1 GyUN2021 = k × 1 Gy (gray), was estimated by comparing incidences and dose dependences of thyroid cancers in Fukushima and Chernobyl after nuclear disasters. Results: The ratio of the observed cases /expected cases from cancer registry: of about 60 in Fukushima prefecture, was higher than the ratios observed after the Chernobyl accident. The thyroid doses estimated by UNSCEAR were corrected by adding a baseline dose to recover the severely underestimated ingestion dose. The conversion coefficients were: k =60~70 from the comparison of the excess absolute risks (EAR) and their dose dependences in Fukushima and in Chernobyl, and k =10~180 from the comparison of excess relative risk per gray (ERR/Gy) in Fukushima with those in Chernobyl. The thyroid doses might have been underestimated by about 1/50~1/100 in UNSCEAR 2020/2021. Conclusion: The dozens-fold increase of childhood thyroid cancer cases after the Fukushima nuclear accident was found to arise from radioactive iodine exposure comparable to that in Chernobyl.

Area Dose–Response and Radiation Origin of Childhood Thyroid Cancer in Fukushima Based on Thyroid Dose in UNSCEAR 2020/2021: High 131I Exposure Comparable to Chernobyl

“…If a strong positive correlation was found between the incidence rate of thyroid cancer and thyroid dose, any factors not associated with dose, including overdiagnosis, were unlikely to cause a high incidence of thyroid cancer. This applies exactly to the present case because strong positive correlations were found between incidence rate and thyroid dose in the first and second TUEs [ 17 , 18 , 19 , 20 , 21 , 22 , 29 , 30 , 32 ]. In addition to the observed dose–response relationship, much evidence was found of a radiation origin of PTC in Fukushima.…”

“…The intercept r0 at zero dose decreased from positive to negative as ingestion dose increased. Hence, we added a base line dose (BLD) to ingestion dose so that the intercept r0 of IR agreed with the expected natural incidence rate from the cancer registry [ 30 ].…”

“…The risk coefficient EAR/Gy UN2021 or ERR/Gy UN2021 of Fukushima should be used, not that of Chernobyl, or thyroid dose calibrated to Gy in Chernobyl should be used. The ERR/Gy UN2021 of Fukushima was derived from the linear individual dose response of the relative risk [ 29 , 30 ].…”

“…In our preceding studies, the thyroid cancer incidence of three individual external dose groups in the second TUE demonstrated a linear response to individual dose in the 0.5–2.5 mSv external dose range [ 29 , 30 ]. In this study, we quantitatively evaluated the dose–response relationship between the incidence rate of thyroid cancer (IR) per person-years (PY), IR/10 4 PY of areas, and thyroid dose estimated in the UNSCEAR 2020/2021 report.…”

“…The increased childhood thyroid cancer in Fukushima was found to arise from radioactive iodine exposure, which was comparable to that in Chernobyl. The IR-dose plot of the four areas and three individual dose groups, low (<1 mSv), middle (1-2 mSv), and high (≥2 mSv), in the second TUE [29,30] is shown in Figure 3B for BLD = 0.003 Gy UN2021 and k = 1 (blue rhombus). The observed EAR/10 4 PY Gy UN22021 of 143 (95% CI: 122, 165) in the second TUE is approximately 60 (=143/2.32) times the average EAR/Gy value of 2.32 after the Chernobyl accident (Table 3).…”