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

Abstract

The FMU and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) concluded that the high incidence of thyroid cancer after the Fukushima nuclear accident was not the result of radiation exposure, but rather might have been overdiagnosis based on the low thyroid dose estimated in the UNSCEAR 2020/2021 report. In this study, the origin of increased PTC in Fukushima was examined based on the thyroid dose estimated by UNSCEAR. The dose-response relationship of the incidence rate per person-years (PY) was analyzed for four areas in Fukushima prefecture via regression analysis. The linear response of the annual incidence rates to thyroid dose in the first six years showed that the dominant origin of childhood thyroid cancer was radiation exposure. Excess absolute risk (EAR) proportionally increased with thyroid dose, with an EAR/10⁴ PY Gy of 143 (95%CI: 122, 165) in the second TUE (p < 0.001), which is approximately 50-100 times higher than the EAR/10⁴ PY Gy ≒ 2.3 observed after the Chernobyl accident. This suggests an underestimation of the thyroid dose by UNSCEAR of approximately 1/50~1/100 compared with the thyroid dose for Chernobyl. The increased childhood thyroid cancer in Fukushima was found to arise from radioactive iodine exposure, which was comparable to that in Chernobyl.

Keywords: 131I exposure; Chernobyl; Fukushima; UNSCEAR 2020/2021; area dose–response; childhood thyroid cancer; excess absolute risk; overdiagnosis; radiation-induced PTC.

Figure 1

Map of 4 areas in the Fukushima health management survey (FHMS)-division of Fukushima prefecture.

Figure 2

(A) Response of thyroid cancer incidence rates IR1 and IR2, and (B) odds ratio of the incidence rates, OR1 (95% CI) and OR2, to thyroid dose estimated for 10-year-old children in UNSCEAR 2020/2021 for four areas (1–4) in the FHMS (Fukushima health management survey)-division. Regression lines, formulae, and p-values for linear dose coefficients are provided.

Figure 3

(A) Thyroid dose–response of IR/10⁴ PY of thyroid cancer of 4 areas in FHMS-division in the first TUE for BLD = 0.0025 Gy^UN2021 (dose unit in Fukushima based on UNSCEAR 2020/2021). Blue rhombus: for 1 Gy^UN2021 = 1 Gy (k = 1), and orange square: for 1 Gy^UN2021 = 125 Gy (k = 125). The average linear response of IR (≒EAR) on thyroid dose after the Chernobyl accident (unit Gy) is shown by black line (y = 2.32x + 0.04). (B) Thyroid dose–response of IR/10⁴ PY in the second TUE for four areas (1–4) and three individual dose groups (h, m, and l) for BLD = 0.003 Gy^UN2021. Blue rhombus: for 1 Gy^UN2021 = 1 Gy (k = 1). Orange square for four areas, and green square of orange frame for three dose groups are for 1 Gy^UN2021 = 60 Gy (k = 60). The average linear response of IR on thyroid dose after the Chernobyl accident is shown by black line (y = 2.32x + 0.04). Regression lines, formulae, and p-values for linear dose coefficients are given.