Effects of Thyroid Size and Short-lived Radionuclides on Measurement Values in Pediatric Thyroid Screening: Simulation Studies

Following the 2011 accident at the Fukushima Daiichi Nuclear Power Plant, simplified thyroid screening was carried out in children to determine internal exposure to radioactive iodine. Because NaI(Tl) scintillation survey meters - originally designed for environmental radiation - were used, quantifying radioiodine accumulated in the thyroid involved considerable uncertainty. This study investigated factors impacting measurements, specifically thyroid size (volume) and the contribution of short-lived radionuclides other than ¹³¹I (132I, 133I, and 135I) using Monte Carlo simulations with the PHITS code. For 1-y-old children, comparison between standard and minimal thyroid volumes showed that volume differences combined with variations in soft tissue thickness could cause the thyroid equivalent dose to differ by up to 76.5%. Furthermore, considering short-lived radionuclides revealed that at a ¹³¹I thyroid-equivalent dose of 100 mSv, the actual dose could reach 131.7 mSv. Thus, a screening level was used in the simplified method to effectively detect ¹³¹I. As individual differences in thyroid volume exist even among children of the same age, contributions from radionuclides other than ¹³¹I may not be negligible, impacting screening level use. The results suggest that to assess thyroid radiation dose more accurately, it is necessary to consider both age and individual thyroid volume differences, which can cause measurement errors up to 76.5%. Regarding dose contribution from short-lived radionuclides, estimates based on relative proportions of radioactive materials adhered to evacuees' clothing at the time of the accident indicated a maximum impact of 31.7%.