Abstract
Radioactive iodine released in nuclear accidents may accumulate in the thyroid and by irradiation enhances the risk of cancer. Radioiodine uptake into the gland can be inhibited by large doses of stable iodine or perchlorate. Nutritional iodine daily intake may impact thyroid physiology, so that radiological doses absorbed by the thyroid as well as thyroid blocking efficacy may differ in Japanese with a very rich iodine diet compared to Caucasians. Based on established biokinetic–dosimetric models for the thyroid, we derived the parameters for Caucasians and Japanese to quantitatively compare the effects of radioiodine exposure and the protective efficacy of thyroid blocking by stable iodine at the officially recommended dosages (100 mg in Germany, 76 mg in Japan) or perchlorate. The maximum transport capacity for iodine uptake into the thyroid is lower in Japanese compared to Caucasians. For the same radioiodine exposure pattern, the radiological equivalent thyroid dose is substantially lower in Japanese in the absence of thyroid blocking treatments. In the case of acute radioiodine exposure, stable iodine is less potent in Japanese (ED50 = 41.6 mg) than in Caucasians (ED50 = 2.7 mg) and confers less thyroid protection at the recommended dosages because of a delayed responsiveness to iodine saturation of the gland (Wolff–Chaikoff effect). Perchlorate (ED50 = 10 mg in Caucasians) at a dose of 1000 mg has roughly the same thyroid blocking effect as 100 mg iodine in Caucasians, whereas it confers a much better protection than 76 mg iodine in Japanese. For prolonged exposures, a single dose of iodine offer substantially lower protection than after acute radioiodine exposure in both groups. Repetitive daily iodine administrations improve efficacy without reaching levels after acute radioiodine exposure and achieve only slightly better protection in Japanese than in Caucasians. However, in the case of continuous radioiodine exposure, daily doses of 1000 mg perchlorate achieve a high protective efficacy in Caucasians as well as Japanese (> 0.98). In Caucasians, iodine (100 mg) and perchlorate (1000 mg) at the recommended dosages seem alternatives in case of acute radioiodine exposure, whereas perchlorate has a higher protective efficacy in the case of longer lasting radioiodine exposures. In Japanese, considering protective efficacy, preference should be given to perchlorate in acute as well as prolonged radioiodine exposure scenarios.
Highlights
Nuclear fission reactions generate several hundred radionuclides, most of them with very short decay half-lives (Buddemeier 2018)
The values are slightly larger than in similar previous simulations for the same radioactivity exposure (358 mSv vs. 316 mSv) (Rump et al 2019) as the maximum transport capacity used in the Michaelis–Menten kinetics is derived from the first order rate constants for low iodide concentrations that slightly differ in the studies (0.9758 d−1 as indirectly derived from the data from Ramsden et al (1967) vs. 0.8316 d−1 in our previous studies)
Indicate an increased thyroidal sensitivity to iodine saturation (Wolff–Chaikoff effect) in Caucasians compared to Japanese, resulting in a higher efficacy of iodine blockade against acute radioiodine exposure
Summary
Nuclear fission reactions generate several hundred radionuclides, most of them with very short decay half-lives (Buddemeier 2018). These are responsible for the extremely high activities in the mushroom clouds after the detonation of nuclear weapons and the very high dose rates emanating from early fallout. With increasing distance from the detonation point and dilution of the activity, the impact of external irradiation decreases and the risks emanating from the incorporation of radionuclides and internal contamination relatively increases (Simon et al 2006). The fission reaction is basically the same, but processes are more protracted in time, and the mechanisms involved in a power plant accident may vary (e.g., power surge accident in Chernobyl, coolant deficiency accident in Fukushima) (Imanaka et al 2015). The radioactive mixtures released may quantitatively differ (Wohni 1995; Imanaka et al 2015)
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