| Summary: | In this study, we expanded on a previously published population-based case-control study on subjects exposed to iodine-131 (<sup>131</sup>I) from Chernobyl fallout at age ≤18 years using improved individual <sup>131</sup>I absorbed thyroid doses. We further studied the impact of iodine deficiency and other selected host risk factors on <sup>131</sup>I-related thyroid cancer risk after childhood exposure. We included 298 thyroid cancer cases and 1934 matched controls from the most contaminated regions of Belarus and the Russian Federation. We performed statistical analysis using conditional logistic regression models. We found a statistically significant linear quadratic dose-effect association between thyroid cancer and <sup>131</sup>I thyroid dose in the range up to 5 grays (Gy). Self-reported personal history of benign nodules, any thyroid disease except thyroid cancer, family history of thyroid cancer, increased body mass index, and deficient stable iodine status at the time of the accident were statistically significant risk factors (<i>p</i> < 0.05 for each factor) for thyroid cancer after adjustment for thyroid <sup>131</sup>I dose effect. Subjects who received stable iodine supplementation in the years after the accident had a significantly lower <sup>131</sup>I-related risk of thyroid cancer. Our findings are important for thyroid cancer prevention, and for further improvement of medical surveillance in the affected populations.
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