Thyroid nodules as related to absorbed dose from iodine-131 in a Ukrainian cohort following the Chernobyl accident

Abstract

The Chernobyl accident in 1986 exposed thousands of people to various radioactive isotopes of iodine, particularly iodine-131 (I-131). In subsequent years after exposure, a large increase in thyroid cancer among those exposed as children and adolescents was found, particularly in Belarus, Russia, and Ukraine. Since the natural history of thyroid nodules is not clear, we examined the relationship between the presence of thyroid nodules 12-14 years after the accident and measurement-based individual I-131 thyroid dose estimates. The cohort consisted of 13,008 individuals who were under the age of 18 years on April 26, 1986 and who resided in contaminated areas of Ukraine and had an examination between April 1998 and December 2000. Individual estimates of radiation dose to the thyroid were calculated based on radioactivity measurements made shortly after the accident, environmental transport models, and interview data. Excess odds ratios (EORs) were estimated using logistic regression models. At least one pathologically confirmed thyroid nodule was observed in 263 individuals (2.02% of the cohort). There was a linear-exponential relationship between I-131 exposure and thyroid nodule risk in an EOR model (p-value: 0.00460) with the EOR increasing at lower doses before plateauing and declining at higher doses. The linear EOR over the whole dose range was 0.89 (95% CI: 0.42, 1.64) for prevalence of all nodules and 0.41 (95% CI: 0.09, 0.95) for prevalence of non-neoplastic nodules (excluding thyroid cancer and follicular adenoma). The EORs under 5 Gy for prevalence of all nodules and prevalence of non-neoplastic nodules were 1.08 (95% CI: 0.53, 1.95) and 0.57 (95% CI: 0.17, 1.22), respectively. The EOR per Gy varied significantly by oblast of residence but not by sex, age, iodine status, or thyroid volume. Exposure to radioactive iodine was strongly associated with an increased risk of thyroid nodules among those exposed at ages less than 18 years. Thyroid nodule risk showed a nonlinear relationship with dose received but overall was lower than thyroid cancer risk in previous studies of the same cohort. This suggests that I-131 exposure may be more effective in inducing thyroid cancer or follicular adenoma than non-neoplastic thyroid nodules.