Thyroid cancer risk after thyroid examination with 131I: a population-based cohort study in Sweden

Ionizing radiation is the only established cause of thyroid cancer, though the effect of diagnostic administration of (131)I on thyroid cancer risk appears minimal. The annual number of thyroid examinations using radioiodine is currently 5 per 1,000 individuals worldwide, so this issue is of public health importance. Our objective was to evaluate the excess risk of thyroid cancer following a range of known doses of (131)I administered for diagnostic purposes. We conducted a nationwide, population-based cohort study in Sweden including all 36,792 individuals who received (131)I for diagnostic purposes during 1952-1969 and were alive and free of thyroid cancer 2 years after exposure. Accrual of person-time at risk commenced 2 years after the first (131)I administration. Follow-up for cancer was to the end of 1998. Standardized incidence ratios (SIRs) were calculated as the ratio between the observed and expected numbers of thyroid cancers. Estimates were stratified by previous exposure to external radiation therapy to the neck, reason for thyroid examination, (131)I dose, sex, age at exposure and time since exposure. Thyroid cancers (n = 129) were diagnosed during 886,618 person-years at risk. Excess thyroid cancers were observed only among the 1,767 patients who reported previous external radiation therapy to the neck [SIR = 9.8, 95% confidence interval (CI) 6.3-14.6] and among those originally referred due to suspicion of a thyroid tumor (SIR = 3.5, 95% CI 2.7-4.4 for 11,015 patients without previous external radiation therapy). The 24,010 patients without previous exposure to external radiation therapy to the neck who were referred for a reason other than suspicion of a thyroid tumor received an estimated dose to the thyroid of 0.94 Gy. Among these patients, 36 thyroid cancers were observed compared to 39.5 expected (SIR = 0.91, 95% CI 0.64-1.26). We found no evidence that administration of (131)I for diagnostic purposes increases risk of thyroid cancer. However, our study included few patients under age 20, so the results apply primarily to exposure among adults. Our data suggest that protraction of dose may result in a lower risk than brief X-ray exposure of the same total dose.