Incidence of malignant thyroid tumors in humans after exposure to diagnostic doses of iodine-131. I. Retrospective cohort study.

Between 1952 and 1965, 10,133 patients received an average of 60 mu Cl of 131I for diagnostic examinations of the thyroid gland. The examinations were mainly done on adults, and only 5% of the patients were younger than 20 years at the time of examination. Of the patients, 9 appeared in the Swedish Cancer Registry between 1958 and 1977 with a diagnosis of malignant tumor of the thyroid gland more than 5 years after the administration of 131I. Inasmuch as the expected number of malignant thyroid tumors computed from Swedish cancer incidence figures was 8.3, there was no elevation of the incidence of malignant thyroid tumors in these patients receiving diagnostic doses of 131I. The patients were examined because of a suspected thyroid dysfunction and therefore represented a selected group. The investigation is being continued to analyze further parameters that could have a bearing on these results.