CANCER RISKS AFTER DIAGNOSTIC DOSES OF I-131 WITH SPECIAL REFERENCE TO THYROID-CANCER

Between 1951 and 1969 a total of 35,074 patients less than 75 years of age (mean = 44 years) were examined with diagnostic doses of I-131. The mean administered activity of I-131 was 52-mu-Ci and the radiation dose to the thyroid gland was on the average of 0.5 Gy. The cohort was matched with the Swedish Cancer Register for the years 1958-1984. During this period, 3746 cancers occurred more than 5 years after the I-131 examination, and the resulting standardized incidence ratio (SIR) was 1.01 (95% confidence interval [CI] = 0.98 to 1.04). SIR for thyroid cancer was 1.18 (95% CI = 0.88 to 1.56). The risks for both cancer of all sites and for thyroid cancer were highest 5 to 9 years after examination (SIR = 1.07 and 2.06, respectively) and did not differ from unity thereafter. With greater-than-or-equal-to 10 years of follow-up, risk was not statistically associated with the dose of I-131.