Thyroid cancer risk among patients exposed to diagnostic doses of iodine-131

The level of risk associated with Iodine 131 is not well define but appears lower than equivalent doses of x-rays. To provide quantitative data on the risk of thyroid cancer following Iodine 131 exposure, 34104 patients surviving ≥5 years after Iodine 131 administration between 1950-69 for diagnostic purposes were studied. The mean thyroid dose was estimated to be 1.1 Gy (range 0-40.5). Overall, 67 thyroid cancers occurred in contrast to 49.7 expected based on rates from the general population (SIR = 1.35 -Standardized Incident Ratio-; 95% CI 1.05-1.71 -Confidence Intervals-). However, the excess rates was based entirely among patients referred because of a suspicion of a thyroid tumour whose risk was significantly higher than for those referred for other reasons. (Author). 20 refs