TUMORS AFTER RADIOTHERAPY FOR THYROID-CANCER - A CASE CONTROL STUDY WITHIN A COHORT OF THYROID-CANCER PATIENTS

A case-control study of Swedish thyroid cancer patients was conducted to evaluate the possible influence of I-131 treatment and external radiotherapy on the risk of developing a subsequent cancer. Both cases and controls derived from a cohort of Swedish thyroid cancer patients treated with I-131 (n = 834) or by other means (n = 1 121). Thirty-six breast, 13 stomach, 12 kidney, and 5 bladder cancers were found more than 2 years after I-131 treatment/thyroid cancer diagnosis. Individual, absorbed dose in the organs was calculated by using ICRP tables, administered activity of I-131, and 24-h I-131 uptake. In studying the effect of I-131 and external radiotherapy no statistically significant dose-response relationships were found for cancers of the breast, stomach, bladder or kidney. When the absorbed dose from I-131 was analyzed separately the risks remained essentially the same. The present follow-up time and the relatively low absorbed dose that the patients received from I-131 and external radiotherapy necessitate studies with a longer follow-up time or a larger patient material before more firm conclusions can be made.