LEUKEMIAS AND CANCERS FOLLOWING I-131 ADMINISTRATION FOR THYROID-CANCER

We studied 1771 patients treated for a thyroid cancer in two instituti ons. None of these patients had been treated with external radiotherap y and 1497 had received I-131. The average I-131 cumulative activity a dministered was 7.2 GBq, and the estimated average dose was 0.34 Sv to the bone marrow and 0.80. Sv to the whole body. After a mean follow-u p of 10 years, no case of leukaemia was observed, compared with 2.5 ex pected according to the coefficients derived from Japanese atomic bomb survivors (P = 0.1), A total of 80 patients developed a solid second malignant neoplasm (SMN), among whom 13 developed a colorectal cancer. The risk of colorectal cancer was found to be related to the total ac tivity of I-131 administered 5 years or more before its diagnosis (exc ess relative risk = 0.5 per GBq, P = 0.02). These findings were probab ly caused by the accumulation of I-131 in the colon lumen. Hence, in t he absence of laxative treatment, the dose to the colon as a result of I-131 administered for the treatment of thyroid cancer could be highe r than expected from calculation of the International Commission on Ra diological Protection (ICRP). When digestive tract cancers were exclud ed, the overall excess relative risk of second cancer per estimated ef fective sievert received to the whole body was -0.2 (P = 0.6).