Long-term comparative cancer mortality after use of radio-iodine in the treatment of hyperthyroidism, a fully reported multicenter study.

BACKGROUND The Cooperative Thyrotoxicosis Therapy Follow-up Study reported cancer mortality after use of radio-iodine (131I), surgery, and other treatments in patients with hyperthyroidism, started during 1946-1964 and followed through 1968. In 1984, the National Cancer Institute initiated a continued follow-up (FU) of the same cohort through 1990, to assess potential carcinogenic effects of 131I through 1990, a potential FU ranging from 26 to 44 years. RESULTS With an initial cohort of 35,630 hyperthyroid patients, a mean entry age of 46 years and 738,831 patient-years of FU, the 2950 observed cancer deaths were only minimally over the 2858 expected cancer deaths from US population rates, after exclusion of first-year deaths. In the 64.7% of patients receiving 131I, the total cancer deaths were significantly above expected (258 versus 208), only at durations of 1-5 years, not thereafter. Thyroid cancer deaths were significantly elevated, but the number was very small, only 24 deaths in all exposures, as compared with 5.89 expected. The overall thyroid cancer mortality was higher in cases of toxic nodular goiter than in patients with Graves' disease. CONCLUSIONS Despite the small early increase in total cancer deaths and the relatively higher increase in thyroid cancer deaths (only 18 excess deaths), the authors conclude that therapy with the doses of 131I used is a safe procedure. The interpretation of cause-specific excess mortality is greatly influenced by the prevalence of the cause of death.