Increased cancer incidence after radioiodine treatment for hyperthyroidism

BACKGROUND. Concerns remain about risk of cancer after radioactive iodine (RAI) treatment for hyperthyroidism, especially in organs that concentrate iodine. The objective was to assess the long-term cancer risk from RAI treatment for hyperthyroidism. METHODS. A total of 2793 hyperthyroid patients treated with RAI at Tampere University Hospital between 1965 and 2002, and 2793 age- and sex-matched reference subjects were followed for an average of 10 years through the Finnish Cancer Registry. RESULTS. Cancer incidence among hyperthyroid patients treated with RAI was higher than in the population-based control group (118.9 vs 94.9 per 10,000 person-years, rate ratio [RR], 1.25; 95% confidence interval [CI]: 1.08-1.46). Furthermore, incidence of stomach (RR, 1.75; 95% CI: 1.00-3.14), kidney (RR, 2.32; 95% CI: 1.06-5.09), and breast (RR, 1.53; 95% CI: 1.07-2.19) cancer was increased among RAI-treated patients. The relative risk of cancer increased with higher RAI dose administered. The increase in cancer incidence was statistically significant in patients treated at the age of 50-59 (RR, 1.44; 95% CI: 1.05-1.97) or older than 70 years (RR, 1.39; 95% CI: 1.05-1.82). There was a 5-year latent period after the RAI treatment before the cancer incidence began to differ between the RAI-treated hyperthyroid patients and the control group. CONCLUSIONS. Cancer incidence, especially cancer of the stomach, kidney, and breast, was higher in patients treated with RAI for hyperthyroidism. Cancer 2007;109:1972-9. (C) 2007 American Cancer Society.