Kitahara CM, et al. - In this cohort study with 18,805 patients with hyperthyroidism who were treated with radioactive iodine, researchers examined the association of absorbed dose of radioactive iodine with overall and site-specific cancer mortality. This cohort study is a 24-year extension of a multicenter study (Cooperative Thyrotoxicosis Therapy Follow-up Study) that has followed up US and UK patients diagnosed and treated for hyperthyroidism for nearly 7 decades, starting in 1946. A modest but statistically significant positive dose-response relationship for risk of death was seen for all solid cancers (6% increase in risk per 100-mGy dose to the stomach), breast cancer (12% increase in risk per 100-mGy dose to the breast), and all solid cancers excluding breast (5% increase in risk per 100-mGy dose to the stomach).