Occurrence of Newly Diagnosed Thyroid Cancer Is Not Increased After Radioactive Iodine Therapy for Graves' Disease.

[CONTEXT] An analysis based on a large number of detailed patient data was performed to accurately assess the impact of radioiodine therapy (RAIT) for Graves' disease (GD) on thyroid cancer.

[OBJECTIVE] The purpose was to investigate whether the occurrence of newly diagnosed thyroid cancer is increased after RAIT for GD, and whether thyroid cancer following RAIT has poor prognosis.

[METHODS] A total of 13 874 eligible patients diagnosed with untreated GD were retrospectively analyzed. The incidence rates of newly developed thyroid cancer by each treatment method for GD was evaluated using the person-year method. Among the 23 179 patients who underwent RAIT for GD, including those with recurrent GD or a history of prior treatment for GD, those who developed thyroid cancer after RAIT were analyzed. Logistic regression analysis was performed to investigate the risk factors for thyroid cancer.

[RESULTS] A total of 2273 cases underwent RAIT, 287 underwent surgical treatment, and 11 314 were treated with medication only, and new-onset thyroid cancer was identified in 8 patients in the RAIT group and 39 in the medication-only group. A total of 107 218 person-years were observed, but no significant difference was observed in the incidence rate of thyroid cancer with treatment for GD. Of the 23 179 cases that underwent RAIT, 17 developed thyroid cancer. Sixteen were diagnosed with papillary thyroid carcinoma (PTC), of which 15 were microcarcinomas. Logistic regression analysis did not identify any significant risk factors for thyroid cancer development.

[CONCLUSION] In this study, the incidence of new thyroid cancer cases did not increase following RAIT for GD. Most thyroid cancers that developed after RAIT were micro-PTCs, with no evidence suggesting a poor prognosis.