Risk of Breast Cancer in Patients with Thyroid Cancer Receiving or Not Receiving 131I Treatment: A Nationwide Population-Based Cohort Study.

UNLABELLED An increased risk of second primary malignancy after (131)I therapy has been reported. The objective of this study was to determine the risk of breast cancer in patients with thyroid cancer receiving or not receiving radioiodine treatment in Taiwan. METHODS This nationwide population-based cohort study was conducted using data obtained from the Taiwan National Health Insurance Database from 2000 to 2011. A total of 10,361 female patients with thyroid cancer (3,292 did not receive (131)I treatment and 7,069 received (131)I treatment) were enrolled, and 41,444 female controls were frequency-matched to the thyroid cancer patients in a 1:4 ratio by age (5-y age group). A Cox proportional hazards model was applied to estimate the risk of breast cancer in thyroid cancer patients receiving or not receiving (131)I treatment in terms of hazard ratios and 95% and 98% confidence intervals. RESULTS The incidence rates of breast cancer in patients with thyroid cancer receiving (131)I therapy, those not receiving (131)I therapy, and controls were 18.9, 17.7, and 13.1 per 10,000 person-years, respectively. Compared with patients with thyroid cancer treated with a cumulative (131)I dose of 4.44 GBq or less, the risk of breast cancer was not significantly increased in those treated with a cumulative (131)I dose of more than 4.44 GBq (adjusted hazard ratio, 0.78; 95% confidence interval, 0.50-1.21, P = 0.26; 98% confidence interval, 0.45-1.33, P > 0.02). CONCLUSION The greatest increased risk of breast cancer in patients with thyroid cancer is associated with the fact that the patient has thyroid cancer regardless of (131)I administration. However, (131)I further increased that risk but not as much as just having thyroid cancer.