Risk of Second Primary Malignancies after External Beam Radiotherapy for Thyroid Cancer

Aim: To investigate the association between external beam radiotherapy (EBRT) and the incidence of second primary tumors in patients with thyroid cancer. Materials and Methods: Data were extracted from the Surveillance, Epidemiology, and End Results 9 database. The study cohort included patients diagnosed with thyroid cancer between 1973 and 2017. Risk factors for second primary malignancies were identified with Cox proportional hazards models. Propensity score-matched analyses were used to assess the association between EBRT and second primary malignancies. Results: Out of 72,392 patients with thyroid cancer, 7,684 (10.6%) developed a subsequent primary malignancy. Propensity score-matched analysis demonstrated patients receiving EBRT were more likely to develop second primary malignancies [30-year cumulative incidence=35.3% (95% confidence interval (CI)=30.4-39.8% vs. 28.1% (95% CI=27.0-29.2%); hazard ratio=1.17 (95% CI=1.03-1.33)]. Conclusion: In patients with thyroid cancer, EBRT is associated with an increased incidence of second primary malignancies.