Factors predictive of the development of hypothyroidism after intensity-modulated radiation therapy for pharyngeal cancer.

BACKGROUND: Hypothyroidism is a common adverse event after radiotherapy for head and neck tumors and the incidence need to be re-evaluated because of using intensity-modulated radiotherapy (IMRT). AIMS/OBJECTIVES: Confirm the dose-volume effect of IMRT for pharyngeal cancer on hypothyroidism. MATERIALS AND METHODS: This was a retrospective analysis of patients underwent IMRT for pharyngeal cancer from June 2011 to May 2018. Patients were classified into group A (thyroid stimulating hormone (TSH) <5µU/ml), group B (5< =TSH < 10), and group C (10< =TSH) based on TSH over 36 months post-radiation. Radiation dose, thyroid volume, and the proportion of the thyroid that received X Gy or greater (Vx) were measured. RESULTS: Fifty-two patients were included in this work. Hypothyroidism developed in 33/52 (63%) patients, 13 in group B and 20 in group C. The mean radiation dose to the thyroid was 49.4 Gy and the median time until hypothyroidism was 39 months after irradiation. Hypothyroidism was significantly related to neck dissection (ND) and radiation dose to the thyroid. Patients whose thyroid received 45 Gy or more (V45) >67% had a significantly higher incidence of hypothyroidism. CONCLUSIONS AND SIGNIFICANCE: Patients with pharyngeal cancer who had ND and V45 to the thyroid >67% are at risk of hypothyroidism.