Radiation-Induced Hypothyroidism after Radical Intensity-Modulated Radiation Therapy for Oropharyngeal Carcinoma

Abstract Purpose To evaluate two published normal tissue complication probability (NTCP) models for radiation-induced hypothyroidism (RHT) on a large cohort of oropharyngeal carcinoma (OPC) patients who were treated with IMRT. Methods OPC patients treated with retrievable IMRT plan/dose DICOMs and available baseline and follow-up thyroid function tests were included. Mean dose (Dmean) to the thyroid gland (TG) and its volume were calculated. The study outcome was clinical HT at least 6 months post-RT, which was defined as grade ≥ 2 HT per CTC-AE grading system (symptomatic hypothyroidism that required thyroid replacement therapy). Regression analyses and Wilcoxon rank-sum test were utilized. Receiver Operating Characteristic (ROC) curves and area under the curve (AUC) for the fitted model were calculated. Results 360 OPC patients were included. The median age was 58 years. Most tumors (51%) originated from the base of tongue. IMRT-split field was utilized in 95%, and median RT dose was 69.96 Gy. 233 patients (65%) developed clinical RHT that required thyroid replacement therapy. On multivariate analysis higher Dmean and smaller TG volume maintained the statistically significant association with the risk of clinical RHT, (p Conclusion Volume and Dmean of the TG are important predictors of clinical RHT and shall be integrated into NTCP models for RHT. Dmean and thyroid volume should be considered during the IMRT plan optimization in OPC patients.