Dose-Volumetric Parameters for Predicting Hypothyroidism after Radiotherapy for Head and Neck Cancer

Abstract

Purpose: To investigate predictors those affect the development of hypothyroidism (HT) after radiotherapy (RT) for head and neck (HN) cancer, focusing on radiation dose-volumetric parameters, and to determine the appropriate radiation dose-volume threshold of thyroid gland. Materials and Methods: A total of 114 patients with HN cancer whose RT fields included the thyroid gland were analyzed. Purpose of RT were either definitive (n=81) or postoperative (n=33). Thyroid function was monitored before RT and after completion of RT at 1 month, 6 months, 1 year, and 2 years. A diagnosis of HT was based on TSH value greater than the maximum value of laboratory range, regardless of symptom. In all patients, dose volumetric parameters, such as V10 to V50 (at intervals of 5 Gy), and mean/maximum/minimum dose of thyroid were analyzed. Results: Median age at diagnosis was 56 years (range, 14-77). 50 patients underwent 3-dimensional conformal RT, whereas 64 patients underwent intensity-modulated RT. Sixty-five patients received concurrent chemotherapy. Median follow-up duration was 13 months (range, 6-29). Forty-five percent of the patients were diagnosed as HT after median 8 months (range, 1-24). There were no significant differences in the distribution of age, gender, tumor site, surgery, RT technique, and chemotherapy between no-HT group and HT group. In univariate analysis, V20 to V50 results were significantly associated with HT. From ROC analysis, V45 was an optimal value for HT prediction, and V45 of 50% was a threshold value. If V45 was <50 %, the cumulative incidence of HT at 1 year was 19.7%, whereas the incidence was 50.3% if V45 was ≥ 50% (p= 0.024). Conclusion: The V45 may predict risk of developing HT after RT for HN cancer, and a V45 of 50 % was a useful dose-volume constraint for thyroid.