Thyroid V40 Predicts Primary Hypothyroidism After Intensity Modulated Radiation Therapy for Nasopharyngeal Carcinoma

Abstract Purpose Primary hypothyroidism (HT) is a common occurrence after intensity modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC). The aim of this study is to investigate the various clinical and thyroid dosimetric parameters that could predict the risk of primary HT after IMRT and to determine useful thyroid dose constraints to guide radiotherapy planning. Methods & Materials From September 2009 to August 2012, 102 clinically euthyroid NPC patients were included in this study. All patients were treated with IMRT and randomized to induction chemo followed by concurrent chemo-IMRT or concurrent chemo-IMRT alone. Thyroid function was evaluated by measuring TSH (thyroid stimulating hormone) and fT4 (free thyroxine) at each annual follow up visit. Various clinical and dosimetric parameters (e.g. V40 – percentage of thyroid volume receiving >40Gy) were obtained. Univariate and multivariate logistic regression analyses were performed to identify predictors of HT. Results Median follow up was 48.8 months. Among the 102 patients, 44 (43.1%) patients developed HT within 2 years post radiotherapy. The median time to HT was 36.7 months (range: 24.9 – 49.0 months). The 1-year and 2-year cumulative incidence rate of HT was 33% and 44.5%. Univariate analysis revealed that younger age, early T stage, minimum dose to the thyroid gland, V40 and V45 were associated with HT. On multivariate analysis, younger age (p=0.017), early T stage (p=0.005) and V40 (p=0.009) remained statistically significant. Patients with V40>85% had significantly higher cumulative incidence rates of HT than patients with V40≤85% (p=0.007). Conclusions Thyroid V40 is predictive of primary HT following IMRT for NPC and V40≤85% can be a useful dose constraint to adopt during IMRT planning without compromising tumor coverage.