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

Purpose: To investigate the various clinical and thyroid dosimetric parameters that could predict the risk of primary hypothyroidism (HT) after intensity modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC) and to determine useful thyroid dose constraints to guide radiation therapy planning.

Methods And 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 chemotherapy followed by concurrent chemo-IMRT or concurrent chemo-IMRT alone. Thyroid function was evaluated by measuring thyroid-stimulating hormone and free thyroxine at each annual follow-up visit. Various clinical and dosimetric parameters (eg, V40 [percentage of thyroid volume receiving >40 Gy]) 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%) developed HT within 2 years after radiation therapy. The median time to HT was 36.7 months (range, 24.9-49.0 months). The 1-year and 2-year cumulative incidence rates of HT were 33% and 44.5%, respectively. 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=.017), early T stage (P=.005), and V40 (P=.009) remained statistically significant. Patients with V40 > 85% had significantly higher cumulative incidence rates of HT than patients with V40 ≤ 85% (P=.007).

Conclusions: Thyroid V40 is predictive of primary HT after IMRT for NPC, and V40 ≤ 85% can be a useful dose constraint to adopt during IMRT planning without compromising tumor coverage.