Dosimetric predictors of Hypothyroidism after Radical Intensity Modulated Radiation Therapy for Nasopharyngeal Carcinoma

Abstract

PURPOSE/OBJECTIVE(S): We investigated for dosimetric predictors of hypothyroidism after radical intensity modulated radiation therapy (IMRT) for nonmetastatic nasopharyngeal carcinoma (NPC). MATERIALS/METHODS: Patients with nonmetastatic NPC treated with radical IMRT from 2008 to 2013 were reviewed. They were regularly monitored clinically with serum thyroid function tests before and after IMRT. Univariable and multivariable analyses were performed for demographic and dosimetric predictors of biochemical and clinical hypothyroidism. RESULTS: A total of 149 patients fulfilled eligibility criteria. After a median follow-up duration of 3.1 years (range, 1.3-8.2 years), 33 patients (22.1%) and 21 patients (14.1%) developed biochemical and clinical hypothyroidism, respectively. Eight patients (24.2%) who had biochemical hypothyroidism developed clinical hypothyroidism later. Univariable and multivariable analyses revealed that volume of the thyroid (P=.002, multivariable), VS60 (the absolute thyroid volume spared from 60 Gy or less) (P<.001, multivariable), and VS45 (P<.001, multivariable) of the thyroid were significant predictive factors of biochemical hypothyroidism. The freedom from biochemical hypothyroidism was longer for those whose VS60≥10 mL (mean 90.9 months vs 62.6 months; P<.001) and VS45≥5 mL (mean 91.9 months vs 65.2 months; P=.001). Similarly, multivariable analyses revealed that VS60 (P=.001) and VS45 (P=.003) were significant predictors of clinical hypothyroidism. The freedom from clinical hypothyroidism was longer for those whose VS60≥10 mL (91.5 months vs 73.3 months, P=.002) and VS45≥5 mL (91.5 months vs 75.9 months, P=.007). Receiver-operating characteristics (ROC) analyses revealed that the area-under-the-curve (AUC) value for VS60 was significantly higher than that for VS45 for both biochemical (0.751 vs 0.660; P=.009) and clinical hypothyroidism (0.718 vs 0.607; P=.007), respectively. CONCLUSION: VS60 and VS45 of the thyroid significantly predicted post-IMRT hypothyroidism and should be considered important priorities of dose constraints during IMRT optimisation for NPC.