Dosimetric Predictors of Hypothyroidism After Radical Intensity-modulated Radiation Therapy for Non-metastatic Nasopharyngeal Carcinoma

Abstract

Aims: To investigate dosimetric predictors of hypothyroidism after radical intensity-modulated radiation therapy (IMRT) for non-metastatic nasopharyngeal carcinoma (NPC).

Materials and methods: Patients with non-metastatic NPC treated with radical IMRT from 2008 to 2013 were reviewed. Serum thyroid function tests before and after IMRT were regularly monitored. Univariable and multivariable analyses were carried out for predictors of biochemical and clinical hypothyroidism.

Results: In total, 149 patients were recruited. After a median follow-up duration of 3.1 years, 33 (22.1%) and 21 (14.1%) patients developed biochemical and clinical hypothyroidism, respectively. Eight (24.2%) patients who had biochemical hypothyroidism developed clinical hypothyroidism later. Univariable and multivariable analyses revealed that the volume of the thyroid (P = 0.002, multivariable), VS60 (the absolute thyroid volume spared from 60 Gy or less) (P < 0.001, multivariable) and VS45 (P < 0.001, multivariable) of the thyroid were significant predictors of biochemical hypothyroidism. The freedom from biochemical hypothyroidism was longer for those whose VS60 ≥ 10 cm3 (mean 90.9 versus 62.6 months; P < 0.001) and VS45 ≥ 5 cm3 (mean 91.9 versus 65.2 months; P = 0.001). Similarly multivariable analyses revealed that VS60 (P = 0.001) and VS45 (P = 0.003) were significant predictors of clinical hypothyroidism. The freedom from clinical hypothyroidism was longer for those whose VS60 ≥ 10 cm3 (91.5 versus 73.3 months; P = 0.002) and VS45 ≥ 5 cm3 (91.5 versus 75.9 months; P = 0.007).

Conclusions: VS60 and VS45 of the thyroid should be considered important dose constraints against hypothyroidism without compromising target coverage during IMRT optimisation for NPC.