Dosimetric predictors of hypothyroidism in nasopharyngeal cancer patients treated with intensity-modulated radiation therapy

Abstract

Objective To investigate the mobidity and risk factors for primary hypothyroidism (HT) in nasopharyngeal cancer (NPC) patients treated with intensity-modulated radiotherapy (IMRT). Methods 113 NPC patients with complete clinical information who received IMRT from 2008-2010 were retrospectively analyzed. Thyroid function assessments before and after IMRT were periodically monitored. Various clinical and dosimetric parameters were obtained including Dmin, Dmax, Dmean, V30, V35, V40, V45, V50, V55, V60, V65, V70(thyroid gland), PDmin, PDmax, PDmean(pituitary gland). Univariate and multivariate logistic regression analyses were performed to identify predictors of HT. Results After a median follow-up period of 62 months, 41 patients (36.3%) had clinical HT, and 28 patients (24.8%) developed subclinical HT. Univariate analysis revealed that younger age, mean dose to the thyroid gland, V40, V45, V50, V55, V60 were correlated with developing HT (all P<0.05). On multivariate analysis including patient, tumor, and treatment variables, younger age (P=0.002) and V50(P=0.002) remained statistically significant. We found that the cutoff value of V50(50%) may be an valuable evaluation marker of HT.Combined with age to predict the HT, the area under ROC curve is 0.728.The endpoint event rate of the patients whose level of V50 is above 50% and age level below 45 years were 79.3%, whlie the date in patients whose level of V50 is below 50% and age level above 45 years was 31.8%. Conclusions Thyroid V50 above 50% is predictive of primary HT after IMRT for NPC patient, Our results suggested that restricting V50<50% during IMRT planning may facilitate the reduction in incidence of HT for the younger patients. Key words: Hypothyroidism; Nasopharyngeal neoplasms/intensity-modulated radiotherapy; Receiver operating characteristic curve