A multivariable normal tissue complication probability model for predicting radiation-induced hypothyroidism in nasopharyngeal carcinoma patients in the modern radiotherapy era

Radiation-induced hypothyroidism (RHT) is a common long-term complication for nasopharyngeal carcinoma (NPC) survivors. A model using clinical and dosimetric factors for predicting risk of RHT could suggest a proper dose-volume parameters for the treatment planning in an individual level. We aim to develop a multivariable normal tissue complication probability (NTCP) model for RHT in NPC patients after intensity modulated radiotherapy (IMRT) or volumetric modulated arc therapy (VMAT). The model was developed using clinical data and dose-volume data of thyroid and pituitary gland based on a standard backwards stepwise multivariable logistic regression analysis, then internally validated using 10-fold cross validation. The final NTCP model consisted of age, pre-treatment thyroid stimulating hormone (TSH) and mean thyroid dose. The model performance was good with an area under the receiver operating characteristic curve of 0.749 on an internal (200 patients) and 0.812 on an external (25 patients) validation. The mean thyroid dose at ≤ 45Gy was suggested for treatment plan owing to an RHT incidence of 2% versus 61% in the > 45Gy group.