Background:Gross target volume of primary tumor (GTV?P) is very important for the prognosis prediction of patients with nasopharyngeal carcinoma (NPC), but it is unknown whether the same is true for locally advanced NPC patients treated with intensity?modulated radiotherapy (IMRT). This study aimed to clarify the prognostic value of tumor volume for patient with locally advanced NPC receiving IMRT and to ifnd a suitable cut?off value of GTV?P for prognosis prediction. Methods:Clinical data of 358 patients with locally advanced NPC who received IMRT were reviewed. Receiver oper?ating characteristic (ROC) curves were used to identify the cut?off values of GTV?P for the prediction of different end?points [overall survival (OS), local relapse?free survival (LRFS), distant metastasis?free survival (DMFS), and disease?free survival (DFS)] and to test the prognostic value of GTV?P when compared with that of the American Joint Committee on Cancer T staging system. Results:The 358 patients with locally advanced NPC were divided into two groups by the cut?off value of GTV?P as determined using ROC curves: 219 (61.2%) patients with GTV?P≤46.4mL and 139 (38.8%) with GTV?P>46.4mL. The 3?year OS, LRFS, DMFS, and DFS rates were all higher in patients with GTV?P≤46.4mL than in those with GTV?P>46.4mL (allP<0.05). Multivariate analysis indicated that GTV?P>46.4mL was an independent unfavorable prognostic factor for patient survival. The ROC curve veriifed that the predictive ability of GTV?P was superior to that of T category (P<0.001). The cut?off values of GTV?P for the prediction of OS, LRFS, DMFS, and DFS were 46.4, 57.9, 75.4 and 46.4mL, respectively. Conclusion:In patients with locally advanced NPC, GTV?P>46.4mL is an independent unfavorable prognostic indi?cator for survival after IMRT, with a prognostic value superior to that of T category.