Prognostic value of masticatory muscle involvement in nasopharyngeal carcinoma patients treated with intensity-modulated radiation therapy

•Our research is the first study to evaluate the validity of the current T-staging system (8th edition of the UICC/AJCC staging system) after the addition of revisions regarding Masticatory muscles (MM) involvement.•Our results showed that the decision to down-stage MP as T2 in the 8 th edition staging system is valid. However, tumors with LP involvement were more rational classified into a higher T-staging category than tumors with only-MP involvement. In 2017, the 8th edition of the nasopharyngeal carcinoma (NPC) staging manual published by the Union for International Cancer Control/American Joint Committee on Cancer (UICC/AJCC) defined new rules regarding the involvement of anatomical masticatory muscles (MM). The purpose of this study is to evaluate the validity of the current T-staging system after the addition of revisions regarding MM involvement. We performed a retrospective review of 608 patients, who were newly diagnosed with NPC and treated with intensity-modulated radiotherapy (IMRT) between January 2008 and March 2010. We found MM involvement in 398 NPC cases (65.4%). We observed a significant difference in overall survival (OS; 75.6% vs. 86.7%, P = .043) and in local relapse-free survival (LRFS; 81.5% vs. 88.6%, P = .048) between patients with and without MM involvement, but no difference in distant metastasis-free survival (DMFS; 80.3% vs. 85.0%, P = .147) or disease-free survival (DFS; 77.9% vs. 81.6%, P = .081). Significant differences in OS and LRFS were also found among different types of MM involvement. Compared with patients who had concurrent involvement of the lateral pterygoid muscle (LP), patients with tumors invading only the medial pterygoid muscle (MP) had a higher OS and LRFS rates, according to univariate and multivariate analysis (P