Prognostic value of grading masticator space involvement in nasopharyngeal carcinoma according to MR imaging findings.

PURPOSE: To derive a suitable method for grading masticator space invasion in nasopharyngeal carcinoma on the basis of magnetic resonance (MR) images and to determine its prognostic value in patients undergoing intensity-modulated radiation therapy. MATERIALS AND METHODS: After institutional review board approval and informed consent were acquired, 808 patients with nasopharyngeal carcinoma who were treated with definitive intensity-modulated radiation therapy were analyzed retrospectively. The anatomic sites of masticator space involvement were identified with MR imaging. Overall survival, local relapse-free survival, and distant metastasis-free survival were calculated by using the Kaplan-Meier method and were compared by using the log-rank test. Potential prognostic factors were identified by means of multivariate analysis. RESULTS: Masticator space involvement was diagnosed in 163 of 808 patients (20.2%). Patients with lateral invasion (involvement of the lateral pterygoid muscle of the masticator space and beyond) had significantly poorer overall survival and distant metastasis-free survival than those with medial invasion (involvement of the medial pterygoid muscle of the masticator space) (P = .035 and P = .026, respectively). Furthermore, their overall survival, local relapse-free survival, and distant metastasis-free survival were significantly poorer compared with patients with stage T2 or T3 disease (all P ≤ .023) but similar to patients with stage T4 disease. The grade of masticator space involvement was an independent prognostic factor for overall survival, local relapse-free survival, and distant metastasis-free survival (all P ≤ .023). CONCLUSION: Masticator space involvement in nasopharyngeal carcinoma should be graded as medial (stage T2 disease) or lateral (stage T4 disease). This can facilitate staging of nasopharyngeal carcinoma and may be a suitable prognostic indicator of survival.