To identify the prognostic value and staging categories of masticator space involvement in nasopharyngeal carcinoma (NPC) as determined with anatomic criteria ("anatomic masticator space involvement") to determine whether it is necessary for the masticator space described with criteria of the American Joint Committee on Cancer to exclude the medial and lateral pterygoid muscles.

This retrospective study was approved by the institutional review board, and the requirement to obtain informed consent was waived. NPC was diagnosed in 924 patients with magnetic resonance (MR) imaging, and medical records and imaging studies were retrospectively analyzed. The overall survival, local relapse-free survival, and distant metastasis-free survival of these patients were analyzed by using the Kaplan-Meier method; differences were compared by using the log-rank test.

The frequency of anatomic masticator space involvement was 19.7% (182 of 924 patients). With use of multivariate analysis, anatomic masticator space involvement was identified as an independent prognostic factor for overall survival (P = .02). In addition, anatomic masticator space involvement was associated with a marginally significant decrease in local relapse-free survival (P = .05); however, it had no effect on distant metastasis-free survival. There were no statistically significant differences between different degrees of anatomic masticator space involvement with regard to overall survival and local relapse-free survival in NPC (P = .34 and P = .54, respectively) or between T3 or T4 disease and anatomic masticator space involvement with regard to overall survival and local relapse-free survival (P = .63 and P = .44, respectively).

Anatomic masticator space involvement affects the overall survival and local relapse-free survival of patients with NPC. When masticator space involvement includes the medial and lateral pterygoid muscles, the disease should be classified as stage T4.