T4a classification for oral cancer using mandibular canal invasion instead of bone marrow invasion in the TNM staging system

BACKGROUND AND PURPOSE: In oral cavity cancers, tumors invading the bone marrow of the mandible or maxilla have traditionally been classified as T4a in the TNM staging system. This study evaluated the impact of bone invasion on T classification and survival in patients with squamous cell carcinoma of the oral cavity. MATERIALS AND METHODS: We retrospectively reviewed and restaged 2,143 patients treated between 2001 and 2019. Bone invasion was categorized as no bone invasion, non-mandibular canal (MC) bone marrow invasion, or MC invasion. Disease-specific survival (DSS) was the primary endpoint. Prognostic performance was assessed using Kaplan-Meier analysis and Cox proportional hazards models. Discriminative ability was evaluated with Harrell's concordance index (C-index) and calibration analysis. RESULTS: Among 505 T4a tumors, 271 (53.7%) were classified as T4a solely due to bone marrow invasion, and their survival outcomes were comparable to those of T3 tumors. In multivariate analysis, MC invasion was independently associated with worse DSS, whereas non-MC bone marrow invasion was not. Based on these findings, we developed two revised versions of the MC-T classification for oral tumors, replacing bone marrow invasion with MC invasion as a T4a criterion in the 8th edition UICC/AJCC T classification. Both MC-T classifications showed superior prognostic discrimination, with higher C-indices than the original system. CONCLUSIONS: Patients with T4a oral tumors classified solely by bone marrow invasion had survival outcomes similar to those with T3 tumors. The proposed MC-T classification improves prognostic accuracy by more effectively stratifying T4a tumors.