Tumor Depth of Invasion (Tumor > 4 cm/Depth > 10 mm and Depth > 20 mm) and Through Cortex/Skin Invasion are Both Valid Criteria for Classifying Tumors as pT4a in AJCC 2018 Oral Cavity Cancer Staging System

Background: According to the AJCC third to seventh edition staging manuals (1988–2010), the presence of through cortex and/or skin invasion in oral cavity squamous cell carcinoma (OCSCC) identifies T4a tumors. The AJCC eighth edition (2018) introduced a depth of invasion (DOI) > 20 mm as a criterion for pT4a. Subsequently, a revision maintained that tumors > 4 cm with a DOI > 10 mm should be classified as pT4a. We sought to analyze the prognostic impact of the three distinct criteria identifying pT4a disease. Methods: We examined 667 consecutive patients with pT3-4 buccal/gum/hard palate/retromolar SCC who underwent surgery between 1996 and 2016. pT1/pT2 (n = 108/359) disease were included for comparison purposes. Results: The 5-year outcomes of patients with pT1/pT2/without (n = 406)/with tumor > 4 cm/DOI > 10 mm (n = 261), pT1/pT2/DOI ≤ 20 mm (n = 510)/> 20 mm (n = 157), and pT1/pT2/without (n = 305)/with through cortex/skin invasion (n = 362) were as follows: disease-specific survival (DSS), 98%/89%/79%/65%, p < 0.001, 98%/89%/78%/59%, p < 0.001, and 98%/89%79%/69%, p < 0.001; overall survival (OS), 90%/79%/63%/51%, p < 0.001, 90%/79%/63%/42%, p < 0.001, and 90%/79%/65%/52%, p < 0.001. In pT3-4 disease, a tumor > 4 cm/DOI > 10 mm was an independent adverse prognosticator for 5-year DSS rate, DOI > 20 mm was an independent adverse prognosticator for 5-year DSS and OS rates, whereas through cortex/skin invasion independently predicted 5-year OS rates. Conclusions: All of the three criteria (tumor > 4 cm/DOI > 10 mm, DOI > 20 mm, and through cortex/skin invasion) identify high-risk patients, which should be reflected in further revisions of pT4a classification in OCSCC.