Margin-To-Depth-Of-Invasion Ratio: A Paradigm Shift in Oral Cancer Margin Assessment.

OBJECTIVE: Surgical margin status and depth of invasion are established prognostic factors in oral squamous cell carcinoma, yet an adequate margin remains undefined. A fixed 5 mm threshold is applied to tumors of widely varying depth. We examined the margin-to-depth-of-invasion ratio, the closest surgical margin divided by the depth of invasion, as a tumor-proportional alternative. METHODS: We synthesized two retrospective studies: a single-institution cohort of 911 patients with early-stage disease, and a nationwide registry cohort of 7420 patients with node-negative disease across all tumor classifications, each stratified by margin distance and by ratio (below 0.5 versus 0.5 or greater). RESULTS: A ratio below 0.5 predicted higher local recurrence among non-positive margins (odds ratio 2.81). Five-year disease-free survival was 80.8% with clear margins, 76.3% with close margins and a ratio of 0.5 or greater, and 65.2% with close margins and a ratio below 0.5. Nationwide, both subgroups with a ratio of 0.5 or greater showed superior survival regardless of margin distance. Postoperative radiotherapy improved disease-free survival only when the ratio was below 0.5. CONCLUSIONS: The margin-to-depth-of-invasion ratio reframes margin adequacy as proportional rather than absolute. A ratio of 0.5 distinguishes high-risk from low-risk margins and identifies candidates for postoperative radiotherapy.