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.