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Resection of the primary tumor with negative margins is the gold standard treatment for squamous cell carcinoma of the oral tongue (SCCOT). A microscopically positive surgical margin is clearly associated with a higher risk for local recurrence, whereas a negative margin has traditionally been defined as greater than 5.0 mm clearance from the tumor, with lesser margins arbitrarily designated as close. The precise cutoff at which the risk for local recurrence with a close margin approximates that of a microscopically positive margin remains unclear.To determine whether the arbitrarily defined close margin (<5.0 mm) would portend as high a risk for local recurrence as a positive margin after resection of SCCOT.In this retrospective study, head and neck pathologists reviewed archived tumor specimens from 381 patients with SCCOT who underwent primary surgical resection at a tertiary care center from January 1, 2000, through December 31, 2012. Data were analyzed from November 15, 2015, to January 5, 2016. Time-dependent receiver operating characteristic curve analysis was used in patients who did not have a microscopically positive margin to determine an optimal margin cutoff for local recurrence-free survival (LRFS). Pathologic factors were assessed for LRFS in a multivariate Cox proportional hazards regression model.The primary end point was evaluation of the margin distance associated with LRFS.Among the 381 patients included in the analysis (222 men [58.3%] and 159 women [41.7%]; mean [SD] age, 58 [14.7] years), the optimal cutoff associated with LRFS was determined to be 2.2 mm. This cutoff was compared with the traditionally accepted cutoff of 5.0 mm. Patients with a margin of 2.3 to 5.0 mm had similar LRFS as patients with a margin of greater than 5.0 mm (hazard ratio [HR], 1.31; 95% CI, 0.58-2.96), and all other comparisons were significantly different (HR for positive margin, 9.03; 95% CI, 3.45-23.67; HR for 0.01- to 2.2-mm margin, 2.83; 95% CI, 1.32-6.07). Based on this result, negative margins were redefined as those with a clearance of greater than 2.2 mm. In a multivariate model adjusting for pathologic factors, positive margins (adjusted HR, 5.73; 95% CI, 2.45-13.41) and margins of 0.01 to 2.2 mm (adjusted HR, 2.00; 95% CI, 1.13-3.55) were the variables most significantly associated with LRFS.In this study, local recurrence-free survival was significantly affected only with surgical margins of less than or equal to 2.2 mm in patients with SCCOT. This new definition of close margins stratifies the risk for local recurrence better than the arbitrary 5.0-mm cutoff that has been used.