The extent of surgical safety margin (gross tumor border to resection margin) in oral cancer surgery remains unclear, and no study has determined the differential impact of close surgical margin and microscopic extension according to primary tumor size in oral cancers.

We retrospectively analyzed the clinical data of 325 patients with surgically treated oral cavity squamous cell carcinomas to determine the effect of a close surgical margin (<5 mm) (cSM₅) on local recurrence. In addition, the depth of microscopic tumor infiltration was determined in 90 available surgical specimens.

The cSM₅ was not related to the risk of local tumor recurrence in early-stage oral cancer, while it significantly increased the rate of local tumor recurrence in resectable advanced-stage oral cancers (hazard ratio 3.157, 95 % confidence interval 1.050–9.407, p = 0.041). Addition of postoperative adjuvant radiation to early-stage tumors with cSM₅ did not further reduce the local recurrence rate compared to surgery alone. The depth of microscopic tumor extension from the gross tumor border was significantly associated with primary tumor thickness (ρ = 0.390, p < 0.001) and tumor sizes (ρ = 0.308, p = 0.003), which was a median (range) of 0.84 (0.14–2.32) mm in T1, 1.06 (0.20–4.34) mm in T2, and 1.77 (0.13–4.70) mm in T3–4.

The cSM₅ was a significant risk factor for local recurrence only in advanced oral cancers, but not in early-stage tumors, where microscopic tumor extension was not beyond 3 mm in T1 tumors. Thus, the extent of surgical safety margin can be redefined according to the primary tumor size.