Surgical Margins Required for the Complete Excision of Canine Oral Melanomas

Oral malignant melanoma (OMM) is the most common canine oral neoplasm. Complete excision is recognised as the mainstay of treatment in the absence of distant metastasis, but recommended surgical margins are unvalidated. This study investigates the surgical margins required for complete excision of canine OMMs. We hypothesised that surgical margins 10-15 mm wide and a qualitative deep margin (bone, fascia or full-thickness tissue excision) would be adequate for complete local excision of OMMs. Margins required to excise canine oral melanocytomas are also unknown; a secondary objective was to investigate this. Cases were retrospectively collected from two referral centres and a veterinary pathology laboratory. Histologic diagnosis, en bloc excision of gross tumour, surgical margin and histologic margins were required for inclusion. Tumours were categorised based on surgical margin width (Group A < 10 mm, Group B 10-15 mm and Group C > 15 mm). A qualitative deep margin was required for Groups B and C, but not Group A, to include marginal excision cases. Histologic margins were classified by the R tumour classification scheme. Twenty-eight tumours were included, comprising 25 OMMs and three melanocytomas. R0 rates for OMMs were 56% (10/17) in Group A, 100% (7/7) in Group B and 100% (2/2) in Group C. Surgical margins ≥ 10 mm were significantly (p = 0.03) more likely to result in R0 classification. The three melanocytomas were all classified as R0, with < 10 mm surgical margins. Results suggest that surgical margins 10-15 mm wide and a qualitative deep margin may be adequate for complete excision of OMMs.