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Wide surgical excision with adequate lateral and deep margins has historically been the primary treatment of choice for most MCTs. The deep surgical margin is a qualitative margin rather than a quantitative margin. Fascia and collagen‐dense tissues are good barriers to tumor infiltration. The deep margin should include at least one fascial plane deep to the tumor that has not been invaded by the tumor. This margin should be removed en bloc with the tumor so that tumor contamination is not encountered during the surgery. The appropriate lateral surgical margin is grade and tumor size dependent. Historically, many MCTs have been treated with ‘surgical dose’ that is greater than required for local control. Simpson et al. (2004) reported that a 2 cm lateral margin and a deep margin of one fascial plane were adequate for complete excision of grade I and II MCTs in dogs. In fact, a 1 cm lateral margin was able to obtain tumor‐free margins in 75% of grade II and 100% of grade I cutaneous MCTs. In another study, a 2 cm lateral margin and one deep facial plane excision were successful in completely excising 100% of grade I and 89% of grade II MCTs (Fulcher et al. 2006). A similar local recurrence rate and de novo development rate were observed compared to previous reports with a 3 cm margin. Investigators concluded that excision of grade I and II MCTs with 2 cm margins might minimize complications associated with larger local tumor resection (Fulcher et al. 2006).

Wide surgical margins do not appear to be a prerequisite for a successful long‐term outcome in dogs with well‐differentiated cutaneous MCTs (Murphy et al. 2004). A proportional size model for surgical margins has been proposed where the lateral margins are equivalent to the widest diameter of the MCT, with an upper limit of 2–4 cm (Pratschke et al. 2013; Chu et al. 2020; Saunders et al. 2020). Using the proportional margins approach, 85–95% of tumors had complete excisional margins and local recurrence of 0–3%.

Complete removal of grade I or II cutaneous MCTs, even with narrow histologic margins, is associated with successful outcome without adjuvant therapy. Narrow (≤3 mm) histologic margins are likely adequate to prevent local recurrence of low‐grade MCTs (Schultheiss et al. 2011). In one study, the width of the tumor‐free margins on histology was not prognostic for local recurrence for completely excised tumors (Donnelly et al. 2015). High‐grade tumors have significant risk of local recurrence (36%) regardless of histologic margins width (Donnelly et al. 2015). Adequate margins for grade III MCTs have not yet been determined; thus 3 cm lateral and at least one fascial plane deep margins are recommended.

Intraoperative real‐time assessment of surgical margins has the strong advantage of allowing the surgeon to know where incomplete margins are and to take appropriate measures to rectify this at the surgery table without necessitating an additional surgery at a later time. Two methods that provide assessment of surgical margins intraoperatively described in veterinary surgery are fluorescence‐based imaging and optical coherence tomography (see Novel diagnostic imaging techniques in soft tissue sarcomas section). Using a fluorescent‐based imaging technique, sensitivity and specificity of the imaging system for identification of cancer (soft tissue sarcomas and mast cell tumors) in biopsies have been reported to be 92% for both. Although responsive to antihistamines, hypersensitivity to the fluorescent agent was seen in 53% of dogs and the risk needs to be considered in light of the potential benefits of this imaging system in dogs (Bartholf DeWitt et al. 2016). Optical coherence tomography‐guided pathology sections of canine mast cell tumors were able to detect incompletely excised MCT near the surgical margin with a sensitivity of 90% and specificity of 56.2% in one study (Dornbusch et al. 2020).

The excised specimen should be submitted in toto and not in sections. The anatomical relationship between the deep fascial plane and lateral margins should be preserved with sutures to help orient the pathologist, and the deep and lateral margins should be inked (ideally with separate colors). There is a significant amount of shrinkage artifact that occurs with each step of sample processing (Milovancev et al. 2018). Tissue shrinkage occurs mainly in the skin tissues (24%) compared to the tumor tissue (4%) (Upchurch et al. 2018). Mean histologic margins have been reported to be 35–42% smaller than the surgical margins (Risselada et al. 2015). Clinicians should take these factors into account when interpreting histologically reported margins relative to surgical margins.