Figure 4.2 (a) Preoperative margins marked on skin with marking pen. (b) En bloc excision of cutaneous mass. Skin incision and excision plane extends at least one fascial plane beyond the deepest layer of tumor. (c) En bloc excision of cutaneous mass.

      (Images courtesy of Dr. Simon Kudnig).

      Wide resection is removal of the tumor with complete margins of normal tissue in all directions. Local recurrence is unlikely after this extent of surgery. For skin tumors, an appropriate example would be the excision of mast cell tumors on the trunk with 2–3 cm lateral margins and at least one fascial plane deep.

      Radical resection is removal of an entire anatomical structure. Local recurrence is unlikely. An appropriate example for skin tumors where radical resection would be appropriate would be amputation for a stage III soft tissue sarcoma of an extremity. Radical surgical resection requires a thorough knowledge of regional anatomy, concurrent and postoperative side effects, and reconstructive procedures.

      Any tissue that the tumor touches or invades must be removed with a margin of normal tissue in the curative setting. Fat is a poor barrier to tumor invasion, so wider margins may be required, particularly as a deep margin for skin tumors. Fascia and bone are most often effective barriers to tumor growth, so a deep margin of muscle fascia is preferred. When possible, it is recommended that dissection occurs through normal tissue planes and the tumor could be removed en bloc. Higher‐grade tumors generally require a more aggressive approach using larger surgical margins.

      Surgical margins should be planned with closure of the wound taken into consideration, but completeness of tumor resection and size and quality of surgical margins should not be compromised to facilitate closure. From a surgical oncology perspective, it is preferable in most cases to deal with a large wound rather than an incomplete malignant tumor resection. A wide variety of reconstructive surgical options using skin flaps, grafts, or secondary intention wound closure techniques can be used as appropriate for the surgical situation.

      Mohs micrographic surgery has been described in a pilot study for total margin assessment of cutaneous tumors in veterinary oncology (Bernstein et al. 2006). This technique requires specialized surgical training in the horizontal sectioning technique and frozen sections to obtain margin evaluation in real time.

      Margin Evaluation

After local tumor resection, the surgeon has an important role communicating with the pathologist to help identify the specimen margins that are most likely to be close or incomplete. This can be achieved by providing detailed information on the pathology submission form, including pertinent clinical history as well as attempting to maintain normal tissue architecture and using ink and/or sutures to orient the tissues.

      The surgeon is responsible for interpreting the pathology report in the context of the surgery performed. The pathology report should contain three important pieces of information for the surgeon that will help to dictate whether adjuvant therapy is required: the histologic diagnosis, the tumor grade if appropriate, and the adequacy of surgical margins. If surgical margins are incomplete, the entire surgical scar is assumed to be contaminated with tumor cells and larger surgery or adjunctive treatments such as radiation therapy or chemotherapy are indicated.

      Because the entire surgical scar is assumed to be contaminated with tumor cells when performing the second surgery where the goal is to remove the residual tumor cells left behind, the entire surgical scar needs to be removed. In one study evaluating the second surgery for soft tissue sarcomas in dogs (Bacon et al. 2007), it was suggested that if wide excision of the previous scar was not possible (because of anatomic location for example), excision of the scar with narrow margins appeared to be beneficial. However, it is not just the scar that is contaminated but the entire previous surgical bed. Therefore, excising the entire surgical bed should be the goal of the second surgery. One study concluded that the extent of the surgical bed is, as expected, proportional to the extent of the first surgery. The mean maximum lateral extension was 6, 14, 18, and 26 mm for 2‐, 4‐, 6‐, and 8‐cm skin defects, respectively. Even so, removing a scar with 3 cm margins around it can lead to an incomplete excision of the surgical bed in some cases when the skin defect was more than 4 cm (Cunningham and Skinner 2020).

      Reconstructive Procedures

      Reconstructive procedures may be indicated after tumor removal to reconstruct skin deficits, depending on the anatomical location and extent of surgical resection. Reconstructive procedures may include local or pedicle skin flaps, delayed primary closure or secondary closure, free skin grafting or free microvascular cutaneous, and muscle or myocutaneous grafts. Detailed descriptions of these reconstructive procedures are beyond the scope of this chapter, and the reader is referred to several texts that describe these procedures in detail (Pavletic 2003, 2018; Swaim 2003; Kirpensteijn et al. 2013).

The surgeon should plan the reconstructive procedure before the definitive tumor excision by marking the margins of tumor excision and the proposed flap site with a marking pen prior to surgery (Figures 4.3a–e).

      Consideration should also be given to the degree of resection required in a circumstance. For example, a grade I MCT located on an extremity could be treated appropriately with a more conservative 1 cm lateral margin resection. An ulcerated grade III MCT with regional lymph node metastasis may be treated with a palliative marginal resection, and a grade II MCT on the pelvic region may require wide excision and an axial pattern flap reconstructive procedure.

      If a reconstructive procedure is used, the donor site and recipient tumor resection site should be treated as two separate surgical fields, with separate gloves and instruments used for each site to decrease the risk of contaminating the donor site with tumor cells. The donor site should be completely closed before moving to the recipient tumor resection site.

      Reconstructive flapping procedures can be used successfully in combination with radiation therapy. The severity of complications was reduced when flapping procedures were used as part of the planned therapy as opposed to those used to correct a complication or failure of radiation therapy (Séguin et al. 2005).

      Open