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The guidelines for surgical margins depend on tumor type, anticipated biological behavior, tumor grade, anatomical location, and adjoining normal tissue types (also see Chapter 1 for further discussion).

Surgical resection margins for skin tumors are described as intracapsular, marginal, wide, or radical based on the system developed by Enneking for musculoskeletal tumor excisions (Enneking et al. 1980).

Intracapsular resection is defined as a debulking or cytoreductive procedure that leaves behind clinically evident macroscopic tumor. Local recurrence for malignant tumors is assured unless surgery is followed by radiation therapy or other adjunctive therapies. These surgeries are often performed for palliation of clinical signs.

Marginal excision is immediately outside the pseudocapsule of the tumor, leaving behind microscopic tumor in the case of malignant invasive disease. Local recurrence is likely without repeat surgical excision or adjuvant therapies. A common example of this type of excision for skin tumors is “shelling out” soft tissue sarcomas that appear well encapsulated but are removed through a pseudocapsule of compressed tumor cells, leaving microscopic tumor cell projections in the surgical periphery.

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.