As a general rule, if a primary tumor can be completely excised with acceptable morbidity, surgery is the best choice of treatment. The first attempt at surgical excision always offers the best opportunity to completely remove the tumor. Locally recurrent tumors often are more difficult to remove than the initial tumor because of more extensive involvement of normal tissues in the region and distortion of normal tissue planes by scar tissue. For tumors that are large, fixed, or located adjacent to critical normal structures, preoperative CT or MRI may be helpful in planning the surgical excision.

The usual objective of surgery is to obtain wide surgical margins in all directions surrounding the tumor, that is, to remove the tumor with a grossly visible intact cuff of surrounding normal tissue. There is no universally appropriate margin width, and adequate margins vary from tumor to tumor and location to location. Tumors with a high probability of local recurrence (e.g., high-grade soft tissue sarcomas or mast cell tumors and feline mammary carcinomas) should be removed with 2–3 cm margins if possible. Many other malignancies can safely be removed with 1–2 cm margins. The necessary margin often depends in part on the type of tissues that are adjacent to the tumor. For example, fascial planes generally provide a good physical barrier to tumor growth, so that excision of an intact fascial plane below a tumor is an excellent way to optimize the chance of a complete excision. Subcutaneous fat is poorly resistant to tumor growth and should always be aggressively excised with the tumor mass.

A marginal excision refers to “shelling out” a tumor, or excising it just outside its pseudocapsule. Because the pseudocapsule often consists of compressed cancers cells, marginal excisions risk leaving microscopic quantities of tumor cells in the patient and are associated with higher rates of local recurrence than wide excisions. As a general rule, marginal excisions should be avoided unless postoperative radiation therapy is being considered.

All excised tumors should be submitted for histopathologic examination and margin analysis. The accuracy of margin analyses can be optimized by inking the excised specimen to allow the pathologist to distinguish true surgical margins from artifactual margins created during tissue processing. Sutures may be placed in the surface of the excised specimen to guide the pathologist to areas of particular concern. Because pathology labs typically prepare only four or five slides from a given specimen, a report of complete margins does not necessarily imply that an excision was complete. A report of incomplete margins means the resection was histologically incomplete in at least one location. While overall recurrence rates are consistently greater for tumors with incomplete margins than for tumors with complete margins, owners should be aware that tumors with complete margins can recur locally and, conversely, many tumors with incomplete margins do not recur. Following a report of incomplete margins, options include close monitoring (if an appropriate re-excision will be feasible should local recurrence develop), immediate wide excision of the surgical scar, or postoperative radiation therapy.