Biopsy
An incisional biopsy will provide a larger tissue sample for histopathological examination compared to a needle core biopsy. Biopsy samples from both the central area of the skin mass, as well as at the margin between tumor and normal skin are recommended to assess local invasiveness. The biopsy tract should be planned so that it is in a location that can be excised with the definitive surgery. Contamination of normal tissues with tumor cells during biopsy can occur and result in local recurrence if the biopsy tract is not removed (Enneking and Maale 1988; Gilson and Stone 1990). Ideally, the surgeon who will perform the definitive surgical procedure will perform the incisional biopsy. A longitudinal biopsy orientation is recommended for extremity skin tumors, as most masses in this region will be excised with incisions parallel to the long axis of the limb. Histologic grade can differ between incisional biopsy and definitive surgical biopsy with incisional biopsy results more frequently underestimating rather than overestimating grade (Perry et al. 2014; Shaw et al. 2018). Based on these findings, grade as determined by pretreatment biopsy should be interpreted with caution.
Excisional Biopsy
Excisional biopsy involves removal of the skin tumor with a margin of normal tissue in all planes. The potential advantage of an excisional biopsy is that it provides both a diagnosis and definitive treatment in one surgical episode. Excisional biopsy is best suited for small masses with benign or low‐grade features of malignancy on FNA cytology in anatomical locations that allow for wide resection. Inappropriate use of excision biopsy can result in incomplete surgical margins compromising the optimal treatment options for a patient. Bacon et al. (2007) reported on 41 cases of unplanned excisional resection of soft tissue sarcoma skin masses that resulted in incomplete resection. Only 41% of cases in that study had preoperative FNA cytology performed, and 59% percent of cases did not have a presurgical biopsy procedure, highlighting the need for appropriate preoperative diagnostic evaluation.
Regional Lymph Node Assessment
All regional lymph nodes should be assessed by palpation to assess size, firmness, and adherence to underlying structures and FNA cytology regardless of size as part of the evaluation of a cutaneous mass. The sensitivity and specificity of FNA cytology for diagnosis of metastatic disease in lymph nodes in solid neoplasms is 91–100% and 91–96%, respectively, compared to histopathology of the entire lymph node (Langenbach et al. 2001; Ku et al. 2017). Factors reported contributing to discrepancies between cytology and histology include focal distribution of metastases and poorly defined criteria for metastatic mast cell tumors (Ku et al. 2017). False‐positives results with cytology were more common with mast cell tumors and melanomas (Ku et al. 2017). Carcinomas are reported to metastasize to regional lymph nodes more frequently than sarcomas (Langenbach et al. 2001). Lymph node size is not predictive for metastatic status. Incisional or excisional biopsy and histologic assessment of the regional lymph node is the optimal approach to lymph node assessment.
Identification and biopsy of the first draining regional lymph node, the sentinel lymph node (SLN), is important in the prediction of survival for a variety of cancers in human and veterinary oncology (Tuohy et al. 2009; Beer et al. 2018). The anatomically closest regional LN is not necessarily the SLN, so SLN mapping is recommended. The sentinel lymph node can be identified using a variety of techniques including lymphoscintigraphy (Worley 2014), CT lymphography (Brissot and Edery 2017; Grimes et al. 2017; Majeski et al. 2017; Rossi et al. 2018), and methylene blue. SLN mapping and sampling allows identification of microscopic metastatic disease that would otherwise have been undetected. In such circumstances, clinical stage changes and consequently additional therapy is recommended that would have otherwise not been offered. This can lead to an improved oncologic outcome (Worley 2014).
Preoperative Diagnostic Imaging
Diagnostic imaging is used to evaluate for evidence of metastatic disease as part of the staging process. Three‐view thoracic radiographs or CT are used most commonly to evaluate for pulmonary metastases and thoracic lymph node involvement, and abdominal ultrasonography or CT for evaluation of abdominal lymph nodes and intraabdominal metastases.
Imaging of the primary cutaneous mass, using ultrasonography, CT, or magnetic resonance imaging, provides detail on the degree of local invasion, particularly at the deep margin that facilitates appropriate surgical anatomical margin planning. This is particularly important when major reconstructive procedures are required to achieve local tumor control. Examples of skin tumors where this is particularly useful are those overlying the thoracic cavity, head and neck or pelvis, and any other area with important anatomical structures (Figure 4.1).
Treatment Options for Skin Tumors
Appropriate treatment options in an individual case are based on the tumor type and degree of local tumor disease, the results of staging tests, the presence or absence of metastases, and the overall condition of the patient. Most solid skin and subcutaneous tumors can be treated successfully with surgical resection. Surgery includes tumor removal by means of excision or local ablative therapies, such as cryosurgery, electrosurgery, and surgical lasers. Surgery can be used as the sole treatment modality or in combination with chemotherapy, radiation therapy, or other adjunctive treatments.
Figure 4.1 (a, b) Noncontrast and contrast CT scan imaging of an interscapular vaccine‐associated sarcoma in a cat used to plan deep surgical resection margins.
Radiation therapy can be used as an effective primary local therapy or as an adjunctive treatment in combination with surgery. Squamous cell carcinoma, basal cell carcinoma, cutaneous lymphoma, and mast cell tumors (MCTs) are the most radiation‐sensitive skin tumors.
Chemotherapy is the preferred treatment option for some of the round cell tumors, such as lymphoma, transmissible venereal tumor, and some mast cell tumors.
Principles of Surgical Excision
The goal of surgical excision of malignant skin tumors is to achieve wide and complete en bloc excision of the primary tumor surrounded by a margin of normal tissue in three dimensions (Figures 4.2a–c). The extent of the surgical margin will depend on the tumor type and location. More conservative margins are appropriate for removal of benign skin tumors. En bloc surgical resection requires removal of any tissue that the tumor is in contact with, which may require removal of fascia, muscle, subcutaneous fat, or even bone. The first surgery generally provides the best opportunity to achieve local tumor control. Surgical excision of a skin tumor should be done with aseptic surgical techniques and sterile instruments. Gentle tissue handling and maintenance of blood supply with minimization of dead space and tension at the surgery site are important surgical principles to maintain during removal of cutaneous tumors. Any previous biopsy site should be removed with the resected tissues. Ideally, complete surgical excision of the tumor without entering the tumor capsule should be done to avoid tumor seeding at local or distant sites. Veins should be ligated early in the procedure to prevent hematogenous spread of tumor cells especially in large tumors.
Surgical instruments, drapes, and gloves should be changed immediately, and intraoperative lavage should be done, if the tumor is entered inadvertently or if an intracapsular resection is done and the change should be performed routinely after malignant tumor excision.
Postoperative surgical drains should be avoided as they can potentially contaminate the normal tissues through which they pass with tumor cells; however, they should be considered if surgery results in a large dead space or is in a high‐motion anatomical