information gathered should include duration of presence of the mass and the rate of growth, as well as any change in growth rate. Knowledge of any previous aspiration cytology, biopsy, or histology results of the current or other tumors is important and may affect prognosis and treatment decisions. Details of any concurrent diseases are equally important, as they could include signs of paraneoplastic syndromes associated with the skin mass (e.g. gastrointestinal ulceration associated with mast cell tumors). A thorough physical examination is part of a routine clinical examination. A complete blood count, serum biochemistry, and urinalysis are part of a minimum database for many patients with cutaneous masses, who are often middle aged to older, to assess for concurrent pathologies before administration of general anesthesia and surgical intervention.
Tumor Staging
Clinical tumor staging determines the extent of the primary tumor and the presence of local or disseminated metastases. The results of accurate staging tests including physical assessment of the mass and fine needle aspiration (FNA) cytology provide information on whether further clinical staging by incisional or excisional biopsy of the mass, regional lymph node evaluation, and diagnostic imaging to assess for metastatic disease are indicated. This is most important for skin tumors that have a high metastatic potential. The tumor type, histologic grade, and clinical TNM stage should ideally be determined before surgical intervention to allow the formation of an appropriate treatment plan and to provide the client with a realistic expectation of prognosis.
Assessment of the Cutaneous Mass
The anatomical location of the mass and three‐dimensional measurements to determine the tumor volume should be recorded on a tumor map in the medical record. Cutaneous masses should be carefully palpated to determine consistency and the degree of fixation to underlying structures. Diagnostic imaging may be required to assess the degree of tumor invasion into deeper structures.
FNA Cytology
All skin and subcutaneous masses should have FNA cytology performed as part of the diagnostic process before surgical intervention. Most skin masses are easily accessible and amenable to this cost‐effective procedure, which can provide a rapid diagnosis and differentiate benign from malignant disease in most cases. FNA cytology provides information on tumor type (e.g. round cell vs. epithelial vs. mesenchymal cell types), but often the specific cell of origin type or tumor grade cannot be determined because the cytomorphology is not distinctive and no tissue architecture information is available. Round cell and epithelial tumors tend to exfoliate better and are more likely to provide a diagnostic sample compared to mesenchymal tumors. For diagnosing cutaneous neoplasia, FNA cytology had a sensitivity of 89.3%, a specificity of 97.9%, a positive predictive value of 99.4%, and a negative predictive value of 68.7% compared to histology (Ghisleni et al. 2006). If cytologic evaluation of a cutaneous mass is not diagnostic or knowledge of the tumor grade will influence the surgical dose, an incisional or needle core biopsy is indicated to obtain enough tissue to determine the histologic tumor type and grade to determine appropriate treatment options.
Biopsy
A pretreatment biopsy is indicated in the following situations:
If the type of treatment will be altered by knowledge of tumor type. For example, cutaneous lymphoma (chemotherapy) vs. FSA (surgery)
If the extent or surgical dose of treatment would be altered by knowledge of tumor grade, for example, grade III soft tissue sarcoma on an extremity (amputation or wide resection plus or minus adjunctive therapy) vs. grade I soft tissue sarcoma on an extremity (marginal resection)
If tumor location is in an anatomical location with difficult or limited reconstructive options
When knowledge of tumor type and grade affects the owner’s willingness to pursue treatment based on prognosis (Ehrhart 2005)
Biopsy techniques for skin tumors include needle core, incisional, and excisional biopsies (for technique description see Chapter 1).
Needle Core Biopsy
Disposable 14‐ or 16‐gauge needle core biopsy needles provide enough tissue for a pathologist to make a histologic diagnosis and, in most cases, provide a grade of skin tumors. Multiple (3–6) biopsy samples should be obtained to maximize the probability of an accurate diagnosis. Generally, the needle core biopsy can be done as an outpatient procedure with only local anesthesia and/or light sedation required.
Needle core biopsy is reported to accurately predict surgical biopsy with an overall sensitivity of 95.5%, specificity of 96.6%, and a positive predictive value of 95.5% (Aitken and Patnaik 2000). Thus, needle core biopsy performed before surgical excision of cutaneous masses can facilitate surgical planning and reduce the need for multiple surgical procedures.
Incisional 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
