and epithelial skin tumors of domestic animals (Goldschmidt and Shofer 1998). Cutaneous tumors can be broadly classified histologically by the tissue of origin into epithelial, adnexal, mesenchymal, round cell, or melanocytic tumors.
1 Epithelial – Epithelial tumors comprise basal cell tumors, papilloma, squamous cell carcinoma, and subungual (nail bed) tumors.
2 Adnexal – Adnexal tumors arise from the adnexal structures of the skin. They include sebaceous gland tumors (adenoma or adenocarcinoma), ceruminous gland adenoma, perianal tumors (adenoma, adenocarcinoma), adenocarcinoma of the apocrine glands of the anal sac, sweat gland tumors, hair follicle tumors, trichoepithelioma, pilomatrixoma, meibomian gland adenoma, and intracutaneous cornifying epithelioma (ICE).
3 Mesenchymal tumors – Mesenchymal tumors originate from connective tissue and are often located within or invade the subcutis and skin. Malignant mesenchymal tumors are referred to as soft tissue sarcomas. Mesenchymal skin tumor types include lipo(sarco)ma, fibro(sarco)ma, hemangio(sarco)ma, myxo(sarcoma)ma, and peripheral nerve sheath tumors (PNSTs) (neurofibro(sarco)ma and malignant schwannoma). Hemangiopericytomas and feline injection site‐associated sarcoma (FISAS) are also included.
4 Round cell – Tumors of round cell populations that are normally resident within the dermis and subcutis. Examples of cutaneous round cell tumors include mast cell tumors, histiocytomas, plasmacytoma, lymphoma, and transmissible venereal tumor.
5 Melanocytic – Most cutaneous melanocytic tumors are benign. Common anatomical sites are the eyelids, face, trunk, and extremities. The biological behavior of melanocytic tumors varies with anatomical location, with those located in the oral cavity and subungual sites are more likely to be malignant and associated with a poorer prognosis.
More detailed descriptions of the biological behavior and characteristics of individual tumor types can be found in other veterinary oncology textbooks (Vail et al. 2019; Meuten 2016). Mast cell tumors (MCTs) and soft tissue sarcomas (STS) are covered later in this chapter. Within individual tumor types, tumors can be classified based on biological behavior, histologic grade, and clinical stage. Biological behavior describes the growth characteristics of the tumor, histologic grade, and the degree of tumor differentiation. Clinical stage is described by the WHO Tumor Node Metastasis (TNM) system (Owen 1980) (Table 4.1). Staging is based on the size and invasiveness of the local primary tumor (T), spread to regional lymph nodes (N), and presence or absence of distant metastases (M). The TNM is modified with specific criteria for different tumor types. The TNM staging system characterizes the pattern and extent of spread from the local tumor which can be correlated to prognosis and can therefore guide appropriate treatment decisions.
General Approach to the Diagnosis and Staging of Skin Tumors
Preoperative Assessment
History and Physical Examination
A thorough history of the clinical course of the skin tumor is essential. Pertinent 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.