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Cutaneous tumors involve the skin or subcutaneous tissues. The World Health Organization (WHO) has a detailed histologic classification scheme for mesenchymal 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.