10 most frequent tumors had a mean age of 8.3 years, with a range of 2 months to 19 years (Pakhrin et al. 2007).
In another canine study, Kaldrymidou et al. (2002) examined specimens of 174 dogs diagnosed with cutaneous neoplasms with comparable findings. Thirty‐one types of neoplasm were diagnosed, among which mast cell tumors (13.8%), hepatoid gland adenomas (9.8%), lipomas (5.7%), and histiocytomas (5.7%) were the most common. The prevalence of epithelial, mesenchymal, lymphohistiocytic, and melanocytic tumors was 47.7%, 40.8%, 8.6%, and 2.9%, respectively. Potentially, malignant neoplasms were less frequently recorded than benign neoplasms. The tumors were either single (80.5%) or multiple (19.5%) and located on the head and neck (18.4%), the body and trunk (49.4%), the limbs (25.9%), or at multiple sites (6.3%). In a multivariable logistic regression model, the odds of tumor malignancy linearly increased with increasing age of the dog by a factor of 1.1 per year.
A study examining the existence of a possible overrepresentation in tumor incidence, reported a group of 2242 Dutch Golden Retrievers that seemed to have an increased risk in tumor development compared to other breeds. Combining individual tumors from both the cytological and the histopathological data‐set resulted in an annual estimated incidence rate of 2242 for 100 000 dog‐years at risk regarding the tumor development in general. The most common cytological tumor diagnoses were “fat, possibly lipoma” (35%), mast cell tumor (21%), and non‐Hodgkin lymphoma (10%). The most commonly diagnosed tumors by histology were mast cell tumor (26%), soft tissue sarcomas (11%), and melanoma (8%) (Boerkamp et al. 2014).
Miller et al. (1991) reported a total of 340 cases of cutaneous neoplasia were diagnosed in 340 of 3564 cats that were examined by biopsy or necropsy during a 41‐month period from 1 January 1986 through 31 May 1989. Eighteen types of tumors occurred but four types comprised 77% of the cases. These were basal cell tumors, 89 cases (26%, mean age 10.3); mast cell tumors, 72 cases (21%, mean age 8.6); squamous cell carcinomas, 52 cases (15%, mean age 11.6); and fibrosarcomas, 50 cases (15%, mean age 10.2). For each of these four types of tumors, peak number of cases occurred in cats older than 10 years. Mast cell tumor was the only tumor diagnosed in cats younger than one year. The head was the most common site for basal cell tumors, mast cell tumors, and squamous cell carcinomas. The legs were the most common location of fibrosarcomas (FSAs). Siamese cats had approximately three times as many mast cell tumors as statistically expected but only one‐fourth as many squamous cell carcinomas. Breed predilection for other skin tumors was not apparent. Sex predilection was not detected for any skin tumor (Miller et al. 1991).
Table 4.1 WHO TNM staging system for tumors in animals.
| Primary tumor (T) | |
| T0 | No evidence of neoplasia |
| T1 | Tumor < 1 cm diameter, not invasive |
| T2 | Tumor 1–3 cm diameter, locally invasive |
| T3 | Tumor > 3 cm diameter or evidence of ulceration of local invasion |
| Node (N) | |
| N0 | No evidence of nodal involvement |
| N1 | Node firm and enlarged |
| N2 | Node firm, enlarged, and fixed to surrounding tissues |
| N3 | Nodal involvement beyond the first station |
| Metastasis (M) | |
| M0 | No evidence of metastasis |
| M1 | Metastasis to one organ system (e.g. pulmonary metastasis) |
| M2 | Metastasis to more than one organ system (e.g. pulmonary and hepatic metastases) |
Classification
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.
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
