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4 Skin and Subcutaneous Tumors
Erik G.H. Wouters, Sebastiaan (Bas) A. van Nimwegen, Stewart Ryan, and Jolle Kirpensteijn
Skin Tumors General Principles
Introduction
Prevalence
In dogs, approximately 25–30% of all neoplasms are reported to arise in the skin (Dobson et al. 2002; Villamil et al. 2011; Gruntzig et al. 2015). In a study evaluating 2952 canine biopsy specimens, 748 (25.34%) were canine cutaneous tumors. Thirty‐eight different types of cutaneous tumors were identified and categorized into epithelial and melanocytic tumors (56.95%), mesenchymal tumors (38.90%), and hematopoietic tumors (4.14%), all located in the skin. Among these, 69.25% were benign and 30.75% were malignant (Pakhrin et al. 2007; Bronden et al. 2010).
The top 10 most frequently diagnosed cutaneous tumors were epidermal and follicular cysts (12.70%), lipoma (11.36%), mast cell tumors (8.82%), cutaneous histiocytoma (7.49%), basal cell tumors (6.82%), sebaceous gland adenoma (6.68%), sebaceous gland hyperplasia (5.08%), hepatoid gland adenoma (3.61%), apocrine adenocarcinoma (3.07%), and fibroma (2.81%). These mentioned tumors comprise 68.45% of all cutaneous tumors and were distributed on the trunk (30.08%), head and neck (20.9%), extremities (19.14%), anal and perianal area (8.59%), and tail (3.91%). Dogs with the 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