Etiology

Several external agents and biologic factors are recognized to be important in the development of certain tumors of the skin but for the majority the etiology is unknown. Long-term exposure to ultraviolet light (especially UVB) is implicated in the development of squamous and basal cell tumors in unprotected, non-pigmented and lightly haired skin. Papilloma viruses may, in some cats, give rise to viral plaques and these can progress to form ‘Bowenoid’ in situ carcinoma which can in turn progress to invasive squamous cell carcinoma (SCC).

Pathology

The skin is a complex organ essentially comprising two layers:

Clinical examination

A presumptive diagnosis of a solitary skin tumor may be made on the clinical examination, including visual inspection and palpation. Skin tumors that present as multifocal lesions may be harder to distinguish from other dermatological conditions. In both cases skin tumors must be differentiated from other skin lesions (Box 20-1).

A useful clinical approach to a lesion affecting the skin is to determine the location of the lesion, i.e., whether it is epidermal, dermal or subcutaneous, as this will give some indication of its origin and thus its possible histogenesis. The distinction between solitary and multiple lesions is also of clinical value. However, a definitive diagnosis can only be made upon histopathological examination of representative tissue.

Blood analysis

Routine hematological and biochemical analysis of blood is not generally very helpful in the diagnosis of skin tumors, although some skin tumors may be associated with hematological or paraneoplastic complications and these investigations would be indicated in the general evaluation of such tumors.

Imaging techniques

Radiography of the primary tumor is not usually necessary unless the lesion is invasive and sited close to bone (e.g., SCC of the digit). Radiography of the thorax and ultrasound evaluation of abdominal organs (liver, spleen, and kidneys) is important in the clinical staging of malignant tumors. Both techniques may be used to assess the internal iliac and sub-lumbar lymph nodes for staging malignant skin tumors sited on the hind limbs and perianal region.

Biopsy and fine needle aspiration

Cutaneous masses are easily accessible for fine needle aspiration and this is a quick, minimally invasive and useful technique for assessing any mass within the skin. In some cases (e.g., mast cell tumors, cutaneous lymphoma) cytology may provide a diagnosis, although histological examination of the tumor may still be required to assess the grade of the lesion. Fine needle aspiration of local lymph nodes is also useful to assess metastatic spread.

Skin punch, needle, or incisional biopsies are required for a definitive histological diagnosis. Collection of a representative sample of tissue is important. Areas of ulceration and necrosis should be avoided and it is important to collect a deep enough sample of the tumor. Careful planning of the biopsy procedure should be made so that seeding of tumor cells through normal tissue planes is avoided. Definitive surgery should include the biopsy tracts, as they are regarded as contaminated. Excisional biopsy (i.e., local resection of the whole mass) is not generally recommended, except as a diagnostic procedure in a case with multiple skin lesions. More details on biopsy techniques can be found in Chapter 14.

Staging

Clinical staging of primary tumors of the skin is by clinical examination and imaging of body cavities where indicated by the nature of the primary tumor. The primary tumor is assessed on the basis of its size, infiltration of the subcutis, and involvement of other structures such as fascia, muscle, and bone (Box 20-2). For more information on tumor staging see Chapter 14.

Papilloma/cutaneous papillomatosis

Cutaneous papillomatosis is a viral disease that is rare in dogs and very rare in cats, but papilloma viruses may be implicated in the etiology of some carcinomas of the feline skin.

Basal cell tumor/carcinoma

This tumor is relatively common in the cat and usually affects middle-aged to older animals. It is reported to account for 14– 26% of feline skin tumors1,2 or for 34% of feline epithelial tumors.³

Tumors are usually solitary, discrete, firm, well-circumscribed masses, sited in the dermis and subcutis of the head and neck (Fig. 20-1). The overlying skin may be ulcerated. Most tumors are small, 0.5–2.0 cm in diameter, although on occasion they may reach up to 10 cm in size. Some feline basal cell tumors contain abundant melanin pigment and may therefore be confused on gross inspection with melanoma. Various subtypes have been described based on the histological appearance, e.g., solid, cystic, adenoid, and medusoid. Diagnosis can be made on cytological examination; however, the occurrence of cells with characteristic basal cell-like appearance has been shown to be an inconsistent finding in cats.⁴

Tumors of the adnexae

Benign adnexal tumors are relatively common in the dog but appear to be less common in the cat and they include:

In one survey of feline follicular tumors in Italy trichoblastoma was reported as the most common form, representing 26% of cases. Follicular tumors and tumor-like lesions together represented 8% of all skin tumors in the cat.⁶ All these tumors are slow growing, non-invasive and benign in their behavior. Malignant tumors arising from the adnexal structures of the skin are extremely rare in the cat.

Squamous cell carcinoma

Squamous cell carcinoma is the most common malignant skin tumor in the cat, accounting for 15–50% of all skin neoplasms. It usually affects older animals and there is no known breed predisposition, although white-coated cats have a higher incidence. SCC is a locally invasive tumor that infiltrates the underlying dermal and subcutaneous tissue. Metastasis tends to be via the lymphatic route but the incidence of metastasis is low.

Feline cutaneous SCC most frequently occurs on the head, affecting particularly the pinnae, nasal planum, and eyelids (Figs 20-2 to 20-4). The etiology in the majority of cases is related to long-term exposure of unprotected skin (non-pigmented, hairless regions) to ultraviolet light, with lesions initially of solar dermatitis progressing to actinic keratitis (actinic carcinoma in situ) to carcinoma in situ to invasive SCC, often over quite a long course of time. Ultraviolet-related p53 lesions have been identified in feline SCC. Clinically these lesions are crusty, scabby, ulcerated to plaque-like. Diagnosis is made by tissue biopsy and histology. Staging beyond local disease is not usually necessary.

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