Tumors of the Skin and Subcutaneous Tissues

Chapter 30 Tumors of the Skin and Subcutaneous Tissues

Janet L. Peterson

The skin and subcutis are the most common sites of neoplasia in the dog, accounting for approximately 30% to 40% of all tumors. In the cat, only 20% of all tumors originate in the skin and subcutis, making it the second most common site of origin. Most canine skin tumors are benign, whereas most feline skin tumors are malignant. Owners often discover skin tumors while petting or grooming their animals.

Skin tumors should not be ignored. Advising owners to “watch” the tumor rather than to seek a diagnosis could be life-threatening to their dog or cat.

The most common skin tumors in the dog and cat are shown in Table 30-1.

Dog breeds at an increased risk for skin tumors include the basset hound, boxer, bull mastiff, Scottish terrier, and Weimaraner; there is no apparent breed predilection in the cat. In general, skin and subcutaneous tumors are more common in older dogs and cats. Common tumors in younger dogs are shown in Table 30-1. The etiology of most canine and feline tumors is unknown; however, some tumors have been associated with defined etiologic agents, such as those listed in Table 30-2.

Table 30-2 ETIOLOGIC AGENTS

Etiology	Tumor Associations
Viruses	Canine squamous papilloma
	Warts
	Feline sarcoma virus—associated multiple fibrosarcomas
Solar and ionizing radiation	Squamous cell carcinoma
Hormones	Perianal adenoma
Thermal injuries	Squamous cell carcinoma
Thermal injuries	Mast cell tumor
Genetic factors	Various tumors
Immunologic compromise	Feline lymphoma
Age	Many tumors

Physical examination may assist the clinician in formulating a list of potential tumors in the small animal patient. A basic working knowledge of the anatomic location and tissue of origin of various tumors can assist the clinician in making educated decisions. Additionally, fine-needle aspiration helps make a definitive diagnosis without requiring an incisional or excisional biopsy.

TUMOR CLASSIFICATION

Skin tumors can be classified by tissue of origin (Table 30-3), anatomic location (Table 30-4), and stratum of origin (Table 30-5). Miscellaneous tumors affecting the skin are listed in Table 30-6.

Table 30-6 MISCELLANEOUS TUMORS AFFECTING THE SKIN

Intracutaneous cornifying epithelioma (keratoacanthoma)

Trichoepithelioma and pilomatrixoma (tumors of the hair follicles)

Epidermal inclusion cyst (epidermoid, epidermoid cyst)

DIAGNOSIS

General Principles

• Diagnosis and characterization of a skin mass is important for several reasons. Knowledge of the tumor type before surgical excision allows the clinician (1) to plan an appropriate surgical approach (e.g., in a mast cell tumor that requires 3-cm margins beyond the tumor edges), (2) to consider radiation therapy (e.g., non-resectable mast cell tumors), or (3) to institute medical treatment (e.g., vincristine chemotherapy is usually curative for transmissible venereal tumors).

• To detect the presence of metastatic disease before surgery, evaluate every enlarged regional lymph node cytologically (fine-needle aspiration) or histologically. Additionally, consider lymph node excisional biopsy at surgery for more accurate staging.

• Perform thoracic radiographs (three views) on all animals with suspected (or confirmed) malignant tumors to detect potential pulmonary metastases. Perform abdominal radiography and/or abdominal ultrasonography in those cases in which dissemination to the abdominal organs, cavity, or multicentricity is suspected (e.g., mast cell tumor or hemangiosarcoma). Evaluate skeletal structures on thoracic and abdominal radiographs for metastatic disease.

Diagnosis and staging of a skin tumor before surgery are critical to ensure complete excision and to determine appropriate additional treatment.

Fine-Needle Aspiration Cytology

Equipment

• 12-ml syringe

• 22- or 25-gauge needle

• Glass slides

• Diff-Quick or Wright-Giemsa stain

Cytologic Classification of Neoplasia

Cytology can be used to classify masses as neoplastic or non-neoplastic. If the mass is not neoplastic, then the decision must be made as to whether it is inflammatory or non-inflammatory. Masses also can be classified as mixed, as in the case of a neoplastic mass with a necrotic center and associated inflammation. Infectious agents also may be identified cytologically (e.g., bacteria, fungi, or protozoa).

Cytologic examination may allow classification of tumors into one of three categories: epithelial, mesenchymal, or round cell neoplasm (includes epithelial, mesenchymal, and melanocytic tumors).

Epithelial Tumors

Epithelial cells cluster because of desmosomes. Aspiration cytology reveals round to polygonal cells with basophilic cytoplasm (with and without vacuoles) and cell-to-cell association. The latter may be absent in squamous cell carcinomas.

Mesenchymal Tumors

Mesenchymal tumors arise from connective tissue, including fibrous tissue, muscle, fat, and blood vessels. They do not exfoliate well on fine-needle aspiration; however, when they do exfoliate, they appear as individual cells or in small groups of cells, with a spindle to polygonal shape.

Round Cell Tumors

On fine-needle aspiration, round cell tumors tend to exfoliate as single cells with the exception of transmissible venereal tumor cells, which usually clump. The cells have distinct cell membranes like epithelial cells. Some cells may have cytoplasmic granules (i.e., mast cells, melanoma cells, and large granular lymphocytes).

Round cell tumors include lymphoma, melanoma, histiocytoma, transmissible venereal tumor, mast cell tumor, and plasma cell tumor. Additionally, squamous cell carcinomas and basal cell tumors can resemble round cell tumors cytologically.

Excisional Biopsy

Excisional biopsy implies that all or most of the tumor or mass is removed and submitted for histopathologic examination. This approach is indicated for small, easily excisable masses. Margins of excised tissue should be at least 1 cm in all cases. All excised tumors must be properly fixed. Label each tumor by location if there are multiple tumors, and finally, send all tumors to a qualified veterinary pathologist for examination. Principles of surgical biopsy of tumors are described in Chapter 26.

Perform histopathologic examinations on all excised skin masses, no matter how “benign” they appear. Appropriate treatment necessitates a definitive diagnosis.

Staging

Clinical staging is helpful in characterizing the extent of the disease. Obtain thoracic radiographs (left and right lateral and ventrodorsal views) when malignancy is suspected. Perform abdominal radiography or ultrasonography for tumors such as cutaneous hemangiosarcoma and mast cell tumor to evaluate for splenic and hepatic involvement (metastases or primary tumor).

SELECTED TUMORS OF THE

SKIN AND subcutaneous tissue

Epithelial Neoplasia

Papilloma

Origin and Etiology

Papillomas (squamous papillomas, squamous cell papillomatosis, warts, and cutaneous papillomatosis) originate from the squamous epithelium. Papillomas have a DNA viral etiology in puppies (see Chapter 16), but the etiology is unknown in older dogs.

Description

Papillomas may appear as cauliflower or wart-like growths that are usually well encapsulated. They can be sessile or pedunculated and may bleed if traumatized. They can occur as a single tumor (usually non-viral etiology) or as multiple tumors (usually viral etiology) in the skin, mucous membranes, or mucocutaneous regions.

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Tags: Saunders Manual of Small Animal Practice

Aug 27, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

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