Canine and feline skin tumors (Part 1)
Due to its complex structure a large variety of tumors may arise in the skin (Table 1), as can secondary (metastatic) tumors. Approximately 2/3 of all canine cutaneous tumors are solitary, benign lesions originating from the epithelium or from adnexal structures, whilst in the cat malignant tumors are more common than benign ones. The etiology of most skin tumors is still unknown, but roles of UVB light exposure, hormonal and viral etiologies are demonstrated in some tumors. This article provides an overview of the more common skin tumors affecting dogs and cats but excludes soft tissue sarcomas.
A presumptive diagnosis of a solitary skin tumor may be made on clinical examination, but multifocal lesions may be harder to distinguish from other dermatological conditions. Fine needle aspiration (FNA) 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 is still required to grade 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 and is discussed elsewhere in this issue. Excisional biopsy (local resection of the whole mass) should only be performed after careful consideration; it can have disastrous consequences if used inappropriately and the opportunity for curative treatment may be lost forever. Excisional biopsy is appropriate in cases where prior knowledge of tumor histology/ grade would not alter the surgical approach.
Specific cutaneous tumors
Squamous cell carcinoma
Squamous cell carcinoma (SCC) is one of the more common malignant cutaneous tumors in the dog and the most common in the cat, usually affecting animals > 10 years with no breed predisposition. SCC can occur anywhere in the skin in the dog, whereas lesions are more common on the head of the cat, affecting the nasal planum (Figures 1,2), pinnae, eyelids and lips. SCC may be productive, forming a friable, papillary growth, or erosive, forming an ulcerated lesion, and must be distinguished from inflammatory or infective lesions.
Prolonged exposure to UVB light is recognized to be an important factor of actinic solar dermatitis, carcinoma in situ and eventually invasive SCC, but some may be caused by papilloma viruses, thermal injury or chronic inflammation.
Behavior, treatment and prognosis
Actinic keratosis is a pre-invasive lesion which may develop to pre-malignant carcinoma and then SCC. These are locally invasive tumors that infiltrate the underlying dermal and subcutaneous tissue. They are usually differentiated and metastasis (usually slow) tends to be via the lymphatic route, but the incidence is variable at other sites, e.g. in the nail-bed of the digit (Figure 3) it can behave much more aggressively. Canine tumors of the nasal planum are locally aggressive and frequently metastasize to the draining lymph nodes. Wide local surgical excision is the treatment of choice and if achieved, the prognosis is favorable. SCC is moderately radiosensitive and radiotherapy may be indicated as an alternative or adjunctive treatment in cases where adequate surgical resection is not possible. Locally applied Strontium 90 has been reported for treatment of superficial lesions (1). Photodynamic therapy (PDT) has been used successfully for treatment of early, superficial lesions of the nasal planum in cats and provides a good alternative to surgery and radiotherapy (2).
A multicentric SCC in situ has been described in cats (3). Biologically this is a premalignant lesion which is histologically similar to Bowen’s disease in humans (sometimes referred to as Bowenoid carcinoma). This tends to affect middle aged to older mixed breed cats with lesions developing in haired, pigmented regions of the skin. A papilloma virus may be implicated in the etiology, and the lesion can progress to invasive SCC.
SCC of the nail-bed region of the canine digit is an aggressive tumor and invasion and destruction of the distal phalanx is common. Amputation of the affected digit(s) is the treatment of choice but these tumors may metastasize to the local and regional lymph nodes and distantly; the prognosis is guarded. A phenomenon of SCC affecting several digits has been reported in large breed, black dogs e.g. standard poodles, giant schnauzers (4). These tumors appear to have a lesser tendency for metastasis but may affect multiple digits on different feet.
Melanocytic tumors are relatively uncommon. In dogs, cutaneous melanoma principally affects older animals and is most common in breeds with heavily pigmented skin, e.g. the Scottish terrier. Older cats are also affected but there is no sex or breed predisposition. Grossly, tumors may appear as flat, plaque-like to domed masses, up to 2 cm in diameter, sited within the dermis. They are usually dark brown to black and quite well defined. Malignant tumors may attain larger size, contain less pigment and frequently ulcerate. In the cat cutaneous melanoma must be distinguished from the more common pigmented basal cell tumor.
Behavior, treatment and prognosis
Tumor site seems to be an important factor governing the behavior of cutaneous melanoma. Most melanocytic tumors of the canine skin are slow growing and benign. Tumors arising on the digits and at mucocutaneous junctions (e.g. eyelids and lips) are more aggressive with a higher incidence of metastasis to local lymph nodes, the lungs and other organs.
Wide surgical excision is the treatment of choice for benign dermal melanoma and the prognosis following complete surgical excision is good. Surgical excision is also indicated for local control of malignant tumors but the prognosis in such cases is guarded to poor because of the risk of metastasis. There is no proven role for chemotherapeutics in the control of malignant melanoma. Investigations into the canine melanoma vaccine, using human tyrosinase, are still ongoing (5).
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This article was previously published in 2012.