Introduction

Superficial dermal and subcutaneous masses are common in companion animals. They are the most common tumor in dogs representing approximately one third of all tumors. Skin and subcutaneous tumors in cats are the second most common tumor and overall represent approximately one quarter of the tumors in cats. It is important to recognize and differentiate benign from potentially malignant masses. In the dog approximately 20-30% are malignant, whereas 50-65% are malignant in the cat. The problem can be that some dermal masses with malignant potential are mistaken for benign lesions. Benign tumors are typically slow growing and have been present for months to years with little change. They are commonly well circumscribed, freely moveable and non-painful. Malignant tumors are more likely to be rapidly growing, have ill-defined margins, infiltrative, and may be ulcerated. The following will provide a review of skin masses as well as a systematic approach to diagnosis and treatment. This is not intended to be an exhaustive review of the subject but will touch on the most common clinical conditions encountered and treatment options.

The most common skin tumors in dogs and cats :

Etiology

The etiology for specific skin/subcutaneous tumors is often unknown. There are some instances wherein a specific causative event has been documented. Sunlight exposure (UV irradiation) has been associated with the development of squamous cell carcinoma in cats, and with cutaneous hemangioma, hemangiosarcoma, and squamous cell carcinoma in dogs. Papillomas in young dogs are of viral origin, and are more rarely documented in older dogs. Fibrosarcomas in cats have been associated with vaccination; a now well documented phenomenon. Feline leukemia virus has been associated with the development of cutaneous lymphoma.

Diagnostics and Clinical Evaluation

The physical examination should include a description of the size, consistency, location and any other defining characteristics of a mass. Three-dimensional measurements using calipers or other measuring device should be recorded in the patients' record. In the event that treatment is delayed then further characterization of the mass and its behavior will be possible if the original dimensions are available for comparison.

The accessibility of skin/subcutaneous masses allows for aspiration cytology and/or tissue biopsy. An initial aspirate may provide a definitive diagnose and obviate the need for a more invasive procedure. For instance, aspiration cytology of a lipoma may provide enough information and other than charting the location of the lipoma further diagnostics are not indicated. There are exceptions even in this situation that should be considered. For instance there are subcutaneous masses that on aspiration appear to be simple lipomas but are actually infiltrative lipomas and will require a more aggressive approach. Other examples of aspirates that may provide sufficient information include epidermal inclusion cysts and sebaceous adenomas/hyperplasia. Other potentially malignant tumors that are readily identified on cytology include mast cell tumors and melanomas. Even when the aspirate is diagnostic a biopsy may be important. For instance, for mast cell tumors, grade, an important prognostic factor, is obtained based on tissue biopsy and not on cytologic examination. A number of biopsy techniques are available depending on the size and location of the mass. Punch biopsy is the technique used most frequently. Even for subcutaneous masses an incision can be made in the skin and a punch biopsy obtained from the underlying mass. Other techniques include incisional biopsy using a scalpel blade, or core needle biopsy. One important question is should it be an incisional or an excisional biopsy. Depending on the size, location, and ease of resection an excisional biopsy may be appropriate. For larger masses in more difficult locations a biopsy should be done first and a definitive course of treatment determined based on the results of histopathologic examination. When an excisional biopsy is performed it is imperative that the specimen be marked and submitted as a whole such that the pathologist can evaluate the surgical margins for adequacy of resection.

If the regional lymph node is enlarged or otherwise altered on physical examination then an aspirate should be obtained. Sometimes in the face of a normal sized lymph node an aspirate is obtained only after the determination has been made that the skin/subcutaneous mass is one that could potentially metastasize to the regional lymph node. For instance, mast cell tumors typically metastasize first to regional lymph node and close attention is critical to staging the loco-regional disease.

In some instances further evaluation should be conducted prior to definitive treatment. Complete blood work to assess overall health status, and thoracic radiographs to determine if there is any evidence of distant metastasis may be indicated.

Tumor imaging may be important particularly for deeper, more invasive, and larger more extensive tumors. Ultrasonographic examination may provide information on the extent and nature of the lesion. For instance ultrasound of a cutaneous/subcutaneous hemangioma or hemangiosarcoma may reveal fluid filled spaces within the mass that may alter the diagnostic approach and treatment. A contrast enhanced computed tomographic scan of a vaccine-associated sarcoma often will more adequately describe the extent of local disease and guide therapeutic decisions.

Skin/Subcutaneous Tumors

The following discussion will provide an overview of the most common skin/subcutaneous tumors encountered, and are broken down by histopathologic type into epithelial, round cell and mesenchymal tumors.

Epithelial tumors

1  Papillomas - cutaneous papillomas, warts, squamous cell papilloma. These cauliflower like growths with small frond like structures occur in the dog but are rare in cats. In young dogs they are typically multiple while in older dogs they are usually solitary. They can occur on the head, eyelids, feet and in and around the mouth. Viral associated papillomas are contagious but treatment is usually not necessary as they will regress within approximately 3 months. Surgery or cryotherapy are sometimes necessary and can be very effective.

2  Squamous cell carcinoma. This is a common skin tumor particularly in the cat. They are usually located on nonpigmented skin and in areas that are not covered with hair. In the cat lesions occur most commonly on the head including the nasal planum, eyelids, temporal region, and pinnae. Multiple lesions are present in 30% of cats. There is typically a progression of lesions from actinic keratosis, to squamous cell carcinoma in situ to squamous cell carcinoma. In the dog the sites of occurrence include nail bed, scrotum, nose, legs, and perianal region. Sun associated lesions have been reported in dogs in areas of nonpigmented skin of the flank and abdomen in dalmatians, beagles, whippets, and white English bull terriers. Squamous cell carcinoma can present as a proliferative or ulcerative/erosive lesion. Early lesions most notably in cats appear to be small pinpoint scabs that may even heal then recur. Tumors can be locally invasive but are late to metastasize. Early lesions can be addressed effectively with a range of treatment modalities. Options include excision, cryotherapy, photodynamic therapy, and Strontium-90 plesiotherapy or external beam radiation therapy. Situations that are more problematic include dogs with multiple large lesions; and digital squamous cell carcinoma which has a greater metastatic potential than SCC at other cutaneous sites.

3  Basal cell tumors. Basal cell tumors are benign; and basal cell tumors are the most common skin tumor in cats. They are usually solitary, well circumscribed, firm hairless, elevated masses that are freely moveable. Tumors are most commonly located on the head, neck and shoulders in both species. In the cat they can be pigmented, cystic or solid and may be ulcerated. Treatment entails surgical resection effecting local control.

4  Sebaceous gland tumors. This group of tumors include sebaceous hyperplasia, sebaceous epithelioma, sebaceous adenoma, and sebaceous adenocarcinoma. They are relatively common in dogs and less common in cats. Sebaceous hyperplasia is the most common and may be solitary or multiple wart-like growths that are usually < 1 cm in diameter. In the dog, miniature schnauzers, beagles, poodles and cocker spaniels are commonly affected. All of the sebaceous gland tumors can be treated effectively with surgery alone.

5  Sweat gland tumors. Apocrine sweat gland cysts in dogs are raised, round, fluctuant intradermal lesions that contain a clear watery fluid. Apocrine sweat gland adenomas and adenocarcinomas are relatively rare. They are solitary, raised, solid masses and approximately 50% are ulcerated. There is often evidence of lymphatic invasion on histopathologic examination but metastasis is relatively uncommon. Wide surgical resection is the treatment of choice.

6  Perianal tumors. Perianal adenomas (circumanal gland origin) represent the majority of perianal tumors and are most common in the male dog. Intact males are at risk for development of this tumor due to the effect of androgens. Castration alone can at times be curative effecting reduction in the perianal tumor with removal of the androgen source. They occur on the hairless skin around the anus, are raised, sometimes multiple masses that may become ulcerated and secondarily infected. Castrated males are more likely to have the malignant counterpart, perianal adenocarcinoma which may require more aggressive management.

Round cell tumors

1  Mast cell tumors. Mast cell tumors are common in both the dog and cat. In the dog histopathologic determination of tumor grade in conjunction with a number of other prognostic factors aids in determining the potential course of the disease. Tumor grading has not been useful in cats. The majority of dogs have a solitary skin mass (raised, dermal, may be erythematous, can be ulcerated, typically < 3 cm in diameter) located on the trunk or extremities, although up to 14% of dogs will have more than one lesion at initial presentation. The subcutaneous form of mast cell tumors in the dog can be problematic as they often feel like a lipoma but should be aspirated to rule out the possibility of a mast cell tumor. Cats more frequently have multiple lesions at presentation, and are more commonly located in the head and neck region. The treatment of choice is surgical resection but additional treatment such as local radiation therapy and/or chemotherapy are frequently and effectively utilized.

2  Histiocytoma. Histiocytomas are common in dogs. They are typically solitary, fast growing, dome-shaped dermal masses with a hairless or ulcerated surface. Aspiration cytology is usually diagnostic. These are typically benign tumors that may spontaneously regress in approximately 3 months. Alternatively surgical resection is curative.

3  Plasmacytomas. Also referred to as extramedullary plasmacytomas. Tumors can occur anywhere on the body involving haired skin, and have been identified at mucocutaneous junctions as well as in the oral cavity and other mucosal sites. They are usually raised, smooth, solitary nodules. They are typically benign and can be treated effectively with surgical resection. They are also responsive to radiation therapy. Alternatively, medical management is possible (melphalan and prednisone).

4  Transmissible venereal tumor. Abbreviated TVT. TVT's typically involve mucosal membranes (genital tract or nasal cavity) and are sexually transmitted. They are typically red, proliferative growths that can potentially metastasize to the regional lymph node. Aspiration cytology is usually diagnostic. Treatment is most commonly systemic chemotherapy with vincristine administered weekly until 1-2 weeks after complete resolution of the lesion. Surgical resection is typically not indicated as this tumor is quite responsive to chemotherapy, as well as radiation therapy.

5  Melanoma. Cutaneous melanomas are relatively uncommon in the dog and more so in the cat. Melanomas are typically round, raised, and darkly pigmented. Tumors can be benign or malignant and can be located anywhere on the body. Specific locations are typically associated with more aggressive behavior with a higher metastatic potential, e.g., digital melanomas, and melanomas located at a mucocutaneous junction. The vast majority of melanomas in dogs involving haired skin are benign and surgical resection is curative.

Mesenchymal tumors

1  Lipoma. Lipomas are benign fatty tumors that are common in dogs and far less common in cats. Often dogs that have one lipoma will ultimately develop multiple tumors. It is important to aspirate and confirm that it is a lipoma as opposed to a possible mast cell tumor. Surgical resection is only indicated when due to the location and size it is mechanically causing a problem, e.g., an axillary lipoma that is causing problems with the mobility of the forelimb. Another possibility for a tumor that on aspiration appears to be a lipoma is what is known as an infiltrative lipoma, a benign tumor that looks like normal fat but that infiltrates surrounding normal tissue most commonly muscle. Treatment in this situation may require a combination of surgery and radiation therapy.

2  Hemangiopericytoma/Fibrosarcoma. Fibrosarcomas are typically locally aggressive with infiltration into the surrounding normal tissues but less likely to metastasize. Tumors are often subcutaneous but can grow and extend through the skin resulting in ulceration of the overlying skin. Tumors can be a solitary mass effect, multi-nodular and often will have obvious attachments to the surrounding normal tissues. Tumor imaging, particularly for vaccine-associated sarcomas in cats is important to identify the extent of disease and for treatment planning. Treatment options include wide surgical resection, or combination approaches with surgery and radiation, +/- chemotherapy. Hemangiopericytomas are a common soft tissue sarcoma in dogs that have a very low metastatic potential. Tumors will typically recur after local resection.

3  Hemangiosarcoma. Cutaneous hemangiomas are raised, red dermal lesions that are benign and cured by surgical resection. Cutaneous/subcutaneous hemangiosarcomas have some malignant potential with metastasis possible particularly once the tumor involves and invades the underlying muscle. Ultrasound examination of a mass that on aspirate revealed only blood may be of benefit to identify the blood filled cavities that could result in significant hemorrhage should an incisional biopsy be performed. Treatment entails local surgical resection and as indicated systemic chemotherapy with a doxorubicin based chemotherapy protocol.