Melanoma

TL;DR. While many skin melanomas in dogs are benign, those occurring in the mouth, toes, or on cats are typically highly aggressive cancers requiring prompt veterinary intervention.

Cutaneous melanomas on the haired skin of dogs are frequently benign, but any new dark nodule should be evaluated by a veterinarian.

What is it?

Melanoma, or melanocytic neoplasia, is a type of tumor that arises from melanocytes—the specialized cells responsible for producing pigment (melanin) in the skin, hair, and eyes. These tumors can develop anywhere melanocytes are present, including the skin, the oral cavity, the nailbeds, and the eyes.

In dogs, the behavior of these tumors depends heavily on where they are located. The majority of melanomas found on haired skin are benign (often referred to as melanocytomas) and do not spread. However, melanomas that develop in the mouth (oral cavity), on the toes (digital), or where the skin meets moist tissues (mucocutaneous junctions) are typically highly malignant, aggressive, and prone to spreading rapidly to other parts of the body.

In cats, melanocytic tumors are relatively uncommon. However, unlike in dogs, when a cat develops a melanoma, it is almost always considered malignant and carries a high risk of spreading. Understanding the location and nature of these masses is critical for determining the appropriate medical response.

Causes & risk factors

Unlike melanoma in humans, which is strongly linked to ultraviolet (UV) radiation from sun exposure, sun exposure does not appear to be a primary cause of melanoma in dogs and cats. Because these tumors frequently arise in haired skin or deep within the oral cavity, other factors are at play.

Veterinary researchers believe that genetic susceptibility, breed predispositions, and alterations in oncogenes (genes that can transform a cell into a tumor cell), tumor suppressor genes, and the body's immune surveillance system are the primary drivers. Melanomas are most common in older dogs and are rare in older cats. Dogs with heavily pigmented skin are also at a higher risk for developing digital (toe) melanomas.

Signs to watch for

Melanomas can vary widely in appearance. While many are dark brown or black, some do not produce pigment at all (known as amelanotic melanomas) and may appear pink or flesh-colored.

• Fleshy mass on the gum, tongue, or palate (Common)
• Solitary, well-circumscribed, dome-shaped, firm, brown to black, hairless, or wartlike growth on the skin (Common)
• Ulceration or bleeding of the mass (Occasional)
• Amelanotic (non-pigmented, pink or flesh-colored) lesions (Occasional)
• Secondary bacterial infection of the nail bed (paronychia) (Occasional)
• Deformed or shedding nails (Occasional)
• Plaque-like flat tumors on the skin (Occasional)

Digital melanomas often present as a swollen toe, a deformed nail, or a mass at the nailbed, and are typically highly malignant.

How vets diagnose it

If your vet suspects a melanoma, they will begin with a thorough physical examination, paying close attention to the local lymph nodes.

• Fine-Needle Aspiration (FNA) and Cytology: Your vet may insert a small needle into the mass to collect cells for microscopic evaluation. While cytology can often identify melanocytic cells, it cannot always reliably differentiate between a benign melanocytoma and a malignant melanoma.
• Histopathology (Gold Standard): A tissue biopsy or complete removal of the mass is required for a definitive diagnosis. A veterinary pathologist will examine the tissue structure under a microscope to determine whether the tumor is benign or malignant, assess its mitotic index (how fast the cells are dividing), and check if the surgical margins are clear.
• Staging Tests: Because malignant melanomas metastasize early, your vet will recommend staging tests before planning treatment. This typically includes three-view thoracic radiographs (chest X-rays) to check for spread to the lungs, an abdominal ultrasound, and potentially a CT or MRI scan to evaluate the extent of local tissue involvement and regional lymph nodes.

Treatment options

Treatment for melanoma is multi-modal, focusing on both local control of the primary tumor and systemic control of potential metastasis.

Local Control

• Surgery: Complete surgical excision with wide, clean margins is the primary treatment for localized tumors. For digital melanomas, this typically requires amputation of the affected toe. For oral melanomas, partial jaw removal (mandibulectomy or maxillectomy) may be necessary to ensure all cancer cells are removed.
• Radiation Therapy: If a tumor cannot be completely removed with surgery, or if it is located in an area where surgery is not feasible (such as certain parts of the mouth), radiation therapy is highly effective at shrinking the tumor and controlling local disease.

Systemic Control

• Chemotherapy: For systemic control of malignant melanomas, chemotherapy drugs such as carboplatin (an antineoplastic platinum alkylating agent) may be used. However, chemotherapy has shown limited success in significantly extending survival times for highly malignant melanomas.
• Immunotherapy (Melanoma Vaccine): A therapeutic DNA vaccine is available for dogs with oral melanoma. Rather than preventing the disease, this vaccine is administered after local control (surgery or radiation) is achieved. It trains the dog's immune system to target a protein found on melanoma cells, helping to destroy microscopic cancer cells that may have spread.

According to a leading veterinary internal medicine reference:

"Surgery and/or radiation therapy for local control. For systemic control, carboplatin chemotherapy has been used with limited success. A vaccine recently has been released; initial reports indicate increased survival when used in a microscopic disease setting."

Prognosis

The prognosis for pets with melanoma is highly variable and depends almost entirely on the tumor's location and whether it is benign or malignant.

Cutaneous (skin) melanomas in dogs are mostly benign (melanocytomas) and carry an excellent prognosis with complete surgical removal. Conversely, oral, digital, and mucocutaneous melanomas in dogs, as well as almost all melanomas in cats, are highly malignant. These aggressive forms carry a guarded to poor prognosis due to their high rate of early metastasis to the regional lymph nodes and lungs. Early detection and aggressive multi-modal therapy offer the best chance of extending a good quality of life.

Prevention

Because sun exposure is not a major causative factor in dogs and cats, lifestyle modifications like sun avoidance or sunscreen do not prevent melanoma. Prevention relies entirely on early detection. Regularly inspect your pet's skin, paws, and oral cavity. Routine veterinary exams are crucial for identifying suspicious masses before they have the opportunity to spread.

When to call your vet

You should schedule a veterinary appointment if you notice any new lumps, bumps, or pigment changes on your pet's skin, lips, or toes.

Contact your vet immediately if you observe any of the following red-flag signs:

• Difficulty breathing, rapid breathing, or persistent coughing (which can indicate spread to the lungs)
• Sudden bleeding from the mouth or difficulty eating
• Severe lameness, swelling, or discharge from a toe
• A rapidly growing or ulcerated mass

For specific breeds

Certain breeds have a documented or suspected genetic predisposition to developing melanocytic tumors. If you own one of these breeds, routine screening of their skin, toes, and mouth is highly recommended:

• Scottish Terrier
• Airedale Terrier
• Doberman Pinscher
• [English Cocker Spaniel](/p/breeds/englishcockerspaniel_dog)
• Poodle
• Irish Setter
• [Miniature Schnauzer](/p/breeds/miniatureschnauzer_dog)
• Standard Schnauzer
• Golden Retriever
• Cocker Spaniel

Sources

• Cowell and Tyler's Diagnostic Cytology and Hematology of the Dog and Cat, 5th Edition, p. 88.
• Small Animal Dermatology: A Color Atlas and Therapeutic Guide, p. 423, 489.
• Small Animal Internal Medicine, 5th Edition, p. 463.