Overview of Malignant Melanoma in Dogs
Malignant melanoma is a tumor arising from melanocytes, which are the cells that produce pigment. Although there is no known cause of malignant melanoma, the predisposition of many dog breeds makes many researches believe there is a genetic predisposition for this disease.
Malignant melanoma can originate from different areas in the body, most often the oral cavity, skin, and digits. The aggressiveness of the tumor and the likelihood of the metastasis vary with the tumor location. Any organ may be affected by a metastatic melanoma (tumor that has spread from a primary site).
Melanoma is more commonly in dogs than cats and primarily affects middle-aged to older pets (often 9 to 12 years). Black dogs may be predisposed. Male dogs are more commonly affected.
The Scottish terrier, Boston terrier, Airedale terrier, cocker spaniel, boxer, springer spaniel, Irish setter, Irish terrier, chow chow, Chihuahua, and Doberman pinscher are the most common breeds affected by melanomas of the skin and toes. The poodle, dachshund, Scottish terrier and golden retriever are the most common breeds affected by melanomas located in the mouth. Other breeds affected include the giant schnauzer and miniature schnauzer, Golden retriever, and Gordon setter.
What to Watch For
Tumors occur most commonly in the skin, digits and in the mouth. The tumors may be pigmented (black) or un-pigmented.
In patients with cutaneous melanoma:
Solitary (single) growths that may or may not be pigmented or dark in color, most commonly on the face, truck, feet, and scrotum in dogs
In patients with the oral form:
Halitosis (bad breath)
Bleeding from the mouth
Patients with advanced disease may experience difficulty breathing due to metastasis (spread) to the lungs.
Diagnosis of Malignant Melanoma in Dogs
A complete blood cell count (CBC), biochemical profile, and urinalysis should be performed in all cases, and are most often within normal limits.
Screening chest X-rays, although often within normal limits, may be of benefit in older patients, and/or may reveal evidence of pulmonary (lung) metastasis.
Fine needles aspirates retrieve cells for analysis with a small needle and syringe and may be helpful in diagnosing malignant melanoma.
Evaluation of the associated lymph nodes by fine needle aspirate/cytology is recommended as well.
Abdominal ultrasound may be indicated in patients with lesions that occur in or on the hind legs.
Biopsy of the mass is necessary for a definitive diagnosis of malignant melanoma.
Additional tests to help determine the type of tumor or the overall malignancy. Tests may include immunohistochemical staining may confirm the type of tumor or evaluation of the mitotic index may help determine the malignancy associated with that particular tumor.
Treatment of Malignant Melanoma in Dogs
Treatment of choice is surgical removal of the tumor. Wide surgical margins are recommended. Melanoma involving the nail bed or digit often requires amputation of the digit. Melanoma involving the oral cavity often necessitates radical mandibulectomy, or removal of the a part of the associated lower jaw, or maxillectomy, which is the removal of the part of the associated upper jaw.
Adjunctive (concurrent) chemotherapy is recommended if surgical excision is incomplete or the mass cannot be removed surgically.
Radiation therapy may of benefit in certain cases.
Chemotherapy is often indicated in addition to surgery and/or radiotherapy due to the aggressiveness of the tumors and the high rate of metastasis. Commonly used drugs include carboplatin or cisplatin
A new vaccine, called Canine Melanoma Vaccine DNA by Merial, has been given a conditional license for the treatment of stage II or stage III oral melanomas in dogs. The vaccine works by injecting a protein into the body that alerts the body’s immune system to the presence of the melanoma tumor protein tyrosinase. Use of the vaccine has shown very promising results so far by increasing survival times. Research is ongoing and there are several studies under development. The vaccine is recommended every 2 weeks for 4 doses and a booster vaccine at six-month intervals.
Immunotherapy to regulate the immune system may be of benefit in selected cases.
Home Care and Prevention
Prognosis is generally guarded and early detection is very important. Twenty to fifty percent of cutaneous melanomas in dogs are malignant. Those occurring in the scrotum, digit, or oral cavity are most often malignant. Aggressive and radical surgery greatly increases survival times and decreases reoccurrence rates.
Contact your veterinarian if there is recurrence of the melanoma or change at the surgical site. Return for follow up as directed by your veterinarian.
There is no preventative care for malignant melanoma.