What is Canine Melanoma?
By Dr. Suzanne Rau, DVM, DACVIM (Oncology) | Oncology
Melanomas are tumors of pigment cells in dogs that can be malignant. If you suspect that your dog has a melanoma, your veterinarian can make recommendations regarding further steps that can be taken. We interviewed Lillie Davis, DVM, DACVIM (Oncology) and Suzanne Rau, DVM, DACVIM (Oncology) to ask them several important questions about canine melanoma.
What is melanoma?
Melanoma is a tumor of melanocytes, or pigmented cells in the body. Malignant melanomas in dogs can be an aggressive cancer. We worry about both about the growth of the local tumor, as well as the potential for this tumor type to metastasize, or spread, to places like the local lymph nodes and lungs. Melanoma is the most common cancer found within the oral cavity in dogs.
Where are melanomas frequently found in dogs?
The malignant form of melanoma tends to occur in the oral cavity, mucocutaneous junctions (regions of the body where non-haired areas meet the haired, such as the lips, vulva, and anal regions), and digit/nailbed. Canine cutaneous (skin) melanomas often are behaviorally benign. However, on rare occasion, features seen on the biopsy of a cutaneous melanoma may confirm that it is malignant.
What do melanomas look like?
Melanomas in dogs often appear as pigmented masses that may start to bleed or ulcerate as they get larger. In some instances, however, the tumor may be amelanotic (without dark pigment), and actually appear pink in color. Melanoma should be a consideration for any mass found within the mouth of a dog, though a tissue biopsy is required to confirm the presence of a melanoma.
What breeds are at higher risk for developing melanomas?
Any breed of dog has the potential to develop melanoma. The prevalence of melanoma is higher in certain breeds, however, including cocker spaniels, schnauzers, poodles, chow chows, and golden retrievers.
What is staging for melanoma and how are staging tests performed?
Staging for malignant melanoma is determined by the size of the oral mass and whether it has spread to the lymph nodes or lungs. This can be determined through oral exam, lymph node assessment (via cytology or biopsy evaluation), and imaging of the lungs through chest x-rays or a CT scan. An abdominal ultrasound may also be recommended to evaluate for more distant spread within the abdomen.
The stage of a melanoma has a very strong correlation with prognosis. Tumors less than two centimeters without spread (stage I) can have survival times of 15-18 months on average with surgery alone. Once tumors are two cm or larger, the prognosis with surgery alone decreases. Tumors two to four centimeters (stage II) have median survival times of approximately six months with surgery alone, while tumors larger than four centimeters or with lymph node spread (stage III) have a prognosis of three to four months with surgery alone. The poorest prognosis is associated with disease that has already spread to the lungs (stage IV). Patients with stage IV likely have a prognosis of one to two months.
What are the different forms of treatment for melanoma in dogs?
Treatment for canine melanoma is focused on both controlling the local tumor, as well as addressing the concern for metastasis.
For local control, surgery and/or radiation therapy are the most common options implemented.
The canine melanoma vaccine, Oncept, is recommended in addition to surgery and/or radiation to try to prevent future metastasis (see below).
Chemotherapy is generally not used in first line therapy for malignant melanomas in dogs. Melanomas are often resistant to chemotherapy, and responses achieved are typically not durable. Chemotherapy is usually reserved as a rescue option for tumors that have recurred or started to metastasize in the face of the above therapies.
How does the canine melanoma vaccine work?
Until recently, treatment of canine malignant melanomas was limited to surgery, radiation, and chemotherapy. However, an additional systemic treatment that has shown promise is the canine melanoma vaccine developed by Merial, called Oncept. The vaccine alerts the immune system to the presence of the melanoma tumor cells (particularly a protein called tyrosinase), so that the dog’s own immune will hopefully try to attack the cancerous cells
The canine melanoma vaccine is designed to be administered in an initial vaccination series of one dose every two weeks for a total of four doses. Following the initial series, one booster dose is administered every six months. The vaccine works best when there is adequate local control of the melanoma, though responses have been seen in dogs with gross disease burden. The vaccine can therefore be used alone, or in combination with surgery or radiation as recommended for each patient. The vaccine is currently available to patients through boarded specialists in oncology and internal medicine.
In a recently published study, 75 percent of dogs with stage II and III disease treated with adequate local control in combination with the melanoma vaccine were alive over 15 months. This is in contrast to dogs treated without the vaccine that historically have a median survival times in the three to six month range for stage II and III disease. The melanoma vaccine has been shown to be effective for canine malignant melanomas in other locations, as well.
How often should owner do check-ups once melanoma treatment has been completed?
The medical oncologist overseeing your dog’s melanoma treatment will make recommendations about an appropriate monitoring schedule for your particular pet, as recommendations may vary for each individual.
For dogs whose tumors appear to be locally controlled and have not yet metastasized, rechecks every three months are recommended, along with a melanoma vaccine booster every six months. At each recheck, it is recommended to have a physical exam performed to evaluate for local tumor recurrence, as well as to palpate the draining lymph nodes in the region. Chest x-rays are also obtained to evaluate the lungs for evidence of cancer spread.