Melanoma is the most common tumor found in the mouth of dogs, and the second most common tumor found on the digits (toes). There is a predisposition for male dogs and certain breeds seem to be overrepresented, including Scottish terriers, Cocker Spaniels, Gordon Setters, Chow Chows, and Golden retrievers. They are locally invasive tumors, often infiltrate deep into the bone (of the jaw or toe), and have a high rate of metastasis (spreading). Cats can also get melanomas but it is much less common than in the dog.
What causes this type of cancer in dogs and cats?:
- Damage to the DNA of skin cells (especially by UV light) has been shown to cause melanoma in people. The cause of melanoma in dogs and cats is less clear, since many of the melanomas occur in areas not directly exposed to UV light.
What are the common signs of this cancer?:
- Oral melanomas: The presence of a noticeable swelling in the mouth is the most common sign. Increased salivation, facial swelling, weight loss, bad breath, pain, inability to eat, dropping food from the mouth, and loose teeth are also common. These masses may be pigmented (black) or pink to white in color.
- Digit (toe) melanoma: Swelling of the digit (similar to a toe infection) is often the first noticeable clinical signs. Some dogs will have loss of the toe-nail or limping on that leg. The tumors often are black in color.
How is it diagnosed?:
- Melanomas can be diagnosed with a fine needle aspiration and cytology-- a small needle is inserted into the tumor and some cells are removed from it and then evaluated. Since the mouth is a very sensitive location, most animals need some mild sedation for a needle aspiration. When located on the toe, sedation is typically not necessary.
- Occasionally a needle aspiration is inconclusive, so a biopsy is required-- a small piece of tissue is removed (typically under sedation or anesthesia) and sent to the laboratory for a pathologist to look at under the microscope. Biopsies typically require anesthesia for both the mouth and toes.
- All patients with a diagnosis of melanoma should have the local lymph nodes and lungs evaluated for evidence of metastasis. The local lymph nodes should be sampled with a fine needle aspiration and cytology (or biopsy under anesthesia). Radiographs or a CT scan should be used to evaluate the lungs for spread of the cancer. Occasionally an abdominal ultrasound will be recommended to evaluate the abdominal organs (i.e. spleen, liver, etc.) for evidence of metastatic disease.
How is this cancer treated?:
- Good local control of the primary tumor is very important in the therapy of melanomas. For oral melanomas, it can sometimes be difficult to remove the entire tumor without removing bone. Most dogs (> 85%) will continue to eat, drink, chew, and play with toys. For digital melanomas, the entire toe is removed with the tumor. A good wide aggressive surgical approach minimizes the chance for local recurrence.
- The pharmaceutical company Merial has developed a new treatment called the melanoma vaccine (Oncept). It is a new DNA vaccine treatment that stimulates your dog’s immune system to fight the melanoma. This is not traditional chemotherapy and is tolerated extremely well by most dogs.
Will chemotherapy be recommended for my dog?
- Chemotherapy has been used in dogs that have tumors that are not surgically excisable, not responsive to radiation therapy or when metastatic disease is present. Novel therapy with small molecules that block tyrosine kinase receptors have also shown some promise.
Does radiation therapy play a role in the treatment of melanomas?
- Additional local control options for an incompletely excised (removed) tumor include a second surgery, if possible, or radiation therapy. Radiation therapy can also be used post-operatively (if tumor cells were left behind and cannot be removed) or if a tumor is too large to remove. Melanomas tend to be very responsive to radiation therapy.
What is the prognosis for dogs and cats with this cancer?:
- The average survival time of untreated dogs is only a few months.
- Dogs with an oral melanoma that have good local control (surgery +/- radiation therapy if indicated) followed by the Merial melanoma vaccine have an improved median survival time of about 1.5-2 years (with many living much longer).
- Dogs with digit melanomas have an average survival time of about 1 year with surgery alone. The melanoma vaccine, however, can dramatically improve survival times.
What is on the horizon for this cancer?:
- The FDA has recently approved a drug called Yervoy – a drug that “unblocks” the immune system and allows it to work better. This drug may be adapted to be used in dogs in the future.