by Dr. Gail Mason, DVM, MA, DACVIM
Staff Internist, Portland Veterinary Emergency & Specialty Care
Malignant melanoma Is a relatively common cancer in dogs, especially those with significant amounts of skin pigmentation. Melanomas are relatively rare in cats. The most common location for canine melanoma is the haired skin which can appear to be small brown to black masses but also can appear as large, flat, and/or wrinkled masses. Primary melanomas can also occur in the oral cavity, the nail bed, footpad, eye, gastrointestinal tract, and other places in the body. The biological behavior of malignant melanomas is exceedingly variable, but often extremely aggressive and eventually metastatic.
The most common oral malignancy in the dog is malignant melanoma. This tumor is more commonly seen in heavily pigmented breeds and is primarily a disease of older dogs.
Most dogs with oral cancer have a mass in the mouth which is first noticed by the owner. Pets with oral tumors will typically have symptoms of increased salivation, facial swelling, bleeding from the mouth, weight loss, foul breath, difficulty swallowing, and/or pain when opening the mouth. Loose teeth can also be an indication of bone destruction by a tumor.
Diagnosis of malignant melanoma is determined by a tissue biopsy. In the case of oral tumors, a thorough evaluation of the mouth in a sedated or anesthetized patient is critical to evaluate the type and extent of the tumor and to retrieve tissue for a biopsy. Palpation and needle aspirates of adjacent lymph nodes are commonly procured at this time as well.
The anatomic site of the melanoma is highly, although not completely, predictive of local invasiveness and metastatic propensity. Melanomas involving the haired skin, which are not adjacent to mucosal (lip) margins, often behave in a more benign manner. In these cases, adequate surgical removal is often curative. This differs from oral mucosa melanomas which are highly malignant with a high degree of local invasion as well as early metastatic potential. Unfortunately, this mirrors the pattern seen in humans.
Dogs with melanomas of the digits (toes) without lymph node involvement, have reported median survival times of 12 months with a 2-year survival rate of 57% with surgery alone.
Surgical removal of malignant melanoma by a specialty surgeon continues to be the most effective local treatment. For more biologically benign cutaneous tumors, the tumor can be removed with 1-2 cm skin margins (the amount of healthy tissue surrounding the lesion) with a favorable chance for cure. One should never wait for it to “grow back” to seek treatment. For tumors involving a digit, the removal of that entire digit is almost always recommended to obtain margins on a more biologically aggressive melanoma. For oral melanomas, it is important to be as aggressive as possible to provide the best long-term outcome. Melanomas involving the gums or jaw often require the removal of at least a portion of the jawbone to remove the tumor in its entirety. The amount of bone that must be removed can generally be judged by the results of the CT scan. It is important to note that surgery on the upper and lower jaws of dogs can be highly successful, functional, and cosmetic. Because the area has a large blood supply, healing takes place quickly. Unplanned or limited attempts at removing the tumor should not be made as this will often leave residual disease and result in rapid tumor recurrence. Your surgeon will discuss this in detail with you prior to pursuing this avenue of treatment.
Radiation therapy can play an important role in the management and treatment of canine and feline oral melanomas. It is generally used for the purpose of achieving local and regional tumor control when complete removal is not possible or practical. Most radiation oncologists will treat both the primary tumor and the regional lymph nodes. Pets tolerate radiation therapy extremely well and do not manifest "radiation sickness" as do humans. It does not however prevent metastatic disease. The median survival times for dogs treated with radiation therapy range from 4.5-14.7 months. Chemotherapy does not seem to play an important role in the management of dogs with malignant melanoma.
Immunotherapy represents a potential systemic therapeutic strategy for the treatment of malignant melanomas. The Oncept® Melanoma Vaccine has been shown to be a successful innovation in extending remission times up to 3 times longer than those previously achievable after the local disease is removed/controlled. It was originally approved for use in dogs with oral tumors only. However, experience has shown that it has merit for both dogs and cats with melanomas located anywhere in the body. This vaccine is administered trans-dermally (needle-free), to a hind limb muscle of the patient. Vaccination provokes the patient's immune system into manufacturing antibodies against a biological enzyme called tyrosine kinase. Though we are not certain of the reasons, many tumors in dogs (especially melanomas) manufacture large amounts of tyrosine kinase which the tumor requires for its growth. Blockade of the tyrosine kinase as well as formation of specific immune cells (T cells) can offer a significantly better outcome for patients with malignant melanoma. The vaccine is extremely safe and is initially given as a series of 4 vaccinations which are 2 weeks apart. This is called the induction phase. This is followed by a single Oncept® vaccine booster every 6 months.