Melanoma in Pets

Melanoma in dogs and cats is a common tumor we treat at Arizona Veterinary Oncology. This tumor type is often quite different than in people and there are different forms of the tumor even within dogs and cats.

Oral malignant melanoma is the most common malignant oral tumor in the dog.

This tumor is far more common in dogs than it is in people if you eliminate people that smoke or chew tobacco (oral tumors are common in people that do that). We don’t know why dogs get these tumors but we suspect it is because they take toxins into their mouth far more often than we do. That said, we have looked at common toxins like carpet cleaners, pesticides and herbicides and have not been able to prove that they increase the risk for this tumor.

Most often a mass is found in the mouth because of a visible swelling on the outside or because there is bleeding from the mouth. These tumors can be black (from the melanin in the cells) or can be pink. Often these tumors have an irregular surface, much like a cauliflower. They can occur anywhere inside the mouth, including the tongue and the tonsils. These tumors are highly malignant, with some reports suggesting rates of spread being as high as 90% at the time of diagnosis (i.e. whether we can find it in other organs or not, most of the time it is there). Because of this, our treatment options must not only treat the oral tumor, but also the potential spread to other places. As with most tumors, surgery is our first choice when it is possible and when it will not severely impact the quality of life of your pet. Without removing bone, surgery may not be able to remove the entire tumor. A veterinary surgeon or dentist may be needed for more extensive procedures. If the entire tumor is not removed, further local treatment will be required (usually radiation) or the tumor will probably regrow.

Sometimes surgery is not possible at all because of the location or the size of the mass.  In these cases, radiation becomes our best chance at control.

Radiation for melanomas in dogs and cats is a bit different than other tumors.

It has been shown that melanomas do better when larger doses of radiation are delivered over an extended period of time (rather than a small dose every day like many of our other tumors). We most often use a once a week treatment for four weeks at Arizona Veterinary Oncology. With that treatment, hair loss in the area of the treatment is common. Sometimes blister like lesions may form on the skin or inside the mouth in the treatment field. If this occurs, it is generally not until near the end of the treatments. It often persists for a week or so after the treatment and then heals quickly after that. Mild pain and drooling may occur.

We use antibiotics and pain medicines as necessary.

The hair usually grows back but may take a few months and may grow back white. The role of stereotactic radiosurgery is still being investigated but on paper, this tumor should be the most responsive to this treatment. Stereotactic radiosurgery delivers a large dose of radiation to the tumor while sparing the normal tissues better than other treatments. We hope this will result in even better tumor control locally. For the metastatic disease (i.e. the disease that has spread) we must use something that treats the entire body. Chemotherapy for melanoma has not been very effective. Because of this, we recommend the Oncept® Melanoma Vaccine. The vaccine helps the body identify the abnormal cells so that the immune system can attack the cells (because cancer cells are our own cells, the body does not attack them despite the fact they are reproducing abnormally). While the vaccine will not treat the local tumor, it has shown to prolong life when the local tumor was controlled with surgery and/or radiation. For more information about the vaccine, please refer to www.petcancervaccine.com.

The most common prognostic factors for oral melanomas are size and location. Tumors that were over 2 centimeters in size did worse than those less than 2 centimeters (just under one inch). Tumors that are closer to the front of the mouth do better than those in the back of the mouth (likely because they are found sooner and because we have more treatment options). Melanoma in the tongue or the tonsil carry a much worse prognosis.‍

Dermal melanomas in dogs occur in the haired skin. We do not know why these tumors occur.

These are often smaller and darker (black) masses in the skin. In most cases these tumors are benign (i.e. they do not spread to other locations). Surgery is the treatment of choice for these tumors and very often the surgery is curative.

Ungal (nail-bed) melanomas are more aggressive than dermal melanomas. These tumors have a higher risk for spread to other locations. Very often these tumors start as a “broken nail that won’t heal.” Over time, the mass becomes more apparent. Pink, bleeding, irregular tissue is present. It is very hard to distinguish squamous cell carcinomas of the nail-bed and melanomas of the nail-bed. Most often, surgery to amputate the toe is the first step. Locally, this can be curative and no other local treatment is required.  We do recommend the Oncept® Oral Melanoma Vaccine for these cases to try and control the distant disease. Less information is known about these tumors but longer survivals than oral melanomas can often be seen.

Because of the high risk of spread of melanoma, staging tests like chest X-rays, abdominal ultrasound and lymph node aspirates are often recommended.

Melanoma in dogs and cats is a treatable disease and we would appreciate the opportunity to help your pet.