Oral Malignant Melanoma in Dogs
What is Oral Malignant Melanoma?
Melanoma is the most common malignant oral tumor in dogs. It is locally invasive (meaning it readily invades adjacent normal tissues) and has a moderate rate of spread to other places in the body (called metastasis).
Diagnostic tests are recommended to determine which sites in the body are involved. These tests include:
- Complete blood count (CBC)
- Chemistry panel
- Urinalysis (U/A)
- Lymph node aspiration cytology
- Thoracic radiographs
Treatment is aimed both at controlling the tumor in the site where it originates (local treatment) and delaying the onset of metastasis (systemic treatment). The treatment of choice for a locally invasive tumor is complete surgical resection. In the oral cavity, this often requires aggressive procedures with removal of a portion of underlying bone. If surgical excision with adequately wide margins is not possible, then surgical debulking in conjunction with radiation therapy or radiation therapy alone is recommended. Radiation therapy consists of 19 treatments given on a Mon-Fri schedule. Radiation is associated with a median disease free interval of approximately 7-10 months. In a study of 140 dogs treated with radiation therapy at NCSU, median time to first event for all dogs was 5 months, however three prognostic factors were identified which influenced the time to first event. The risk factors identified were: microscopic > macroscopic disease, lack of bony invasion > bony invasion, and location rostral to PM4 > caudal location. For dogs with none of the identified risk factors, median time to first event was 21 months as compared to 11, 5, and 3 months for dogs with 1, 2, and 3 identified risk factors, respectively. Potential side effects of radiation include oral mucositis and moist desquamation that generally develops midway through treatment and lasts 2-4 weeks before resolving. During this time, patients are managed with anti-inflammatory analgesic medications as needed. There is also the potential, though unlikely, for late irreversible changes to underlying bone, muscle, nerve.
Due to the potential for metastasis, adjuvant carboplatin chemotherapy can be considered. This drug is given at 3-week intervals for a total of four-six treatments. Chemotherapy drugs work by targeting fast-growing cells and cancer cells divide faster and abnormally compared to normal cells. Because of this, chemotherapy preferentially destroys cancer and spares normal tissues. There is the potential for some normal tissues with rapid growth rates (intestinal lining, bone marrow, hair cells) to be transiently damaged by chemotherapy. However, our goal in treating cancer in animals is quality of life, so the dosages are lower than those used in people, and we do not normally see significant toxicity with chemotherapy. There is always some risk when a drug is given for the first time, and the oncologist will discuss what types of side effects may develop and what you should watch for. If the pet has no problems the first time they receive a drug, they should have no problems with subsequent treatments of that drug. If the pet experiences any side effects, we address them as needed and lower the dosage of the causative drug for future treatments to prevent recurrent problems.
Another medication to consider is piroxicam. This oral Non-Steroidal Anti-Inflammatory Drug (NSAID) has anti-inflammatory and analgesic properties as well as possibl e anti-tumor activity against carcinomas. This medication should be given with food and the pet owner instructed to watch for signs of gastrointestinal upset (vomiting, change in stool, etc.). If such signs develop, piroxicam should be discontinued for 3-5 days. Restarting of therapy can then be considered in conjunction with the stomach-protectant misoprostil.
If metastasis beyond the lymph node is confirmed or definitive therapy not an option, chemotherapy and/or palliative radiation therapy can still be considered. Palliative radiation therapy consists of two consecutive twice-daily doses of radiation therapy and is aimed at alleviating pain, inflammation, and swelling associated with the tumor. This treatment course is repeated in one month if no disease or symptom progression is noted.
Proulx DR, Ruslander DM, Dodge RK, et al. A retrospective analysis of 140 dogs with oral melanoma treated with external beam radiation. Vet Radiol Ultrasound 2003;44(3):352-359.