In general, the oral melanoma is the most aggressive form of melanoma in dogs. It is the most common oral cancer we see in dogs.
The behavior of the cancer can vary depending on the size when removed, how aggressive it appeared on the biopsy sample, and if your surgeon was able to remove it all. The pathologist who read the biopsy can report about the aggressiveness of the tumor. Once you've had a few tests done to see if the cancer has already spread (staging), you can assign a grade to the tumor. The lower the grade, the better. It will be important to have a better idea about what to expect for your dog if you have the cancer staged. Staging is straight forward, and will consist of aspirating both lymph nodes under the jaw and evaluating for cancer cells, performing a full cbc/chem/electrolyte/urinalysis screen, and taking chest x-rays to look for any spread (metastasis). For some cases, an abdominal ultrasound may also be indicated.
Staging will probably cost a few hundred dollars, but will help you determine your best course of action.
Unfortunately, oral melanomas tend to recur locally and metastasize to lymph nodes, the brain, lungs, and other tissue. Treatment with surgery alone yields an average median survival time of 150-318 days, with only 35% surviving to 1 year. Stage 1 dogs do better, with average survival 511 days, but stage 2 survive only 165 days (median). Stage 3 and 4 cases have even shorter life expectencies. Radiation treatment and radical surgical procedures can improve survival outcome significantly.
The good news is that recently Merial has introduced a vaccine for dogs with melanoma that has been very helpful, and is showing to dramatically extend survival times. This vaccine can only be administered by a veterinary oncologist, so you will need a referral from your vet to see a specialist for this type of treatment.
So far the data has shown median survival times for melanoma with the use of the vaccine is as follows:
Stage I > 939 days with 92.8% survival
Stage II > 908 days with 79% living 1 year and 63% living 2 years
Stage III > 1646 days with 77% living 1 year, 65% living 2 years and 57% living 3 years
Stage IV = 293 days with 40.5% living 1 year and 18.8% living 2 years
More information on the vaccine can be found at www.merial.com.
If left untreated, it is difficult to say how things will progress. Locally, tumors may recur and grow, causing pain and difficulty eating. Spread of the disease can cause swollen lymph nodes, coughing, difficulty breathing, and severe weight loss. Pain management will be important if the disease progresses.
Please let me know how thing go. If I can answer any other questions, don't hesitate to ask. I'll be keeping you and your spaniel in my thoughts.