Oral Tumors in Dogs - Melanoma
What is an oral melanoma?
Oral melanoma ("malignant melanoma" or "melanosarcoma") is a tumor of melanin pigment producing cells (melanocytes) in the mouth. These cancers are rapidly growing and are rarely completely cured by surgical removal. Regrowth in the mouth and spread to other parts of the body (metastasis) are common.
How are oral tumors caused?
Although the exact cause is unknown, cancer is often the culmination of many factors including heredity, diet, and environment.
Environmental factors, such as ultraviolet radiation from the sun, are commonly associated with the development of melanoma in people and can occur in areas of the body that have not been directly exposed. We do not currently know if this is the case in dogs.
What we do know is that melanomas behave differently when they occur in different parts of the body.
Why has my dog developed melanoma?
Melanocytes are fragile cells and are easily damaged. Cancer mutations develop in response to cell damage and, because the more divisions a cell undergoes the more probable is a mutation to occur. We do not know why specific dogs get melanomas.
Is melanoma a common oral tumor?
Melanomas are the most common malignant oral canine tumor – the gums and inside of the mouth are the most common sites. Male dogs and dogs with a lot of pigment seem to be more predisposed to developing melanoma, as are Poodles, Doberman Pinchers and Cocker Spaniels. Studies on breed prevalence have varied between countries, but the average age of affected dogs is 11 years.
Malignant melanomas are rare in cats.
"Melanomas are the most common
malignant oral canine tumor – the gums and
inside of the mouth are the most common sites."
How will melanoma affect my dog?
Melanoma tumors appear as swellings on the gums and are often black in color. The swellings frequently ulcerate and bleed and may become secondarily infected, causing bad breath. Bad breath is characteristic of melanomas but can occur with other tumors or with tooth and periodontal gum disease. Other common signs of an oral tumor include drooling, difficulty eating, bleeding, displacement or loss of teeth, facial swelling, pain and swelling of the local lymph nodes.
How is an oral melanoma diagnosed?
Characteristic visual appearance of melanomas give a clue as to the tumor type.
X-rays (and CT scan when available) may be useful to detect whether tumors have invaded the bones, which will guide surgical decisions. If there’s bone loss adjacent to the tumor, the prognosis is usually poor.
An accurate diagnosis of melanoma tumors requires microscopic examination of tumor tissue. Cytology, the microscopic examination of a small sample of cells, may be used to diagnoses these tumors in some cases.
For a definitive diagnosis, prediction of behavior (prognosis) and assessment of the completeness of a tumor removal, veterinarians will rely on the results of a microscopic examination of tissue (histopathology). Histopathology also rules out other cancers. Your veterinarian will submit either a small part of the mass (biopsy) or the whole tumor to a specialized laboratory, where a veterinary pathologist will examine and diagnose the lesion. If your veterinarian submits the entire mass, the pathologist may be able to indicate whether the cancer has been completely removed.
"An accurate diagnosis of melanoma
tumors requires microscopic examination
of tumor tissue."
Malignant oral melanomas often invade the jaw bone. For complete removal, they need wide surgical margins, usually including excision of substantial parts of the jawbone.
What types of treatment are available?
Surgical removal is the standard method of treatment for all oral tumors. If the tumor is invasive, it may be difficult to remove completely so large pieces of the jawbone may need to be taken out (hemimaxillectomy or hemimandibulectomy). If your dog requires one of these complex and extensive surgeries, your veterinarian may refer you to a specialist. The surgeon may want to have a CT scan performed (if available) to determine how much tissue to remove to achieve complete excision.
"Surgical removal is the standard method
of treatment for all oral tumors."
Melanomas do not respond well to chemotherapy or radiation therapy, but in 2007, the first therapeutic vaccine for the treatment of canine oral melanoma was conditionally approved for use in the United States by the USDA. To date the results are most promising when combined with surgery. The vaccine is only available for use by veterinary oncologists, but over time it may become more widely available. Your veterinarian will advise you of its availability in your area.
Can this cancer disappear without treatment?
The body's immune system is not effective at making these tumors regress. Treating secondary infections and healing ulcers will help reduce superficial swelling and discomfort, but it will not cure the cancer. Very occasionally, a spontaneous loss of blood supply to the cancer can make parts of it die but the dead tissue will still need surgical removal.
How will I know how this cancer will behave?
The histopathology report indicates how it is likely to behave. The veterinary pathologist usually adds a prognosis that describes the probability of local recurrence or metastasis (distant spread), and if the entire mass was submitted for examination, will also assess the completeness of excision.
When will I know if the cancer is permanently cured?
The outlook for dogs with oral melanomas is guarded as more than half of these tumors tend to invade the underlying bone. Approximately 70% of tumors spread to local lymph nodes (glands) and the lungs. Survival is unrelated to sex, site (those in the front of the mouth carry a better prognosis than those in the back), rate of growth, histological type, amount of pigment or cancer size.
Tumor stage (how far it has spread) is correlated with survival time. If there is no involvement of the local lymph nodes and no X-ray evidence of lung tumors, survival time is improved by surgery. Partial removal of the jaw reduces local recurrence of the tumors but does not always prevent the spread elsewhere.
Are there any risks to my family or other pets?
No, these are not infectious tumors. They are not transmitted from pet to pet or from pets to people.
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