Melanoma is a form of cancer in which the pigment-producing cells of the skin known as melanocytes multiply in an erratic fashion eventually invading the tissues that surround them. Malignant melanoma refers to those melanomas in which the cancerous melanocytes spread from the original tumor and travel via blood and lymph vessels to lymph nodes and distant organs. Once established in an organ, additional tumors develop that cause the ultimate death of the animal. Melanomas are characterized as skin lesions with irregular borders and display a wide variety of coloration. Although found primarily on the skin, in the dog, they can also be found in the oral cavity. In fact, malignant melanoma is the most common oral malignancy in the dog and it accounts for 30-40% of all oral tumors. The mouth is the fourth most common tumor site in dogs. Unfortunately, if found on the lips or in the mouth, melanoma is nearly always malignant; that is, the tumor has a high propensity for early local invasion and early metastasis.

CLINICAL SIGNS:

A lesion with irregular borders and variable colors. The lesion may be brown or black and it may also have shades of red, white or blue. They grow rapidly. If located in the mouth, you might see increased salivation, mouth odor, weight loss, bloody discharge, difficulty eating and loose teeth.

DIAGNOSIS:

Complete physical exam including palpation of lymph nodes.
Blood work should include a complete blood count and serum chemistry. Secondary bacterial infection is common with oral malignant melanomas which could cause an elevated white blood cell count.
Chest x-ray. A common site for metastasis is the lung.
Biopsy. This may also serve as a treatment because the entire tumor with 2-3 centimeters of healthy tissue surrounding it should be removed, if possible. This may not always be possible if the tumor has invaded the adjacent bony structures. The biopsied tissue should be examined by a pathologist. There can be some confusion at this point because 30% of melanoma biopsy specimens do not contain melanin. Your veterinarian may also choose to perform a needle aspiration of nearby lymph nodes or to surgically remove one or more lymph nodes to determine if the melanoma has metastasized.

TREATMENT:

Surgery to remove all or as much as the tumor as possible. If the melanoma has arisen from the oral cavity and has invaded the jaw, your veterinarian may recommend that part of the jaw bone be removed as well. This is generally recommended only if there is no evidence of metastases.
Radiation to promote shrinkage of the tumor.
Chemotherapy is not a mainstay of treatment because it has not been found to affect distant metastases in most patients. Combination chemotherapy, including dacarbazine, has helped some patients. Consultation with a board-certified veterinary oncologist is warranted.

PROGNOSIS:

Melanoma is the most malignant of all the skin cancers and therefore has the poorest prognosis. If found in the mouth, the prognosis is even more grim. Approximately 25% of dogs diagnosed with oral melanoma will survive for one year; 75% will not survive even this long. The future does hold promise that genetic therapies directed at stimulating the dog’s own immune system to attack and destroy tumor cells may be developed. There is presently research in this area.

REFERENCES:

Hogge, G. Melanoma research. Animal network, website. Madison, WI: University of Wisconsin, 1997, August.

Kirk. Kirk’s current veterinary therapy XII: small animal practice. Philadelphia: W. B. Saunders Co.; 464, 470, 514, 693.

Kitchell, B. & Ehrhart, N. Topics in Small Animal Oncology. 1993. Paper presentation given in Rockford, IL; 1997, Jan.

Madewell, B. R. Cancer. In: Siegal, M., ed. UC Davis school of veterinary medicine book of dogs: a complete medical reference for dogs and puppies. New York: HarperCollins, 1995; 415.

Ogilvie, G. Selected topics: veterinary oncology. Fort Collins, Co.: Colorado State University.