Melanomas, Squamous Cell Carcinomas, and Mast Cell Tumors in Dogs

Mastocytomas (Mast Cell Tumors)

Mast cell tumors are common, accounting for 10 to 20 percent of skin tumors in dogs. About half of them are malignant. Brachycephalic breeds, such as Boxers, Boston Terriers, and Bulldogs, have a higher incidence. However, mast cell tumors can occur in all dogs. In Bernese Mountain Dogs, mast cell tumors are especially common and are inherited as a polygenic trait.

The mean age for dogs to develop mast cell tumors is 9 years. Both sexes are equally affected. Multiple tumors are present in 10 percent of cases. Look for these tumors on the skin of the trunk and perineum, lower abdomen, foreskin of the penis, and hind legs.

Mast cell tumors vary greatly in appearance. The typical tumor is a multinodular growth that appears reddish, hairless, and ulcerated. It is impossible to tell by appearance whether the tumor is benign or malignant. Some growths may be present for months or years, then suddenly enlarge and metastasize to the regional lymph nodes, liver, or spleen. Others grow rapidly right from the start. Still others may be completely under the skin and look like a lipoma. For that reason, all new lumps should be checked by your veterinarian.

Mast cell tumors release histamine and other substances that cause stomach and duodenal ulcers. In fact, up to 80 percent of dogs with mast cell tumors may be suffering from ulcers. Dogs with intestinal symptoms should be evaluated for ulcer disease and treated accordingly (see Stomach and Duodenal Ulcers).

Treatment: The World Health Organization has established a system for staging mast cell tumors based on the size of the tumors, the number present, the degree of local involvement, and the presence or absence of metastases. Early-stage (favorable) tumors are treated by complete local excision with a margin of normal tissue. Larger tumors that cannot be removed with adequate tissue margins are treated with surgery plus prednisone and/or radiation therapy. Chemotherapy and/or immunotherapy have been of benefit in treating late-stage disease.