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.
Squamous Cell Carcinomas
These tumors are induced by exposure to the ultraviolet radiation in sunlight, and occur on lightly pigmented areas of the body, including the underside of the belly, trunk, scrotum, nail beds, nose, and lips.
One variety of squamous carcinoma appears as a hard, flat, grayish-looking ulcer that does not heal. Another appears as a firm red patch, and still another as a cauliflowerlike growth. There may be hair loss around the tumor because of constant licking.
Squamous carcinomas invade locally and metastasize at a late stage to the regional lymph nodes and lungs.
Treatment: Complete surgical removal is the treatment of choice. When this cannot be accomplished due to widespread involvement, radiation therapy can be used. Light-skinned dogs should avoid being in the sun at peak hours of UV exposure-generally 10 a.m. to 2 p.m.
Melanomas arise from melanin-producing cells in the skin. They are more common in Scottish Terriers, Boston Terriers, and Cocker Spaniels. These brown or black nodules are found on darkly pigmented areas of skin, particularly on the eyelids. Rarely, you will find an unpigmented melanoma. They also occur on the lips, in the mouth, on the trunk and limbs, and in the nail beds.
Melanomas on the skin are usually benign; those in the mouth are highly malignant. About 50 percent of nail bed melanomas are malignant and metastasize. Metastases occur in the regional lymph nodes, lungs, and liver.
Treatment: The melanoma must be removed surgically, along with a margin of normal tissue. Recurrence is common and difficult to treat. The outlook is extremely poor for melanomas in the mouth.
Perianal growths are usually benign cancers that appear around the rectum, especially in unneutered male dogs. Rarely, these will be malignant perianal adenocarcinomas. Siberian Huskies are predisposed to this type.
Anal sac gland tumors are a separate entity. These tumors are often aggressive and malignant. They arise from the anal glands on either side of the rectum and often metastasize quickly. These cancers are also associated with an increase in calcium in the blood.
Treatment: Neutering and/or surgical removal may be curative for perianal growths. Multimodal therapy is the best way to fight anal sac gland cancer: surgery, radiation, and chemotherapy.
This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.
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