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
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
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.