Prognosis of Mast Cell Cancer in Dogs

By Rebecca Bragg

Because mast cell tumors can mimic so many other forms of cancer, vets call them "the great imitators," according to VCA Animal Hospitals. Typically, the first sign is one or more bumps on or just under a dog's skin. When caught early, these tumors often can be removed surgically, thereby curing the dog without the need for further treatment. After malignancies have started to spread, the prognosis depends largely upon the grade and stage of the cancer at the time of diagnosis and where the tumors are located.

Mast Cell Tumors: Signs and Symptoms

Dogs and people often develop similar types of cancer and respond to the same drug treatments but mast cell cancer, which accounts for about 20 percent of canine skin cancers, is rare in humans. Mast cells, part of the body's immune system, occur in all tissues and organs but the largest concentrations are in the skin, respiratory and digestive tracts. Even though an itchy skin lump that may bleed and vary in size from one day to the next may be the first sign of the disease, other symptoms can have no apparent connection to the skin. When mast cells, reservoirs for many biochemicals with different systemic functions, are inflamed, these substances can be released into the body, knocking such vital functions as heart rate and blood pressure dangerously out of kilter, says the National Canine Cancer Foundation.

Some Breeds Are More Susceptible

Dogs of any age, breed or mixed breed can develop mast cell cancer but some breeds appear to be more susceptible than others. Brachycephalic -- or wide-skulled, short-headed -- breeds, including boxers, Boston terriers, English bulldogs and pugs, are over-represented in the statistics. Rhodesian ridgebacks, beagles, Labrador and golden retrievers and schnauzers also may be at increased risk but mast cell cancer is rare in German shepherds. Average age of onset is between 8.5 and 9.5 years.

About Grades, Stages and Locations

Grading and staging are two related processes that both assign numerical values, usually ranging from one to three, to predict how mast cell cancer is likely to behave, thereby helping vets decide on treatment options and probable outcomes. Grading measures the likelihood of malignancy, while staging indicates how far, if at all, the tumor is believed to have spread. In both cases, the lower the numbers, the more reason to heave a sigh of relief. Where cancers are located also plays a role in projected outcomes. When tumors are on the limbs, the prognosis tends to be much brighter than if they're in the nail beds, genital region, mouth or muzzle, according to Dogs whose tumors are affecting internal organs such as the spleen or bone marrow have the poorest prognosis.

Prognosis Depends Upon Severity

Although grade I tumors can be large and difficult to remove, they're considered benign, meaning that they haven't spread. Most canine mast cell tumors fall into this category and, after the tumor and surrounding tissue are removed, the vast majority of dogs are cured. The outlook for grade II tumors, which have spread below the skin to encompass other nearby tissues, is less optimistic. One survey cited by VCA Animal Hospitals found that with surgery alone, only 44 percent of dogs in this category were still alive four years after diagnosis, although survival rates may improve when surgery is combined with chemotherapy, the hospital chain notes. Grade III tumors are so aggressive, and already have spread so far, that even with surgery, the prognosis is very poor and chemotherapy seldom helps much. Fewer than 10 percent of affected dogs survive more than a year and many succumb to the disease within 14 weeks.