Pet Health Library

Mast Cell Tumor in Dogs

Mast cell tumor on a dog's hind leg

Background

Mast cells are a type of white blood cell that play a key role in a dog’s immune system. They contain granules that release histamine and other chemicals involved in allergic reactions and fighting off infectious agents like parasites and bacteria. When mast cells become cancerous they form a mass called a mast cell tumor (MCT).

Mast cell tumors are the most common malignant skin tumor found in dogs. They may be seen in dogs of any age but occur most commonly in dogs who are 8 to 10 years old. They can develop on any part of a dog’s body, including internal organs, but the limbs, lower abdomen, and chest are the most common sites. If these tumors spread, they typically spread first lymph nodes near the tumor followed by the liver and spleen.

Risk Factors

Any dog can develop a mast cell tumor, but the following breeds are at a greater risk:

  • American Staffordshire terriers
  • Boxers
  • French bulldogs
  • Golden retrievers.
  • Labrador retrievers
  • Pugs
  • Rhodesian ridgebacks
  • Shar-peis

Signs

Mast cell tumors vary in appearance. Some look like raised, hairless bumps on or just below the surface of the skin. Others appear as red, ulcerated, bleeding, bruised, or swollen growths. Some tumors stay the same size for months or years, some may spontaneously grow and shrink, while others grow rapidly over days or weeks. Additional signs such as vomiting, loss of appetite, and dark tarry stool typical of intestinal bleeding are also possible.

Diagnosis

Most mast cell tumors can be diagnosed with a simple needle aspirate of the tumor. This quick procedure is done while the dog is awake, and involves inserting a thin, hollow needle into the mass to collect a sampling of cells, which are then examined under a microscope.

Treatment

Surgical removal of mast cell tumors is the mainstay of treatment. In order to remove the entire tumor, the surgical recommendation is to remove a 1 ¼ – inch margin in all directions around the tumor. If a biopsy indicates the tumor has not been completely removed or if the tumor cannot be removed, veterinary oncologists will likely recommend radiation therapy to try to control the tumor.

After the mast cell tumor is removed, it is sent to a veterinary pathologist for biopsy. The pathologist will make a variety of assessments, including how abnormal the cells look, how much the cell invades into normal tissue, how many cells are dividing, and what the center or nucleus of the cell looks like. The pathologist then “grades” the tumor by assigning it a number:  1, 2, or 3. The more abnormal the cells are, the higher the grade. The lower the grade, the more favorable the prognosis. Grade 1 tumors usually require less treatment and grade 3 tumors usually need chemotherapy because they have a form of the disease that will most likely spread throughout the body. The treatment protocol for dogs with a grade 2 mast cell tumor falls between grades 1 and 3, as determined by a veterinary oncologist.

Prevention

Dogs who have had one mast cell tumors are at greater risk for developing additional tumors. Early detection and removing tumors when they are small and have not spread increases the likelihood of treatment success. For that reason, it is important to check your dog regularly for lumps and agree to a fine needle aspirate If your veterinarian suggests one for your dog.

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