Treating Mast Cell Tumors in Dogs

Atlantic Veterinary Internal Medicine & Oncology offers state-of-the-art treatment options for dogs with mast cell tumors at its Annapolis animal hospital. 

Mast cells are special blood cells that are normally involved in the body’s response to allergens and inflammation. When these cells mutate, they develop into a mast cell tumor. Mast cell tumors are commonly found on the skin, but other sites include the lymph nodes, spleen, liver, bone marrow, and intestines. Mast cell tumors are often raised, red, hairless masses that can wax and wane in size.

Mast cells contain substances within their granules (the most common ones are histamine and heparin) that can cause local inflammation (redness) and occasionally bleeding. Due to the effects of the granules, most patients will be started on anti-histamines and antacids for supportive care.

Treating Mast Cell Tumors in Dogs: A Mast Cell Tumor on the Stomach of a Dog

Any new lump or bump should be evaluated by your veterinarian. There are several breeds that are at a higher risk of developing mast cell tumors in their lifetime, including Boxers, Bulldogs, Pit Bulls, Pugs and Boston Terriers, but dogs of any breed can be diagnosed with a mast cell tumor.

Treating Mast Cell Tumors in Dogs: A Microscope Image of Mast Cells


A mast cell tumor is usually diagnosed by fine needle aspiration. This procedure entails inserting a needle into the mass and aspirating back some cells. The cells that are removed are put on a slide and sent out to the laboratory for the pathologist to review.

Mast cell tumors vary in behavior. Some are slow-growing and less aggressive, while others grow quickly and cause discomfort. Initial evaluation of a dog with a mast cell tumor usually involves additional diagnostics to determine the extent of disease (also known as staging the disease). The most common sites for metastasis are the draining lymph nodes, liver, spleen, and bone marrow.

Additional diagnostics include:

  • Bloodwork (complete blood count and chemistry panel)
  • Urinalysis
  • Chest X-rays
  • Regional lymph node aspiration
  • Abdominal ultrasound to evaluate the liver, spleen and lymph nodes
  • Possible aspiration of the spleen

In some cases, we may also recommend a bone marrow aspirate. There are additional tests that can be done to try and predict the behavior of the mast cell tumor. This can include testing for a mutation, called c-kit, or performing a mast cell tumor panel.

Mast cell tumors are graded in two ways:

  1. Patnaik scale: Tumors are graded I-III. Grade I tumors are the least aggressive, Grade II tumors are the most common, and Grade III tumors are the most aggressive.
  2. Two Tier Scale: Tumors are graded high or low. Low-grade mast cell tumors are considered locally aggressive. High-grade mast cell tumors are not only locally aggressive, but are likely to spread to other areas of the body.

Treating Mast Cell Tumors

Treatment options for cutaneous mast cell tumors may include surgery, radiation therapy, chemotherapy and/or supportive care.

The gold standard of treatment is surgery to remove the mass. Mast cell tumors can be invasive and therefore, the surgeon will want to be aggressive by taking a large margin both around and under the tumor to try and ensure complete removal. Unfortunately, even when a large margin is taken, some of the tumor cells can still be left behind. When a mast cell tumor is not completely removed, there is a concern that there will be recurrence along the scar. If incomplete margins are found, then additional surgery or radiation therapy may be needed.

The goal of radiation is to try and clean up the residual disease that remains to slow the recurrence rate. Fortunately, radiation is very effective at preventing/slowing local mast cell tumor recurrence. Radiation therapy is administered while the patient is under anesthesia and consists of a daily treatment, Monday through Friday, for 3-4 weeks in a row. Studies indicate local control times of 2-5 years for patients treated with surgery and radiation.

Surgery and radiation therapy are local treatments that do not affect the spread of the tumor. Chemotherapy is beneficial for patients with a mast cell tumor that has already spread, is too large for surgery, or when there is a high risk for metastasis. Chemotherapy can be used with surgery or alone, but it is much more effective if the tumor has been removed.

Fortunately, chemotherapy is well-tolerated by most patients. Side effects may include upset stomach, a decreased white blood cell count, and possible thinning of the fur coat. Fewer than 10% of patients treated will experience significant side effects that warrant a visit to the animal hospital for outpatient care, and less than 1% will experience any life-threatening side effects or need to be hospitalized.

The prognosis for cutaneous mast cell tumors depends on the grade of the tumor, its location, whether or not it is positive for the kit mutation, and the presence of metastasis. Once a patient develops a mast cell tumor, they are at risk for developing others in the future. The majority of mast cell tumors can be successfully treated when found early.

The goal of any treatment option is to help maintain a good quality of life for your pet for as long as we can. Your oncologist will be able to discuss the treatment options available and what is best for your pet. Call us today in Annapolis at (410) 224-0121.