Cutaneous Mast Cell Tumors in Dogs

What are mast cell tumors?

Mast cell tumors are among the most commonly reported canine skin tumors. While any dog may develop a mast cell tumor, there are certain breeds who are predisposed including English Bulldogs, Boxers, Pugs and retrievers.

Mast cells are a type of immune cell involved in the mediation of allergic responses, especially in the skin. A mast cell tumor may present as an ulcerative lesion or swollen lump underneath the skin. It is not uncommon for these tumors to increase and decrease in size. Mast cells may lead to inflammation, local bleeding, swelling and gastric upset. We may recommend placing your pet on over the counter drugs like Benadryl or Pepcid to counter these effects.

How are mast cell tumors diagnosed?

Despite the different types of tumors, mast cell tumors can be readily diagnosed by fine needle sampling (also known as fine needle aspirate). In some cases, a CT scan or a biopsy may be recommended to further asssess the tumor.

Once a diagnosis has been made, the course of treatment will be determined by the grade and stage of the tumor. The stage indicates if the tumor has spread (metastasis). To help stage a mast cell tumor, the following diagnostics may be recommended:

  • Blood count
  • Chemistry panel
  • Urinalysis
  • Abdominal ultrasound
  • Lymph node
  • Liver and spleen aspirate

The grade is determined by biopsy (histopathology) and refers to the appearance of the tumor under the microscope. The pathologist will describe the features of the mast cells and report if the tumor was completely removed during surgery.

How are mast cell tumors treated?

Surgery remains the mainstay treatment for mast cell tumors, however, it depends upon the grade and stage of the tumor. For grade I and most grade II mast cell tumors, wide surgical excision is generally considered to be a curative treatment. Radiation therapy and chemotherapy will likely be recommended for patients with high risk mast cell tumors.

Chemotherapy protocols can include the use of oral steroids (such as prednisone) and oral or IV chemotherapy drugs. These drugs may be given as the sole therapy or as combined therapies based on your pet’s disease.

About 80% of patients undergoing chemotherapy tolerate their treatment with no side effects. About 15% experience mild side effects that can be controlled at home. 5% of patients become sick enough to require hospitalization with supportive care and antibiotics.

Palliative Options

Steroids, in conjunction with anti-histaminic drugs and anti-acid drugs, can be given to help your pet. Chemotherapy can shrink mast cell tumors and slow down further distant spread.

Palliative radiation can be used alone or in addition to chemotherapy. Radiation can be very effective in reducing the size of mast cell tumors. This would involve giving one treatment under anesthesia once a week for four treatments. The response rate, when combined with oral chemotherapy, is about 75% with half of the patients showing a complete response for several months (average of 11 months). Radiation can cause side effects on the skin, mucous membranes in the mouth (it looks like inflammation) and hair loss at the radiation site.

What is the prognosis?

The prognosis for patients with mast cell tumors may vary according to the grade of the tumor, presence of distant spread and the therapy pursued. Your oncologist will review all of your pet’s diagnostic results and will discuss the prognosis with you.


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