Canine Mast Cell Tumors

What are Mast Cell Tumors?
Mast cell tumors are one of the most common types of cancer in dogs. They develop from particular cells of the immune system called mast cells, which normally treat inflammation and allergic reactions in a pet’s body. There is not one single cause of mast cell tumors. Fortunately, the majority of mast cell tumors are localized to one spot. However, rarely they will spread to lymph nodes, the blood, the spleen, the liver, the lungs, the bone marrow or other skin locations. If the tumor spreads, or metastasizes, to other areas of the body, multi-modality therapy may be required.

Symptoms vary depending on what organ is affected. For most dogs, mast cell tumors are not a painful cancer. In fact, many dogs with mast cell tumors are taken to their veterinarian because the owner feels a lump in or under the skin. Some dogs will have other non-specific signs such as decreased appetite, increased respiratory rate, vomiting, diarrhea or a sluggish activity level if other organs are affected.

Mast cells have inflammatory mediators in them that can cause shock-like signs (severe acute lethargy, collapse, decreased appetite, pale gums and vomiting) and/or stomach ulcers (decreased appetite, vomiting “coffee ground” particles and dark, tarry, stool) if the cells are disturbed and the mediators leak out.

In most pets a diagnosis can be made with a needle aspiration of the tumor (a procedure
performed with the patient awake using the same size of needle used to give a typical vaccine). Normally, the sample can be evaluated and a diagnosis made at the initial appointment; however, the sample will also be sent to a lab for confirmation.

After the tumor is removed, a biopsy is performed on the tumor. This will provide the oncologist more information about your pet’s mast cell tumor, such as grade (1, 2 or 3), which sometimes affects prognosis. Additionally, it allows the pathologist to determine the margin of normal cells around the tumor, to help the oncologist determine if all the cells were likely removed or not.

Evaluation of the Body Systems
Clinical staging involves completing a set of diagnostic tests to determine if the mast cell tumor has spread to other parts of the body. These tests can include any of the following: routine blood work, chest X-rays, abdominal ultrasound, fine needle aspiration or biopsy of any enlarged local lymph nodes and bone marrow aspiration.

Treatment recommendations and prognosis of mast cell tumors depend on a few important factors, such as tumor grade, tumor location, surgical margins and whether there are other organs affected. Your oncologist will evaluate all these factors and inform you of the best treatment and specific prognosis.

Treatment almost always includes surgical removal of the primary tumor when possible. Radiation therapy is recommended for tumors that have not been completely removed or that cannot be removed based on size or location. Chemotherapy is recommended for grade 3 tumors, if there is evidence of spread, or if the tumor cannot be removed.

Radiation therapy is given as a small dose of powerful X-rays for approximately four weeks. Potential side effects can include hair-loss or redness and ulceration at the treatment site. Dogs do not get systemically ill with radiation therapy like people. If a tumor in the mouth is being treated, hyper-salivation and inflammation of the gums may develop that can affect appetite and energy. These localized side effects typically begin during the final week of treatment or the week following treatment, may progress slightly and take about two weeks to completely resolve.

Combination chemotherapy protocols that utilize more than one drug at a time are the most successful in fighting cancer. The chemotherapeutic, Palladia™, is showing great promise in the treatment of canine mast cell tumor. The protocol chosen by the oncologist depends on a number of factors. Potential side effects include mild and self-limiting stomach upset (vomiting and diarrhea) and bone marrow suppression (a drop in the white blood cell level) making your pet susceptible to an infection. Most pets feel normal even if they have a low white blood cell level. Occasionally, antibiotics are recommended to prevent an infection from developing.

Dogs tolerate both radiation therapy and chemotherapy much better than people, and the doses given are much lower than in people. This allows the oncologist to reach their goal of maintaining an excellent quality of life for each patient throughout their treatment.

For more information on this subject, speak to the veterinarian who is treating your pet.