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Mast Cell Tumors (Mastocytoma) in Cats

By: Dr. Jeffrey Philibert

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Depending on their grade of differentiation, mast cell tumors often can be effectively treated. Treatment for mast cell tumors may include one or more of the following:

  • Surgery. In this effective method of treatment for many mast cell tumors, wide surgical excision removes one to two inches of normal-appearing tissue in all three directions around Grade I or II mast cell tumors. This may be all that is necessary for effective treatment.

    In cats with especially large tumors or tumors located in regions of the body that are difficult to manage surgically, referral to a specialist in veterinary surgery may be advisable.

    The surgeon also should remove any enlarged lymph nodes in the area of the tumor because the tumor may have spread to these local lymph nodes. If any of the removed lymph nodes contains tumor cells, follow-up chemotherapy will be needed.

    Cats with incompletely excised tumors should be referred to a veterinary oncologist for consultation on additional treatment options. Some Grade II and all Grade III tumors behave in a malignant fashion and may require chemotherapy to manage the disease.

    For cats with Grade I mast cell tumors, complete surgical excision carries an excellent prognosis with more than 90 percent of affected cats alive without disease after 4 years.

    For cats with Grade II mast cell tumors, there is a wide spectrum of biological behavior, but approximately 50 percent of affected cats survive for 4 years without tumor recurrence.

    For cats with Grade III mast cell tumors, the prognosis is poor with surgical excision alone, and less than 10 percent of affected animals survive for 4 years. It often is impossible to completely excise Grade III tumors.

  • Radiation therapy. A beam of radiation is directed at the tumor or the region of the body from which the tumor has been removed surgically. Your pet will likely be referred to a specialty clinic or university because this specialized therapy is available only at selected locations. Radiation therapy may be recommended to shrink large tumors that initially cannot be treated by surgical excision alone. Surgical excision of such tumors may be feasible after they have shrunken in size in response to radiation therapy.

    Radiation therapy also may be recommended after surgery if the removal of the tumor was thought to have been incomplete based on the pathology report that tumor cells were observed in specimen margins. Radiation therapy can be a very effective treatment for incompletely excised Grade I and II tumors with control rates approaching 90 to 95 percent at five years. Radiation therapy may also be used in conjunction with surgery and chemotherapy to treat animals with Grade III mast cell tumors. In this situation, the life span of affected animals may be prolonged and their quality of life improved.

  • Chemotherapy may be recommended for animals with mast cell tumors that already have metastasized or tumors that have a high potential to metastasize (Grade III mast cell tumors). Your veterinarian likely will refer you to a specialist in veterinary oncology if your pet requires chemotherapy. Many different chemotherapy drugs have been used to treat pets with mast cell tumors including vinblastine, lomustine (CCNU), prednisone and cyclophosphamide.

    Drugs used in chemotherapy have potentially serious toxic effects and should only be administered by veterinarians experienced in their use.

    The success of chemotherapy in treating pets with malignant mast cell tumors has been mixed and overall chemotherapy should be viewed as palliative, which means it will improve your pet's quality of life and prolonging survival.

    Mast cell tumors contain several chemical mediators of inflammation that can cause adverse reactions in your pet either in association with the tumor itself or when the tumor cells die during chemotherapy and release these inflammatory mediators. Potential adverse effects include stomach ulceration, generalized itchiness and, in unusual instances, massive release of inflammatory mediators with a potentially fatal allergic reaction called anaphylaxis. Additional drugs to counter the effects of the inflammatory mediators released by mast cell tumors may be prescribed for your pet. These drugs include anti-histamines such as diphenhydramine, H2-blocking drugs such as cimetidine, ranitidine or famotidine and ulcer-coating agents such as sucralfate.

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