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Resource Library: Mast Cell Tumors, Canine

Mast cells are a part of the body's immune system. They help to coordinate a protective response to foreign substances that invade through the skin. Mast cells contain histamine which are released when the mast cell is activated by exposure to the foreign material. The most well known example of mast cells at work is the response that occurs to a mosquito bite. The redness, swelling and itchiness is due to the release of those chemicals from mast cells.

A mast cell tumor occurs when mast cells becomes cancerous. Mast cell tumors have a wide range of behavior, from very benign to highly malignant. The grade, location and past behavior of a tumor can be used to predict the future behavior of a tumor in an individual dog. The grade is determined by a pathologist examining a surgical biopsy of the tumor under a microscope. Grade 1 tumors are relatively benign and usually treated with surgery alone. Grade 2 tumors are moderate in their degree of malignancy. Grade 3 tumors are very aggressive and metastasize frequently (>90%). Regarding tumor location, tumors that are on limbs do not spread often, but they can be difficult to remove completely because the tight skin on the limbs prevents removal of a large amount of tissue around and deep to the tumor. Tumors on the trunk are usually intermediate in their degree of malignancy, but surgical removal can be less challenging due to the extra folds of skin available in that area to achieve wide margins around and beneath the mass. Tumors at muco-cutaneous junctions (lips, eyelids, rectum, toenails, etc.) tend to be very aggressive. Regarding past behavior, as would be expected, rapidly growing tumors and tumors that have spread to more than one area have a poorer prognosis.

Surgery is the treatment of choice for mast cell tumors in areas were a resection is feasible. In areas were the mass can not be removed without interfering with function such as a large tumor of the foot, then radiation therapy can be very rewarding as most of these tumors are sensitive to radiation. The goal of surgery is to remove the tumor in its entirety (surgical margins free of tumor cells). If surgery removes the tumor in its entirety, no further therapy is recommended for grade 1 masses. Close and frequent monitoring is essential to identify recurrence locally or spread to the regional lymph nodes as this can occur in low to intermediate grade mast cell tumors in a small number of cases . If the tumor is a grade 2 or 3, follow up chemotherapy or local radiation therapy is recommended. If the surgical margins are not free of tumor cells, whether the tumor is a grade 1, 2 or 3, follow up therapy with radiation therapy is recommended to decrease the chances of local recurrence.

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