Mast Cell Tumors


Mast cells are found in loose connective tissue. Connective tissue serves to connect and support other tissues and organs. It is found throughout the body in mucous membranes, around blood vessels and nerves, around body organs and in the dermal region of the skin. Scattered throughout this loose connective tissues are various cells, some being mast cells. Mast cells play an important role in the body’s immune response. They contain several kinds of chemicals. One is heparin, which prevents blood from clotting in the blood vessels. Two others are histamine and serotonin. They act to dilate or widen small blood vessels. Mast cells play an important role in the body’s immune response. A type of antibody, IgE adheres to the surface of mast cells. When a foreign substance is identified by the IgE, the mast cell releases its chemicals. These chemicals affect the local blood vessels to allow delivery of specific white blood cells which arrive to eliminate the foreign substance. Unfortunately, these chemicals are also responsible for the annoying symptoms of an allergic reaction. Swelling, redness and itching can all be attributed to the contents of the mast cells. Mast cell tumors are inappropriate proliferations of these cells.

Mast cell tumors are the most common malignant skin tumor in the dog. They account for 16-21% of all skin tumors. The cancerous growth of mast cells can have several names: mast cell tumor, mastocytoma, mast cell sarcoma. If the mast cell tumor has spread or metastasized, the disease may be called mastocytosis. The most common sites for development of mast cell tumors are the skin of the trunk, perineal area and legs. Other less common sites are the head and neck. It is possible for more than one mast cell tumor to develop at the same time in different locations.


Mast cell tumors of the skin appear in two different forms. The most common is a mass between 1 and 10 centimeters in diameter which is well-formed, raised and firm to the touch. It may appear reddened and welt-like. The border of the tumor may look like a blister and the center may be yellowish in color and look ulcerated. The second skin form of the disease is less common. This is a soft, poorly-formed lesion that usually has hair on it. It does not look ulcerated or reddened and it can be mistaken for a lipoma. Sometimes the mast cell tumor may resemble both types. Although mast cell tumors feel like discrete masses, microscopically most will extend well beyond the palpable borders. The size of the tumor itself is not an indicator of malignancy or spread to distant locations.


Diagnosis of mast cell tumors can be made by examination of a fine needle aspiration, but excisional biopsy is essential if accurate histologic grading is desired. Histologic grading is necessary in order to determine the correct treatment plan. In addition to the biopsy, recommended tests include a complete blood count, serum chemistry panel, buffy coat examination, chest x-rays, abdominal x-rays or ultrasound, bone marrow biopsy and urinalysis. Malignant mast cell tumors spread by way of blood and lymph vessels. Fine needle aspiration of regional lymph nodes can help determine if this spread has occurred.

Stage I: One tumor confined to the skin. No regional lymph node involvement.
Stage II: One tumor confined to the skin. Regional lymph nodes are involved.
Stage III: Multiple tumors or one large, invasive tumor. Regional lymph nodes may or may not be involved.
Stage IV: Tumor has metastasized to a distant site or it has recurred with metastases.


Treatment for mast cell tumors depends on the histologic grade and stage of the disease. Higher grade tumors are more likely to have metastasized. The result will be a shorter expected survival time. Surgery is the mainstay of treatment. Tumors are surgically removed along with a wide, deep border of normal tissue of at least 3 centimeters. This can be curative for the lower grade tumors. If the tumor is large and cannot be completed removed surgically, radiation therapy may be able to control the disease. Chemotherapy to prevent recurrence of mast cell tumors or their metastases has not been proven effective.


There is a fairly good prognosis for dogs treated for low grade mast cell tumors. Stage III and IV tumors will have a poorer prognosis. In one study, 93% of dogs with Grade I tumors treated with surgery alone, survived about four years, while only 47% of dogs with Grade II tumors and 6% of dogs with grade III tumors survived this long with surgery as the only treatment. This same study as well as another demonstrated that the combination of surgery and radiation therapy could improve the prognosis.


Mast cell tumors have not been shown to be genetically transmitted. Their etiology is presently unknown. On rare occasions, mast cell tumors have been associated with chronic inflammation or the application of skin irritants. Evidence remains lacking for a viral cause.


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Withrow, S., MacEwen, G. Small animal clinical oncology, 2nd ed. 1996.