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Canine Mast Cell Tumors

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Canine Mast Cell Tumors

††††††††††† Mast cell tumors comprise 20-25% of all cutaneous and subcutaneous tumors in dogs, making them the most prevalent canine skin tumor.† Mast cell tumors may occur in the intestines, liver, spleen, and bone marrow at a much lower prevalence.† Mast cell tumors most frequently develop in dogs at an average age of eight years.†

††††††††††† The clinical signs of mast cell tumors are variable and dependent on the location and grade of the tumor.† Mast cell tumors can be found on all areas of the skin.† Mast cell tumors most commonly appear as small raised nodular masses that vary from firm to solid on palpation, but no typical morphology of mast cell tumors exists.† Mast cell tumors can imitate the appearance of most other skin tumors, making diagnosis based on physical exam difficult.† Most mast cells are solitary tumors, but approximately 10% are multicentric or infiltrative.†

††††††††††† Other clinical signs of mast cell tumors relate to the release of mediators stored in their intracellular granules or cytoplasm.† These mediators include histamine, heparin, proteolytic enzymes, and other cytokines.† The most common sign is gastric ulceration via histamine, which can manifest as vomiting, diarrhea, or anemia.† Similarly, local ulceration near the cutaneous neoplasm may develop as a result of inflammation and edema.† Delayed wound healing may be caused by the proteolytic enzymes and the activation of macrophages via histamine to release fibroblastic suppressor factor.† In addition, coagulation abnormalities due to heparin release or hypotensive shock due to a massive histamine release may occur.

††††††††††† A diagnosis of a mast cell tumor can be accomplished through cytological examination of a fine needle aspirate of the tumor.† Cytology of a typical mast cell tumor will reveal a large number of discrete round cells with abundant, small, uniform, basophilic cytoplasmic granules located intracellularly and extracellulary.† The granules may stain poorly with Diff Quick“ stains.† The nucleus will appear round to oval, but may be hidden in heavily granulated cells.† Mast cells can vary greatly in appearance, and some tumors may contain agranular mast cells.† Varying numbers of eosinophils and neutrophils may also be scattered throughout the smear.

††††††††††† Preliminary staging of the tumor should include palpation of the local lymph nodes, which are the most common sites of metastasis.† The liver and spleen should also be palpated because hepatomegaly and splenomegaly are common with disseminated mast cell neoplasia.† Cytology of the regional lymph nodes and bone marrow, as well as an enlarged liver or spleen, can reveal increased numbers of mast cells signifying metastasis.† Greater than 10 mast cells per 1000 nucleated cells in the bone marrow is indicative of neoplastic infiltration. †.

††††††††††† The preferred treatment for a mast cell tumor confined to the dermis with no nodal involvement is complete excision with a wide margin of at least 3 cm.† Although most tumors palpate as discrete masses, most mast cell tumors are not discrete but surrounded by small numbers of neoplastic cells.† Histologic examination of the tissue removed is imperative to confirm the diagnosis, grade the tumor, and evaluate completeness of excision.† If histology reveals a poor margin of excision, a second aggressive surgery is the treatment of choice.† If a proper margin of excision cannot be obtained, treatment usually involves surgery to remove as much of the tumor as possible followed by radiation or chemotherapy.† No protocol has been agreed upon or proven to be markedly effective for malignant tumors, multiple tumors, and tumors that cannot be excised.† A treatment plan for these tumors usually involves corticosteroids as well as chemotherapeutic drugs and radiation therapy.† Palliative treatment with cimetidine and histamine blockers can help prevent gastric ulceration and some of the other secondary effects of the tumor.

††††††††††† Mast cell tumors are graded histologically based upon the number of granules, mitotic index, and cellular characteristics of malignancy.† The grading scale divides mast cells into three groups.† The distribution within these groups is:† 40% differentiated, 40% intermediately differenti-ated, and 20% undifferentiated.† Prognosis is excellent for a solitary well differentiated tumor that can be easily excised with good surgical margins.

- by Matt Renninger, Class of 1999

- edited by Janice Lacey, DVM

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