Mast cell tumor
MCTs are one of the most common neoplastic diseases in dogs and have an extremely variable biologic behavior ranging from a benign mass to a fatal metastatic disease.
Canine cutaneous MCTs commonly present as a solitary neoplastic mass in the skin and/or subcutaneous tissue of older dogs, with mean age of onset of approximately 9 years of age. There is no reported sex predilection.
These tumors usually originate in the skin but primary tumors have also been reported in the spleen, kidney, gastrointestinal tract, oral cavity, and bone. Most dogs present with a single cutaneous lesion. The tumors can vary greatly in size, shape, and gross appearance. Dogs also may present with other systemic signs of disease resulting from histamine release by mast cells.
The most common are gastrointestinal signs such as vomiting resulting from ulceration, bleeding, delayed wound healing, and hypotensive shock. Systemic disease also may be seen with metastasis to the lymph nodes, spleen, liver, and bone marrow.
Diagnosis may be tentatively ascertained by clinical appearance of the tumor, but histology is required for definitive confirmation.
Histologically, mast cell neoplasms are graded as well differentiated (grade I), intermediate differentiation (grade II), and poorly differentiated (III). Proliferation markers are commonly used for prognostication of these tumours.
Grade I and grade II mast cell tumors are readily identified by the abundance of metachromatic (purple) cytoplasmic granules that sometimes obscure nuclear morphology. Poorly differentiated or anaplastic mast cell tumors have fewer granules that are finer in appearance. They may have indistinct cytoplasmic boundaries, anisokaryosis, anisocytosis, and increased mitotic activity. Due to cellular fragility, extracellular granules may be present in the background on the aspirates. Eosinophils may or may not be present.
Wide surgical resection is the treatment of choice.
Histological grade is the most important prognostic factor.
Adjunct radiation therapy should be considered in cases of histologically metastatic tumor.
For grade II mast cell tumors, chemotherapy should be considered, with drugs such as toceranib, paclitaxel or vincristine recommended in combination with prednisolone, omeprazole and diphenhydramine.
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