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Mast cell tumors (MCTs) are one of the most common skin tumors in dogs. Although some breeds are predisposed, such as Boxers and Golden Retrievers, MCTs can develop in dogs of any breed. Middle-aged to older dogs have a higher risk of developing MCTs, but young dogs can also be affected.
Mast cell tumors can resemble almost any lesion of the skin or subcutaneous tissue, such as erythematous (red) areas, nodules (bumps), masses (lumps), benign or malignant skin tumors (lipomas, hemangiopericytomas, etc), so a cytologic evaluation (needle aspirate) of any cutaneous or subcutaneous lesion is always indicated. Cytologically, MCTs consist in a population of round cells with intracytoplasmic purple granules. Although the behavior of MCTs is unpredictable without treatment, the histologic (microscopic) grade is helpful in establishing their behavior; thus a histopathology study (biopsy) has to be performed in all cases. Microscopically, MCTs are graded as well-, moderately, and poorly differentiated (grades 1, 2, and 3, respectively).
In general, well-differentiated (grade 1) MCTs have a low metastatic (ability to spread) potential, while moderately- and poorly-differentiated MCTs have a higher metastatic potential to regional or distant lymph nodes (lymph glands), spleen, liver, bone marrow, and other organs.
The clinical evaluation of a dog with a MCT includes physical examination, a complete blood cell count, biochemical profile, urinalysis, fine needle aspirate from the lesions or affected sites for cytology evaluation, abdominal ultrasonography, and, sometimes, thoracic radiography if thoracic lymphadenopathy is suspected.
The treatment is variable depending on the histopathologic grade and extent of disease; complete surgical excision is frequently curative in dogs with solitary grade 1 or 2 MCTs. When surgical excision is incomplete (there is evidence of tumor cells at the surgical site) 3 options are available: to perform another surgery immediately,if possible, or to use radiation therapy or chemotherapy
When MCTs can't be surgically removed, radiation therapy is recommended, although in some cases chemotherapy may have similar effects. In dogs with grade 3 MCTs or with metastatic MCTs, chemotherapy is the treatment of choice. In general a combination of lomustine (CCNU) and prednisone provide very good results with low toxicity (side effects).