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  News from the Maryland Veterinary Medical Association                                                   Fall 2012

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Canine Mast Cell Tumors
by Janet Peterson, DVM, DACVIM (oncology)

Mast cell tumors are a relatively common tumor (especially involving the skin) in dogs. They can “masquerade” and look like many other tumor types. They can vary from relatively small and insignificant tumors to very aggressive, large and lethal tumors.

The most important diagnostic is a fine needle aspirate and cytology of the mass to assist in surgical planning, although grading cannot be accomplished without histopathology. Other recommended presurgical diagnostics include routine CBC, chemistry profile, and urinalysis, and abdominal ultrasound. Diagnostics that might be considered include fine needle aspirate and cytology of the draining lymph nodes, chest x-rays (primarily to look for lymphadenopathy), and a bone marrow.

Even with grading and extensive testing, it can be difficult to predict how these tumors will behave in an individual patient. With the traditional grading system, there are three grades (I, II, and III) of mast cell tumor which can roughly predict the behavior of the tumor (I being the most benign form of the tumor). There is a newer histologic grading system simplifying the categories to a high and low grade. Additionally, there is an additional panel of stains which may assist in more accurately predicting the aggressive nature of a particular mast cell tumor. This may be especially helpful with some mast cell tumors in terms of prognosis and possible adjunctive treatment considerations.

Aggressive surgery is often the primary treatment consideration for mast cell tumors and may be considered curative for grade I mast cell tumors and some grade II mast cell tumors (with adequate margins---greater than 0.5 to 1 cm). Radiation therapy might also be considered.

Recent advances in chemotherapy have also occurred. There are now two relatively new veterinary chemotherapy drugs available which are both very promising: Palladia™ (toceranib phosphate) and Kinavet CA1® (masitinib mesylate). Candidates for this type of chemotherapy typically include dogs with higher grade mast cell tumors and dogs with multiple mast cell tumors. These drugs are receptor tyrosine kinase (RTK) inhibitors. Inhibition of RTKs on endothelial cells, pericytes, and tumor cells disrupts multiple processes necessary for tumor growth.

Palladia inhibits the activity of VEGFR-2, an RFK expressed on endothelial cells. It inhibits the activity of PDGFR-B,an RTK expressed on pericytes and inhibits the RTK KIT on tumor cells. KIT is commonly dysregulated in canine MCT. Other chemotherapy considerations in these situations might include vinblastine, CCNU (lomustine), and prednisone. Label information on the veterinary drugs is provided by the manufacturer. PublishingImages/Palladia_PI.pdf

Consultation with a veterinary oncologist may be helpful in diagnosis and management of mast cell neoplasia. To locate a veterinary oncologist in Maryland please reference the MVMA’s specialist locator or resources offered by the American College of Veterinary Internal Medicine

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