Mast Cell Tumors in Dogs

Canine mast cell tumors are the most common type of skin tumor in the dog, accounting for 16-Mast Cell Tumors in Dogs21% of skin tumors.  They occur most commonly in older dogs, but can occur in dogs of any age.  Boxers, Boston terriers, Staffordshire bull terriers, Labrador retrievers, golden retrievers, Weimeraner, beagles, shar-pei, and schnauzers have been reported to be predisposed to this type of tumor, but they occur commonly in mixed breed dogs as well.  Mast cell tumors can present in a number of different ways.  Most are single mass lesions, but in approximately 12% of cases, they can present as multiple in masses.  The cause of mast cell tumors is largely unknown.  However, some mast cell tumors are known to exist due to changes in a particular tyrosine kinase receptor on mast cell tumors called c-kit.

Behavior of Mast Cell Tumors in Dogs

Mast cell tumors have a wide range of behavior, which is largely dependent on the grade of the tumor.  Mast cell tumors are generally graded one, two, or three (I, II, III) depending on how aggressive the tumor is, with grade three being the most aggressive.  They can mimic any type of skin lesion and can develop anywhere on the body.  Grade I mast cell tumors tend to be solitary, slow growing, soft consistency, with occasional hair loss and rarely ulcerated.   Grade III mast cell tumors tend to be rapidly growing, large masses that can become ulcerated.  Tissues in the area of the tumor tend to look inflamed and fluctuant.  Grade II mast cell tumors fall in between these two extremes.

All mast cells contain granules that have biologically active molecules such as heparin (an anti-clotting agent), histamine, proteases, and cytokines.  These molecules play important roles in a mast cell’s normal function in the body as a mediator of inflammation.  Simple manipulation of a mast cell tumor may result in swelling and redness associated with release of histamine by the tumor resulting in changes that mimic an acute allergic reaction.  Mast cell tumors can appear to increase and decrease in size in response to release of histamine by the tumor.

Diagnosis of Mast Cell Tumors in Dogs

Any skin or subcutaneous mass could be a mast cell tumor and a diagnosis cannot be made Dogs and Mast Cell Tumorsbased on external appearance.  A diagnosis of mast cell tumor is most commonly made based on fine-needle aspirates and cytological evaluation with a microscope.  Although cytology provides a quick and easy means of diagnosing a mast cell tumor, the grade of a mast cell tumor can only be determined by histopathology, not with cytology.  Mast cells are round cells that typically contain granules in the cytoplasm that stain purplish-red that make the cells easy to identify as mast cells.

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Dr Stephen Atwater

Stephen W. Atwater, DVM, MS, Diplomate ACVIM (oncology) is a board-certified veterinary oncology specialist. His professional interests include utilizing emerging therapies for difficult to treat cancers.

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