Mast Cell Tumors in Dogs: Diagnosis

Mast cell tumors are typically diagnosed on the basis of fine-needle aspiration cytology.  Once aVet Diagnosing Mast Cell Tumors in Dogs mass is determined to be a mast cell tumor cytologically, whether or not additional tests are indicated prior to surgery is dependent on the presence or absence of negative prognostic factors (see section on prognostic factors of mast cell tumors) and if the tumor is of a size and in a location that allows easy removal with wide margins.  Diagnostic tests involve procedures to determine where the cancer may exist in the patient, a process called staging of the cancer.  It may also involve obtaining a small amount of tumor tissue to grade the tumor prior to definitive treatment.

Diagnostic testing of a dog with a mast cell tumor with no negative prognostic features

If there are no negative prognostic factors and the tumor can be easily excised with wide margins, then additional diagnostic tests are unlikely to provide evidence of cancer in other locations.  This is particularly true if there is no indication of spread of the cancer to the regional lymph node (which may require cytological evaluation of the lymph node to confirm).  The more practical approach is to proceed with surgery and have the tissue evaluated by the veterinary pathologist to determine the grade of the tumor and how complete the removal appears to be based on the margins.  Margins are an estimation of the width of normal tissue between the outer edge of the tissue that was removed from the patient and the tumor itself.  Margins of 1.0 cm or more are preferred, however, there are many reports of mast cell tumors in dogs that were more narrowly excised, which never recurred.

Diagnostic testing of a dog with a mast cell tumor with negative prognostic features

If the tumor is not easily excised with wide margins or negative prognostic factors exist, then additional diagnostic tests to stage the pet are indicated.  The findings of these tests allows the veterinarian to better advise owners about the potential outcome of their pet after treatment.  It also allows the veterinarian to prepare the owner for the extent of treatment that would be indicated to best treat the disease. This allows the owner to anticipate potential cost and determine what is the most reasonable approach to deal with the disease based on expected outcome.  Treatment of more aggressive mast cell tumors typically involves a combination of surgery, radiation therapy and chemotherapy.

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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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