Ask a UW Veterinarian: Treating Mast Cell Tumors



This expert response comes from David Vail, professor and Barbara A. Suran Chair in Comparative Oncology at the UW School of Veterinary Medicine.

Question: Is it still the best to have mast cells in dogs removed? Thank you for any information on how to handle the diagnosis. —Melissa, Rockton, Illinois

Answer: Mast cell tumors (MCT) are the most common malignant tumor in dogs. Thankfully, most MCTs (more than 80 percent) are of low or intermediate grade and curable with surgery alone. So, you are correct that surgical removal is the treatment of choice in most cases.

For those 20 percent of mast cell tumors that are higher grade, surgery by itself may not be curative as they are more likely to spread to other parts of the body (metastasize) or recur following surgery.

Generally, if a mast cell tumor is at a location that allows surgical removal with fairly wide (approximately one inch) normal tissue borders (“margins”), surgery is performed. A sample is submitted to a pathologist to determine the tumor grade and whether the surgical margins are free of cancer cells.

If the margins are free of cancer and the tumor is not high grade, no further treatment is generally necessary other than active surveillance. This includes a recheck examination every three months for one and a half years after surgery.

If the pathologist finds that the mast cell tumor is of high grade, then there is a higher likelihood of metastasis or local recurrence, and further diagnostics and treatment (e.g., chemotherapy) following surgery should be discussed with a veterinary oncologist.

If the pathologist finds that tumor cells were left behind after surgery, then either further surgery (revision surgery) or radiation therapy is often effective in clearing the remaining cancer cells.

If surgery is not possible initially, alternatives may be necessary in hopes that the tumor can be shrunk to a size that surgery can be performed or to control the tumor as much as possible. Alternatives to surgery, which should be discussed with a veterinary oncologist, include radiation therapy, chemotherapy, investigational clinical trials, or in some cases, injections with substances that destroy tumor tissue.

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This article appears in the summer 2022 issue of On Call magazine.

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