Mast cell tumors, or MCTs, are tumors that often appear on the skin, but may arise anywhere in the body. They occur most commonly in dogs, with certain breeds affected more frequently, including boxers, Boston terriers, bullmastiffs and golden retrievers. Cats, horses, pigs and cattle also can have MCTs. The tumors may be malignant or benign, but veterinarians should treat all MCTs due to their unpredictable biology. Several treatment methods exist, including chemotherapy.
MCTs occurring on the skin are variable in appearance; they may be present as single or multiple masses, and may be smooth or ulcerated. They are more common on the trunk, perineum and limbs of the body. They may show redness when palpated due to a release of histamine from the tumor. Pets.ca list the internal or systemic symptoms of MCTs as including vomiting, diarrhea, weight loss, bloody stools and rapid pulse.
The treatment method of choice for MCTs is surgical removal, including wide margins around the tumor to cut down on the possibility of the tumor spreading. Radiation therapy may follow surgery in cases where complete removal of the tumor is not possible, and is beneficial in cases where the tumor has not spread to other areas of the body. In cases where MCTs have spread to multiple sites, chemotherapy drugs such as prednisone, lomustine, vinblastine and cyclophosphamide will augment surgery and radiation. Canada's Pet Information Center website reports that veterinarians will use these drugs individually or in combination with one another. Veterinarians have also been using a drug called Palladia, approved by the FDA in 2009, to treat canine MCTs, but they only use it where other treatments have failed.
Which veterinarians use chemotherapy drugs and how they administer them will vary from case to case. Some prescribe drugs orally in tablet form, as is the case with prednisone. Others may require a series of injections given at regular intervals, requiring scheduled veterinary appointments. Yet, others may require slow drug infusions by IV, requiring the dog to be in clinic for the day.
Side effects of chemotherapy drugs in pets tend to be less severe than those experienced by humans, due to the different types and lower doses of drugs used. Most commonly seen side effects include vomiting, diarrhea, decreased appetite, increased susceptibility to infections and variable hair loss most commonly seen in the face and tail. According to the OncoLink website, these side effects will usually abate within a few days.
Veterinarians generally reserve chemotherapy for use in tumors that are of advanced stage and have spread to other areas of the body. These tumors are the most difficult to treat and in many cases will not respond favorably to chemotherapy. Prednisone is inexpensive, well tolerated and safe; and so veterinarians often prescribe it. They consider other chemotherapy drugs more heavy-duty as these carry greater risks of side effects and are more costly.