More about Mast Cell Tumors

I’ve recently discussed some of the basic information about diagnosing canine cutaneous mast cell tumors and the inherent challenges related to this particularly frustrating cancer. So what do we do once we know we are dealing with this chameleon of tumors? As mast cell tumors are so unpredictable in their behavior, each patient must be approached on an individual basis and treatment recommendations can vary remarkably from case to case.


The most straightforward example would be a dog presenting with a solitary mast cell tumor. With rare exception, in such cases, surgical removal with wide margins is the treatment of choice. We recommend the surgery entail the removal of 2-3 cm of “normal” appearing skin surrounding the tumor, and one layer of tissue below the tumor.


Owners are often surprised when I show them exactly how wide and deep these surgical margins should be in a quantitative sense. However, this is the best way to ensure the entire tumor is removed in order to limit the potential for regrowth of the tumor, and/or assure cells are not left behind that could spread to distant sites in the body.


Such wide surgical margins may translate into biopsy margins of only a few millimeters (meaning only a small region of “normal” tissue is present between the last visible tumor cell and the edge of tissue where the scalpel blade cut). When a biopsy returns, we hope to see more than 5 millimeters of clear tissue in all directions – anything less is generally considered an incomplete excision. It’s very important the biopsy include surgical margins so oncologists know what to recommend to owners.


Even if a dog presents with more than one mast cell tumor at the same time, surgery will be the recommendation. Sometimes it can be tough to know “how many tumors are too many”, and I must use my best judgment as to when to recommend intervention with medical therapy instead of surgery.


Radiation therapy plays a large role in the treatment of canine mast cell tumors, primarily for tumors unable to be entirely removed with surgery.


In its most simplistic form, radiation therapy entails bombarding the remaining tumor cells with high-energy beams of radiation. Treatments are usually administered daily, and each is performed under a short period of anesthesia. Dogs tolerate radiation therapy very well, and side effects are usually limited to some transient changes within the skin, although this will vary depending on the location of the tumor.


Radiation therapy is most effective when used after surgery, but in some cases it can be used prior to surgery (e.g. for very large tumors or tumors in regions where surgery is not feasible.) This tends to be a more palliative option, and the best outcomes occur when radiation is combined with chemotherapy (see below).


Chemotherapy has a role for mast cell tumors, but is often less effective than surgery or radiation therapy. I recommend chemotherapy for all grade 3 mast cell tumors, any tumor has already metastasized to a distant site, and for some cases of narrowly excised “high-risk” grade 2 tumors (though the role of chemotherapy for such cases remains somewhat controversial).


Chemotherapy can also be used to treat dogs who present with multiple mast cell tumors at the same time, or who have tumors too large to be removed surgically.


There are typically two main avenues of chemotherapy for treating mast cell tumors in dogs: the more “traditional” chemotherapy drugs (e.g. CCNU, vinblastine, prednisone), and the newer class of drugs called tyrosine kinase inhibitors (Palladia and Kinavet).


Traditional chemotherapy drugs work by causing damage to DNA within cells, without regard to whether the cell is a tumor cell or a healthy cell. This is the reason for some of the side effects seen with chemotherapy, including adverse gastrointestinal signs and lowered white blood cell counts.


The mechanism of action of tyrosine kinase inhibitors (TKI’s) is very different. These drugs work primarily by inhibiting the action of a receptor on the surface of mast cells that is mutated in about 20-30% of tumors. When the receptor is mutated, it causes uncontrolled cell division, leading to tumor growth.


TKI’s can also work by inhibiting the growth of blood vessels to tumor cells (this is called anti-angiogenesis therapy). This mechanism of action is separate from the previously mentioned mechanism, which means tumors without the specific receptor mutation may still have a good response to treatment.


TKI’s are orally administered medications given chronically at home. Dogs need to have “steady state” levels of these drugs in their blood stream to continually keep the receptor turned off. The receptor is present on other cells in the body, so side effects can occur with TKI’s as well, but are generally fairly limited in their spectrum.


The take home messages for canine mast cell tumors are:

  • They are very unpredictable in their behavior.
  • The biggest predictor of behavior is the grade of the tumor, which can ONLY be determined via biopsy.
  • Staging tests are important to look for spread of disease and should include labwork, regional lymph node aspirates, an abdominal ultrasound, and in some cases, a bone marrow aspirate.
  • Surgery is the mainstay of treatment for most dogs.
  • Radiation therapy and chemotherapy play roles for dogs with mast cell tumors – consult a veterinary oncologist to be sure you know all the options available for treating your dog!

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