What is a mast cell tumor?


Mast cell tumors are cancerous tumors arising from mast cells, a kind of white blood cell that is part of your pet’s immune system.  When they function properly, mast cells, rich in granules containing histamine and heparin (an anticoagulant), release their contents at sites where there is an allergen present.  If you’ve ever watched a mosquito bite swell and turn red, you’ve seen the results of properly functioning mast cells.  It is not known what causes mast cells to turn cancerous and to form tumors in dogs and cats.


What do mast cell tumors look like?


Though some veterinarians rule out the possibility of a mast cell tumor by the growth’s appearance and where it is located on the pet, the practice is not supported by science or data.  Mast cell cancerous tumors can take any shape or form.  They can look like raised, rubbery lumps on the skin, or can be located beneath the skin. They can appear reddish, whitish, smooth, or bumpy.


Are mast cell tumors common in dogs and cats?


Mast cell tumors account for 16-20% of all skin tumors in or on dogs and cats.


What breeds of dogs and cats are most at risk for mast cell disease?


Any dog or cat can acquire mast cell disease, but it is most commonly seen in Beagles, Boston Terriers, Rhodesian Ridgebacks, Boxers, Pugs and all kinds of Retrievers.  In cats, the disease is often seen in the Siamese breed.


How are mast cell tumors diagnosed?


Fine Needle Aspirate


Veterinarians diagnose mast cell tumors by looking at the tumor’s cells under a microscope. Tumor cells are acquired in a process called fine needle aspiration.  During this procedure, a small hypodermic needle is inserted into the mass and a nearly imperceptible amount of tissue is withdrawn.  Your pet will not require any sedation for this procedure because the process is minimally invasive.  Once the sample is acquired, it is placed on a microscope slide, stained, and observed under a microscope.  Mast cells readily absorb stain and appear as light purple circles with more darkly stained, pepper-like grains .  The presence of mast cells in the sample is a positive sign of mast cell tumor disease.


Cytology or histopathology


It is not uncommon for veterinarians to have mast cell tumors evaluated by veterinary specialists.  In general terms, cells with more diffuse borders and nucleic abnormalities are deemed to be more aggressive than those mast cell tumors where the cells have clearer borders and less nucleic abnormalities.


Additional Diagnostics


If the mass is positive for mast cells, your veterinarian will likely order an ultrasound or radiograph of your pet’s chest and abdomen to look for more extensive signs of cancer.  He or she may also order a complete blood count, a blood chemistry, and urine test all of which will help determine if the disease has progressed beyond the primary tumor.


What is the prognosis for a dog or cat that is diagnosed with a mast cell tumor?


The prognosis for a pet diagnosed with a mast cell tumor is determined by the number of tumors found on the pet, the aggressiveness of the cancer associated with the tumors, and the extent to which the disease has spread throughout the body.


How are mast cell tumors on dogs and cats treated?


Provided that the cancer is localized and that the extent of the disease is limited, the first course of action is surgery to remove the tumor.  Because mast cells are known to have a ‘halo’ of cancerous cells surrounding the tumor itself, veterinarians will remove the tumor with a 3 to 4 centimeter margin to reduce the chances of the tumor returning and /or spreading.


If there is more than one tumor and/or lymph node involvement, your veterinarian may recommend radiation therapy or chemotherapy in addition to the surgery to remove the primary tumor(s).


Patients may also receive steroid therapy as part of the their treatment.


My dog or cat has some skin growths, what should I do?


As we stated earlier, approximately 16-20% of all skin masses are mast cell tumors.  The treatment for mast cell disease is straightforward and the prognosis is usually good provided that the cancer is not too aggressive and caught early.  Please bring your pet it for an examination.  We’ll be able to provide you excellent advice on what you should do to address your pet’s skin growths.

What If I Can’t Afford Treatment?


Cancer treatment can be expensive.  We have put together a resource that will help you understand your finance options.




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