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The “411” on Mast Cell Tumors

By Gail D. Mason, DVM, MA, DACVIM | May 01, 2019

Mast cell tumors are common in dogs and are most frequently found in the superficial layers of the skin on any part of the body. These tumors account for over 11- 21% of all cutaneous tumors in dogs. Normal mast cells are part of the immune defense system against invading organisms. They specifically participate in the battle against parasites and allergens within tissues of the body. The mast cells contain granules of inflammatory chemicals which are normally released when they bond to an “enemy.” When released, these chemicals cause inflammation, pain, swelling, and dilated blood vessels (think “bee sting”). This phenomenon explains why mast cell tumors can be often noted to “change sizes” as swelling increases and then subsides.

In some animals, these cells can form a mass or tumor. The granules in the abnormal cells are unstable and can release their chemicals somewhat at random. The chemicals can have both local and more distant effects on the patient. Mast cell tumors do not have a specific appearance, but often cause the dog to scratch the area. Varying degrees of redness, swelling, and ulceration can result. The tumors usually appear as a singular swelling though dogs can have recurrent and/or multiple swellings.

Diagnosis: Since mast cell tumors (MCTs) can have a variable appearance, any new nodule or mass on your dog should be evaluated by your veterinarian. Since they can generally be recognized and diagnosed via a simple office procedure (needles aspiration and cytology), the choice to instead “just watch it” is not recommended. Though the MCT may appear as a small “bump,” it has the ability to reach below the surfaces in “tentacle” form. Such tumors arising on the face, in the mouth, or in the area of lower abdomen or genitals can have more aggressive biological behavior. Early intervention provides the best outcomes.

Don’t Panic! If your veterinarian has just informed you that your dog likely has a MCT, what do you do? If a needle aspirate has been done, then treatment can be planned accordingly. Whenever surgery CAN be performed, it is considered the most precise and efficacious option. Because of the “tentacle effect,” simply shelling the mass out will likely leave malignant cells behind. Ideally, the surgeon plans to remove the mass with 2 cm “margins” (of unaffected, healthy tissue) in all directions (including deep to the mass) for the best chance of cure. If the mass is large, unstable, or in a problematic location, your veterinarian may consider referring your dog to a specialty (board-certified) surgeon who is highly experienced in oncology. The entire surgical sample should be submitted to a pathologist who can best determine if the surgical margins are adequate.

The pathologist will also “grade” the tumor (I, II, or III) based on its characteristics of malignancy which often correlate with expected biological behavior and overall prognosis.

Grade I MCTS: These are the most common type and the most benign in behavior. They are referred to as “well-differentiated” as they look and act similar to normal body mast cells. Complete removal generally results in an excellent prognosis for cure (94% of patients survive longer than 5 years after surgery).

Grade II MCTS: These are “moderately differentiated” with the potential for more aggressive behavior (recurrence or spread to other sites). Within this category, there are other laboratory tests that can be performed on the tissue sample to help predict future behavior (such as nuclear staining, assessing mitotic index or rate of cellular division in the sample, Patnaik and Kiupel scoring). Surgical planning is crucial, and additional tumor “staging” may be warranted. The latter term refers to investigating other parts of the body for evidence of tumor spread (xrays, ultrasound, blood testing) which help to guide treatment options.

Grade III MCTS: These poorly differentiated tumors are the least common, but are the most aggressive. They readily spread to local and regional lymph nodes and can cause the dog to be severely ill. Surgery alone will not likely result in a cure, and referral to a veterinary oncologist (or internist) is highly recommended. Sadly, only 6% of these patients survive 5 years after diagnosis.

Other options: Although surgical removal of the tumor (with adequate margins) is the ideal first step, it may not be the only recommendation. When surgery is not feasible, corticosteroids, chemotherapy agents, and tumor enzyme “inhibitors” can also help control MCTS progression. This is referred to as managing the disease. Lastly, radiation can also be utilized (and dogs do not get “radiation sickness” like humans) for specific instances. These tools are also used in patients that have incomplete margins or are at risk for developing tumor recurrence or spread. The terms sound frightening, but remember that knowledge is power!

It is important to consider that 1) dogs who develop more than one MCT in a lifetime do not necessarily have a worse prognosis than those that don’t; 2) Grade I or Grade II MCTS do not “morph” into Grade III tumors over time. Lastly, the great majority of dogs with a diagnosis of MCT can live happy, extended lives even if ongoing therapy is required.