As you may have read in our previous blog on lumps and bumps, not all skin masses need to cause a panic. Quite often, worrisome bumps turn out to be harmless. There are, of course, some lumps that should be addressed, and mast cell tumors are one of these.

Mast cell tumors are one of the most common skin tumors in pets and are often called “the great pretender,” as they can look like just about anything. They have a wide range of sizes, shapes and locations, making them difficult to definitively diagnose without the help of fine needle aspirates or biopsies.

What is a mast cell tumor?

First, let’s start with a little bit of background information. Mast cells are a normal part of our body’s immune system, playing a key role in inflammatory responses. Mast cells contain granules that are full of heparin (an anticoagulant, or blood thinner) and histamine, and these granules release their contents in response to injury. Mast cells are important in keeping our body safe from invaders as well as during allergic reactions, thanks in part to the histamine granules. A great example of mast cells in action is the red swelling you get almost immediately after a mosquito bite.

Mast cells tumors are a result of uncontrolled proliferation of mast cells. They are most common in the skin and underlying subcutaneous tissues in dogs and cats, though the spleen can also be a primary site for mast cell tumors in cats. As mentioned before, mast cell tumors can occur anywhere on the skin and can be small or large, haired or non-haired. A classic presentation is a mass that seems to vary in size, being small one day, large the next, and returning to its small size the following day. This swelling is due to the release of histamine from the mast cells. Mast cell tumors may be painless or may cause discomfort.

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What to do?

If you find a lump on your pet, your veterinarian will want to take a sample of the mass. Often mast cells are easily detected on a non-invasive fine needle aspirate sample. Mast cell tumors should always be surgically removed and submitted to a laboratory where they can be further identified. Mast cell tumors are graded on a three point scale, though most tumors fall in to Grade II.

Grade I tumors are very well defined and are the least aggressive. After surgical removal with clean margins, further treatment is not needed.

Grade II tumors are intermediate in both definition and aggression. This grey area makes it hard to determine the course of treatment after surgical removal. Other prognostic factors must be considered.

Grade III tumors are poorly differentiated and highly aggressive. Additional treatment will be necessary after removal.

Treating the tumor

Unfortunately, surgical removal may not remove the tumor in its entirety. Other times, mast cell tumors are in a place that would make surgical removal difficult, like a lower limb or foot. For these cases, radiation therapy is indicated. Radiation can shrink or kill the remaining tumor cells. Chemotherapy can be used in cases of Grade II tumors with poor prognostic factors and is indicated in Grade III tumors as well.

Whether or not you choose to pursue these additional treatments is highly personal. Many owners have reservations with regards to chemotherapy and radiation. Talk to your veterinarian or veterinary oncologists about your hesitations prior to making a decision regarding these treatments. If you made the decision to protect your pet with Petplan before he developed cancer, his cancer treatments can be covered for life, allowing you to focus not on the cost of treatment but on your pet’s care.

Remember, if you notice a new lump on your dog or cat, especially if it is rapidly growing or seems to change size, have it checked out by your veterinarian.

Dec 11, 2012
Pet Health

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