Diagnosing Mast Cell Tumors

by Dr. Susan Ettinger

When should you see an oncologist for a mast cell tumor (MCT)? I recommend you get an oncologist involved early. But before you see me, you need to know what a MCT looks like, and how they are diagnosed.

Most dogs are not feeling or acting sick when they are diagnosed with MCT. Usually the first sign of illness is when you or your vet find lump, or mass, on or just under the skin. Most MCT tumors look like raised, hairless, pink bumps – but their appearance can vary widely. Because of this, MCT is called “the great impersonator.” Tumors can look like benign skin tags, or harmless lipomas. The tumor can be ulcerated (an open sore), swollen, and inflamed – or relatively benign looking.

It can also be confusing because MCT can get bigger and bigger, and then smaller – without any seeming rhyme or reason. We don’t typically think of tumors waxing and waning on their own. The reason these tumors do is because histamine is involved. Histamine causes swelling, and as it is released from the tumor cells, it causes inflammation and swelling. When it dissipates, the swelling and inflammation resolves. So it’s not the tumor changing in size, but the inflammation associated with the tumor.

Tumors can be found anywhere, but the trunk and the limbs are the most common locations. Some tumors are present for months or even years with little change in growth or appearance. Others appear suddenly and grow very rapidly.

Some MCT masses itch, so your dogs may scratch or lick them. If your dog chews, scratches, or bangs the tumor against the ground, it can release inflammatory chemicals from the granules. This is called degranulation, and the release of histamine can cause a localized swelling that looks like a hive.

Massive degranulation can also cause system-wide symptoms, like full-body swelling or in very severe cases, anaphylaxis (shock). This is uncommon with a single, smaller MCT. It’s more likely when there is a very large tumor, or if there is metastasis (spread) to internal organs.

Because of the wide variation in MCT appearance, I strongly recommend that every skin mass be aspirated for a microscopic evaluation. Without this evaluation, you might not catch MCT early.

MCT are typically diagnosed with a fine needle aspirate, which has proven very reliable in confirming the diagnosis. No one – not even experienced oncologists like me – can be sure that a suspicious lump is benign just by feeling it, which is why every skin and subcutaneous mass (those just below the skin) should be aspirated. If the mass cannot be aspirated, a surgical biopsy should be considered.

But for the majority of MCT, a simple and inexpensive fine needle aspirates will typically confirm MCT.  After the aspirate confirms MCT, the next step is typically surgery, especially if we are dealing with your dog’s first MCT and there is only one. If possible, the goal of surgery is to remove the entire visible tumor along with a wide (usually 2 to 3 cm on all sides and a layer of tissue below) margin of surrounding normal tissue. Then the removed tumor is biopsied. The surgical margin must be confirmed with the pathologist’s report. “Clean margins” on the report will mean that the tumor was removed completely. (Remember, there is always a chance that cancer cells remain that we can’t detect with our current technology – but in general clean margins are a good sign.)

The biopsy report will also provide the MCT grade, which we will discuss in the next blog. Grading MCT tumors has always been a challenge, because they are so changeable. There is the classic, 3-tiered system (grade 1, 2, and 3), but also a newer 2-tiered system that rates tumors are “low grade” or “high grade.” We’ll talk more about this in the next blog. We will also discuss who should do the surgery, when should staging like abdominal ultrasound be done, should you include a buffy coat in staging tests, and what the implications are if your dog has more than one MCT.


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