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Mast Cell Tumors in Dogs - One of These is Not Like the Other, Don't Ask Me Why, Go Ask Your Mot

The skin is the largest organ of the body, so it makes sense that a large majority of tumors occur in the skin and subcutaneous (under the skin) tissue. Lipomas, which are benign tissues of the fat, occur most frequently, but mast cell tumors are a close second.

Mast cell tumors are made up of mast cells, hence the name. These cells are part of the immune system, and are responsible for the allergy response. They are round cells that have purple granules in them. The granules contain many important factors such as heparin, which prevents blood clotting, and histamine, which is what causes allergic reactions.

Mast Cells with Red Blood Cells

Mast cells with red blood cells

Mast cells do not unfortunately have a typical appearance. Most often, they affect the tissue of the superficial skin, but even there, the appearance varies. They can be a small mass with a minimal amount of hair loss.

Small mast cell lesion

They can also be a large mass that has become ulcerated.

Ulcerated mast cell lesion

They can even be a mass that is located under the skin.

Mast cell lesion under the skin

Wherever the location, they are usually easy to diagnose. A fine needle aspirate sample will often yield a large number of cells to view under the microscope, and because of their unusual appearance, the cells are often easy to identify.

Once a mast cell tumor has been found, there are a couple of options as to how to proceed. The most common place a mast cell tumor will spread to is the nearest lymph node. After it spreads to the lymph node, it has the ability to spread to other areas of the body. The most frequently affected areas include the liver, spleen, other lymph nodes, and the bone marrow.

If we find that the mast cell tumor is confined to the skin or subcutaneous tissue, surgical removal is recommended. This will allow the veterinarian to submit the tissue for grading. Knowing the grade of the mast cell tumor is one of the best ways we have to know how the mast cell tumor is going to behave. There are a couple of different ways that a mast cell tumor is graded. Historically, we used a three category system which classified the mast cell tumor as grade I, grade II, or grade III. Grade I tumors are considered benign, and surgery is the best option for control. Grade III tumors are malignant with the highest chance of spread. These should all receive chemotherapy to help prevent or slow the spread. The problem is, what do we do with a grade II? Some will be more benign like a grade I, while others will be very malignant, like a grade III.

Margins for a mast cell tumor removal

Margins for a mast cell tumor removal

Luckily, we now have a couple of ways to predict how these tumors will behave. One way is to use a new grading system in which the tumors are classified as high or low. The low grade tumors will include all of the grade I tumors and most of the grade II, while high grade tumors include all of the grade III tumors, and some of the grade II. This still sounds a little confusing, but trust me, this new system has cleared up a low of questions as to how a mast cell tumor patient should be treated.

Another way that we know how to proceed after a mast cell tumor has been removed is knowing the mitotic index. When the pathologist is examining the tumor under a microscope, they can visualize how many cells were actively dividing. They then count the number of these cells present in 10 random fields of the tumor sample. We want that number to be less than 5, as those dogs typically have very long life expectancies. If the number is greater than 5, we expect that the tumor will behave more aggressively, with the chance of regrowth and spread being increased. As a general rule, tumors with a mitotic index of less than 5 won’t require treatment other than surgery, while those with a mitotic index greater than 5 will benefit from additional treatments, possibly including radiation and/or chemotherapy.

In regards to the chemotherapy, there are a few different options. One of the most widely used traditional chemotherapy protocols involves giving the dog an injection of a drug called vinblastine in combination with an oral steroid. This is a protocol that most dogs handle very well, and we see very few side effects associated with this treatment. Some protocols will include an oral chemotherapy such as cyclophosphamide or lomustine.


One of the newer cancer treating drugs available to dogs is Palladia. This medication works differently than other chemotherapy drugs in the instead of causing fatal damage to the cell’s DNA, it works on cutting off communication to the tumors cells by blocking signaling pathways. It also works to prevent new blood vessels from forming, which are important for continued tumor growth and spread. This is a fantastic drug to have in our cancer treatment toolbox, but it unfortunately isn’t a cure for the disease. It also isn’t indicated for all patients with mast cell tumors; however, when it is indicated we can see some amazing results.


So, in summary, mast cell tumors have a varied appearance and behavior, and may or may not need additional treatment after surgical removal. It is always important to have the lab evaluate any skin tumor that is removed from your pet, as if we don’t know what it is, we don’t know if any additional treatment is indicated. This is especially important with mast cell tumors. If you are concerned about a new skin mass on your dog, please take them to be examined by your primary care veterinarian.

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