Mast Cell Tumors in Dogs

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Mast Cell Tumors in Dogs

Mast cell tumors are a common cancer in dogs and cats. Most are located on the skin or in the tissue layer below the skin, called the subcutaneous tissue. Mast cell tumors are considered a Great Mimicker and can have a wide range of appearances. Any lump or bump you find on your dog that persists for over a week should be examined by a vet. A small sample of the growth should be obtained for evaluation under the microscope. There is no way to know just by looking or feeling a growth what it is, so additional testing is always needed. Continue reading for more information on mast cell tumors and treatment options.

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What exactly is a mast cell tumor?

A mast cell tumor (MCT) is a growth that contains histamine, heparin, growth factors, and other substances. Many mast cells contain a gene mutation to the c-kit gene that allows them to be more proliferative.

Mast cell tumors most commonly develop on the skin or subcutaneous tissue, however, they can also primarily arise on internal organs such as the liver and spleen. In cats, mast cells typically develop on the head and neck, especially around the base of the ears. In dogs, mast cells can develop anywhere and even appear like discolored areas of skin on the nose.

If the mast cell tumor is injured, such as a hard bump during play, it can release a large amount of histamine. This can trigger an anaphylactic reaction, lead to stomach ulcer formation, delay wound healing, and even potentially affect the ability to control bleeding.

The part of the mast cell tumor that is the visible bump is just a small part of the tumor. This type of cancer sends out tendrils that can spread 3cm or more away from the visible growth. Think of an octopus and its tentacles - the head of the octopus is the tumor and the tentacles are the invisible spread that is present. Mast cell tumors can also spread to regional lymph nodes.

Mast cell tumors often fluctuate in size. So if you notice a lump or bump on your pet that is large one day, then small the next, be sure to get it evaluated by your vet as soon as possible.

How do I know if my dog or cat has a mast cell tumor?

Any lump or bump you find on your pet that persists for more than a week should be evaluated by your vet. The size and location should be measured and charted, and a sample can easily be taken by Fine Needle Aspiration (FNA). This is a quick and simple sample collection where a needle is inserted into the growth and a cluster of cells is obtained. The collected cells are placed onto a microscope slide, stained, and then examined. Most mast cell tumors have a classic appearance under the microscope of cells full of small purple circles. Some mast cells are just dark cells and special staining at an outside laboratory may be needed to confirm the diagnosis.

My vet has diagnosed a mast cell tumor on my dog. What do I do now?

After diagnosing a mast cell tumor, your vet will likely recommend starting your dog on an antihistamine like diphenhydramine (an H1 blocker), an antacid like famotidine (H2 blocker), and possibly a steroid. These medications can help shrink the tumor down to a more manageable size prior to surgery and also reduce the risk of anaphylactic reactions and GI ulceration.

Once the diagnosis of a mast cell tumor has been made, surgical removal or biopsy is the next step. Mast cells have the potential to be very aggressive and you cannot tell this based on an FNA sample. Your vet will need to remove all or part of the tumor and submit that tissue to the laboratory for additional testing. These tests will determine if the entire growth was removed and how aggressive that mast cell tumor was.

Since you now know mast cell tumors will spread out further than the visible growth, your vet will want to remove the tumor with approximately 3cm of healthy appearing tissue all around it. So even a small bump will have a large incision and section of tissue taken out to try to ensure all the cancerous cells have been removed.

This can be complicated or impossible to do in certain locations, such as when the tumor is present on the nose, muzzle, paws, or lower limbs. There is just not enough extra tissue in these locations to remove all that is needed.

Also, remember that mast cell tumors can cause delayed wound healing. This can lead to the surgical site opening and having to heal slowly by “second intention” with regular bandage changes. This doesn’t mean your vet didn’t do a perfect job in surgery. It’s just one of the annoying things mast cell tumors can do.

Additional Testing Options for Mast Cell Tumors in Dogs

Since mast cell tumors aren’t always localized to the skin or subcutaneous tissue, your vet may also recommend an ultrasound of the abdomen to look for any growths on the liver, spleen, or other internal organs. FNA samples can be obtained if anything looks concerning. Radiographs (x-rays) of the chest may also be recommended to evaluate the lungs, heart, and lymph nodes for anything abnormal.

Once the tumor has been removed, your vet will submit the tissue to the lab for additional testing. Ideally, the tumor should be tested for the c-kit mutation, as these tend to be more aggressive types. The lab will also look for any tumor cells at the edge of the tissue to see if the entire tumor was removed or if anything microscopic was left behind that warrants further treatment.

Your vet may also remove, biopsy, or FNA the lymph nodes in the region of the mast cell tumor at the time of surgery. If mast cells are found in the lymph node tissue, it means the tumor has spread and additional treatment is needed.

Additional Treatment Options for Mast Cell Tumors in Dogs

If the mast cell tumor is highly aggressive or was not able to be completely removed, a referral to a veterinary oncologist for radiation therapy or chemotherapy is a good next step.

There are also two drugs available, Palladia and Kinavet, to use as adjunctive treatment if your dog’s mast cell tumor has the c-kit gene mutation.

Finally, there is a new treatment option that has recently become available called Stelfonta. This is an injection given into the mast cell tumor to help shrink it and slow the growth. It’s a good option for tumors that cannot be fully removed due to their location and have not metastasized (spread to other areas) yet.

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Mast Cell Tumors in Dogs

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Published: 1/21/2021
Last updated: 9/12/2021

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