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Mast Cell Tumor: This is One Lump You Can't Ignore

Story at-a-glance -

  • Mast cells are found in tissues throughout the body, but they are especially prevalent in the skin, respiratory and GI tract. They play a role in allergic responses, non-allergic skin disease, wound healing and tissue remodeling. They also increase stomach acid production.
  • When mast cells replicate in higher than normal numbers, mast cell tumors can develop. These tumors can occur in both dogs and cats, and while some are benign, most are malignant. Dogs usually develop a single tumor; cats tend to develop multiple tumors.
  • In cats, mast cell tumors of the skin are usually curable with surgery. Internal tumors carry a much poorer prognosis.
  • In dogs, prognosis for recovery depends on where the tumor is located, its grade, and the treatment given.

In this video, Dr. Karen Becker discusses mast cell tumors in dogs and cats.

By Dr. Becker

Today I want to discuss mast cell tumors in dogs and cats.

Mast cells are found in all tissues of the body, but they are in especially high numbers in the skin, respiratory tract, and gastrointestinal tract.

Mast cells contain histamine and heparin.

They play a role in allergic responses, non-allergic skin disease, wound healing and tissue remodeling.

They can also increase stomach acid production.

Mast Cell Tumor Development

When mast cells replicate in higher than normal numbers, a mast cell tumor can form.

Pets with mast cell tumors can also have complications like stomach problems from the overproduction of histamine and excessive bleeding from the release of heparin.

Mast cell tumors occur in both dogs and cats. Although some are benign, most of these tumors are malignant (cancerous).

Mast cell tumors vary widely in their appearance, shape, and size.

Dogs usually develop a single tumor. More cats than dogs develop multiple tumors.

If your pet has a mast cell tumor on the skin, there'll be a bump or lesion of some kind. Sometimes it's a raised pink bump that looks like a number two pencil eraser on the surface of the skin. Sometimes the tumor will be a less-defined mass that feels like a lump under the skin.


Definitive diagnosis of a mast cell tumor is made through physical examination and testing, including tumor aspiration or biopsy.

If the fine needle aspirate reveals mast cells, the surgeon will be prepared to take large margins around the tumor, which reduces the likelihood of leaving tumor cells behind. I think this is really important.

The tissue that is removed is sent to a pathologist for staging or grading. This will let your vet know how extensive the disease is, and what type of treatment is needed.

Mast Cell Tumors in Cats

In kitties, mast cell tumors are most often seen in the skin of the head or neck, but they can occur anywhere in the body. Cats with these tumors are usually middle age or older (4+ years of age), but any cat can develop a mast cell tumor, including kittens.

Interestingly, Siamese cats are at higher risk than other breeds. They develop a specific type of tumor called a histiocytic mast cell tumor.

In the majority of feline cutaneous (skin) mast cell tumors, the treatment is removal of the entire tumor with surgery. Usually, surgical intervention is curative, meaning one round of surgery removes the entire problem.

Frequently in cats, mast cell cutaneous lesions are benign. In some cases, surgery isn't even needed because the mast cells resolve on their own.

Unfortunately, kitties with mast cell tumors on the inside of their bodies -- typically in the GI tract or the spleen -- carry a much poorer prognosis than tumors occurring on the skin.

Canine Mast Cell Tumors

In dogs, mast cell tumors are most often found on the trunk, limbs, and in between the toes.

The tumors occur more often in some breeds than others. Boxers, Boston terriers, English bulldogs, bull terriers, fox terriers, Labs, doxies, and Weimaraners are all at increased predisposition to develop mast cell tumors.

Prognosis for dogs depends on the tumor location, the extent of the tumor, the grade, and the type of treatment given.

Mast cell tumors of the skin are very different in dogs than cats. Surgery to remove the tumor is less invasive in cats, and the prognosis for a full recovery is much better in cats than in dogs.

Mast cell tumors with generally poor prognosis are those on the muscle, around the mouth or in internal organs, in the bloodstream or bone marrow, and ulcerated tumors. Mast cell tumors that cause GI ulceration or are large, fast-growing, or recurring also carry a much poorer prognosis.

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Treatment of Mast Cell Tumors in Dogs

Historically, mast cell tumors have been graded on a scale of I to III in dogs -- with grade III being the most serious and carrying the worst prognosis for recovery.

Grade I mast cell tumors generally have an excellent curative rate as long as the whole thing is removed. If a dog goes 30 weeks post-surgery for a grade I tumor with no recurrence of tumors during that time, he is considered cured.

Even with aggressive surgery, the recurrence rate for a grade II mast cell tumor is about 20 percent.

The majority of dogs with grade III malignant mast cell tumors will experience spread of the tumor. Only about 10 percent of these dogs live past a year of surgery.

Recently, a 2-tier histologic rating system1 for canine cutaneous mast cell tumors has been proposed to more accurately predict biologic behavior.

This rating system puts mast cell tumors into a low-grade or a high-grade category. Low grade tumors carry a good prognosis for full recovery; a high grade tumor carries a poor prognosis.

Additional resources:

In addition to histopathology, which means looking at the cancerous tissue microscopically, vets also now have the option to perform cell-proliferation analysis through Michigan State University's diagnostic lab2,3. I recommend this.

This new technology analyzes three markers to assess the risk of systemic disease. Veterinarians using the technology to help formulate a treatment plan will have a much more accurate blueprint of what's really going on in a mast cell patient's body.

If Your Pet Has Been Diagnosed with a Mast Cell Tumor

If your pet has been diagnosed with a mast cell tumor, I recommend you work with an integrative or holistic vet to reduce the risk of recurrence. There are supplements that can naturally help reduce mast cell degranulation and reduce histamine release.

And I recommend you eliminate carbohydrates, which are pro-inflammatory foods, from your pet's diet. I also recommend supplementation with a rich source of omega-3 fatty acids such as krill oil.

And I absolutely recommend that you never again vaccinate a mast cell patient.


+ Sources and References