Canine Mast Cell Tumors

A common canine cancer, described
By Susan Tasaki, Content reviewed by Stephen Gardner DVM, DABVP, February 2021
mast cell cancer in dogs

As they age, most dogs will develop a lump or bump, or a whole series of them. It can be tempting to ignore them, or to write them off as just another old-dog thing. While that’s what they often are, they should always be checked out by your veterinarian. Canine mast cell tumors are one of the five cancers most frequently diagnosed in dogs.

What is it?

Part of the immune system, mast cells’ job is to respond to inflammation and allergic reactions. Among other things, mast cells contain granules of histamine, which are released in allergic/hypersensitivity reactions (think hay fever). They are found in connective tissues, particularly those close to the surface. When mast cells proliferate at a tissue site, a tumor may result.

Accounting for about 20 percent of all skin tumors, mast cell tumors can be locally invasive, which sometimes makes them difficult to remove completely.

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Signs and symptoms.

Any lump that can be seen or felt, or both, should be checked. When a mast cell tumor involves other organs (generally, the spleen, lymph nodes or liver), vomiting, diarrhea, and loss of appetite may also occur.

How is it diagnosed?

To diagnose a mast cell tumor—particularly, to differentiate it from a lipoma (a benign fatty tumor that grows just beneath the surface of the skin)—a veterinarian will use a thin hypodermic needle to obtain a sample from the lump. This technique is called fine needle aspiration and is a quick, well-tolerated and inexpensive way to determine the nature of the lump, although it’s not necessarily definitive. The sample is then examined under a microscope. If the sample doesn’t provide enough cells or is otherwise inconclusive, the veterinarian may recommend that the lump be biopsied.

Two systems are used to grade a mast cell tumor: the three-tier Patnaik and the newer two-tier Kiupel. Dr. Janet Patterson-Kane, Morris Animal Foundation’s chief scientific officer, considers the Kiupel system to be superior. “The two-tiered grading system is a significant improvement in terms of predicting how mast cell tumors may behave, and better agreement between pathologists … should increase the confidence of veterinarians who are making decisions as to how to treat these tumors.”

To determine whether mast cell disease is localized or systemic, X-rays, ultrasound and bone marrow cytology (in which a needle is used to withdraw a small sample of liquid bone marrow material) may be recommended.

How is it treated?

Mast cell metabolism is inhibited by corticosteroids, such as prednisone, which is why steroids are sometimes used in the management of allergic reactions. Prednisone given orally can be helpful to shrink mast cell tumors. Another approach is to inject a related steroid, such as triamcinolone, directly into the mast cell tumor at several sites to decrease the size of the tumor prior to surgical removal.

For non-metastatic, skin-based mast cell tumors, a more recent therapy involves the use of tigilanol tiglate, a drug derived from the Australian blushwood plant. It stimulates the rapid destruction of mast cells within seven days of being injected into a tumor. This may avoid the need for surgery but does create a wound that needs to be managed during the healing process.

Grades 1 and II tumors are usually surgically removed; when caught early and removed with a wide margin of healthy tissue, prognosis is good. That being said, grade II tumors can present a dilemma; they may behave like grade I tumors or the more malignant grade III tumors. The latter are always surgically removed.

In cases where removal needs to be followed by another therapy, radiation is more commonly used than chemotherapy. Chemo tends to be reserved for grade III tumors, although their response to it is unpredictable. Some dogs will require a second, more aggressive surgery. A dog who’s had one mast cell tumor is likely to develop another; these tend to be new tumors rather than evidence that the original tumor metastasized.

Mast cell tumors also affect humans, and drugs that have been developed for use in people have shown promise when used with canine patients. Toceranib phosphate (Palladia), the first tyrosine kinase inhibitor approved by the Food and Drug Administration for treatment of canine mast cell tumors, and masitinib (Masivet) both seem to be well-tolerated in managing these tumors and slowing their progression.

Are certain breeds predisposed?

Though any breed or mix may develop mast cell tumors, flat-faced breeds such as Boston Terriers, Boxers, Pugs, Bulldogs and Retriever breeds seem to be predisposed. Both males and females are equally likely to develop them.

Photo: Hyunwon Jang / Unsplash

Susan Tasaki is a The Bark contributing editor.