Mast cell tumors can mimic many different lumps and bumps on your dog, so it is important to have any new lump checked out by your veterinarian.

By Kate Eldredge Basedow, LVT
December 22, 2020
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Mast cell tumors (MCT) are the most common skin tumor in dogs, and make up about 20 percent of skin tumors in our canine companions. These tumors are made up of the mast cells that give them their name.

Mast cells are a normal part of your dog’s immune system. They are white blood cells that contain granules of chemicals such as histamines. When mast cells are exposed to an allergen, they degranulate and release the histamines and other compounds that play a role in the allergic response by causing itchiness and swelling.

A mast cell tumor is an abnormal gathering of mast cells that replicate and divide out of control. While small amounts of histamines are useful, mass degranulation from a mast cell tumor can cause negative effects all over the body, even anaphylaxis (a life-threatening allergic reaction).

Mast cell tumors generally form as bumps on or just under the skin, but can also occur in lymph nodes, internal organs such as the liver or spleen, and in the bone marrow.

What Causes Mast Cell Tumors in Dogs?

As with many cancers, mast cell tumors are caused by a variety of environmental and genetic factors that all interact and influence each other. We often don’t know for certain why a particular dog gets mast cell tumors.

Though any dog breed or mix can develop mast cell tumors, some breeds are at a higher risk including:

Mast Cell Tumor Signs and Symptoms

Mast cell tumors are masters of disguise. The Washington State University Oncology Service says, “MCT can look like just about anything, ranging from benign-appearing lumps (such as a lipoma), to more angry or ulcerated lumps, masses with a stalk or focal thickenings in the skin.” They may be reddened and on top of the skin, or may grow just under the skin as subcutaneous masses that you cannot see directly. They can even look like a simple insect bite.

Some mast cell tumors grow slowly over time, while others seem to pop up overnight and grow rapidly. They can also wax and wane, swelling when the mast cells degranulate. Handling the tumor can cause degranulation, causing the tumor and surrounding areas to feel itchy as well as to grow larger. As your dog chews at his itchy skin, he can trigger more degranulation.

Credit: Wojciech Kozielczyk / Getty

If the MCT has degranulated and released histamines into the bloodstream, your dog may show additional symptoms such as:

  • Enlarged lymph nodes
  • Ulcers
  • Vomiting
  • Poor appetite
  • Lethargy
  • Tarry stools (sign of intestinal bleeding)
  • Diarrhea
  • Anaphylactic shock (in severe cases) 

Less commonly, the tumor itself can metastasize to other sites in the body, leading to a variety of symptoms. Because mast cell tumors can be so sneaky, it is important to have any new lump checked out by your veterinarian.

Diagnosing and Staging Canine Mast Cell Tumors

Diagnosing a mast cell tumor can be done by your veterinarian with a quick procedure called a fine needle aspirate (FNA). Your vet will insert a needle into the lump and suck out some material, which is then put on a slide to examine under a microscope. Mast cell tumors are easy to identify because of their granules.

The next step is to stage the tumor, or determine how severe it is. This is often done by taking a biopsy and sending it out to a lab for a histopathology report. The pathologist will examine the biopsy under a microscope. Other things that can be useful for grading and determining the best treatment plan include lymph node aspirates, radiographs (x-rays), blood work, and a bone marrow biopsy, all to evaluate if the cancer has spread.

Treatment for Mast Cell Tumors in Dogs

The good news is, mast cell tumors can often be treated and even completely cured. There are a variety of treatment options available, depending on your individual dog’s case.

Surgical Removal

Surgical removal is the treatment of choice. Because MCTs can fluctuate in size and are invasive to the surrounding tissues, the surgeon will take wide margins to try to ensure that the entire tumor is removed. The tumor can be sent out for a histopathology exam to double check. For low-grade tumors that don’t show any evidence of spread throughout the body, complete surgical removal can cure it.

Radiation Therapy

Radiation therapy is useful for tumors that either are not fully removed during surgery or can’t be completely removed due to their location.

Chemotherapy

The Washington State University Oncology Service says, “Chemotherapy is sometimes used to treat mast cell tumors, but chemotherapy is usually reserved for dogs with grade III tumors; mast cell tumors are notoriously unpredictable tumors with regards to response to chemotherapy.”

Stelfonta Injection

Newer targeted therapies that utilize some of the genetic mutations associated with mast cell tumors are in the works. The latest is Stelfonta (tigilanol tiglate injection), which just received approval from the FDA for use in dogs in November 2020. This drug can be injected directly into the mast cell tumor. 

According to the FDA, “Stelfonta works by activating a protein that spreads throughout the treated tumor, which disintegrates tumor cells.” Stelfonta can only be used for mast cell tumors on the surface of the skin or those that are under the skin but located below the dog’s elbow or hock. These limitations are in place because it can form a wound at the tumor’s site and cause tissue sloughing. This treatment had a 75-percent success rate in clinical trials.

Other Treatment Options

Your veterinarian may also prescribe medications to address some of the secondary effects of mast cell tumors. These include prednisone, antihistamines (such as diphenhydramine), and antacids.

If your dog is unable to undergo surgery or other treatment options, palliative care is an option to keep her comfortable. This usually consists of antihistamines to counteract the effects of degranulation of the mast cells and pain medications. This will not slow the progression of the cancer, but can keep your dog feeling good and enjoying her life.

What to Do if a Dog’s Mast Cell Tumor Bursts

Some mast cell tumors may become ulcerated or bleed. While this can be messy and may be painful, it is usually not an emergency. Cover the tumor with a light bandage until your dog can be seen by your veterinarian. Excessive bleeding may require an emergency visit.

To prevent bleeding, prevent your dog from chewing or scratching at a mast cell tumor. These tumors can be itchy because of the histamine release, so you may need to use an Elizabethan collar (cone) to prevent chewing and scratching, which can make the swelling and itchiness worse.

Life Expectancy With a Canine Mast Cell Tumor

Dogs with low-grade tumors that can be completely removed surgically or treated with radiation following surgery have an excellent prognosis, with studies showing survival times upwards of three to five years.

Tumors that have metastasized throughout the body have a more guarded prognosis depending on the extent of the spread. Dogs with cancer that has only spread to the nearest lymph nodes can still have a good prognosis if treated with a combination of surgery, radiation, and/or chemotherapy.

High-grade tumors have a guarded prognosis, and survival times may only be a few months even if the dog receives treatment.

Mast cell tumors can behave unpredictably, so the prognosis given for your dog may not reflect what actually comes to pass.

Dogs can develop new mast cell tumors even if the first tumor was considered cured through surgery. Growing a new tumor is not the same as metastasis, and the new tumor may look or behave differently from the first one. Dogs may also have multiple mast cell tumors at the same time that arose independently. For these dogs, it is prudent to take samples from multiple sites to determine the grade of each tumor. About 60–70 percent of dogs with mast cell tumors only develop one tumor.

Thankfully, mast cell tumors are one of the more treatable types of cancer in dogs. You can work with your veterinarian to determine the best treatment plan and will likely get several more quality years with your beloved companion.