Mast Cell Tumor (Mastocytoma) in Dogs

What Are Mast Cell Tumors in Dogs?

Mast cell tumors in dogs are the most common form of canine skin cancer. Mast cells are white blood cells found throughout the body and are part of the immune system. They are responsible for the allergic response and can release histamine as well as other compounds.

Mast cells become cancerous when they begin dividing abnormally and grow into tumors. Mast cell tumors can be easily mistaken for other skin lesions, like warts or benign lumps.

They can appear in any shape, firmness, size, or location. However, mast cell tumors in dogs are often firm, solitary, slow-growing masses of the skin. In some cases, mast cell tumors can also cause severe allergic (anaphylactic) reactions in dogs. 

Because they are so good at mimicking other skin conditions, it’s usually impossible to identify a mast cell tumor just by looking at it—even for your veterinarian. 

Mast cell tumors are more common on middle-aged dogs, and certain breeds like Boxers and Boston Terriers are more at risk.

Because of this, it’s a good idea to have your vet look at any unusual skin masses that appear on your dog. 

Mast Cell Tumor Symptoms in Dogs

Mast cell tumors in dogs may not cause any symptoms at all. To help prevent more serious disease, vets recommend routinely checking your dog over to look for any masses on the skin. Make an appointment to see your vet if you find:  

• A new skin mass 

• A known mass that has changed in size, texture, or color 

• An unexplained allergic reaction or hives

Some masses may appear small and easily movable in (or just under) the skin with little swelling. More aggressive tumors can appear as larger, hairless sores. 

Often, mast cell tumors will change size. They can go from being very small to very large when they are bumped. When a tumor is bumped or agitated, it may go through a process called degranulation. This means that the tumor releases all its inflammatory material at once. These tumors can even get bigger and smaller over the course of the day, which is unique to this type of tumor.

Some mast cell tumors in dogs will result in body-wide symptoms, including strong allergic reactions. Degranulation can cause symptoms such as swelling and redness in the affected area or a more severe anaphylactic reaction that includes: 

• Weakness 

• Nausea 

• Vomiting 

• Diarrhea 

• Swelling of the face and limbs 

• Collapse and death (rare)  

Causes of Mast Cell Tumors in Dogs

Like other types of cancer in dogs, mast cell tumors are likely caused by a mix of genetic and environmental factors. There is a receptor on the surface of mast cells called c-kit. Current research suggests that mast cell tumors are the result of a mutation in this c-kit receptor.

Certain breeds are more likely to get mast cell tumors. These breeds include:  

• Boxers 

• Pugs

• Pit Bull Terrier

• Boston Terrier 

• Bulldogs

• Retriever breeds (such as the Golden Retriever) 

• Rhodesian Ridgeback

• Bullmastiff

• Weimaraner

• Schnauzer

How Veterinarians Diagnose Mast Cell Tumors in Dogs

Your veterinarian will create a skin chart of all known masses. They will collect cells from the masses to look at under the microscope using a fine needle aspiration (FNA).

Mast cells are usually very easy to identify under the microscope and this is the primary way these tumors are diagnosed. Your vet will likely recommend performing a biopsy on the more concerning masses.

Once a mast cell tumor has been diagnosed, several more tests are recommended to help determine if the cancer has spread.

Your vet may want to do lymph node aspirates to collect cells from the regional lymph nodes to look for spread of the mast cell cancer. X-rays, blood work, and/or an ultrasound of the abdomen may be recommended to look for any evidence that the cancer has spread to the liver or spleen.

Mast cell tumors that are more aggressive are likely to spread (metastasize) to other locations outside of the original tumor. 

It’s helpful to know if the tumor has spread before moving forward with choosing the best treatment. Because most mast cell tumors are low- or intermediate- grade and have not spread, some pet parents may elect to go straight to surgery and skip staging.

Grading and Staging of Mast Cell Tumors in Dogs 

When your vet removes the tumor, they will want to send it off for histopathology (biopsy). This will allow them to tell if the tumor is completely removed and determine how likely it is to come back or spread.

Grading helps determine the behavior of the tumor. Low grade tumors are less likely to spread or return, while high grade tumors are more likely to metastasize and grow back.

Staging is done to figure out how much the cancer has already spread.

With lower (zero or one) staging, the mast cell tumor has not spread to other locations. Mast cell tumors most commonly metastasize (spread) to the lymph nodes, spleen, and liver.

Higher stages (three or four) correlate with more spread of the cancer to other sites.

Treatment of Mast Cell Tumors in Dogs

Staging can be helpful in determining the course of treatment.

For example, in a tumor that hasn’t spread, you would be more likely to surgically remove it. Compare that to a cancer that has already spread to other locations. This dog would be better helped by chemotherapy or other system-wide treatment.

Fortunately, most mast cell tumors are low to intermediate grade, with high grade aggressive mast cell tumors being much less common. This is good news because most low- and intermediate-grade tumors are less likely to grow back after they’ve been completely surgically removed.

High-grade masses that have spread aggressively to the lymph nodes and internal organs may require more treatment than just surgery. Your vet may recommend consulting with a veterinary oncologist about chemotherapy for additional treatment in these cases.

If surgery is not an option because the tumor is too big to remove, then radiation therapy may be considered.

Recovery and Management of Mast Cell Tumors in Dogs

Recovering from surgical removal of a low-grade mast cell tumor in dogs usually requires two weeks of rest, pain medications, antihistamines like Benadryl®, and wearing a recovery collar. After two weeks, your veterinarian will remove the sutures and your dog can resume normal activity.  

For several months after surgery, it’s important to watch the area where the mass was removed to see if it comes back, although recurrence is unlikely for most low-grade tumors. Surgery in most cases is curative, and dogs usually can live their natural lifespan. 

For high-grade tumors, recovery after surgery will be the same, but your veterinarian will likely suggest pursuing additional treatment such as radiation or chemotherapy to prevent any tumors or cancer cells that have spread from causing more problems.  

Survival time in dogs with high-grade mast cell cancer following radiation or chemotherapy usually ranges from 10 months to two years, depending on the tumor and the therapy.

Without any treatment, the lifespan of dogs diagnosed with high-grade, aggressive mast cell tumors averages about four months. 

Prevention of Mast Cell Tumors in Dogs

Prevention of mast cell tumors in dogs is not something pet parents have any control over since it’s largely something encoded in a dog’s DNA.

The best thing a pet parent can do is catch mast cell tumors early.

Regularly feel your dog's body and monitor them closely for any lumps or bumps. If new masses are found, get your dog into your regular veterinarian for them to collect cells from the mass and rule out mast cell cancer.

Remember, the smaller the mass at the time of diagnosis and surgery, the easier it will be for your vet to remove all of it.