Mast Cell Tumor (Mastocytoma) in Dogs

What are Mast Cell Tumors?

Mast cell tumors are the most common form of skin cancer found in dogs. Located in connective tissue throughout the body, mast cells are white blood cells that are part of the immune system. They are responsible for the allergic response.  

Mast cells become cancerous when they begin dividing abnormally and grow into tumors. Mast cells tumors are easily mistaken for other skin lesions, like warts or benign lumps. They can appear in any shape, firmness, size, or location. In most cases, however, they are 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. They are more common on middle-aged dogs, and certain breeds like Boxers and Boston Terriers are more susceptible, so it’s always a good idea to have your vet look at any unusual skin masses that appear on your dog. 

Symptoms of Mast Cell Tumors

Mast cell tumors may not cause any symptoms at all. To help prevent more serious disease, vets recommend checking your dog every few months 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 or color 

• An unexplained allergic reaction or hive 

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. 

In some cases, however, a mast cell tumor will result in symptoms, including strong allergic reactions triggered by the activated immune system. Also, if a tumor is bumped or agitated, it may go through a process called “degranulation.” This means the tumor releases all its inflammatory material at once. 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 (rarely)  

Causes of Mast Cell Tumors in Dogs

The underlying causes of mast cells tumors are not known. Like other types of cancer, it’s likely they are caused by a mix of genetic and environmental factors, but we do not have enough information to recommend any changes to your dog’s environment to prevent mast cells tumors.  

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

• Boxers 

• Pugs 

• Pit Bull / Bull Terrier 

• Boston Terrier 

• Bulldogs 

• Retriever breeds (such as Golden Retriever) 

• Rhodesian Ridgeback 

• Weimaraner 

• Schnauzer 

How Do Vets Diagnose Mast Cell Tumors?

Your veterinarian will likely recommend creating a skin chart of all known masses and then taking a tissue sample using a fine needle aspiration (FNA) or performing a biopsy on the more concerning masses. Many veterinarians will be able to initially identify mast cell tumors by a fine needle aspirate.  

Fine needle aspiration is a simple procedure and usually does not require sedation. It involves placing a needle in the mass to suction out some tissue. Once a few cells are collected, they can be looked at under a microscope and the initial diagnosis can be made by a specially trained veterinarian pathologist.  

At the time of diagnosis, the mast cell tumor will be graded based on how aggressive it is. Low-grade tumors are less aggressive, while high-grade tumors are more aggressive. Tumors that are more aggressive are more likely to spread (metastasize) to other locations outside of the original tumor.  

Treating Mast Cell Tumors

Once diagnosed, mast cell tumors are usually surgically removed and sent to a pathologist for review to determine if the tumor is low grade (less aggressive cancer) or high grade (more aggressive cancer). This is important because treatment recommendations between low- and high-grade tumors are very different.  

Roughly 80% of mast cells are low- to intermediate-grade and unlikely to reoccur after surgery. This means that for a majority of mast cell tumors surgical removal can be curative.  

High-grade masses can spread aggressively to the lymph nodes and internal organs. For these cases, your veterinarian may recommend additional testing such as lymph node aspirates and chest x-rays. They may also recommend consulting with an oncologist about chemotherapy or radiation therapy.  

In some situations when surgery is not an option, veterinarians can use a needle for radiation therapy, but not dogs are good candidates for non-surgical treatments. 

Recovering from Mast Cell Tumors

Recovering from surgical removal of a low-grade mast cell tumor usually requires 2 weeks of rest, pain medications, antihistamines like Benadryl, and wearing an e-collar (“cone of shame”). After 2 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 a dog usually will be able to 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 with radiation or chemotherapy usually ranges from 10 months to 2 years, depending on the tumor and the therapy. Without any treatment, survival averages about 4 months. 

Mast Cell Tumors FAQs

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