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Updated June 11, 2020 – It’s normal for our dogs to get a little lumpy and bumpy as they age, with skin growths being one of the major reasons pet owners seek veterinary care for their pups. Although the majority of these lumps are benign, owners still need to be concerned about the possibility of skin cancer, particularly the most common malignant skin cancer of dogs – mast cell tumors. 

An important part of the immune system

Mast cells are a type of white blood cell and a component of the immune system. They are one of the body’s first line of defenses against invading organisms and assist in wound healing. The majority of mast cells are found in areas that come in contact with the outside environment, such as the skin and the lining of the intestinal tract. But mast cells have a downside as well; histamine is one of the chemicals contained within mast cells, and you’d be correct in suspecting that mast cells are involved in asthma and severe allergic reactions. 

Mast cell tumors are one of the most common types of skin cancer

Mast cell tumors account for 16% to 21% of all tumors found in the skin. They can take many different forms, from a slow-growing isolated mass to ulcerated, itchy, diffuse lesions. Not only do these tumors vary in appearance, they also vary in prognosis. The earlier they are found and dealt with the better – small, discreet tumors that are easily removed surgically have a better prognosis than large, ulcerated masses.

Male and female dogs are equally likely to develop mast cell tumors, but several breeds of dogs are predisposed to mast cell tumors. Brachycephalic breeds (flat-faced dogs) such as Boston terriers, boxers, pugs and bulldogs, as well as golden retrievers and Labrador retrievers, have a higher risk of developing mast cell tumors. However, any breed of dog can develop this skin cancer. 

Grading to help determine treatment and prognosis

Over the years, several grading systems for mast cell tumors were developed to help veterinarians and veterinary oncologists make predictions on tumor behavior and to help guide therapy. Grading helps researchers, veterinarians and pet owners compare apples to apples when conducting clinical trials or speaking with concerned pet parents.

One of the earliest, and most widely used, mast cell grading systems placed mast cell tumors into one of three categories, from least to most aggressive – Grade 1, 2 or 3. If you’ve had a dog diagnosed with a mast cell tumor in the past, you’re probably familiar with this grading system.

Approximately 10 years ago, a new grading system was developed to improve diagnostic consistency among pathologists while providing more accurate treatment and prognostic information to veterinarians and dog owners. Dr. Matti Kiupel of Michigan State University, in collaboration with colleagues from around the world, proposed a new mast cell tumor grading system consisting of just two categories – low grade and high grade. 

“The two-tiered grading system is a significant improvement in terms of predicting how mast cell tumors may behave, and better agreement between pathologists when they look at the same biopsy should increase the confidence of veterinarians who are making decisions as to how to treat these tumors,” said Dr. Janet Patterson-Kane, a Board-Certified Anatomic Pathologist and Morris Animal Foundation’s Chief Scientific Officer. “The Kiupel system is gaining in popularity and use among both veterinary pathologists and oncologists as well as general practitioners, but most pet owners are unaware of the switch, which can lead to confusion when reading older material on this type of tumor. It’s important for dog owners to ask their veterinarian about the grading system if their dog is diagnosed with a mast cell tumor.”

Diagnosis and treatment

The majority of mast cell tumors are diagnosed by fine needle aspirate, which is a simple procedure that can be done by any veterinarian. Once a diagnosis is made, additional tests may be performed to determine if there are any signs of tumor spread, which can influence not only treatment but prognosis.

Surgical removal remains the primary treatment for mast cell tumors. It is important to make sure your veterinarian is comfortable removing these tumors – incomplete removal can lead to recurrence. 

For higher grade tumors, tumors that have recurred at a surgery site, or tumors that are more diffuse, chemotherapy or radiation therapy are used. Your veterinarian or veterinary oncologist will determine the best course of action based on your pet’s individual needs.

Some mast cell tumors can arise from deeper tissues just under the skin. These types of subcutaneous mast cell tumors often are mistaken for fatty masses (lipomas). The good news is that subcutaneous mast cell tumors are slow to spread and have only an 8% chance of returning  after surgical removal. Because subcutaneous mast cell tumors behave differently than mast cell tumors arising in the skin, the common grading systems aren’t used to characterize these particular tumors

Mast cell tumors can be successfully treated and cured in many cases. Small, isolated lower grade tumors have an excellent prognosis if caught early. Unfortunately, the less common high-grade tumors have a very poor prognosis with most dogs dying of their disease in less than one year, even with aggressive chemotherapy.

Morris Animal Foundation – A leader in mast cell tumor research

Morris Animal Foundation has been funding research on mast cell tumors in dogs for nearly two decades. Past studies and current studies primarily are focused on finding new treatment targets for treating high-grade mast cell tumors, and mast cell tumors are a cancer of special interest to the Foundation’s Golden Retriever Lifetime Study. We’re committed to finding a better treatment option for dogs with this type of devastating tumor.

As our dog’s guardians, we want to keep them healthy. The bottom line is that any new lump or bump needs to be investigated by your veterinarian, and any sudden change in a lump or bump previously examined also should be checked. Check out our latest canine cancer studies and see what we’re doing to help our dogs have longer, healthier lives.