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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 commonly found in the skin and 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. Small, discreet tumors that are easily removed surgically have a better prognosis than large, ulcerated masses.

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

Several grading systems for mast cell tumors were developed to try 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.

In the last few years, a new grading system was developed to improve diagnostic consistency between 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 that a new mast cell tumor grading system consisting of just two categories – low grade and high grade. 

“Most veterinary oncologists and veterinary diagnostic laboratories have moved to the two-tiered grading system,” said Dr. E.J. Ehrhart, a Professor in the Department of Microbiology, Immunology and Pathology at Colorado State University and one of the veterinary pathologists assisting with the Golden Retriever Lifetime Study. The new 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 is used. Your veterinarian or veterinary oncologist will determine the best course of action based on your pet’s individual needs.

The good news is that many mast cells tumors can be successfully treated and cured in many cases. Small, isolated lower grade tumors have an excellent prognosis if caught early. Unfortunately, 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 looking at 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. The Foundation is 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.