Updated May 7, 2026 — It's normal for our dogs to get a little lumpy and bumpy as they age, with skin growths being one of the significant 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.
What Are Mast Cells and Why Do They Matter?
Mast cells are a type of white blood cell and a component of the immune system. They are one of the body's first lines of defense against invading organisms and assist in wound healing. Areas that come in contact with the outside environment, such as the skin and the lining of the intestinal tract, contain most mast cells. However, mast cells also have a downside; 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.
How Common Are Mast Cell Tumors in Dogs?
Mast cell tumors account for 16% to 21% of all tumors found in dogs' skin. They can take many forms, from a slow-growing isolated mass to ulcerated, itchy, diffuse lesions. Not only do these tumors vary in appearance, but they also vary in prognosis. Finding and dealing with them earlier yields better results — small, discreet tumors that surgeons can easily remove have a better prognosis than large, ulcerated masses. Male and female dogs are equally likely to develop MCTs. MCTs also tend to grow in middle-aged and older dogs.
Which Dog Breeds Are Most at Risk for Mast Cell Tumors?
Several breeds of dogs have higher rates of mast cell tumors. These include:
- Brachycephalic breeds (flat-faced dogs)
- Boston terriers
- Boxers
- Pugs
- English bulldogs
- French bulldogs
- Golden retrievers
- Labrador retrievers
Some breeds have a lower risk of MCTs. They include:
- German shepherds
- Chihuahuas
- Poodles
- Yorkshire terriers
- Cocker spaniels
However, any breed of dog can develop this skin cancer, so even if your dog might fall into a low-risk category, it's still important to keep a close eye out for any new lumps or bumps.
What Are the Signs and Symptoms of Mast Cell Tumors?
It might seem obvious that finding a bump is the most common sign people notice when it comes to MCTs, but occasionally, other signs might be noted, especially in more advanced forms. Remember, these cells contain chemicals that have wide-ranging effects on the body.
Signs reported in dogs with MCTs include:
- Skin redness around the tumor
- Limb swelling (if the cancer is located on a leg)
- Vomiting, sometimes with blood
- Abdominal pain
- Dark, tarry stools due to stomach ulcers
- Collapse (uncommon)
How Are Mast Cell Tumors Diagnosed?
Unlike many tumors, diagnosing MCTs is usually straightforward. Most veterinarians diagnose mast cell tumors by performing a fine needle aspirate procedure. Other diagnostic tests, such as abdominal ultrasound, aspiration of local lymph nodes and chest X-rays, are sometimes performed to look for cancer spread.
Since surgeons remove almost all mast cell tumors, pet owners must submit the masses to a pathologist for confirmation, examination of the surgical margins (whether the edges are free of tumor cells), and grading (more on this next). No matter how small the mass, it's important not to skip this step!
In addition to determining the grade, veterinarians sometimes use other laboratory tests to provide even more precise diagnostic and prognostic information. Often referred to as "staging," a combination of tests can give pet parents and their veterinarians a more accurate long-term prognosis.
What Do Mast Cell Tumor Grades Mean?
It was evident to many veterinarians years ago that mast cell tumors could exhibit wildly different behavior.
To better understand different types of mast cell tumors, veterinarians and veterinary oncologists developed several grading systems to predict tumor behavior and 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 categorized 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.
About 15 years ago, pathologists developed a new grading system to improve diagnostic consistency while providing veterinarians and dog owners with more accurate treatment and prognostic information. 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.
Many pathologists find that the two-tiered grading system significantly improves their ability to predict how mast cell tumors may behave. A better agreement between pathologists when they examine the same biopsy should increase veterinarians' confidence in making decisions about treating these tumors.
Veterinary pathologists, oncologists, and general practitioners are increasingly using the Kiupel system, which is gaining popularity. However, most pet owners need to be aware of the switch, which can lead to confusion when reading older material on this type of tumor. If their dog is diagnosed with a mast cell tumor, dog owners must ask their veterinarian about the grading system.
Some experts feel that using both grading systems is best for assessing MCTs. This can get complicated, but don't be surprised if veterinarians use both systems to grade your dog's tumor.
Lastly, new grading systems have been recently developed for the fine-needle aspirates described above. The bottom line is that veterinary oncologists will use a combination of techniques to guide pet owners on treatment and prognosis.
How Are Mast Cell Tumors Treated?
Surgical removal remains the primary treatment for mast cell tumors. Ensuring your veterinarian is comfortable removing these tumors is essential — incomplete removal can lead to recurrence.
Also, because these mast cells contain many substances involved in allergic reactions, manipulating them during surgery can cause them to release these substances. This can lead to signs of anaphylaxis (an acute allergic reaction), alter wound healing and lead to other surgical complications. Again, it's essential, especially for extensive or more invasive MCTs, that the surgeon has experience dealing with complicated tumors.
Veterinarians use chemotherapy or radiation therapy for higher-grade tumors, tumors that have recurred at a surgery site, or more diffuse tumors. Your veterinarian or veterinary oncologist will determine the best action based on your pet's needs.
Some mast cell tumors can arise from deeper tissues just under the skin. These subcutaneous mast cell tumors are often mistaken for fatty masses (lipomas). For many years, veterinarians considered this type of mast cell tumor to have a good prognosis. However, several researchers have questioned this view in the last three to four years. Data from multiple studies show a worse prognosis for these tumors than previously reported. Because subcutaneous mast cell tumors behave differently than those arising in the skin, veterinarians don't use the standard grading systems to characterize these tumors.
The good news is that many cases of MCTs can be successfully treated and cured. Small, isolated lower-grade tumors have an excellent prognosis if caught early. Unfortunately, the less common high-grade tumors have an inferior prognosis, with most dogs dying of their disease in less than one year, even with aggressive chemotherapy.
Recent data also suggests that if a dog develops additional mast cell tumors, they tend to have a higher grade when they recur. It's essential to check your dog at least once a month for any new growths if they have a history of MCTs.
What Is the Prognosis for Dogs with Mast Cell Tumors?
Because MCTs are so variable in their behavior, prognosis ranges from cure to almost 100% mortality. And sometimes, even low-grade MCTs can spread.
Regardless of the size of the main tumor, spread to local lymph nodes is associated with a poorer prognosis.
Earlier studies suggested a relationship between the location of the MCT and prognosis, but researchers have called this data into question. The bottom line is that location may not matter regarding long-term prognosis.
Mast Cell Tumors and the Golden Retriever Lifetime Study
Because golden retrievers are a dog breed that commonly gets MCTs, they are a cancer of particular interest to the Foundation's Golden Retriever Lifetime Study.
As of May 2026, more than 130 Study dogs have been diagnosed with low-grade MCTs and 37 with high-grade MCTs. Because some dogs have experienced multiple occurrences, the Study has recorded 160 low-grade diagnoses and 51 high-grade diagnoses in total. Sadly, 13 dogs have passed away from high-grade MCTs.
Our team continues to collect data on all cancers, including MCTs, in the cohort. We're hoping to analyze this information to find more clues to understanding and possibly even preventing MCTs.
What Research Is Morris Animal Foundation Funding for Mast Cell Tumors?
We funded our first mast cell tumor study in 2006 and have invested more than $1.1 million in studying all aspects of mast cell tumor behavior and treatment. Past and current studies primarily focus on finding new treatment targets for high-grade mast cell tumors and learning more about how mast cell tumors form and spread.
Among our most recently funded studies, researchers at the University of California, Davis, are developing a targeted monoclonal antibody therapy for canine MCTs. Monoclonal antibodies (mAbs) are already used to treat many human cancers, but in veterinary medicine they are still new and not widely available. This study focuses on engineering antibodies designed to target the protein KIT, which is present on mast cells but not on other immune cells, allowing the therapy to precisely attack cancer cells while leaving healthy cells unharmed. If successful, this research could be a game changer in the treatment of MCT in dogs and may also serve as a model for treating similar conditions in other species, including cats and humans.
Science to Save Lives Starts With You
Cancer, including mast cell tumors, remains a leading cause of death in adult dogs, and veterinarians have limited treatment options in large part because research funding has not kept pace with the need. Every dollar you give helps close that gap.
For Every Pet That Deserves More Time
Every year, 12 million dogs and cats are diagnosed with cancer. Your gift funds the research that gives them more time. All gifts matched up to $50,000 through June 30.
