Mast cell tumors in dogs: diagnosis, treatment, and prognosis

Mast cell tumors develop from specific cells of the immune system called “mast cells,” which normally treat inflammation and allergic reactions in a dog’s body. The cause of these tumors is currently unknown, and the tumors can develop anywhere on your dog’s body. This is why it’s important to regularly check your dog for lumps or growths and discuss any concerning spots (and symptoms) with your vet.

The symptoms of a mast cell tumor vary depending on what organ is affected. Mast cell tumors are most commonly found on the skin but can be felt in the layer under the skin called the cutaneous layer. Mast cell tumors can spread to the liver, spleen, lymph nodes, and other organs.

The diagnosis often comes after an owner feels a lump under their dog’s skin, but there are other clinical signs and symptoms to watch out for:  

• Enlarged lymph nodes. While this could be due to inflammation or infection, enlarged lymph nodes can also be caused by the spread of cancer.
• Decreased appetite. Another symptom of mast cell cancer can also indicate many other issues. When your vet examines your dog and takes its history, be sure to mention any change in appetite.
• Increased respiratory rate. A breathing rate of 30 or more breaths per minute is considered an increased respiratory rate and your dog should be seen by a vet. See this at-home breathing rate evaluation for help checking your pup’s respiratory rate.
• GI upset. Vomiting and diarrhea are common in dogs that have mast cell tumors. This can be caused by the release of biologically active compounds found in mast cell tumors.
• Low energy. Any change in your dog’s energy level could indicate an issue, so remember to talk to your vet about your dog’s low energy — especially if they’re having any of the other symptoms of a mast cell tumor.

Mast cells also have inflammatory mediators that can cause shock-like signs such as severe acute lethargy, collapse, pale gums, and severe vomiting. If you notice that your dog has pale gums or they have collapsed, you will want to call your vet and get them to the emergency room for treatment as quickly as you can.

Breeds prone to mast cell tumors

While any breed of dog can get mast cell tumors, certain breeds, especially brachycephalic breeds, are more susceptible. Mast cell tumors are particularly common in:

• Boxers. No one knows exactly why boxers are more prone to mast cell tumors, but they do tend to get them more frequently than other breeds. The good news is that their tumors are often (but not always) less severe.
• Bull terriers. The bull terrier is another breed of dog that is predisposed to mast cell tumors. If you have a bull terrier, make sure they are screened for mast cell tumors regularly and that you know the signs of complications from mast cell tumors.
• Boston terriers. Similar to boxers, this is a breed that is prone to getting mast cell tumors. The reason for this is not known.
• Labrador retrievers. Labrador retrievers are another breed that is prone to mast cell tumors and particularly aggressive cases at that. Early detection is essential.

Mast cell tumors are typically diagnosed via a fine-needle aspiration (FNA). Other testing options may include blood and urine samples, ultrasounds, or radiographs. In cases where it’s important to know the aggressiveness of the tumor to best manage it, a surgical tissue sample (biopsy) can be beneficial.

One important thing to know about diagnosing mast cell tumors is that they have been called “the great pretenders” in that they may mimic or resemble other conditions. It can look like something as simple as an insect bite, a wart, an allergic reaction, or another less serious skin tumor. This is why it’s important to seek veterinary care when you find a bump on your dog that’s out of the ordinary.

Progression of mast cell tumors in dogs

Mast cell tumors, like many other cancers, can spread and become progressively worse (and more complicated to treat). Veterinarians use a grading system from 1 to 3, with grade I being the least aggressive and grade III being the most aggressive.  Tumors that fall higher in the grading system (either grade II or III)  have a higher tendency to recur or metastasize.

While these tumors are generally treatable if caught early, there are a few factors that make a dog’s prognosis less favorable. These include:

• The dog is one of the more susceptible breeds.
• The mast cell tumor is located where the skin and mucous membranes connect. An example of this would be a mast cell tumor located on a dog’s gums.
• The tumor is too large to surgically remove or the tumor is in an area where it cannot be removed with clean surgical margins.
• There are numerous mast cell tumors on the skin or it has metastasized to other organs including the liver, spleen, or lymph nodes.

When it comes to treating a mast cell tumor, your dog’s veterinary oncologist will evaluate a variety of factors before making treatment recommendations. They will then be able to inform you of the overall prognosis and best treatment options.

Surgery — A surgical removal is a good option for a mast cell tumor that is localized to one spot and hasn’t begun to spread. Chances of a full recovery are most likely when the whole tumor can be removed with clean surgical margins, meaning no cancer cells remain in the area.

Radiation therapy — A type of radiation therapy called CFRT (conventionally fractionated radiation therapy) can be used alone to treat mast cell tumors or following a surgery where some cancer cells remain.

Chemotherapy — Chemotherapy is used to treat mast cell tumors that have already spread or have a high risk of spreading. Your dog’s oncologist will evaluate these factors when determining the most effective treatment options. Keep in mind if you have an older dog or a dog who is likely to experience potential side effects from chemotherapy treatment, it may be best to discuss and choose a different treatment with your vet.

Palliative therapy — While palliative therapy is not intended to be curative, it can help your dog maintain a quality of life for the remainder of their time with you. In this treatment option, your vet will provide medicines such as antihistamines and steroids to keep your dog comfortable.

Palladia — This is an oral medication that can stunt the growth of mast cell tumors and sometimes help them disappear or shrink in size. This medication is prescribed mostly for dogs that have grade II or III mast cell disease or recurrent cutaneous mast cell disease with or without lymph node involvement.

If your dog has surgery to remove a mast cell tumor, they will need 10-14 days of rest and light activity while they recover. Your vet may send them home with an e-collar and some medication to control the pain and prevent infection while their incision heals.  After about two weeks, your dog should be able to resume normal activity once any stitches have been removed unless your vet tells you otherwise.

When the tumor grade is higher, recovery might take more time or involve additional treatments. These could include a second surgery, chemotherapy, or radiation to eliminate any remaining cancer cells. It’s important to follow your vet’s instructions and stick to the schedule they provide for the most effective treatment.