Mast Cell Tumor In Dogs: Symptoms & Treatments

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You probably pet your dog a lot, so when you notice a lump on their skin, it may be cause for concern. When should you call the vet? Skin lumps and bumps can appear on dogs from time to time for various reasons, ranging from benign lumps like hives due to allergies or lipomas and warts to malignant bumps like melanomas and mast cell tumors. You should have any new lumps on your dog’s skin checked by a vet in case they’re cancerous and may spread. 

Mast cell tumors (MCTs) are common lumps on dogs indicating skin cancer. They make up around 20% of all dog skin tumors and are most common in older dogs. However, any dog at any age can get them. Unfortunately, mast cell tumors are invasive and can spread.1 Knowing about this type of cancer can help you determine when it’s time to seek veterinary care and what to expect if your dog is diagnosed with cancer. 

Benign and malignant skin lumps and bumps that dogs experience

What Is A Mast Cell Tumor?

Mast cells are white blood cells found in the connective tissues throughout the body. They can be under the skin, near blood and lymph vessels, and in nerves, the lungs, and the intestines.2 Mast cells contain histamines, which are chemicals that are responsible for allergic reactions. They’re an important part of your dog’s immune system, determining how it will respond to bacteria, parasites, and allergens.2 

A mast cell tumor occurs when the division of the mast cells cannot be controlled, forming lumps in the skin or other parts of the body, also known as tumors.3 This type of skin cancer can spread to lymph nodes, bones, and internal organs and may occur in any dog regardless of age, sex, or breed. However, some breeds are at a higher risk. 

All mast cell tumors are cancerous, but because mast cell tumors can look like anything from skin tags to insect bites, they are often mistaken for other types of skin issues.4 Therefore, pet parents can’t identify this type of cancer without the help of a vet. 

But what causes mast cell tumors in dogs? Unfortunately, the cause of mast cell tumors in dogs is unknown, but experts speculate genetic and environmental factors cause it.4 

Mast cell tumors can develop because genetic, hereditary, and environmental reasons

What Does A Mast Cell Tumor Look Like?

Mast cell tumors vary in appearance, and lumps are common on dogs. If you notice one while petting or grooming your dog, do not ignore it. Always alert your vet when you find a new lump on your dog, especially if it’s changing in appearance or size. These tumors can be found anywhere on a dog’s body, especially their skin. They also vary in appearance and may be raised or deep and feel either soft or firm.1

Dogs may have one or several tumors, which can vary in size, making them look like bug bites because they’re itchy and inflamed. However, unlike bug bites, mast cell tumors change in size and don’t go away on their own.3

Because MCTs resemble other, non-dangerous lumps and bumps, pet parents may not notice them. However, most dogs only develop one MCT, so any lumps or bumps should be checked by your vet as soon as possible. 

Symptoms Of Mast Cell Tumors In Dogs

Similar to other types of cancer, like dog lymphoma, dogs suffering from mast cell tumors will likely experience some symptoms. Unfortunately, the signs of mast cell tumors in dogs are unpredictable, and some dogs don’t have symptoms other than skin lumps. 

If your dog has a lump, don’t touch it too much because it can be susceptible to degranulation, leading to further complications.Degranulation occurs when the fluid in mast cells is released into the bloodstream, causing further illnesses like stomach bleeding, swelling and redness of the tumor, dangerous drops in blood pressure, and shock.6 

While symptoms of dog mast cell tumors vary, possible signs include:

  • Scratching/licking/biting the surrounding skin
  • Vomiting
  • Diarrhea
  • Bloody stool
  • Lethargy
  • Decreased appetite
  • Swollen lymph nodes 3

Of course, the main symptom of mast cell tumors in dogs is the newly formed skin mass or a mass that has changed in size or appearance.4 Therefore, your dog may appear healthy even when they’re not. A vet should examine lumps and bumps on dogs as soon as possible to determine what they are and how to treat them. Since lumps can be cancerous, taking your dog to the vet early is crucial; the earlier they can be diagnosed, the earlier treatment can begin, and the less likely the tumor will metastasize if it hasn’t already. 

Dog Breeds More Susceptible To Mast Cell Tumors

Mast cell tumors make up around 20% of all dog skin tumors. Although the cause of MCTs is unknown and they can develop for various reasons, several breeds are at a higher risk. A few breeds predisposed to MCTs are:

  • Golden retrievers
  • Labrador retrievers
  • Pugs
  • Boxers 
  • Boston Terriers1

20% of all dog skin tumors are mast cell tumors

Diagnosing Mast Cell Tumors In Dogs

If you find a lump on your dog, they should be taken to your vet as soon as possible for an examination. Since it can be difficult for vets to identify the type of lump on a dog from visual examination alone, they may employ several techniques for diagnosing mast cell tumors in dogs. 

The first step to diagnosing dog mast cell tumors is fine needle aspiration, in which a vet will use a small needle to draw cells from the lump. Looking at the cells under a microscope, they can determine the types of cells in the lump.3 While this method makes it easy to identify MCTs, your vet may also use other diagnostic tests, including:

  • Sampling the local lymph nodes
  • Abdominal ultrasound
  • Bone marrow cytology3

Once your vet has diagnosed your dog with a mast cell tumor, they must determine the grade of the tumor to gauge how aggressive the cancer is.3 Low-grade tumors are less aggressive and less likely to metastasize (spread).4 

Depending on your dog’s symptoms, the vet may also perform CT scans or ultrasounds to look for signs of internal tumors. Additionally, they will perform a comprehensive health exam that consists of a physical exam, blood work, and urine testing. 

Treating Mast Cell Tumors In Dogs

There are many treatment options for dog mast cell tumors. However, the best option for your dog depends on how aggressive the tumor is and location. Most dogs will have the tumor surgically removed, especially those with multiple tumors.3 About 80% of mast cell tumors are low to intermediate grade and won’t return after surgical removal.4A pathologist will test the removed tumor to determine whether or not it was removed properly. 

If the tumor is not successfully removed, vets will seek alternative treatment options, including a second surgery or radiation therapy.3 Depending on the grade of the tumor, they may suggest chemotherapy to prevent the regrowth of the tumor, but it’s not as effective as other forms of treatment. 

Instead, chemotherapy is used for dogs with tumors that have spread or metastasized no matter how aggressive they are and dogs with internal tumors that can’t be surgically removed.3 

Luckily, MCTs are very treatable, but the prognosis depends on the tumor grade and treatment effectiveness. For example, if the tumor has metastasized, it will require more aggressive treatment, which may not cure the cancer.3 

After successful surgical removal, most dogs need a few weeks of rest, medications, and wearing an e-collar before the sutures are removed.4 However, your dog should continue to be monitored throughout their lives because they can get more tumors. In addition, tumors can grow back depending on their grade. 

Treatment for MCTs in dogs is necessary; without it, dogs may only survive for around four months. Prognosis depends on the grade of tumor and treatment type, but the earlier you take your dog to the vet for evaluation, the better their odds of survival. 

Mast Cell Tumors In Dogs: FAQs

How fast do mast cell tumors spread in dogs?

Some mast cells don’t spread, but they can grow slowly over time or grow quickly. Since MCT is an aggressive form of cancer, most dogs die within a few months if they’re not treated. 

How serious are mast cell tumors?

Mast cell tumors are incredibly serious because they’re an aggressive form of cancer. However, they’re also one of the most treatable. Unfortunately, left untreated, mast cell tumors can have serious consequences, including shock, and may progress into higher-grade tumors that lead to death. Therefore, if you spot a new growth on your dog, or a growth they’ve had for a while starts to change in size or appearance, they should be evaluated by a vet. Even if your dog doesn’t have MCT, you’ll give yourself peace of mind by having a professional tell you if your dog needs treatment. 

Can a dog recover from a mast cell tumor?

Dogs can recover from mast cell tumors if they get the proper treatment. High-grade tumors are more aggressive and require more aggressive treatment. Your dog’s prognosis depends on the grade of the tumor and the effectiveness of treatment. Dogs that aren’t treated for their mast cell tumor can die within a few months due to degranulation and metastasis. 

Dogs with low-grade tumors with successful surgical removal have a good prognosis. However, tumors that have spread require chemotherapy and have lower survival rates. Since this type of cancer can spread quickly, you should take your dog to the vet if they have a new lump or bump, even if you believe it’s something minor. 

Boston Terrier leaning on a picnic table and looking to the side

Final Notes

The earlier mast cell tumors in dogs are diagnosed and treated, the better the prognosis. While MCTs in dogs is a highly treatable type of cancer, early diagnosis is key to ensuring your dog's survival. Don’t ignore new lumps and bumps, even if you’ve had other bumps evaluated by your vet before. New lumps could be MCT, so always have them evaluated by a vet as soon as possible to improve your dog’s chances of survival. 

Wondering about lumps and bumps on your dog’s skin? Use Dutch’s online vet care services to evaluate, diagnose, and treat lumps on dogs. Our vets are available when others aren’t, providing you with better access to pet care when you need it most. Try Dutch today. 



  1. Garrett, Laura D. “Canine Mast Cell Tumors: Diagnosis, Treatment, and Prognosis.” Veterinary Medicine (Auckland, N.Z.), Dove, 12 Aug. 2014,

  2. “NCI Dictionary of Cancer Terms.” National Cancer Institute,

  3. “Canine Mast Cell Tumors.” The College of Veterinary Medicine at Michigan State University,

  4. “Mast Cell Tumor (Mastocytoma) in Dogs.” PetMD,

  5. “Degranulation.” Degranulation - an Overview | ScienceDirect Topics,

  6. Mast Cell Tumors in Dogs. Ryan Veterinary Hospital of the University of Pennsylvania,

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