What Is a Dog Mast Cell Tumor?

Published February 24, 2022
Female veterinarian holding a little schnauzer

A mast cell tumor (MST) is a growth that feels like a lump or raised bump beneath your dog's skin. They are very common, and in dogs they are often malignant -- meaning they are cancerous growths. Hearing the term "cancer" might be frightening. However, they are often not painful for dogs, and treatment is available. When veterinarians diagnose a mast cell tumor, they determine the dog's prognosis and treatment plan based on the tumor's grade, the results of a blood panel, whether total surgical removal is possible, and if cancer has spread to other tissues or organs.

What is a Mast Cell Tumor?

Mast cell tumors are one of the most prevalent malignancies in dogs. They arise from immune system cells known as "mast cells," which are immune system cells that dogs' bodies generate to fight inflammation and allergic reactions. Mast cells are mainly present in places that come into contact with the outside world, such as the skin and the lining of the intestines.

Mast cell tumor on the side of a 3 year old Boxer dog

When mast cells divide abnormally at a fast rate, they may grow into cancerous tumors. They can appear in any shape and firmness, and appear in any part of the body. In the majority of cases, however, they are hard, single, slow-growing lumps under the skin. Mast cell tumors in dogs can produce severe allergic reactions in some situations.

It is important to remember that not all growths or raised lumps are automatically mast cell tumors. Many of these are benign -- meaning they are not cancerous. The only way to be sure is to take your dog to the veterinarian.

Cause of Mast Cell Tumors

Mast cell tumors don't have any identified cause, and it is unknown why MSTs are more often malignant in dogs than in other species, such as cats. Genetics, diet, and environmental factors could play a role. Certain breeds are more prone to develop MCTs, including:

Although these breeds are most prone to developing mast cell tumors, they can affect dogs of any breed.


Symptoms differ based on what organs are affected. Mast cell tumors generally aren't painful for dogs. Typically, owners find these tumors when they notice a lump or bump under their dog's skin. If the malignancy has spread, additional clinical signs may be present:

  • Lack of appetite
  • Lethargic behavior
  • Vomiting
  • Diarrhea
  • Coffee ground-like particles in the vomit

Don't wait for symptoms to show up. If you notice anything abnormal on your dog or under their skin, make an appointment with your veterinarian to have it examined.


Mast cell tumors can be diagnosed by fine-needle aspiration. Don't panic. The procedure involves inserting a very thin needle into tissue to biopsy it, and is similar to your dog receiving a vaccine. During the initial appointment, the veterinarian analyzes the sample and determines a diagnosis. However, the sample will be submitted to a laboratory for confirmation.

If a veterinarian diagnoses a mast cell tumor, they will perform what is known as clinical staging, where the cancer is assigned a stage rating to estimate how far it has spread. The tumor is also assigned a grade from one to three (though newer recommendations suggest assigning MSTs only two grades), with grade-two and -three tumors being the most serious. A veterinarian may recommend the following tests:

  • Routine blood panel
  • X-rays
  • Ultrasound
  • Biopsy of any enlarged local lymph nodes
  • Bone marrow aspiration


Most MSTs are removed surgically. The good news is, 90 percent of MSTs in dogs are considered to be low-grade tumors, and the survival rate after removal is very good. Also, low-grade tumors often do not require chemotherapy. However, higher-grade tumors may require more aggressive therapy. Mast cell tumor treatment and prognosis are determined by:

  • Tumor stage and grade
  • Location of the tumor
  • Organs that are affected

Your veterinarian will assess all of these factors and advise you on the best treatment options and overall prognosis. When possible, surgical excision of the main tumor is nearly always part of the treatment. The vet sends the biopsy to a laboratory for margin examination after it is surgically removed. They are looking to see if they extracted all the cancer cells with the surgery.

Generally, veterinarians only recommend radiation and chemotherapy following surgery if they are not able to surgically remove all the cancer cells.

In the meantime, your vet may recommend your dog take Benadryl, which is classified as an antihistamine because it inhibits the effects of histamine in the body. It essentially aids the body's ability to cope with abnormally high histamine levels. Just as with bothersome allergies, mast cell tumors release excess histamine throughout the body, causing some frustrating discomfort. An antihistamine can help ease your dog's symptoms.

When to See the Vet

Take your dog to the vet if you find any lump or bump that's larger than the size of a pea. In most cases, especially if your dog is over the age of 5, the lump or bump is a benign fatty tumor. However, if it is a malignant MST, the sooner it is treated, the better the prognosis and the easier it is to treat.

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What Is a Dog Mast Cell Tumor?