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Mast Cell Tumors

Normal mast cells occur in the skin and other tissues, such as the intestines and respiratory tract. They are part of the immune system and contain large amounts of histamine, heparin and enzymes that break down protein. These substances can be toxic to foreign invaders, such as parasites, and are released when the mast cell is triggered by the immune system.

Mast cell tumors (MCTs) are a common skin tumor in dogs, and they account for about 20 percent of all skin tumors in dogs. For most dogs, the underlying cause promoting tumor development is unknown. Some MCTs are easily removed without the development of any further problems, and others can lead to life-threatening disease.


The appearance of MCTs can vary, ranging from benign-looking lumps to angry-looking or ulcerated lumps, masses with a stalk or focal thickenings in the skin. MCTs may change quickly in size because of reactions around the mass. Some dogs with MCTs may have signs of systemic disease.

Breeds at Risk

  • Breeds with short, wide heads
  • Golden Retrievers
  •  Boston Terriers
  • Labrador Retrievers
  • Boxers
  • Shar-peis
  • English Bulldogs
  • Many other breeds


Proper identification and treatment are very important in controlling these tumors. MCTs occur in a variety of shapes and locations, so a biopsy or needle aspiration (collecting tumor cells through a needle and examining them under the microscope) is necessary to properly identify a growth as an MCT.

Once the tumor is removed, it is graded by a pathologist and classified based on how it is expected to behave. Grade 1 is benign, grade 2 is intermediate and grade 3 is highly malignant. This process helps determine the prognosis and what type of further treatment may be necessary. Clinical Staging is based on how many tumors were present, whether lymph nodes or other tissues are involved and whether the entire tumor was removed.

  •  Stage 0: One tumor in the skin incompletely removed with no lymph node involvement
  •  Stage I: One tumor in the skin with no lymph node involvement
  •  Stage II: One tumor in the skin with lymph node involvement
  •  Stage III: Multiple large, deep skin tumors with or without lymph node involvement
  • Stage IV: One or more tumors with metastasis to the skin and lymph node involvement. This stage is subdivided into substage a, in which there are no other signs, and substage b, in which there are debilitating clinical signs such as vomiting, diarrhea or hemorrhage.


Surgical removal is the mainstay of treatment for MCTs. The removal of normal tissue surrounding the tumor is required to ensure that the cancer is completely removed.

If an MCT cannot be completely removed, radiation therapy is often effective. Or a more aggressive second surgery is possible for some dogs. MCTs are notoriously unpredictable in their response to chemotherapy, so this treatment is usually reserved for dogs with aggressive tumors.

Cellular damage or surgical removal of an MCT may release histamine, heparin and enzymes. The release of large amounts of these substances into the body can significantly affect heart rate, blood pressure and other body functions. To alleviate these clinical problems, some dogs will be treated with medications, such as prednisone, antihistamines or antacids, which help control the secondary effects of MCTs.


Complete surgical removal often cures low to intermediate-grade tumors. Even incompletely removed high-grade tumors treated with radiation therapy have an excellent prognosis, and about 90 to 95 percent of dogs with MCTs have no recurrence within three years of receiving radiation therapy. However, high-grade tumors often spread and result in death.

Current Research

Click here to find out about Morris Animal Foundation–funded research into mast cell cancer.

Morris Animal Foundation urges pet owners not to implement any suggestions on animal health treatments without prior consultation with their licensed veterinarian. If your pet is experiencing health issues, contact your licensed veterinarian. The Foundation funds research to enhance medical options available to veterinary professionals and their patients.