By Dr. Alison Book, DACVIM Oncology

My dog has a mast cell tumor…now what?

In the first post I discussed mast cell tumor basics, but now I want to get to the heart of your questions about MCTs. In the next two posts, I am going to answer the questions that we are most commonly asked by owners who have had a dog newly diagnosed with a mast cell tumor.

Does my dog have a good or bad mast cell tumor (MCT)?

The grade of a MCT tumor is the best determinant of how “bad” it is, or more specifically, the problems it might cause in your dog. After surgical removal and tissue evaluation by a pathologist, the tumor is assigned a grade. The most commonly used scale has three grades of MCTs. As your dog’s oncologists, it is our job to help you interpret the finer details of the pathology report, but here is a basic explanation:

  • Grade I (low grade): If your dog has a grade 1 tumor, it is usually well-differentiated (looks a lot like a normal mast cell) and rarely metastasizes. Most grade 1 tumors are curable by complete surgical removal.
  • Grade II (intermediate grade): If your dog has a grade 2 tumor, it is usually less well differentiated and extends more deeply into surrounding tissues. Grade 2 tumors can spread to local lymph nodes but only occasionally spread throughout the body.
    • This is the most unpredictable grade and also the most confusing for owners. Some pathologists and oncologists have attempted to divide this group into low grade 2 and high grade 2 based on additional factors. The most compelling of these factors is something called the mitotic index which is a measure of the number of cells actively dividing under the microscope (usually using a cutoff of 5). Most low grade 2 MCTs are also curable by complete surgical removal, but high grade 2 tumors may behave more aggressively and require additional treatment.
  • Grade III (high grade): If your dog has a grade 3 tumor, it is poorly differentiated, may replace the skin and underlying tissues, and has a high rate of spread to lymph nodes, liver, spleen, bone marrow and occasionally other locations. Typically these tumors are not cured, but many dogs can have extended remissions if they are caught early and treated aggressively.

While grade is the strongest predictor of how the tumor is likely to behave, there are some other warning signs that may raise concern:

  • Large size or sudden rapid growth
  • Certain locations (oral, penile sheath, nail bed, and some others)
  • Signs of systemic sickness (vomiting, diarrhea, lethargy, decreased appetite)
  • Severe local signs around the tumor (ulceration, bruising, swelling)
  • Documented metastasis before surgery
  • Regrowth at the same site after surgery

We will help you to understand your dog’s unique situation and what that means for him/her.

What does it mean if my dog has multiple mast cell tumors?

If your dog has multiple mast cell tumors at one time, it does NOT mean your dog will have a worse prognosis. These tumors usually develop independently and are not considered to be spread from another MCT. As long as there are low enough numbers that all the tumors can be completely surgically removed, the prognosis is still dependent on the individual tumor characteristics we discussed above.

What kind of tests should my dog have if he/she is diagnosed with a mast cell tumor?

All dogs that are diagnosed with a mast cell tumor should have












Since we usually don’t know how aggressive the tumor is until after it’s surgically removed, additional staging tests (tests that look for evidence of distant spread) should be performed at the recommendation of your dog’s oncologist or veterinarian.

















Stay tuned for more information about treatment and prognosis in Part III.