Mast Cell Tumors in Dogs

Mast cells are immune cells in the skin. These immune cells respond to allergies and irritations in the skin and help the skin heal. Sometimes, however, they begin to multiply abnormally to form a tumor. In fact, Mast Cell Tumors (MCT) are the most common malignant skin tumor of dogs.

Mast Cell Tumors can look like anything from a skin tag to a cyst, but most commonly they look like lumps under the skin. Because they are normally used to respond to allergens, these cells contain a lot of histamine. Histamine is released to alert the rest of the immune system to a foreign presence in the body. (Humans often will take ANTIHistamines like benadryl, to stop that allergic response to foreign allergens- like ragweeds, or pollens). When histamine is released in the body it causes swelling. (Humans with asthma for example, can have swelling in their airways because of histamine release). Because of that, mast cell tumors will often swell after being manipulated, and then shrink back down later.

How to Diagnose

Mast cell tumors are diagnosed by collecting cells with a needle from the mass. They have a characteristic appearance under the microscope because we will see hundreds of cells with dark staining granules containing histamine in them. Veterinarians are very concerned to diagnose MCT, because of the potential to be a highly malignant cancer.

So what should we do after a MCT is diagnosed?

Owners must first decide is whether to have your dog staged first, or have surgery done first. Staging determines if the tumor has spread to rest of the body. Basic staging included a full physical exam, blood work, and a urinalysis. Basic staging is required prior to surgery at the Westgate Pet Clinic. More advanced staging includes an abdominal ultrasound, biopsy of local lymph nodes, possibly chest x-rays (although rarely do MCt’s metastasize to the chest), and a bone marrow biopsy. If your pet were found to have metastasis at the time of staging, chemotherapy is typically initiated prior to surgery. Less the 5% of otherwise healthy dogs diagnosed with a MCT for the FIRST time will have metastasis.


It is very important to be aggressive about removal of the MCT during surgery. Wide normal tissue margins around the tumor must be taken. Because of this, even though the MCT itself may be small, the incision left behind will probably be 4-10 inches long depending on where the tumor was removed from.

After the tumor is removed, it will be sent to a pathologist to be graded, and also to make sure we “got it all”. The traditional grading scale, and what we continue to employ most commonly, is grading the tumor on a scale of I to III. The grade helps us predict how aggressive the tumor is going to be in terms of metastasis and local recurrence. The following is predicted metastasis rate based on grade:

  • Grade I: 1-2% chance of metastasis
  • Grade II: 5-22% chance of metastasis
  • Grade III: 55-90% chance of metastasis
  • Didn’t “get it all”: 18-35% chance of recurrance

Recently, this grading scale has been called into question because these grade assignments are based on the subjective opinion of a pathologist. In one clinical trial evaluating the accuracy of our current grading system, several different pathologists were sent the same set of 60 slides. Only 4 of those 60 slides were graded the same by all of the pathologists involved in the study. Because of the subjectively of the grading system, new, objective criteria for how to assess MCt’s are being investigated.

The two new tests available to further grade the malignancy of a MCT are Mitotic Index, and special Immunohistochemistry stains to test for genetic markers on the cells. These advanced tests are still being evaluated, but most oncologists agree that one of these two should be performed in addition to traditional grading.

Follow Up

If your pet’s tumor is a grade I, no further treatment is indicated. If your pet’s tumor is a grade III, it is highly recommended an oncologist be seen for follow-up recommendations.

If your dog’s MCT is a stage II, there is a 5-22% chance of metastasis, a 5% chance of local recurrence, and a 10% chance of developing another MCT somewhere else on the body. Further staging and other potential treatment options would be discussed.

What's Next

  • 1

    Call us or schedule an appointment online.

  • 2

    Meet with a doctor for an initial exam.

  • 3

    Put a plan together for your pet.

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