By Dr. Alison Book, DACVIM Oncology

Mast cell tumors in dogs:

Mast cell tumors are among the most common tumors found on and under the skin in dogs. As oncologists, we give advice to owners and other veterinarians about these tumors every week! Moreover, mast cell tumors can look just like other skin tumors (even benign fatty tumors called “lipomas”) but they are not the same and can behave quite differently. As a pet owner, it pays to know about mast cell tumors and what to do if your dog is diagnosed with one.

What is a mast cell tumor (MCT)?

A mast cell tumor is a cancerous overgrowth of a normal cell in the body called the mast cell. Mast cells are part of the immune system. They are produced in the bone marrow and become mature cells where they live in the tissues. As a part of the immune system, they are best known for their role in allergic reactions but also participate in the body’s defense against infections. Mast cells can release several chemicals when stimulated that have an effect on the body. Among these are histamine, heparin, seratonin, prostaglandins and other enzymes. These chemicals are normally contained within the mast cell’s granules.

Why do MCTs cause problems?

Mast cell tumors have a wide range of behavior in dogs and not all MCTs will cause all of the following problems. We will talk about how we attempt to predict the behavior of MCTs in dogs in later posts.

  1. As the mast cell tumor grows, it can grow into and disturb surrounding tissues, resulting in discomfort. The ability to surgically remove a MCT often depends on the location and the degree of tissue invasion.
  2. Metastasis (spread) to other parts of the body can affect the function of the organs to which it has spread and also contribute to signs of sickness such as decreased appetite, vomiting, diarrhea, lethargy, etc.
  3. Release of chemicals such as histamine and heparin can cause problems including stomach ulcers, internal bleeding, local swelling and redness, and a range of allergic manifestations such as itching, hives, or changes in breathing.

Where do MCTs occur in dogs?

MCTs are one of the most common skin tumors in dogs and are commonly seen as solitary lumps or masses in or underneath the skin. Occasionally dogs can have multiple masses. MCTs on the skin can look like just about anything and can change in size. You can’t tell that a tumor is an MCT just by looking at it, which is why it is important to have any new lumps on your dog tested regardless of what they look or feel like. Other locations are less common but can include the conjunctiva, nasal cavity, oral cavity, gastrointestinal tract or urogenital tract.

Which types of dogs get MCTs?

MCTs are primarily found in older dogs but they have been reported in dogs as young as three weeks of age. Breeds frequently diagnosed with MCT include Boxers, Boston terriers, Bulldogs, Pugs and Labrador retrievers, but most are reported in mixed breed dogs. Other breeds that appear to develop MCTs more frequently include the Weimaraner, Viszla, Shar-Pei, Cocker Spaniels, Beagles, Schnauzers and Maltese. Boxers and pugs tend to have MCTs with a good prognosis.

How do you diagnose a mast cell tumor?

In most cases, MCTs are easily diagnosed by examination of a fine-needle aspirate of the mass. This procedure is usually painless and involves using a very small needle to extract cells from the tumor and then looking at those cells under a microscope. This is always recommended before surgical removal of a lump because it affects how much normal tissue should be removed around the tumor. In rare circumstances, a biopsy may also be recommended before surgery is performed.

Stay tuned for more information about mast cell tumors in dogs in Part II.