If ever there was a tumor that fits the saying: “one size does NOT fit all”, it is mast cell tumors (MCT). These tumors are common, particularly skin tumors, in dogs. You may know a dog that had a mast cell tumor removed with surgery and went on to live many happy years to never hear from the MCT again. On the other end of the spectrum, you may know a dog with an aggressive MCT that recurred and/or metastasized quickly and lived just a few months.
What gives? Why is there so much variation in outcome? If your dog has a MCT, will it be a “good” MCT or a “bad” MCT? Can that be predicted? Why is there a new grading system? Do the new c-kit inhibitors (Palladia and Kinavet) work? In my next series of blogs, I hope to answer many questions about the complicated mast cell tumor.
First, let’s understand what MCT are. Normal mast cells are a type of immune system cell and play an important role in inflammation. While other immune system cells tend to circulate throughout the body, mast cells don’t – once they mature, they take up residence in specific tissues. While they can be found anywhere, many live in tissues that mark the boundary between the outside and internal environments; for example, the skin, the mouth, the digestive tract, the nasal passages, and the lungs.
Mast cells hold structures inside their cell walls called granules, which are like little sacks. The granules are filled with substances, or cytokines, including heparin and histamine, which help the immune system respond to problems. Heparin is a blood thinner which helps defend the body against foreign invaders, and histamine is a chemical that triggers inflammation. Normal mast cells release these substances when prompted by the immune system. In addition to their role in inflammation, mast cells are involved in allergies, anaphylaxis (systemic inflammation), the healing of wounds, and defense against outside pathogens.
Mast cell tumors are cancerous accumulations of mast cells with a malignant potential. MCT is very rarely found in humans, but it is the most common malignant skin tumor in dogs, accounting for 15-20% of all skin tumors. Mast cell tumors first occur in the skin and the subcutaneous tissues beneath the skin. It’s very rare to find MCT in internal organs without a primary skin tumor, but skin tumors can spread to the regional lymph nodes, the spleen, the liver, other places deep in the abdomen, and to the bone marrow.
What causes MCT? Like most tumors, the exact cause is not known. About one-third of dogs have a genetic mutation in a protein called the c-kit oncogene. Unlike skin cancer in humans, which is often associated with sun exposure, studies have found no link between sun exposure and MCT in dogs. But chronic inflammation of the skin may predispose dogs to develop MCT, as can the repeated application of skin irritants.
We also tend to see it more commonly in the following breeds: Boxers, Boston Terriers, Labradors, Golden Retrievers, Pugs, and Staffordshire Bull Terriers. Boxers tend to develop low and intermediate grade tumors.
MCT are one of the most common tumors I see in my oncology practice at the Animal Specialty Center. When should you see an oncologist for MCT? We will pick up there at my next post. In the meantime, remember there is a lot more information about MCT in the Guide.