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June 11, 2019 – Although many people have never heard of mast cells, we see them in action all the time. Whenever our dogs are stung by a bee or start scratching when pollen counts are high, we’re witnessing mast cells in action.

Mast cells are a type of white blood cell that are key players in allergic reactions. They contain granules that release histamine, heparin and other enzymes involved in allergic reactions and fighting off infectious agents, especially parasites. In some cases, these immune cells can develop into tumors that can be a serious health threat to dogs.

Mast cell tumors (MST) are most commonly found in the skin and subcutaneous tissues and account for 16% to 21% of canine skin tumors. Several breeds are at an increased risk for these types of tumors including:

  • Flat-faced breeds such as Boston terriers, boxers, pugs and English bulldogs
  • Labrador and golden retrievers
  • Cocker spaniels
  • Schnauzers
  • Staffordshire terriers
  • Beagles
  • Rhodesian ridgebacks
  • Weimaraners
  • Shar-peis

Mast cell tumors can take many forms, from small isolated lumps to large ulcerated lesions. Some tumors can be slow growing while others can seem to appear overnight. Because mast cell tumors contain chemicals such as histamine, these masses can sometimes grow or shrink in size, either spontaneously or if they are irritated, due to release of these substances.

These same substances can cause mast cell tumors to be itchy and dogs will sometimes pick, lick or scratch at them. Mast cell tumors also can mimic the appearance of many other skin lumps, including the benign fatty tumors common in dogs, so it’s important for owners to bring any new lumps or bumps to the attention of their veterinarian.

In most cases, it’s easy for your veterinarian to make the diagnosis of a mast cell tumor simply by aspirating the tumor and looking at the cells under a microscope.

Once a diagnosis or mast cell tumor is made, there are several approaches to treating them. Surgery remains the cornerstone of treatment and can be curative if the MCT is low grade (more on this later). The treatment of high-grade mast cell tumors is more difficult and can include local radiation treatment as well as chemotherapy.

High-grade mast cell tumors can spread (metastasize) internally, with the lymph nodes, spleen and liver being the most common sites of metastasis. If your veterinarian suspects high-grade mast cell tumor, it’s important to know if and where tumor cells have spread, which has a major effect on prognosis.

In veterinary medicine, grading systems were developed to try to find an objective way to predict the behavior of mast cell tumors.

The first of these systems, the Patnaik system, was the standard for many years. Some owners will recognize this three grade system if they have a dog diagnosed with mast cell tumors prior to 2011-2012. However, many veterinary oncologists felt that this system fell short when it came to intermediate (or grade 2) tumor prediction.

A two-tier grading system was developed recently and has gained wide acceptance among veterinarians. This system classifies tumors as either low grade or high grade based on several criteria. The new system is easier to understand and studies have shown it to be a good prognostic tool.

Low-grade mast cell tumors often are cured with surgery. If the tumor is large or difficult to remove and there is evidence that cancerous cells were left behind, follow-up local radiation can be very effective in eliminating residual disease. The average survival time for dogs with low-grade mast cell tumors is more than two years.

For high-grade mast cell tumors, the prognosis is grim. Most dogs with high-grade mast cell tumors succumb to their disease within a year of diagnosis even with treatment.

Morris Animal Foundation has been a leader in trying to find new, innovative therapies to treat mast cell tumors in dogs. We recognize that more work needs to be done on all aspects of this disease, from the basic mechanisms of tumor growth and spread to better treatments. Although we’re making steady progress we’ve got a long way to go to unlock the secrets of this common cancer.