Mast Cell Tumors & Skin Cancer in Dogs

Skin tumors are the most common tumors found in dogs. Sadly, their fur isn’t the armor we all thought it to be. But luckily, if caught early, the disease can often be treated successfully. While there are several different forms of skin cancer in dogs, mast cell tumors are one of the most common.

What are Mast Cell Tumors?

Canine mast cell tumors (MCTs) are the most common skin tumors on dogs. Roughly one third of all tumors in dogs are skin tumors, and up to 20 percent of those are mast cell tumors.

Mast cell tumors are cancerous proliferations of mast cells. Mast cells reside mainly in the skin, but are also plentiful in other parts of the body, such as the intestines and lungs. These cells fight against parasitic infection, help repair tissue, form new blood vessels, and are also connected to allergic reactions. Mast cell tumors invade and impair these functions and affect heart rate, blood pressure and more.

Although mostly found in the skin, MCTs can and will spread throughout the body, commonly appearing in the spleen, liver, and bone marrow. The danger from MCTs comes not from the tumors themselves, but the secondary damage caused by the release of chemicals that they produce.

Mast cell tumors are sometimes referred to as “the great imposters,” because there is no way to definitively identify them without a biopsy and pathology report. Although they are most frequently found on the trunks and limbs of dogs, MCTs can vary widely in their size, shape, appearance, texture, and location. Not only are they difficult to recognize, but it is also nearly impossible to predict their course. They can also range in their threat level from completely benign nodules to aggressively malignant.

Since MCTs are so common in canines, it’s important for any dog owner to have a basic understanding of what they are and the threat level they pose to your canine.

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Diagnosis of Mast Cell Tumors in Dogs

The most common way that MCT tumors are detected is using a fine needle aspirate and cytology. Fine needle aspiration involves inserting a fine needle into the tumor to remove a few of the cells in the tumor. From there the cells are sent to cytology to be inspected for a diagnosis.

What do the Grades and Stages of MCTs mean?

Canine mast cell tumors can be relatively harmless, or in more advanced cases, pose a serious threat to the well-being of your dog. To help set a treatment plan, they are measured on a scale of grades and stages.

The “grade” refers to how likely the MCT is to be malignant. The higher the grade, the more serious the tumor, with grade I being the lowest and grade III being the highest.

  • Grade I: Occur in the skin and are considered benign. Although they may be large and difficult to remove, they tend to not spread to other areas of the body. Most MCTs are Grade I.
  • Grade II: Extend below the skin into the subcutaneous tissues. Their cells show some characteristics of malignancy and their response to treatment can be unpredictable.
  • Grade III: Invade areas deep below the skin, are very aggressive, and require more involved treatment.

The “stage” measures how much a tumor has spread in the dog’s body. After surgery, staging is measured based on how many tumors were present, how they affected the underlying lymph nodes, and if all of the tumor was removed.

  • Stage 0: One tumor in the skin incompletely removed, with no lymph node involvement.
  • Stage I: One tumor in the skin, with no lymph node involvement.
  • Stage II: One tumor in the skin with lymph node involvement
  • Stage III: Multiple large, deep skin tumors, with or without lymph node involvement
  • Stage IV: One or more tumors with metastasis in the skin with lymph node involvement. This stage is further divided into those that have no other signs (substage a) and those that have some other clinical symptoms (substage b).

What Causes Mast Cell Tumors in Dogs?

Unfortunately, no one knows for sure what causes mast cell tumors in dogs. But ongoing studies are looking into a few potential causes.

Cancer-causing or not, the sun can certainly harm your dog. Dog noses and, in some breeds, ears, are exposed to the sun’s harmful rays. Also, since they breathe through their mouths, their oral cavity is exposed as well. So don’t think of their fur as armor!

Speaking of which, take your dog’s coat into consideration as well. Dogs with light-colored or thin coats are more susceptible to sun damage, and some studies suggest that MCT development may be associated with golden or red coats. There may also be environmental factors, pre-existing illnesses, and other contributors.

What is known, however, is that there seems to be a genetic component, since some breeds seem to be more susceptible to MCTs than others. These include Boxers, Bulldogs, Pugs, Boston Terriers, Beagles, Bullmastiffs, Bull Terriers, Dachshunds, English Setters, Fox Terriers, Golden Retrievers, Labrador Retrievers, Schnauzers, American Staffordshire Terriers, and Weimaraners. Of these, Boxers are at the highest risk, yet mast cell tumors are often not as aggressive in this breed.

As with any cancer, older dogs are most likely to develop MCTs, with the average age being eight to nine.

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What are the signs and symptoms of MCTs?

Mast cell cancer is most commonly found in tumor form. Mast cell tumors may appear alone or in groups and can be found almost anywhere on a dog’s body. They can be on the surface of the skin or under the skin (subcutaneous). You won’t know an MCT tumor by look or feel, but there are common signs to look for.

At least half of skin tumors on dogs are found in or under the skin on the trunk of a dog’s body. The vast majority of the rest are found on their legs, especially the hind limbs. They are rarely found on the head and neck, and even less commonly found in tissues other than the skin, although it does happen.

One characteristic MCTs do seem to have is a tendency to change in size, due to a process called degranulation. Mast cells contain granules that release various potentially harmful substances into the bloodstream when tumors become agitated, resulting in the waning of these granules, and thus a temporary decrease in the overall size of the tumor. The tumor can get larger or smaller even on a daily basis. A recent rapid growth after months of inactivity is also common. High-gr