Mast cells are part of the body’s immune system, and participate to fight off parasites that may attack the respiratory or intestinal tract or the skin. When the body detects an invader, the mast ells product an antibody called IgE. The IgE covers the mast cell and “grab” the antigen, then releases a chemical harmful to the parasite. Mast cells also play a role in allergic reactions.

Mast cell tumours, also called mastocytomas, MCTs, or mast cell sarcomas, are accumulations of mast cells that become cancerous. About half of all mast cell tumours are malignant, and can spread rapidly throughout the lymph system and throughout the body. Mastocytomas are one of the most common cancers in dogs.

The exact cause of mast cell tumours is unknown, although a genetic component is suspected. Some breeds tend to be more prone than others. There is a possibility that this type of cancer is associated with the red/golden coat colour, and in dogs that suffer from immune over-stimulation in dogs with inflammatory or allergic issues. There may be environmental causes.

Because no one really knows what causes MCTs, there is really no way to prevent them. The only thing to do is to check your pet regularly, and have a vet look at all suspicious lumps or bumps. Older dogs are more prone to this type of cancer; dogs around seven or eight years old seem to be at higher risk.

Usually, mastocytomas appears as a single mass or lump, but they can show in clusters grouped together or spaced out over the body. Dogs with MCTs generally don’t act ill, but owners may notice:

  1. a hairless, red, itchy, ulcerated, or multi-nodular skin mass;
  2. bumps or lumps on or under the skin on the legs, around the genital and anus area, the torso, or underbelly;
  3. a red, circular lumps or masses that are soft on the outside but hard on the inside;
  4. a skin lump that changes in size, shows up suddenly, or suddenly changes in appearance; or
  5. signs of gastric issues like diarrhea, weight loss, vomiting, or bloody stool.

Signs of MCT are similar to other conditions, so any lump, bump, or suspicious appearance of your dog’s skin should be reported immediately to your vet.

Dogs at Risk
Dogs of any gender, breed, or age may develop MCTs, but older dogs (with a mean age of 8 years) do tend to be affected more than younger dogs. Additionally, certain breeds to tend to be at increased risk, such as English Bulldogs, Boxers, Pugs, and other brachycephalic breeds. Others at risk are Labrador Retriever, Staffordshire Bull Terrier, Golden Retriever, Rhodesian Ridgeback, Bernese Mountain Dogs, Shar-Pei, and Jack Russell Terrier.

Testing and Diagnosis
Because some MCTs can look like insect bites, fatty tumours, hives, skin tags, lesions, or weeping sores, most pet owners don’t even suspect there is a problem. Most mast cell tumours are found by veterinarians during routine check-ups.

Mast cell tumours are not usually difficult to diagnose; a smear of cells on a slide will show as bluish-purple when the proper stain is used. In addition to a biopsy, most vets will also run a complete blood count, chemistry panel, urinalysis, and needle aspiration biopsy from the lymph nodes closest to the mass. All of these results can provide a hesitant diagnosis, but an actual diagnosis cannot be done unless the mass is taken and tested.
Once diagnosed, further tests should be done to see if the cancer has spread to other parts of the body. The cancer is also graded and staged:

  1. Grade 1 (benign);
  2. Grade 2 (moderately malignant); or
  3. Grade 3 (very malignant).

Staging refers to the extent to which the cancer has spread at the time it is diagnosed. Stage 1 tumours have clear margins and no spreading. Stage 2 and Stage 3 cancers progressively worse signs of spreading or present as multiple masses. Stage 4 tumours involve systemic metastasis.

Treatment and Prognosis
Treatment may include surgery, chemo, radiation, and/or immunosuppressive steroid treatments. The stage and grade of the tumour(s) will dictate the best treatment options.  Masses located near the scrotum, perineal area, prepuce, ear flaps, toes, oral mucosa, and muzzle tend to be the most aggressive and hard to treat. Tumours in these locations should always be checked for potential metastasis.

Surgical removal of the mass is always recommended, as well as the lymph glands closest to the tumour. Wide surgical margins that include healthy tissue can help to ensure the entire mass was removed. Cancers that are located in a place that makes this impossible will require further treatment such as chemotherapy and/or radiation.

After surgery and other treatment, any new lumps or bumps should be assessed immediately in case of metastasis.

The prognosis depends upon the grade and stage of the cancer when it is diagnosed. Dogs with MCTs in early stages that haven’t spread have excellent prognoses when treated appropriately. Dogs with advanced or multiple MCTs have a guarded to grave prognosis.