What are Mast Cell Tumors in Dogs?

What are Mast Cell Tumors in Dogs?
By Dr. Ed Mapes
Stonebridge Animal Hospital
McKinney, Texas

What are Mast Cell Tumors in Dogs?

Mast cells are involved in the body’s response to allergens and inflammation.They are the source of histamine and proteolytic enzymes (substances that degrade foreign proteins), and also stimulate angiogenesis, or the generation of new blood vessels.They can be found in the skin, intestinal tract, and the lungs; but primary tumors develop most commonly in the skin and may then spread to other tissues.

Mast cell tumors (MST) are now the most common skin tumors seen in dogs, and are among the most frequent of all tumors seen.MST can spread to the liver, lungs, intestinal tract, spleen, and bone marrow, but primary tumors in these areas are relatively rare.

Symptoms of Mast Cell Tumors in Dogs

Mast Cell Tumors in Dogs

Skin MST can form rapidly or may take longer to develop.They are generally a raised lesion on or just beneath the skin, and hair on the masses is often shed.They are often reddened, and can be ulcerated and bleed on occasion.The tumors don’t seem to bother the dogs much, because they rarely chew or scratch at the masses.

Diagnosing Mast Cell Tumors in Dogs

Our diagnostics start with blood and urine samples to determine if any body systems are affected by the masses, and it’s always wise to perform chest radiographs on any patient suspected of having tumors, since tumor spread is often seen in the lungs.Other testing when indicated can include ultrasound imaging, aspiration of swollen lymph nodes for lab examination, and bone marrow aspiration.

If the testing indicates that patients are fit for surgery, removal of the masses is almost always indicated.Resection of masses suspicious of being MST involves taking at least 2 cm of skin and underlying tissue along with the tumor to remove all associated mast cells in the region.This wide resection may not be possible in some cases, as when the tumor appears on a leg (there is not enough spare skin to suture the edges together after removal of the tumor).We may also remove local lymph nodes that are swollen.

True diagnosis of any tumor is best done after removal of the tumor and biopsy of the specimen.Biopsy results identify the type of tumor involved.Pathologists are also able to stage the tumor, in other words tell us its degree of malignancy, likelihood for re-growth and spreading to other tissues, and the likely prognosis for the patient. This information is invaluable for planning possible other treatment options such as further surgery, radiation or chemotherapy.

The National Canine Cancer Foundation classifies Mast Cell Tumors according to grade and stage:

Classification of Mast Cell Tumors in Dogs

Grading Mast Cell Tumors

  • Grade I: Occur in the skin and are considered non-malignant. Although they may be large and difficult to remove, they do not spread to other areas of the body. Most mast cell tumors belong to Grade I.
  • Grade II: Found below the skin into the subcutaneous tissues. Their cells show some characteristics of malignancy and their response to treatment can be unpredictable.
  • Grade III: Originate in areas deep below the skin, are very aggressive, and require extensive treatment.

Staging Mast Cell Tumors

Mast cell tumors should be staged because it gives us an idea about how they have metastasized in the body. A tumor is staged after it is surgically excised and examined, along with the surrounding lymph nodes. The factors on which staging depends include the number of tumors present and lymph node involvement.

  • 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).

Treatment of Mast Cell Tumors in Dogs

Early tumor detection removal provides the greatest odds of successful treatment.The prognosis for cutaneous MCT is dependent upon several factors, including tumor grade, tumor stage, and ability to perform complete surgical excision. Tumors that are lower in grade and stage are not likely to spread throughout the body, but can have some chance of regrowth.No further treatment is usually indicated for these tumors.

Prognosis of Mast Cell Tumors in Dogs

Grade III and Stage II, III, and IV are more likely to metastasize to other tissues and recommendations are usually made for further treatment. For tumors that have spread distantly (beyond the lymph nodes), or those that occur in locations other than the skin (such as the gastrointestinal tract, spleen or liver), the prognosis is generally poor. The goal of treatment for these patients is to maintain a good quality of life for as long as possible with medications to relieve any discomfort or side effects of treatments given.

Comments are closed.