Canine Cancer - Mast Cell Tumors
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| Characteristics / Terms | Hemangiosarcoma | Lymphoma |
| Histiocytomas | Mammary Tumors | Testicular Tumors |
| Mast Cell Tumors | Bone Cancer |
Mast Cell Tumors in Dogs
Canine mast cell tumors account for up to 20% of all skin tumors in dogs. While they often appear small and somewhat insignificant, they can be a very serious form of cancer. Some mast cell tumors are easily removed without the development of any further problems, while others can lead to a life-threatening disease. Proper identification and treatment are very important in controlling these tumors.
Canine Mast Cell Tumors Defined
Mast cells are cells that normally occur in the skin and other tissues, such as the intestines and respiratory tract. They are part of the immune system of the body. They contain large amounts of histamine, heparin, and proteolytic enzymes (enzymes which break down protein). These can be toxic to foreign invaders, such as parasites, and are released when the mast cell is triggered by the immune system.
A mast cell tumor is formed from many of these mast cells. Because of the histamine, heparin, and enzymes present in mast cell tumors, they can create problems when damaged or removed. Large amounts of these substances can be released into the body and have significant effects on heart rate, blood pressure, and other body functions. Sites where the tumors are removed can sometimes refuse to heal and can become difficult to manage.
Dogs At Risk of Developing Mast Cell Tumors
Mast cell tumors can develop in all ages and breeds of dogs. There appears to be a hereditary factor to these tumors most common in Boxers, Boston Terriers, Pugs, English Bulldogs, and other brachiocephalic breeds (those having a short, wide head). Golden Retrievers may also be at increased risk. Most mast cell tumors develop in older dogs, usually those 8.5–9.5 years of age.
The exact cause of mast cell tumors is still unknown. A viral source has been suggested, as well as hereditary and environmental factors. It is quite possible that there are a variety of causes for the development of this tumor. Because this tumor is not found in humans, less research and information has been available for the veterinarian than for tumors that are found in both humans and animals.
Symptoms
The appearance of mast cell tumors can be widely variable. They can be either benign or malignant and can be found on any part of the body. They are found most commonly on the trunk, limbs, and perineal (genital) area, and can be found on the skin or in the underlying tissue. They can be single or multiple and can be smooth, bumpy, or even ulcerated.
Systemic signs, such as vomiting, duodenal ulcers, blood in the stool, and abnormalities in blood clotting, occur in some dogs with mast cell tumors. These signs result from the release of histamines from the active mast cell tumors.
Diagnosis
Since they occur in a variety of shapes and locations, a biopsy is necessary to properly identify a mast cell tumor.
Mast cell tumors are commonly graded and staged (classified) as to how they are expected to behave. This is performed by examining the tumor after it has been removed. The grading and staging helps determine what type of further treatment may be necessary and the prognosis.
Mast cell tumors are "graded" as to how likely they are to be malignant. The higher the grade, the more serious the tumor.
- Grade I: Occur in the skin and are considered benign. Although they may be large and difficult to remove, they tend not to spread to other areas of the body. Most mast cell tumors 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.
In addition to grading mast cell tumors, they are also staged, which is a measurement of how they have spread in the body. A tumor is staged after it is removed and examined, along with the neighboring lymph nodes. Staging is based on how many tumors were present, how involved the lymph node is, and whether the entire 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 subdivided into those that have no other signs (substage a) and those that do have some other clinical signs (substage b).
Prognosis
The prognosis depends primarily upon the grade and stage of the tumor. The lower the grade, the better the prognosis. In addition, dogs with Stage I tumors have the best prognosis, compared to those staged higher. Dogs with tumors on the limbs appear to have the best prognosis, much better than those with tumors in the nail bed, genital areas, muzzle, and mouth. Finally, those dogs with mast cell tumors in the internal organs have the least favorable prognosis.
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