Mast Cell Tumors, Canine - contd.
Mast cell tumors are among the most common tumors in dogs. They may occur anywhere in the body, and may be benign or malignant. Mast cell tumors, or MCTs, arise from mast cells, which are normal components of the body. Mast cells originate in the bone marrow and migrate to various locations throughout the body, especially in connective and vascular tissues. Mast cells in dogs normally contain histamine and heparin; these substances play a significant role in the inflammatory response to various disease processes, and in wound healing. Histamine released from mast cell tumors causes some of the signs and symptoms of the disease, and may produce secondary disease as well.
How a mast cell or clone of mast cells becomes an MCT is not known. A genetic link is strongly suggested by the relatively frequent occurrence of mast cell tumors in certain breeds of dog, especially Boxers. Chronic inflammation has been suspected as a facilitator of tumor formation; mast cells tend to concentrate in chronically inflamed tissues. Viruses have been proposed as a cause based on an experimental model. However, no supporting evidence for a viral cause has been identified thus far.
In dogs, the skin and subcutaneous tissues are the most common locations for mast cell tumors. Up to one out of five MCTs occur in the skin. These tumors are classified as the cutaneous form. These masses arise in or beneath the skin, and vary in size. Palpation of the tumor may result in the release of histamine, which causes local redness, hives and itchiness of the skin. Although MCTs located elsewhere are more likely to be malignant, cancerous, cutaneous-form mast cell tumors are not uncommon in dogs.
The systemic form of MCTs -- called mastocytosis -- is a second class of these neoplasms, or growths. These mast cell tumors form in organs and other deep tissues of the dog, including the intestines, spleen, lymphatics, and other tissues of the recticuloendothelial system. The systemic forms of MCTs are more likely to produce signs and symptoms of systemic disease; gastrointestinal tumors may produce ulceration of the stomach and duodenum, and associated symptoms of diarrhea, vomiting, anorexia, and melena.
Mast cell tumors in the preputial, perineal, and inguinal regions tend to demonstrate more malignant behavior. MCTs can metastasize, or spread, to any part of the dog's body. However, metastatic mast cells will most likely spread to the regional lymph nodes, spleen, and liver. Spread to the lungs is not common.
