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Mast cells are cells that reside in the connective tissues, especially those vessels and nerves that are closest to the external surfaces (e.g., skin, lungs, nose, mouth). Their primary functions include defense against parasitic infestations, tissue repair, and the formation of new blood vessels (angiogenesis). They are also associated with allergic reactions, since they contain several types of dark granules made up of various chemicals, including histamine and heparin, serving biologically to modify immune reactions and inflammation. Mast cells are derived from the bone marrow, and can be found in various tissues throughout the body.
Mast cell tumors (or mastocytomas) are graded according to their location in the skin, presence of inflammation, and how well they are differentiated. Grade 1 cells are well differentiated with a low potential for metastasis; Grade 2 cells are intermediately differentiated with a potential for locally invasive metastasis; and Grade 3 cells are poorly differentiated or undifferentiated with a high potential for metastasis. Differentiation is a determination of how much a particular tumor cell looks like a normal cell; the more differentiated, the more like the normal cell. In general, the more differentiated the mast cell tumor is, the better the prognosis is.
Boxers, bulldogs, pugs, and Boston terriers appear to be more susceptible to mast cell tumors than other breeds. The mean age for the development of this condition is eight years in dogs, though it has been reported in animals less than one year of age.
Symptoms and Types
Symptoms may be dependent on the location and grade of the tumor.
Tumor on the skin or under the skin (subcutaneous), may have been present for days to months
Tumor may appear to fluctuate in size
Recent rapid growth after months of inactive or subtle growth is common
Recent onset of redness and fluid build-up is most common with high-grade skin and subcutaneous tumors
Extremely variable; may mimic or resemble other types of skin or subcutaneous tumors (benign and cancer); may resemble an insect bite, wart, or allergic reaction
Primarily occurs as a single skin mass or subcutaneous mass, but may have multiple masses located throughout the body
Approximately 50 percent of all mast cell tumors are located on the trunk and perineum (the area between the anus and vulva in females, or the anus and scrotum in males); 40 percent are found on the extremities, such as the paw; and 10 percent are found on the head and neck region
Lymph nodes may be enlarged around the area of the tumor and may develop when a high-grade tumor spreads to the lymph nodes
Masses may be itchy or inflamed due to the higher level of histamines in the tumor
Enlarged liver and enlarged spleen are characteristic of wide-spread mast cell cancer
Symptoms are also dependent on the stage of the disease:
Stage 1 is characterized by a single tumor without metastasis
Stage 2 is characterized by a single tumor with metastasis into the surrounding lymph nodes
Stage 3 is characterized by multiple skin tumors, or by a large tumor that has invaded subcutaneously
Stage 4 is characterized by the presence of a tumor, with metastasis to an organ or wide spread mast cell presence in the blood
You will need to give a thorough history of your dog's health, including a background history of symptoms. The history you provide may give your veterinarian clues as to which organs are being affected.
The most important preliminary diagnostic test will be an examination of the cells taken from one of the tumors. This will be performed with a fine needle aspirate and will determine the presence of an abnormal amount of mast cells in the blood. A surgical tissue biopsy will be necessary for definitive identification of both the grade of the cells occupying the mass, and the stage the disease is in. Additionally, your veterinarian may examine a sample from a draining lymph node, from the bone marrow, or from the kidney and spleen. X-ray and ultrasound images of the chest and abdomen will also be component of determining the exact location and stage of the tumor's development.