Mast Cell Tumor (Mastocytoma) in Dogs

A mastocytoma or mast cell tumor is a type of round-cell tumor consisting of mast cells. Mast cell tumors are cancerous (unregulated/abnormal) growths of mast cells. They arise most commonly in the skin although can appear anywhere on or in the body. 

Mast cells originate in the bone marrow and reside in connective tissues of the body. They contain many granules rich in histamine and heparin and are specialized in dealing with allergic reactions. Mast cell, however, also play an important part in defense against parasitic infestations, tissue repair, and the formation of new blood vessels (angiogenesis). 

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.

Mast cell tumors are known as "the great pretenders" for their ability to appear in different forms. They can be outside the skin, under the skin or somewhere else inside the body, appearing like a wart or a soft subcutaneous lump. They can be red, pink or gray among other colors. They can have hairs growing on top of them or be hairless. They can be itchy or never seem to bother the dog at all. They may be isolated to one area or they can spore out and spread all over the body. 

Certain breeds are predisposed to this condition which include Boxers, bulldogs, pugs, and Boston terriers.

Symptoms of mast cell tumors depend on location and grade of tumor. Common symptoms include

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

Vomiting, loss of appetite, and/or diarrhea may occur, depending on the stage of the disease

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

Leave a Comment