They are usually solitary, but in about six percent of cases, there are multiple mast cell tumors (especially in Boxers and Pugs).Manipulation of the tumor may result in redness and swelling from release of mast cell granules, also known as Dariers sign, and prolonged local hemorrhage. In rare cases, a highly malignant tumor is present, and signs may include loss of appetite, vomiting, diarrhea, and anemia. The presence of these signs usually indicates mastocytosis, which is the spread of mast cells throughout the body. Release of a large amount of histamine at one time can result in ulceration of the stomach and duodenum (present in up to 25 percent of cases) or disseminated intravascular coagulation. When metastasis does occur, it is usually to the liver, spleen, lymph nodes and bone marrow. Diagnosis
A needle aspiration biopsy of the tumor will typically show a large number of mast cells. This is sufficient to make the diagnosis of a mast cell tumor, although poorly differentiated mast cells may have few granules and thus are difficult to identify. The granules of the mast cell stain blue to dark purple with a Romanowsky stain, and the cells are medium-sized. However, a surgical biopsy is required to find the grade of the tumor. The grade depends on how well the mast cells are differentiated, mitotic activity, location within the skin, invasiveness, and the presence of inflammation or necrosis.
Grade I – well differentiated and mature cells with a low potential for metastasis
Grade II – intermediately differentiated cells with potential for local invasion and moderate metastatic behavior
Grade III – undifferentiated, immature cells with a high potential for metastasisHowever, there is a significant amount of discordance between veterinary pathologists in assigning grades to mast cell tumors due to imprecise criteria.The disease is also staged according to the WHO system:

Stage I - a single skin tumor with no spread to lymph nodes
Stage II - a single skin tumor with spread to lymph nodes in the surrounding area
Stage III - multiple skin tumors or a large tumor invading deep to the skin with or without lymph node involvement
Stage IV – a tumor with metastasis to the spleen, liver, or bone marrow, or with the presence of mast cells in the bloodX-rays, ultrasound, or lymph node, bone marrow, or organ biopsies may be necessary to stage the disease. Treatment and prognosis
Removal of the mast cell tumor through surgery is the treatment of choice. Antihistamines, such as diphenhydramine, are given prior to surgery to protect against the effects of histamine released from the tumor. Wide margins (two to three centimeters) are required because of the tendency for the tumor cells to be spread out around the tumor. If complete removal is not possible due to the size or location, additional treatment, such as radiation therapy or chemotherapy, may be necessary. Prednisone is often used to shrink the remaining tumor portion.