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 metastasis However, 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 blood X-rays, ultrasound, or lymph node, bone marrow, or organ biopsies may be necessary to stage the disease.
Mastocytoma
Oncocytoma
Histiocytoma
Pineocytoma
Renal oncocytoma
Melanocytoma
Pleomorphic adenoma
Angiomyolipoma