| − | mastocytoma (MAS-toh-sy-TOH-muh) is a growth or lump of mast cells (a type of white blood cell). Mast cell tumors can involve the skin, subcutaneous tissue, and muscle tissue. Also called mast cell tumor. | + | mastocytoma (MAS-toh-sy-TOH-muh) was a growth or lump of mast cells (a type of white blood cell). Mast cell tumors can involve the skin, subcutaneous tissue, and muscle tissue. Also called mast cell tumor. |