mast cell leukemia (mast sel loo-KEE-mee-uh)
A very rare, aggressive (fast-growing) type of leukemia in which too many immature mast cells (a type of white blood cell) are found in the bone marrow and blood. It may develop in patients who have a buildup of mast cells caused by a condition called aggressive systemic mastocytosis. Mast cell leukemia is hard to treat and has a poor prognosis (outcome). It is a type of acute myeloid leukemia.