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Mast Cell Leukemia (MCL)

Approximately 15% of patients with malignant mastocytosis develop MCL. MCL is a rare condition that is characterized by the presence of large numbers of atypical mast cells in the peripheral blood; more than 10% of the white cells observed on a peripheral blood smear are mast cells. Bone marrow smears would demonstrate greater than 20% mast cells. Patients diagnosed with MCL meet the WHO criteria for systemic mastocytosis, listed on the previous page, and exhibit atypical mast cell infiltration in the bone marrow.
The image on the right is a Wright-stained peripheral blood smear from a patient diagnosed with MCL. Notice the large, heavily granulated mast cells that are indicated by the arrows. These cells are not normally present in the peripheral blood. However, they could be mistaken for basophils because of the heavy, basophilic granulation that extends over the nucleus as well as the cytoplasm. Mast cells are up to twice the size of blood basophils, have more cytoplasm, and have round rather than segmented nuclei.