Mast Cell Leukemia

Comments:
Mast Cell Leukemia (MCL): The bone marrow is extremely hypercellular with diffuse-compact clusters of mast cells and depletion of normal hematopoietic precursors and fat cells. The mast cells make up >20% of nucleated cells in bone marrow smears and >10% of leukocytes circulating in peripheral blood. If circulating mast constitute less than 10% of leukocytes, it is labeled as aleukemic variant of MCL.

The mast cells in bone marrow and peripheral blood are highly atypical. The appearance ranges from mature cells with strong metachromatic granules to highly atypical blast-like cells with scant or no metachromatic granules.

Mast cells express chloroacetate esterase, tryptase, and KIT and lack expression of 2D7 and BB1 (positive in basophils). CD25 and CD2 positivity is variable. KIT mutation (D816V) is seen in about 50% of cases.

Differential diagnosis of MCL includes myeloid leukemias exhibiting mast cell differentiation, such as tryptase-positive acute myeloid leukemia, myelomastocytic leukemia, primary basophilic leukemia, and CML with prominent basophilic differentiation.