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9 - Grade/differentiation unknown, not stated, or not applicable
Criteria for systemic mastocytosis met, and bone marrow aspirate shows 20% or more of cells are mast cells or 10% or more mast cells seen in circulating blood, shape of mast cells and nuclei have malignant features. No associated clonal hematologic non-mast cell disease. Aleukemic mast cell leukemia is a rare variant where <10% of circulating nucleated cells are mast cells.
Acute mast cell leukemia is a rapidly progressive disorder with leukemic mast cells in blood and in large numbers in marrow. Mast cells must equal or exceed 20% of all nucleated cells in bone marrow aspirate smears. Bone marrow will show a diffuse, compact infiltrate of mast cells with a marked reduction of normal precursor blood cells and fat cells.