Acute Myelomonocytic Leukemia M4

M4 accounts for 20 to 30 percent of adult and 20 percent of childhood AML cases, depending on the application of esterase staining with fluoride inhibition. Its diagnosis and separation from M2 and M5 require assessment of both peripheral blood and bone marrow smears.89 In bone marrow both granulocytic and monocytic precursors exist in varying proportions (Plate 14-14). The blast cells (types I, II, III) are greater than 30 percent or more, but less than 80 percent, of the nonerythroid cells. The marrow monocytic component must account for 20 percent or more of the nonerythroid nucleated cells but cannot exceed 80 percent (otherwise, the diagnosis is M5). Auer rods are occasionally present. When these findings are present with a peripheral blood monocytosis of 5000/mm3 or greater, the diagnosis of M4 is not in question. If the peripheral monocyto-sis is less than 5000/mm3 a diagnosis of M4 can still be made if the presence of a significant monocytic component (>20 percent) is confirmed by cytochemical stain (e.g., double esterase reaction with NaF or other esterase combinations). Cytochem-ical stains are also necessary for confirming the diagnosis of M4 in the presence of hypogranular neutrophils that resemble monocytes. Dysplastic features involving granulocytic, ery-throid, and megakaryocytic lineages can be identified in approximately 20 percent of patients. Monocytic component can be identified by immunophenotyping using CD14, CD11c, CD11b, and CD68 in conjunction with other granulocyte-restricted antibodies.70

A small percentage of patients (about 5 percent) with M4 are characterized by myelomonocytic bone marrow infiltration with abnormal eosinophils (M4Eo). These eosinophils have large basophilic granules and demonstrate CAE and PAS positivity. Often the nuclei have pseudo-Pelger features (Plate 14-15).

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