Acute Monocytic Leukemia M5a and M5b

Acute monocytic leukemia (AMoL) accounts for about 10 percent of the AML and is clonally expressed in cells committed to differentiation to monocytic pathways.90 AMoL is divided into two morphological subtypes: M5a and M5b.

M5a, or the poorly differentiated subtype, is frequently observed in children; it can be confused with the ALL L3 type. In the bone marrow 80 percent or more of all monocytic cells are monoblasts. Blast cells display abundant deep basophilic cytoplasm, which is often vacuolated and has no or few azurophilic granules and no Auer rods. The nuclei are round to oval, with one or more prominent nucleoli (Plate 14-16). Since the peroxidase reaction may be negative in 40 to 50 percent of such cases and the PAS reaction is often strongly positive with a blocklike pattern, it is very useful to use nonspecific esterase (NSE) staining differential diagnosis. This stain will be strongly positive in more than 90 percent of cases. Occasionally the SBB reaction will be positive in the absence of a peroxidase reaction.

M5b, or the well-differentiated subtype, is defined by the presence of 20 percent or more of abnormal cells being promonocytes with twisted or folded nuclei, gray-blue cytoplasm, and scattered azurophilic granules (Plate 14-17). Rarely, a few cells will contain Auer rods. The percentage of mature monocytes is often much higher in the blood than in the bone marrow. Monocytes with dysplastic features involving the other lineages can also be seen and have an adverse prognostic effect.

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