AML-M5a: The WHO Classification for Acute Leukemia with Promonocytes and Mature Monocytes
AML-M5a (acute monoblastic leukemia) is the WHO classification that describes an acute leukemia with a spectrum of monocyte differentiation, including promonocytes and mature monocytes.
Understanding AML-M5 Classification
The World Health Organization (WHO) classification system for acute myeloid leukemia (AML) incorporates the older French-American-British (FAB) classification while adding genetic, molecular, and clinical characteristics 1. Within this framework:
- AML-M5 is divided into two subtypes:
- M5a (acute monoblastic leukemia): Characterized by predominance of monoblasts (≥80% of monocytic cells) with a spectrum of monocyte differentiation including promonocytes and mature monocytes 2
- M5b (acute monocytic leukemia): Characterized by predominantly differentiated monocytes
Diagnostic Features of AML-M5a
Morphological Features
- Predominance of monoblasts (≥80% of monocytic cells)
- Presence of promonocytes
- Some mature monocytes
- Monoblasts have abundant cytoplasm, fine nuclear chromatin, and prominent nucleoli
Immunophenotypic Features
- Monocytic differentiation is defined by at least 2 of the following 2:
- Positive nonspecific esterase
- CD11c expression
- CD14 expression
- CD64 expression
- Lysozyme expression
Distinguishing from Other AML Subtypes
- AML-M5b: Has <80% monoblasts with predominance of more mature monocytic forms
- AML-M4Eo: Has both granulocytic and monocytic differentiation with abnormal eosinophils
- AML-M3 (microgranular variant): Has hypogranular promyelocytes and is associated with t(15;17)
Clinical Significance
AML-M5a has distinct clinical features:
- May present with extramedullary involvement
- Can have gingival hyperplasia and skin infiltration
- Often associated with hyperleukocytosis
- May have specific genetic abnormalities, particularly involving 11q23 (MLL gene) rearrangements
Diagnostic Pitfalls
When evaluating for AML-M5a:
- Don't rely solely on morphology; immunophenotyping is essential
- Monocytic differentiation requires at least 2 specific markers as outlined in the WHO criteria
- Avoid confusion with M5b, which has more mature monocytic forms
- Remember that the WHO classification has evolved beyond the FAB classification, incorporating genetic and molecular features
The correct answer is B. AML-M5a, which specifically describes acute monoblastic leukemia with a spectrum of monocyte differentiation including promonocytes and mature monocytes 2, 1.