Types of AML Leukemia

The two most commonly used classification schemata for AML, as of 2006, are the older French-American-British (FAB) system and the newer World Health Organization (WHO) system.

The French-American-British (FAB) classification system divided AML into 8 subtypes, M0 through to M7, based on the type of cell from which the leukemia developed and its degree of maturity. This is done by examining the appearance of the malignant cells under light microscopy and/or by using cytogenetics to characterize any underlying chromosomal abnormalities. The subtypes have varying prognoses and responses to therapy. Although the WHO classification (see below) may be more useful, the FAB system is still widely used as of mid-2006.

The eight FAB subtypes are:
•M0 (undifferentiated AML)
•M1 (myeloblastic, without maturation)
•M2 (myeloblastic, with maturation)
•M3 (promyelocytic), or acute promyelocytic leukemia (APL)
•M4 (myelomonocytic)
•M4eo (myelomonocytic together with bone marrow eosinophilia)
•M5 monoblastic leukemia (M5a) or monocytic leukemia (M5b)
•M6 (erythrocytic), or erythroleukemia
•M7 (megakaryoblastic)

The World Health Organization (WHO) classification of acute myeloid leukemia attempts to be more clinically useful and to produce more meaningful prognostic information than the FAB criteria. Each of the WHO categories contains numerous descriptive sub-categories of interest to the hematopathologist and oncologist; however, most of the clinically significant information in the WHO schema is communicated via categorization into one of the five subtypes listed below.

AML with characteristic genetic abnormalities, which includes AML with translocations between chromosome 8 and 21 [t(8;21)], inversions in chromosome 16 [inv(16)], or translocations between chromosome 15 and 17 [t(15;17)]. Patients with AML in this category generally have a high rate of remission and a better prognosis compared to other types of AML.

AML with multilineage dysplasia. This category includes patients who have had a prior myelodysplastic syndrome (MDS) or myeloproliferative disease (MPD) that transforms into AML. This category of AML occurs most often in elderly patients and often has a worse prognosis.

AML and MDS, therapy-related. This category includes patients who have had prior chemotherapy and/or radiation and subsequently develop AML or MDS. These leukemias may be characterized by specific chromosomal abnormalities, and often carry a worse prognosis.

AML not otherwise categorized. Includes subtypes of AML that do not fall into the above categories.

Acute leukemias of ambiguous lineage. Acute leukemias of ambiguous lineage (also known as mixed phenotype or biphenotypic acute leukemia) occur when the leukemic cells can not be classified as either myeloid or lymphoid cells, or where both types of cells are present.

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