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Classification
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The World Health Organization (WHO) classification of acute myeloid leukemia (AML) incorporates and interrelates morphology, cytogenetics, molecular genetics, and immunologic markers in an attempt to construct a classification that is universally applicable and prognostically valid.[1] In the older French-American-British (FAB) criteria, the classification of AML is solely based upon morphology as determined by the degree of differentiation along different cell lines and the extent of cell maturation.[2,3]
Under the WHO classification, the category "acute myeloid leukemia not otherwise categorized" is morphology-based and reflects the FAB classification with a few significant modifications.[2,3] The most significant difference between the WHO and FAB classifications is the WHO recommendation that the requisite blast percentage for the diagnosis of AML be at least 20% blasts in the blood or bone marrow. The FAB scheme required the blast percentage in the blood or bone marrow to be at least 30%. This threshold value for blast percentage eliminated the category "refractory anemia with excess blasts in transformation" (RAEB-t) found in the FAB classification of myelodysplastic syndromes (MDS), where RAEB-t is defined by a marrow blast percentage between 20% and 29%. In the WHO classification, RAEB-t is no longer considered a distinct clinical entity, and is instead included within the broader category "AML with multilineage dysplasia" as "AML with multilineage dysplasia following a myelodysplastic syndrome."[4]
Although this lowering of the blast threshold has been met with some criticism, several studies indicate that survival patterns for cases with 20% to 29% blasts are similar to survival patterns for cases with 30% or more blasts in the bone marrow.[5,6,7,8,9] The diagnosis of AML in itself does not represent a therapeutic mandate. The decision to treat should be based on other factors including patient age, previous history of MDS, clinical findings, disease progression, in addition to the blast percentage, and most importantly, patient preference.
In the following outline and discussion, the older FAB classifications are noted where appropriate.
- AML with characteristic genetic abnormalities.
- AML with t(8; 21)(q22;q22); (AML/ETO).
- AML with inv(16)(p13q22) or t(16;16)(p13; q22); (CBFß/MYH11).
- Acute promyelocytic leukemia (AML with t(15;17)(q22; q12); (PML/RARa) and variants).
- AML with 11q23 (MLL) abnormalities.
- AML with an FLT3 mutation (not in the WHO classification scheme).
- AML with multilineage dysplasia.
- AML and MDS, therapy related.
- Alkylating agent-related AML and MDS.
- Topoisomerase II inhibitor-related AML.
- AML not otherwise categorized.
- Acute myeloblastic leukemia, minimally differentiated (FAB Classification M0).
- Acute myeloblastic leukemia without maturation (FAB Classification M1).
- Acute myeloblastic leukemia with maturation (FAB Classification M2).
- Acute myelomonocytic leukemia (AMML) (FAB Classification M4).
- Acute monoblastic leukemia and acute monocytic leukemia (FAB classifications M5a and M5b).
- Acute erythroid leukemias (FAB classifications M6a and M6b).
- Acute megakaryoblastic leukemia (FAB Classification M7).
- AML/transient myeloproliferative disorder in Down syndrome.
- Acute basophilic leukemia.
- Acute panmyelosis with myelofibrosis.
- Myeloid sarcoma.
- Acute leukemias of ambiguous lineage.
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