20 The WHO system has now largely superseded the French-American-British (FAB) classification. The newer classification reduces the bone marrow leukaemic blast cell percentage differentiating AML from myelodysplastic syndrome (see p. 50) from 30% to 20%. Other key changes include the creation of specific subtypes with non-random cytogenetic or equivalent molecular abnormalities, and the distinction of patients with multilineage dysplasia and also previous chemotherapy. The major FAB subtypes are included in the ‘other’ category with the exception of acute promyelocytic leukaemia (previously FAB M3) which is now in the ‘recurrent translocations’ group due to the inevitable presence of t(15;17). It can be seen (Table 20.1) that occasional cases of AML show megakaryocytic or erythroid differentiation. Gene mutations are likely to become increasingly important in classification. Table 20.1 WHO classification of acute myeloid leukaemia AML with recurrent genetic abnormalities1 AML with inv (16)(p13;q22) or t(16;16) (p13.1;q22) (M4 Eo) AML with t(15;17)(q22;q12) (M3:M3V) AML with inv(3) (q21;q26.2) or t(3;3) (q21;q26.2) AML with myelodysplasia-related changes Therapy related myeloid neoplasms AML not otherwise specified2 AML with minimal differentiation (M0) Acute myelomonocytic leukaemia (M4) Acute monoblastic/ monocytic leukaemia(M4/5) Acute erythroid leukaemia (M6) Acute megakaryoblastic leukaemia (M7) Myeloid sarcoma Myeloid proliferation related to Down syndrome Blastic plasmacytoid dendritic cell neoplasm 1See also Table 2 for genetic changes.
Acute myeloid leukaemia
Classification
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