Classification of acute leukemias.

Because of the increasing recognition of the importance of genetic events to the diagnosis and treatment of the acute leukemias, the proposed new World Health Organization (WHO) classification incorporates genetic aberrations and immunology as major defining features in addition to morphology. In a hierarchal approach, genetic changes have precedence in the acute myeloid leukemias and immunology and genetic changes have precedence in the acute lymphoblastic leukemias. Four major groups of acute myeloid leukemia are recognized: 1) Acute myeloid leukemias with recurrent genetic abnormalities, 2) Acute myeloid leukemia with multilineage dysplasia, 3) Acute myeloid leukemias, therapy related, and 4) Acute myeloid leukemia not otherwise categorized. Two types of acute lymphoblastic leukemia are recognized based on immunologic characteristics: precursor B lymphoblastic leukemia/lymphoma and precursor T lymphoblastic leukemia/lymphoma. Precursor B acute lymphoblastic leukemia/lymphoma is subclassified into prognostic genetic groups. Biphenotypic leukemia is recognized as a form of acute leukemia of ambiguous lineage.