WHO Classification of Hematologic Malignancies
Summary of Key Points
• The World Health Organization (WHO) classification of hematologic neoplasms includes tumors of lymphoid, myeloid, histiocytic, and dendritic cell lineages.
• Each disease is defined as a distinct entity based on a constellation of morphologic, clinical, and biological features.
• The cell of origin is the starting point of disease definition.
• Some lymphomas and leukemias can be identified by routine morphologic approaches. However, for many diseases, knowledge of the immunophenotype and molecular genetics/cytogenetics plays an important role in the differential diagnosis.
• The sites of presentation and involvement are an important clue to underlying biological distinctions. Extranodal lymphomas differ in many respects from their nodal counterparts.
• Many lymphoma entities display a range in cytologic grade and clinical aggressiveness, making it difficult to stratify lymphomas according to clinical behavior. A number of prognostic factors influence clinical outcome, including stage, international prognostic index, cytologic grade, gene expression profile, secondary genetic events, and the host environment.
• The WHO classification includes five major categories of myeloid diseases, all of which are clonal stem cell disorders exhibiting variable maturation in the affected lineages and either effective or ineffective hematopoiesis:
Myeloid (and lymphoid) neoplasms with eosinophilia and abnormalities of PDGFRA, PDGFRB, or FGFR1
