The World Health Organization classification of neoplastic diseases of thehematopoietic and lymphoid tissues

Introduction: Since 1995, the European Association of Pathologists (EAHP) a nd the Society for Hematopathology (SH) have been developing a new World He alth Organization (WHO) Classification of hematologic malignancies. The cla ssification includes lymphoid, myeloid, histiocytic, and mast cell neoplasm s. Design: The WHO project involves 10 committees of pathologists, who have de veloped lists and definitions of disease entities. A Clinical Advisory Comm ittee (CAC) ) of international hematologists and oncologists was formed to ensure that the classification will be useful to clinicians. A meeting was held in November, 1997, to discuss clinical issues related to the classific ation. Results: The WHO has adopted the 'Revised European-American Classification of Lymphoid Neoplasms' (R.E.A.L.), published in 1994 by the International L ymphoma Study Group (ILSG), as the classification of lymphoid neoplasms. Th is approach to classification is based on the principle that a classificati on is a list of 'real' disease entities, which are defined by a combination of morphology, immunophenotype, genetic features, and clinical features. T he relative importance of each of these features varies among diseases, and there is no one 'gold standard'. The WHO Classification has applied the pr inciples of the R.E.A.L. Classification to myeloid and histiocytic neoplasm s. The classification of myeloid neoplasms recognizes distinct entities def ined by a combination of morphology and cytogenetic abnormalities. The CAC meeting, which was organized around a series of clinical questions, was able to reach a consensus on most of the questions posed. The question s and the consensus are discussed in detail below. Among other things, the CAC concluded that clinical groupings of lymphoid neoplasms were neither ne cessary nor desirable. Patient treatment is determined by the specific type of lymphoma, with the addition of grade within the tumor type, if applicab le, and clinical prognostic factors such as the international prognostic in dex (IPI). Conclusion: The experience of developing the WHO Classification has produce d a new and exciting degree of cooperation and communication between oncolo gists and pathologists from around the world, which should facilitate progr ess in the understanding and treatment of hematologic malignancies.