Lymphoid Neoplasms: Classification Systems

Abstract

The principles of the fourth edition “WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues” (Swerdlow et al. et al (2008) WHO classification of tumours of haematopoietic and lymphoid tissues. 4th edn. IARC Press, Lyon) are based on those defined in the “Revised European-American Classification of Lymphoid Neoplasms” (REAL), published by the International Lymphoma Study Group (ILSG) in 1994 (Harris et al. Blood 84:1361–1392, 1994). Thus, the new World Health Organization (WHO) classification must be considered as an updated and modified version of the REAL classification rather than of the older WHO lymphoma classification (second edition) published in 1976 (Mathe et al. Ann Anat Pathol (Paris) 21:285–300, 1976) Disease entities are defined on the basis of the combination of morphological, immunophenotypic, genotypic, and clinical data. The relative impact of these characteristics varies among the different lymphomas, and thus there is “no gold standard” for defining a particular entity. The WHO classification not only encompasses lymphoid tumors but also extends to myeloid, mast cell, and histiocytic/dendritic cell malignancies. The neoplasms are primarily stratified according to their tumor cell lineage, and for each disease entity a cell of origin is postulated according to our current knowledge of cellular differentiation pathways. The classification of lymphoid malignancies recognizes three major categories: B-cell neoplasms, T-/NK-cell neoplasms, and Hodgkin’s lymphomas. B- and T-cell lymphomas are further classified into precursor- and mature neoplasms, the latter being subdivided again according to their clinical manifestation: i.e., into disseminated/leukemic, extranodal, and nodal malignancies. In contrast to previous classifications, the lymphoid neoplasms are grouped neither according to their histological grade (e.g., Kiel classification) (Lennert Acta Neuropathol Suppl Suppl 6:1–16, 1975) nor according to their clinical aggressiveness (e.g., International Working Formulation) (Institute Cancer 49:2112–2135, 1982). However, the histological grade is considered a prognostic factor, which enters into the description of each disease entity. Hodgkin’s disease, now more appropriately termed Hodgkin’s lymphoma, comprises nodular lymphocyte-predominant Hodgkin’s lymphoma and classical Hodgkin’s lymphomas (CHL) of nodular sclerosis, mixed cellularity, lymphocyte-depleted, and lymphocyte-rich subtype. This chapter provides a summary of the lymphoid malignancies and refers the reader to reviews of the evolution of classification systems for myeloid (Vardiman and Hyjek Hematology Am Soc Hematol Educ Program 2011:250–256, 2011; Czader and Orazi Curr Pharm Des 18:3149–3162, 2012), macrophage/histiocytic, dendritic cell, (Kairouz et al. Am J Hematol 82:924–928, 2007) and mast cell disorders (Amon et al. J Dtsch Dermatol Ges 8:695–711, 2010).