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SUMMARY
The classification of malignant lymphomas has been a contentious issue during the past 50 years, undergoing numerous changes during its evolution. The recent World Health Organization (WHO) classification of lymphoid neoplasms has gained worldwide acceptance by both pathologists and oncologists. It provides a list of distinct diseases that are defined by a combination of morphologic, phenotypic, genetic, and clinical features, and attempts to correlate each disease with a cell of origin. Because the classification of lymphomas requires the integration of such diverse information, the diagnosis has become more complex compared to other solid malignancies. As a result, several ancillary studies have become useful in the diagnosis of lymphomas, which require special handling of biopsy material when a diagnosis of lymphoma is suspected. The WHO classification identifies three major categories of lymphoid malignancies: B-cell neoplasms, T and natural killer (NK) cell neoplasms, and Hodgkin lymphoma. Two major categories are identified within the B-cell and T/NK-cell neoplasms: precursor neoplasms and peripheral or mature neoplasms. Unlike previous lymphoma classifications, the WHO classification does not group different lymphomas by clinical outcome or histologic grade. It recognizes that each disease has distinctive clinical features and response to treatment and may have a spectrum of clinical aggressiveness that may correlate with histologic grade or gene expression patterns. The WHO classification recognizes that several of the diseases it describes are heterogenous and likely include two or more distinct diseases that cannot be identified based on current data, and remains open to incorporate new data as they become available. One such source of new data for classifying lymphoma is the study of gene expression profiling by complementary DNA microarray technology, which is providing new insights into the classification of diseases such as diffuse large B-cell lymphoma and chronic lymphocytic leukemia. Proteomic approaches will add further texture to the molecular taxonomy of lymphoma classification.
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Acronyms and Abbreviations
ABC, activated B cell; ALCL, anaplastic large cell lymphoma; ALK, anaplastic lymphoma kinase; DLBCL, diffuse large B-cell lymphoma; EBV, Epstein-Barr virus; FISH, fluorescence in situ hybridization; GCB, germinal center B cell; IGH, immunoglobulin heavy chain; LP, lymphocyte predominant; MALT, mucosa-associated lymphoid tissue; NF-κB, nuclear factor-κB; NK, natural killer; PCR, polymerase chain reaction; PTCL, peripheral T-cell lymphoma; REAL, Revised European-American Lymphoma; WHO, World Health Organization; ZAP-70, zeta-associated protein of 70 kDa.
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HISTORICAL ASPECTS OF LYMPHOMA CLASSIFICATION
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The classification of malignant lymphoma has been fraught with controversy during much of the 20th century, with much needed consensus reached during the past two decades. A detailed discussion of the history of lymphoma classification is beyond the scope of this chapter and can be found elsewhere.1
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From Thomas Hodgkin’s description in 1832 of what became known as Hodgkin’s disease2 to the first half of the 20th century, several types of lymphomas with distinctive morphologic and clinical features were described using a variety of terms, including lymphoma, lymphosarcoma, reticulum ...