Nam H Dang and Fredrick Hagemeister

T-cell lymphoid malignancies are a heterogeneous group of relatively rare diseases that are defined as distinct entities by a constellation of laboratory and clinical characteristics, including morphologic features, immunophenotypes, genetic abnormalities, clinical manifestations, and responses to treatment. The mature T- and natural killer (NK)-neoplasms as defined by the World Health Association (WHO) classification account for approximately 10% of all lymphoid malignancies.1 However, the incidences of certain subtypes vary considerably by race and geography, with non-AIDS-related T-cell malignancies occurring much more frequently in the Far East than in North America. For example, in the Non-Hodgkin's Lymphoma Classification Project,2 9.4% of all cases of non-Hodgkin's lymphoma (NHL) (129 of 1378) were classified as peripheral T-cell lymphomas (PTCL), 33 (2.4%) of which were anaplastic large-cell lymphomas (ALCL). The frequencies of the remaining subtypes of PTCL (96 of 1378, 7%) ranged from 1.5% (Vancouver) to 18.3% (Hong Kong). Although the reason for this observed difference in frequencies is not clear for most subtypes, such variability is related in some situations to an association between a specific virus and the T-cell malignancy, such as the linkage between Epstein-Barr virus (EBV) and with NK/T-cell lymphoma and human T-cell lym-photropic virus 1 (HTLV-1) with adult T-cell leukemia/ lymphoma (ATLL).

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