[World Health Organization (WHO) classification of malignant lymphoma--how is the WHO now?]

The WHO classification of malignant lymphoma presents a list of disease entities well defined by clinical, immunologic and genetic features. The disease entities are primarily exemplified by mantle cell lymphoma (the relative incidence, 3%, related with an aberrant expression of cyclin D1 activated by t(11;14) chromosomal translocation), follicular lymphoma (7%, with BCL2 by t(14;18) translocation), marginal zone B-cell lymphoma of MALT type (8%, with MALT1 by t(11;18) or t(14;18) translocation), adult T-cell leukemia/lymphoma (7%, but 20% in Kyushu, with human T-cell leukemia virus type 1 [HTLV1]), extranodal NK/T-cell lymphoma of nasal type (2%, with Epstein-Barr virus [EBV]), and anaplastic large cell lymphoma (2%, with ALK by t(2;5) translocation). The genetic alterations or agents related with their pathogenesis are of great relevance to the diagnoses of these entities. Conversely, the category of diffuse large B-cell lymphoma occupies the largest percentage (33%), and is heterogeneous, including several subtypes. The diagnoses of lymphoma subtypes, except for the main entities described above, largely depends on the phenotypic analysis, in addition to their clinical manifestations and histologic evaluations. Therefore, much attention should be paid to the diagnostic pitfalls in the phenotypic analysis of T-cell lymphomas in regard to incorrect classification of lymphoma, due to lack of awareness or incomplete immunophenotypic evaluation; incorrect interpretation of immunophenotypic data, due to failure to correlate them with the other laboratory data; and suboptimal quality of immunohistochemical stains, leading to a wrong conclusion.