The classification of lymphomas and leukemias.

Classifications of lymphomas and leukemias have developed from two distinct clinical needs - to understand the natural history of these diseases in order to predict outcome and to make treatment decisions in a rational fashion. The utility of classifications for research on etiology of these diseases has not guided their development in the past. The classification of leukemias and lymphomas has undergone dramatic changes with increasing understanding of the development of the normal immune cells. Historically, the first entity to be recognised was Hodgkin disease. Other malignancies of the lymphatic system were then called 'Non-Hodgkin Lymphomas' (NHL), a distinction that remains valid even today. Cancers that tend not to form distinct masses but usually present with a raised white blood cell count were called leukemias. As knowledge has improved, however, the early juxtaposition of leukemias versus lymphomas has lost relevance, since often the same entity can present in either way. With better understanding the terms 'lymphosarcoma' and 'reticulosarcoma', which were earlier widely applied have been replaced by more precise terminology. Different classifications have been put forward over the years. The 'Revised European and American Classification of Lymphoid Neoplasms' and the derived WHO classification are structured to mirror normal B/T-cell differentiation. In these modern classifications, distinct disease entities are defined based on the combination of morphology, immunological and molecular techniques and clinical features. The proposed major groups of lymphoid neoplasms are B-cell lymphomas/leukemias, T/Natural Killer-cell lymphomas/leukemias and Hodgkin disease. About 20 entities are recognised. This provides for the first time a truly international view of lymphomas and leukemias. It has emerged that such a classification can be used successfully by expert hematopathologists and yields highly reproducible results. It is also clear that no single marker, be it morphology, genetic analysis or immunophenotyping can be used as the 'gold standard' for diagnosis but that a combination of techniques is needed.