[Small cell B-cell lymphomas: guidelines for differential diagnosis]

Similar to the R.E.A.L-System, the small cell B-cell lymphomas of the new WHO classification consist of chronic lymphocytic leukaemia of B cell type, mantle cell lymphoma, follicular lymphoma, lymphoplasmocytic lymphoma/immunocytoma, hairy cell leukaemia, as well as plasmacytoma. The only major difference between the WHO- and the REAL-classification is the consideration of prolymphocytic leukaemia as a single disease entity in the former system. All the above-mentioned lymphomas arise from B cells of varying stages of differentiation and, therefore, often demonstrate architectural, cytological and immunophenotypic characteristics of their normal physiological counterparts. Consideration of tumour cell growth pattern, -cytology, -immunophenotype and -growth fraction, together with the presence and consistency of the reactive cell infiltrate, usually leads to categorisation of a lymphoma in the majority of cases. The molecular biological characteristics of follicular lymphoma and mantle cell lymphoma are the best defined of the small cell B-cell lymphomas. Chromosomal translocations involving the immunoglobulin heavy chain genes and the bcl-2 gene or Cyclin D1 gene, respectively, probably belong to the initial changes in a cell, which, together with several subsequent unidentified genetic alterations, lead to the development of these tumours. Although nodal small cell B-cell lymphomas are usually diagnosed at an advanced stage of the disease, the progression of the disease--with the exception of mantle cell lymphomas--is often indolent. As a result, the small cell B-cell lymphomas were previously considered as "low-grade" Non-Hodgkin lymphomas in the Kiel classification. However, since the progress of a lymphoma subtype can be heterogeneous and since mantle cell lymphomas cannot really be considered as "low-grade" tumours, "umbrella grading" of lymphomas has been discarded in the WHO classification, with emphasis being placed on grading within a lymphoma disease entity. In the following pages, the characteristics important for the diagnosis and categorisation of the small cell B-cell lymphomas will be summarised. Further, we present information regarding the molecular biological and clinical characteristics of these lymphomas.