Both classifications, Kiel and IWF, have their advantages and disadvantages, страница 5

According to the IWF the diffuse subtypes are of intermediate grade of malignancy. The subtype in the Kiel classification with "numerous small centrocytes and a few centroblasts" is categorized in the IWF as "diffuse small cleaved cell lymphoma". According to the IWF nodular subtypes are of low-grade malignancy except for those with "predominantly large cells" [97].

Monocytoid B-Cell Lymphoma

This is a rare lymphoma. The neoplastic cells correspond to the cells of so-called "immature sinus histiocytes", which are of B-cell origin and are referred to as monocytoid B cells (pa-rafollicular) [98-100]. In most cases the neoplastic cells present a pattern of intrasinusoidal distribution. The monocytoid B cells are usually of medium size (Fig. 7) although a large cell variety has been described [101]. Their cytoplasm is moderately abundant and the nuclei are round to oval and somewhat cleaved. Small numbers of monoclonal Ig-positive immunoblasts can also be found. Variable numbers of rather neoplastic plasma cells and a few neutrophils are dispersed among the monocytoid cells: a picture simulating that of reactive immature sinus histiocytosis. The tumor cells are mostly IgM positive. Their immunophenotype corresponds to CD19+, CD20+, CD22+, KiB3+ and CD5-. On paraffin sections the B-monocytoid cells show a characteristic granular cytoplasm positivity for KiM1 P [102].

Lymphomas with monocytoid B-cell appearance at extranodal sites including the gastrointestinal tract, salivary glands and thyroid gland have also been described [103-105]. This can be explained by the close histogenetic relationship of the above categories with each other and with the marginal zone cells [38,104, 106-109]. It seems that circulating B lymphocytes enter the primary follicles and become a normal component of the follicular mantle zone. After antigenic stimulation these cells transform into centroblasts, which then differentiate to centrocytes. Centrocytes coming into contact with the antigen trapped on the surface of follicular dendritic cells differentiate to plasma cell precursors and memory B cells. The former, when leaving the germinal centers, become plasma cells in medullar cords of the lymph nodes or even in the lamina propria of gut mu-cosa, the spleen red pulp and the bone marrow. Memory B cells migrate to the marginal zone and become marginal B cells. The latter usually intermingle with the mantle zone cells, but not in the spleen and mesenteric lymph nodes where they form a rather separate outer zone in the follicular lymphocyte corona [38,108]. After stimulation they may become larger and develop into monocytoid B cells. This development can occur in lymph nodes (parafollicular cells, immature sinus histiocytosis) or at extranodal sites such as mucosaassociated lymphoid tissue (MALT) of the gut, bronchi, salivary glands, thyroid gland and other epithelial organs. Marginal zone cells directly or through the monocytoid B-cell stage are able to differentiate into plasma cells [100, 110-113]. The sites that prefer to home the above precursors of plasma cell genesis depend on special surface receptors on their own surface or in venules and other stromal tissue components [114-116]. Because of the close histogenetic and functional relationship between marginal zone cells and monocytoid B cells, many authors include marginal zone cell lymphomas (Fig. 8) and monocytoid B-cell lymphomas in the same category [117]. Monocytoid B-cell lymphoma is not listed in the IWF. Although these lymphomas often show moderate to high numbers of mitoses and, according to the IWF, could morphologically be categorized as intermediate grade of malignancy diffuse lymphomas (either small or large cell, non-cleaved/cleaved), their prognosis seems to be better in many instances. However, relapses and development of high-grade lymphomas do occur [99,100,113,117-119].