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The subclassification of low-grade B-cell lymphomas remains controversial. A particularly difficult area has been the definition and characterization of the clinical behavior of so-called intermediate lymphocytic lymphoma (ILL) or lymphocytic lymphoma of intermediate differentiation (IDL) and their relationship to the previously wellcharacterized entity centrocytic lymphoma.At an informal meeting in London in April 1991, centrocytic lymphoma, IDL/ILL, and mantle zone lymphoma (MZL), the nodular variant of this lymphoma, were discussed using historical data and findings from studies in progress. It was concluded that at least a subset of cases described as IDL/ILL/MZL is identical to centrocytic lymphoma of the Kiel classification and that this subset has to be distinguished from other lymphomas with a predominance of cleaved, or irregular, small lymphoid cells of B lineage. It was agreed that there is no evidence that the small cleaved cells found in these lymphomas correspond to the centrocytes found in the reactive germinal centers or to their neoplastic counterpart in follicle center cell lymphomas. Instead, the data indicate that the small cleaved cells of centrocytic lymphoma, IDL/ILL, and MZL may be the neoplastic counterpart of cells normally residing in the follicle mantle. We therefore propose that the term “mantle cell” lymphoma be adopted to indicate this distinctive type of non-Hodgkin's (NH) lymphoma.Centrocytic lymphoma (originally termed germinocytoma) was described as a low-grade B-cell lymphoma (18) and subsequently adopted in the Kiel classification (9). This lymphoma, composed of a relatively uniform population of small- to mediumsized cells with irregularly angular or cleaved nuclei, was considered to be the neoplastic equivalent of centrocytes (i.e., small cleaved follicular center or germinal center cells). It has been stressed that this lymphoma (in clear distinction to centroblastic/centrocytic lymphoma) does not show any admixture of centroblasts. Most cases display a diffuse growth pattern, but occasionally a nodular growth pattern reminiscent of primary B follicles is found (17).In the United States a subtype of NH lymphoma composed of small lymphoid cells with an irregular nuclear outline admixed with a variable number of small round lymphocytic cells has been described (3). It was given the name intermediately differentiated lymphocytic lymphoma since it lay morphologically between well-differentiated lymphocytic lymphoma (WDLL) and poorly differentiated lymphocytic lymphoma (PDLL). A relationship to the follicular cuff or mantle was postulated, based on immunologic and enzyme cytochemical data (15,24). Weisenburger et al. (39) described a nodular variant of this lymphoma, with a mantle growth pattern surrounding residual follicle centers, which they called mantle zone lymphoma. Although some studies (11) showed that a mantle zone pattern was not specific to IDL/ILL/MZL, Jaffe et al. (14) proposed that the term mantle zone lymphoma be used for cases both with and without a mantle zone pattern and postulated that this lymphoma was identical in most cases to centrocytic lymphoma.Centrocytic lymphomas and IDL/ILL/MZL are clearly similar in terms of morphology. They have no obvious counterpart in the original Rappaport classification, in which they would probably be diagnosed as WDLL or PDLL. In the NCI-supported Working Formulation study, IDL/ILL/MZL were not included in the classifications being evaluated. Centrocytic lymphomas were analyzed and included within the category of diffuse small cleaved cell lymphomas.Although centrocytic lymphoma was designated as low grade, clinical studies have shown a median survival of <5 years (33). Analysis of clinical data for IDL/ILL/MZL is more difficult, since differing morphologic criteria have been employed, with the exclusion (16) or inclusion (39,40) of cases containing pseudofollicular growth centers. However, when cases with proliferation centers are excluded, survival is comparable to that of centrocytic lymphoma (16,25).