Monocytoid B-cell lymphomas: An assessment of diagnostic criteria and a perspective on histogenesis

To determine which morphologic criteria are most useful in distinguishing reactive from malignant monocytoid B cells (MBCs), we compared 16 monoclonal cases (11 nodal, five extranodal) of monocytoid B-cell lymphoma (MBCL) with 12 cases of various reactive diseases in which MBCs were polyclonal. The results of our study showed that in MBCL the MBC component was the predominant architectural finding and that there was confluence of MBCs in all but one case. In contrast, the MBC component did not predominate in the reactive group (P < .000001) and focal confluence was seen in only one case. A cytologic comparison showed that in MBCL areas there were more large transformed (prominent nucleolated) MBCs (P = .003), a higher mitotic rate (P = .03), and more nuclear irregularities (P = .007) than were present in the reactive group. In addition, evolution to an aggressive histologic type was found in four cases of MBCL. Our results also revealed concomitant multiple, monoclonal, morphologically distinct populations in other compartments (follicular center cells in seven, mantle cells in five, small lymphocytes in five, and plasma cells in 11). These unique findings can be reconciled by postulating (1) that the simultaneous presence of these diverse cytologic types represents morphologic expressions of a B cell whose population is in different phases of its cell cycle and/or its evolution or (2) that the histogenesis of MBCL is possibly from a nodal pluripotent B-stem cell that can differentiate directly into these various cytologic types.