Monocytoid B-cell lymphoma. The biologic and clinical implications of peripheral blood involvement.

Monocytoid B-cell lymphoma (MBCL) is a newly recognized malignant lymphoma that shares clinical and pathologic features with other low-grade B-cell neoplasms, especially small lymphocytic lymphoma and hairy cell leukemia. However, although circulating malignant cells and bone marrow involvement are relatively common in small lymphocytic lymphoma and are characteristic features of hairy cell leukemia, MBCL in the peripheral blood and bone marrow rarely have been described. From 124 patients entered in the MBCL registry, three cases with peripheral blood involvement are described and the clinical and pathologic features in these patients are compared with those of other low-grade B-cell neoplasms. Monocytoid B-cell lymphoma was confirmed by lymph node biopsy in each case. Two patients had lymphocytosis at the time of presentation; the remaining patient presented with pancytopenia. For each patient, phenotypic studies of lymph node and peripheral blood revealed identical monoclonal surface immunoglobulin expression. The morphologic appearance of the circulating MBCL cells was different in each case, varying from a relatively homogeneous population of small lymphocytes to a heterogeneous collection of large and small lymphoid cells. The two patients with lymphocytosis also had extensive replacement of the bone marrow by MBCL; the third patient had diffuse infiltration by MBCL in a normocellular marrow. All three patients had advanced-stage (Stages III or IV) disease, and all required systemic chemotherapy for disease control. The two patients with lymphocytosis had relentless, progressive infirmity despite relatively aggressive treatment regimens. These patients ultimately died of lymphoma 13 and 18 months after initial diagnosis. The third patient is alive and well with stable disease 30 months after coming to the authors' institution. The clinical and pathologic features of the patients reported here reaffirms the placement of MBCL in the spectrum of low-grade B-cell neoplasms. However, unlike small lymphocytic lymphoma and hairy cell leukemia, MBCL only rarely undergoes leukemic conversion. Furthermore, it appears that peripheralization of MBCL occurs primarily in patients with advanced-stage disease and may be indicative of a relentless course and progressive disease despite aggressive chemotherapeutic intervention.