A Tissue Counterpart to Monoclonal B-Cell Lymphocytosis Clinical and Pathologic Features

center dot Context.-B-cell clones discovered in tissue biopsies, without overt lymphoma, may represent a tissue counterpart to peripheral blood monoclonal B-cell lymphocytosis (MBL), herein termed tMBL. Objective.-To characterize the clinicopathologic features of tMBL. Design.-During a 10-year period, we retrospectively identified non-bone marrow/peripheral blood cases with monotypic B cells detected by tissue-based flow cytometry but without an identifiable lymphomatous infiltrate on routine histopathology. We excluded cases with prior diagnosis of chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma or MBL. Results.-Fifty-four cases were identified (35 lymph node, 3 splenic, and 16 soft tissue/viscera). Forty-six cases were CLL-type, 2 were atypical CLL, and 6 were non-CLL. tMBL was detectable by immunohistochemistry in 14 cases (26%, all CLL-type). Concurrent blood flow cytometry, available in 10 cases, showed 4 with low-count MBL (3 CLL-type, 1 with non-CLL-type), 5 with high-count MBL (all CLL-type), and 1 case negative for clonal population. With median follow-up of 51 months, 2 patients had progression of disease (CLL, 68.7 months; and diffuse large B-cell lymphoma, 5.9 months). Patients with immunohistochemistry-detectable tMBL had increased monoclonal B cells per total lymphocyte events (P = .01), morphologic evidence of bone marrow involvement (P = .04), higher white blood cell count (P = .02), and increased absolute lymphocyte count (P = .02). Conclusions.-tMBL spans an immunophenotypic spectrum similar to MBL, is detectable by immunohistochemistry in a minority of cases (often CLL immunophenotype), and is likely systemic in most cases. Development of overt lymphoma is uncommon but may occur, warranting clinical follow-up.