CD5-Positive Mature B-Cell Neoplasm With Large-Cell Morphology, Predominant Bone Marrow Involvement, and a Leukemic Peripheral Blood Presentation: A Case Report.

A 65-year-old man presenting with pancytopenia and circulating large atypical cells was initially suspected of having acute leukemia. Flow cytometric analysis demonstrated a mature clonal B-cell phenotype, and the integrated findings supported a diagnosis of a CD5-positive mature B-cell neoplasm with large-cell morphology, predominant bone marrow involvement, and a leukemic peripheral blood presentation. Based on the available staging data, primary bone marrow DLBCL was suspected but could not be definitively confirmed. The neoplasm was characterized by CD5 expression, a BCL6 rearrangement, and a complex karyotype including monosomy 17. The patient received six cycles of R-CHOP followed by two cycles of systemic high-dose methotrexate as individualized central nervous system prophylaxis. A complete response was confirmed after treatment, with no evidence of recurrence 6 months after treatment completion.