Abstract
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.