SDC1 and plasma cell neoplasm: Immunohistochemistry showed that the atypical large lymphoid cells were strongly CD20-positive with dim CD5 expression, confirming B-cell lineage. The neoplastic cells were restricted to vascular lumina, consistent with an intravascular pattern. They were negative for CD34, CD138, MUM1, CD10, and Cyclin D1, effectively excluding plasma cell neoplasms and other B-cell lymphoma subtypes. Non-neoplastic elements included scattered CD3-positive reactive T cells and CD138-positive plasma cells. E-cadherin highlighted scattered proerythroblasts within the marrow.