CD5-Positive Primary Cutaneous Diffuse Large B-Cell Lymphoma, Leg Type: An Unusual Presentation.

Primary cutaneous diffuse large B-cell lymphoma-leg type (PCDLBCL-LT) is a rare and aggressive lymphoma of elderly patients, typically affecting the lower extremities and characterized by centroblast- and immunoblast-like cells with a non-germinal center phenotype (MUM1+, BCL2+, IgM+, CD10-). CD5-positive cases are exceptionally uncommon and pose a diagnostic challenge, as CD5 expression broadens the differential diagnosis to include other B-cell lymphomas such as mantle cell lymphoma and transformed chronic lymphocytic leukemia. We describe a 70-year-old female patient who presented with a 2-cm cutaneous nodule on the lower extremity. Histopathology revealed a pandermal diffuse infiltrate of atypical large B-cells. Immunohistochemistry showed a CD5+, CD20+, MUM1+, BCL2+, IgM+, cyclin D1-profile, supporting CD5 + PCDLBCL-LT. Fluorescence in situ hybridization for MYC , BCL2 , and BCL6 rearrangements were negative, effectively ruling out double-hit/triple-hit lymphoma. The patient achieved a complete metabolic response after six cycles of R-CHOP chemotherapy, which was maintained for 24 months. However, she subsequently developed extracutaneous relapse involving the soft tissues of the extremities, trunk, and periorbital region, consistent with a progression. Treatment with a rituximab-bevacizumab combination regimen induced a second complete remission, sustained for six months at the time of reporting. Our case highlights the importance of routine CD5 testing in PCDLBCL-LT to identify this distinct subgroup and to guide appropriate differential diagnosis and patient monitoring.