<scp>BCL</scp>‐6‐Positive Cutaneous Mantle Zone Hyperplasia: A Novel Subtype of B‐Cell Cutaneous Lymphoid Hyperplasia

Cutaneous lymphoid hyperplasia (CLH) is a benign lymphoid proliferation that may share features of skin lymphoma. Although reactive germinal centers (GCs) are common in B-cell CLH, cases with mantle zone hyperplasia (MZH), a rare phenomenon reported in reactive lymphadenopathy, have never been documented. We report a case of CLH with MZH showing weak BCL-6 expression, complicating its distinction from cutaneous B-cell lymphoma (CBCL). A 57-year-old man presented with erythematous nodules on the face. Biopsy showed diffuse B-cell infiltrates with lymph follicles. BCL-6 was weakly positive in lymphocytes outside the GCs, raising concerns about CBCLs, such as primary cutaneous follicle center lymphoma (PCFCL). However, the infiltrate outside the GCs expressed IgD and TCL1 and was confirmed to be an expanded mantle zone. The absence of cyclin-D1 expression ruled out mantle cell lymphoma. Although Southern blotting showed weak clonal bands, polymerase chain reaction-based clonality analysis and ultrasensitive RNA in situ hybridization for kappa and lambda chains did not detect clonal B-cell proliferation, leading to the diagnosis of CLH. Herein, we report BCL-6-positive cutaneous MZH as a novel subtype of CLH, highlighting the limitations of BCL-6 positivity in extrafollicular B-cells as an indicator for CBCLs, as it can be observed outside the GCs.