A Rare Case of Cutaneous Crystal-Storing Histiocytosis With Kappa Light Chain Restriction and Unusual BCL6 Expression.

Crystal-storing histiocytosis (CSH) is a rare histopathologic phenomenon characterized by the accumulation of crystalline material within histiocytes, most often associated with lymphoplasmacytic or plasma cell neoplasms. Cutaneous involvement is uncommon and may present diagnostic challenges. We report a 63-year-old woman with indurated plaques involving the face, chest, and upper extremities. Skin biopsies revealed aggregates of epithelioid histiocytes containing intracytoplasmic crystalline material, admixed with a lymphoplasmacytic infiltrate. The histiocytic cells were positive for CD68 and CD163. Plasma cells showed IgG kappa light chain restriction, and molecular studies demonstrated a monoclonal immunoglobulin heavy chain gene rearrangement. Serum studies identified an IgG kappa monoclonal gammopathy, supporting a diagnosis of cutaneous involvement by a B-cell lymphoma with plasmacytic differentiation associated with localized CSH. Weak, nonspecific immunoreactivity with the BRAF V600E antibody was observed; however, next-generation sequencing did not identify a BRAF V600E mutation, supporting interpretation as antibody cross-reactivity. A subset of histiocytes showed weak BCL6 expression of uncertain significance. This case highlights the importance of recognizing cutaneous CSH, emphasizes evaluation for an underlying lymphoproliferative disorder, and illustrates immunohistochemical pitfalls that may complicate interpretation.