Crystal-Storing Histiocytosis of the Duodenum in a Patient With IgG-Lambda Multiple Myeloma.

Crystal-storing histiocytosis is a rare condition characterized by the accumulation of crystalline immunoglobulins within histiocytes, often associated with plasma cell disorders such as multiple myeloma. We report a 70-year-old man with a history of IgG-lambda multiple myeloma, systemic amyloidosis, and end-stage renal disease managed with peritoneal dialysis and long-term lanthanum carbonate therapy for hyperphosphatemia, who presented with gastrointestinal bleeding. Endoscopy revealed nodular erythematous mucosa in the duodenum, and biopsies showed histiocytosis in the lamina propria. The histiocytes showed abundant cytoplasm containing periodic acid-Schiff-positive diastase-resistant intracellular material. Immunohistochemical staining revealed diffuse CD68 positivity and lambda light-chain restriction of the storage material in the histiocytes, supporting a diagnosis of crystal-storing histiocytosis. Differential diagnoses, including Whipple disease, mycobacterial infections, lanthanum deposition, amyloidosis, and xanthoma, were excluded based on histological findings and clinical history. This report highlights the importance of recognizing crystal-storing histiocytosis in uncommon sites such as the duodenum, especially in the context of plasma cell dyscrasias.