A challenging diagnosis: crystal-storing histiocytosis in plasma cell myeloma.

Objectives: Crystal-storing histiocytosis (CSH) is an uncommon finding in plasma cell neoplasms. CSH is thought to be an intralysosomal deposition of secreted paraproteins or immunoglobulins, which usually express kappa immunoglobulin light chains that finally aggregate in crystals. Because of its rarity, CSH in bone marrow often makes diagnosis difficult. Methods: A 57-year-old woman had IgA kappa monoclonal proteinemia and monoclonal proteinuria. In the bone marrow aspirate, plasma cells were initially counted less than what would be expected, whereas histiocytes with intracellular crystals were increased. Then, we used alpha-naphthyl acetate esterase (ANAE) staining to distinguish between true histiocytes and plasma cells. Immunostaining for kappa, CD138, CD56, and CD68 was performed on a bone marrow biopsy specimen. Results: True histiocytes containing crystalline inclusions were stained strongly for ANAE, while unstained cells with intracytoplasmic crystals represented plasma cells. The biopsy specimen revealed diffuse infiltration of CD138-positive plasma cells. We also confirmed the presence of plasma cells, histiocytes, and their crystallized inclusions with the immunostaining. The patient was finally diagnosed with plasma cell myeloma. Conclusions: The diagnosis was challenging; the bone marrow findings resembled features of other histiocytic disorders. The use of immunohistochemistry enabled the diagnosis of CSH in the presence of plasma cell myeloma.