Plasma Cell Myeloma Presenting With Amyloid-Laden Crystal-Negative Histiocytosis.

Objectives: Crystal-storing histiocytosis (CSH) is rare in plasma cell dyscrasias, with only 3 cases reported in the setting of amyloid. No cases of crystal-negative histiocytosis coincident with multiple myeloma and amyloidosis have been reported previously.
Methods: A 58-year-old woman presented with pain due to destructive bone lesions and was found to have plasma cell myeloma (PCM) and marrow amyloid deposition associated with crystal-negative histiocytosis. Differential diagnoses included Langerhans cell histiocytosis, Erdheim-Chester disease, and Rosai Dorfman disease. BRAF mutations were negative, and there was no evidence of paraprotein crystals, arguing against typical CSH.
Results: The patient was treated with bortezomib, cyclophosphamide, and dexamethasone, and she subsequently underwent autologous stem cell transplant and ixazomib maintenance. She achieved complete remission with improvement of her symptoms and preserved remission after following up at 60 months.
Conclusions: We describe a case of crystal-negative histiocytosis associated with PCM. CSH is a rare disorder associated with paraprotein-producing conditions in which immunoglobulins aggregate as intracellular crystals in the lysosomes of organ-specific phagocytic macrophages. Light chain tropism in PCM can also lead to the development of amyloid deposition in organs and, in rare cases, is associated with light chain aggregation as intracellular crystals in macrophages.
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