A 64-year-old woman was referred to our center for a painful swelling of the neck and difficulty swallowing. Physical examination and positron emission tomography/computed tomography imaging revealed a firm submandibular mass with skin changes (panel A). Laboratory and imaging findings met the diagnostic criteria for smoldering myeloma with immunoglobulin G κ-restricted plasma cells. Histopathologic examination of the skin showed fibrosis and numerous histiocytes with abundant cytoplasm containing eosinophilic crystals (panel B arrows; original magnification ×800; hematoxylin and eosin [H&E] stain) immunohistochemically positive for κ light chains, consistent with crystal-storing histiocytosis (CSH). This was further illustrated by electron microscopy demonstrating the presence of crystals within the cytoplasm of the histiocytes (panel C arrows; original magnification ×10 000). Skin biopsy did not reveal plasma cell infiltrates, and bone marrow was not involved by CSH.

CSH is a rare disorder associated with underlying lymphoproliferative or plasma cell disorders. The characteristic morphology should alert the pathologist; electron microscopy also supports the diagnosis. Awareness of CSH is critical, because the disease is associated with high mortality and, in this case, significant morbidity. Therefore, although the standard of care for smoldering myeloma would be observation, the patient was started on melphalan, prednisone, bortezomib, and daratumumab, which resulted in clinical improvement of the skin lesions and decrease in monoclonal protein.