Case studyLocalized pulmonary crystal-storing histiocytosis complicating pulmonary mucosa-associated lymphoid tissue lymphoma presenting with multiple mass lesions

•We report a case of pulmonary CSH complicating pulmonary MALT lymphoma.•Our patient had multiple lung masses consisting of CSH lesions with or without lymphoma.•Analyses of the lesions by electron microscopy and LC/MS are crucial for etiologic analysis.•Abnormal κ chains may crystallize only within plasma cells and histiocytes.

Crystal-storing histiocytosis (CSH) is an uncommon finding in lymphoplasmacytic disorders that presents histiocytes with abnormal intralysosomal accumulations of immunoglobulin light chains as crystals of unknown etiology. A 38-year-old woman with antiphospholipid syndrome had a surgical lung biopsy because of multiple lung mass lesions. In a right middle lobe lesion, lymphoplasmacytic cells had a monocytoid appearance, destructive lymphoepithelial lesions, and positive immunoglobulin heavy chain (IGH) gene rearrangements. A right upper lobe lesion manifested proliferating rounded histiocytes with abundant, deeply eosinophilic cytoplasm and negative IGH gene rearrangements. Electron microscopy and mass spectrometry revealed a case of pulmonary CSH: abnormal proliferation of the immunoglobulin κ chain of a variable region that may be crystallized within plasma cells and histiocytes. We report a rare case of localized pulmonary CSH complicating pulmonary mucosa-associated lymphoid tissue lymphoma with multiple mass lesions. We demonstrate advances in the understanding of the pathogenesis of CSH by various analyses of these lesions.