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Sternal mass core biopsy performed in a 64-year-old man showed extensive involvement by non-neoplastic histiocytes containing abundant eosinophilic amorphous material, occasional crystals, and globules of immune globulin (Panel A and Panel B; Hematoxylin and Eosin stain; original magnification x100 and x500). Immunostains confirmed sheets of CD68 positive histiocytes separated by small, mature appearing plasma cells (Panel C; original magnification x400) . By in situ hybridization, both the plasma cells (Panel D; arrowhead) and the histiocytes (Panel D; arrow) contain cytoplasmic kappa light chain immunoglobulin and negative for lambda light chain immunoglobulin (Panel D; original magnification x400 [inset]).

Crystal-storing histiocytosis is a rare lesion in which crystalline material accumulates in the cytoplasm of histiocytes that are present in soft tissue. The crystal deposits are generated by intralysosomal accumulation of immunoglobulin and are typically monotypic for kappa light chain, though cases of lambda light chain restriction have been reported. Most cases are associated with underlying multiple myeloma, lymphoplasmacytic lymphoma, or monoclonal gammopathy with rare cases being associated with infection or autoimmune disease. The number of histiocytes in the lesion can range from very few to abundant. Lesions with overwhelming histiocytosis, as in our case, can be diagnostically challenging as the reactive histiocytes may obscure the underlying plasmacytic or lymphoplasmacytic neoplasm.