Bone marrow extracellular large geometric crystals in IgG/lambda MGUS.

We describe the peculiar histopathology of the bone marrow in a case of IgG/lambda MGUS. Striking eosinophilic crystals with a rectangular, rhomboid or square shape lay in the interstitium, sometimes in optically empty spaces, but failed to elicit a foreign body giant cell reaction. Their histochemical properties, immunoreactivity for anti-lambda light chain antiserum, and ultrastructural features strongly supported their relationship with the paraprotein synthesized by the monoclonal plasma cells. The crystals were not observed on bone marrow aspirate smear, suggesting that they had formed during trephine biopsy processing or, alternatively, that they had been removed during the smear preparation. We feel that pathologists should be aware of the existence of this type of crystals, which differ from both the amyloid deposits and the proteinaceous material sometimes observed in plasma cell proliferations. Their presence in the bone marrow should alert the clinician to investigate the involvement of other organs with immunoglobulin deposits.