A 27-year-old woman presented with a thymic mass containing cystic spaces and an atypical lymphoid infiltrate with plasma-cell differentiation. Juxtaposed to the lymphoid component were numerous histiocytes with abundant cytoplasm filled with crystalline and/or globular eosinophilic material (panels A-B; hematoxylin and eosin stain, original magnification ×100). CD68 highlighted numerous macrophages and accentuated the crystalline and globoid structures (panel C; CD68 [KP1], original magnification ×400). λ light chain restriction was demonstrated in both lymphoplasmacytic cells and crystalline inclusions (panel D; λ light chain, original magnification ×400). Polymerase chain reaction identified clonal rearrangement of the κ-deleting element locus, consistent with λ light chain expression. MYD88 mutation was negative. The final diagnosis was thymic extranodal marginal zone lymphoma with plasmacytic differentiation and crystal-storing histiocytosis (CSH).

CSH most often represents the accumulation of crystallized immunoglobulin molecules within the cytoplasm of histiocytes. It is encountered in B-lymphoproliferative disorders with plasmacytic differentiation, including myeloma. The pathogenetic mechanism most likely is an alteration in the immunoglobulin molecules conferring abnormal crystallization properties. A majority of the cases show serum or urine paraprotein. Preferential association with any heavy or light chain class has not been noted in published reports. Awareness of this lesion is important because CSH may obscure the underlying neoplasm.