Negative images of crystalline immunoglobulin in crystal storing histiocytosis: A potential cytologic mimic of mycobacteria in smears

Two cases are described of crystal storing histiocytosis (CSH) associated with extranodal marginal zone lymphoma, presenting as lung and subcutaneous masses respectively. Fine-needle aspiration of subcutis and smears prepared from the resected lung masses showed negative images. Cytology slides of both cases were reviewed to identify cytomorphological features for the differential diagnosis between immunoglobulin crystals and mycobacteria. The crystals in CSH consist of straight and needle shaped rods with pointed or angular edges and are more variable in thickness than the uniformly thin mycobacteria. Mycobacteria show a haphazard distribution, whereas crystals are frequently present in parallel arrays. Small lymphoid or plasma cells are identified in the background of CSH, whereas a necrotic and inflammatory background is seen in mycobacteriosis. Additional samples for culture in the case of mycobacteriosis, or flow cytometry and molecular clonality testing in the case of CSH can provide critical data for a definitive diagnosis.