A Rare Case of Tracheal Crystal-Storing Histiocytosis Associated with Marginal Zone Lymphoma of Mucosa-Associated Lymphoid Tissue.

Crystal-storing histiocytosis (CSH) is a rare non-neoplastic histiocytic lesion with abnormal accumulation of immunoglobulin (Ig) light chain. CSH is associated with Ig overproduction by B-lymphoproliferative disorders (B-LPDs) or by persistent inflammatory diseases. Eighteen cases of pulmonary CSH have been reported. However, no case reports of tracheal CSH have been published. In this patient, we found a solitary tracheal tumor in an asymptomatic 60-year-old man on chest computed tomography scan. Histologically, the tumor comprised two different lesions. One lesion showed diffuse proliferation of spindle-shaped histiocytes with abundant eosinophilic granular cytoplasm. With immunohistochemistry, the histiocytic cells were positive for CD68, CD163 and Ig kappa light chain, and the cytoplasm was weakly positive for anaplastic lymphoma kinase (ALK) protein. Fluorescence in situ hybridization indicated no split signals for the ALK gene. Electron microscopy demonstrated many elongated or rhomboid-shaped dense crystals in the cytoplasm of histiocytes. The second lesion showed proliferation of CD20-positive small atypical lymphocytes mixed with Ig kappa chain-positive plasma cells. A diagnosis of CSH and concomitant mucosa-associated lymphoid tissue lymphoma was made. In this patient, unexpected ALK protein was detected in infiltrating histiocytes. Therefore, careful assessment of the ALK protein and gene was necessary to differentiate from other histiocytic disorders.