Online Articles: Case ReportTwo Cases of Crystal-storing Histiocytosis Diagnosed by Morphology, Immunohistochemistry, and Ultrastructural ExaminationVogel, Ashley N. DO*; Casey, James MD*; Kaur, Jashanpreet BDS†; Uppal, Guldeep MD* Author Information *Department of Pathology, Anatomy, & Cell Biology, Thomas Jefferson University, Philadelphia, PA †Shri Guru Ram Das Institute of Dental Sciences & Research, Amritsar, Punjab, India The authors declare no conflict of interest. Reprints: Ashley N. Vogel, DO, Department of Pathology, Anatomy, & Cell Biology, Thomas Jefferson University, 117 South 11th Street, Suite 301, Philadelphia, PA 19107 (e-mail: [email protected]). Applied Immunohistochemistry & Molecular Morphology: January 2021 - Volume 29 - Issue 1 - p e1-e4 doi: 10.1097/PAI.0000000000000765 Buy Metrics Abstract Crystal-storing histiocytosis (CSH) is a non-neoplastic histiocytic proliferation containing crystalline material, usually associated with an underlying lymphoproliferative or plasmacytic disorder. The crystalline structures are typically derived from kappa light chain immunoglobulins. The lesions of CSH are comprised of sheets of histiocytes with abundant eosinophilic cytoplasm containing variably prominent, elongated crystals. This rare phenomenon is important to recognize, as it is known to morphologically obscure an underlying neoplasm. Histologically, the cells of CSH may closely mimic Gaucher cells, as well as the “pseudo-Gaucher” cells sometimes encountered in chronic myeloid leukemia. The distinction between the cells of CSH and that of histologic mimics may be made more definitively through the use of electron microscopy, as the crystalline inclusions seen in CSH display characteristic size, shape, and localization within the cells. Here, we report 2 rare cases of CSH diagnosed by morphology, immunohistochemistry, and ultrastructural examination. The first case presented was diagnosed concurrently with plasma cell myeloma, and the second case discussed was diagnosed in association with marginal zone lymphoma. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.