All That Glitters Is Not Gold: Crystal-Storing Histiocytosis Presenting as a Lung Mass.

Crystal-storing histiocytosis (CSH) is a rare condition often linked to monoclonal gammopathies and lymphoproliferative disorders. It involves the accumulation of crystallized immunoglobulins within histiocytes and may mimic other diseases, especially when presenting in unusual sites such as the lung. We report on a 51-year-old man with chronic cough and fatigue, initially suspected of having an infectious pulmonary process. Imaging revealed a lesion in the right lung. Bronchial biopsy showed large eosinophilic histiocytes with granular cytoplasm. Immunohistochemistry confirmed histiocytic differentiation and light chain-restricted plasma cells, consistent with CSH. Further workup revealed monoclonal IgM kappa gammopathy without bone marrow involvement. A subsequent lung biopsy showed small B-cell non-Hodgkin lymphoma infiltrating the parenchyma. This presentation highlights the importance of including CSH in the differential diagnosis of pulmonary lesions, as well as the critical role of histopathology and immunohistochemistry in identifying underlying hematologic malignancies.