Not So Crystal Clear: Pulmonary Crystal-Storing Histiocytosis Without Haematological Disease: A Unique Radiological Manifestation and Review of the Literature

Background and Clinical Significance: Pulmonary crystal-storing histiocytosis (CSH) without an associated haematological malignancy, lymphoproliferative disorder, plasma cell disorder, or other identifiable underlying condition is exceptionally rare. Long-term radiological data from published cases remain limited, and this case, contextualised by a narrative review of the literature, may expand the recognised imaging spectrum of localised pulmonary CSH. Case Presentation: We report the case of a 59-year-old man with incidental multiple pulmonary lesions identified on CT thorax imaging. The lesions demonstrated an unusual combination of cystic change, cavitation, and surrounding ground-glass opacities. His medical history was significant for bipolar disorder treated with lithium and a 50-pack-year smoking history. Interval imaging showed progression, prompting further investigation and ultimately right upper lobectomy. Histopathological analysis confirmed pulmonary CSH; lesional cells contained crystalloid material and showed CD68 and PAS positivity, with dual kappa and lambda expression on immunohistochemistry. Markers for other differential diagnoses, including Congo red, birefringence, Langerin, and CD1a were negative. Following diagnosis and resection, serial imaging demonstrated fluctuating yet slowly progressive pulmonary abnormalities. No lymphoproliferative or plasma cell disorder has emerged after more than seven years of post-diagnostic surveillance and more than ten years since the initial imaging abnormality. Conclusions: This case demonstrates that the radiological spectrum of localised pulmonary CSH may include cystic, cavitary and ground-glass abnormalities, with subsequent fluctuating yet slowly progressive post-resection evolution. Pulmonary CSH should be considered in the differential diagnosis of unexplained or atypical pulmonary nodules, particularly when histiocyte-rich pathology with intracytoplasmic crystalloid material is identified. Long-term multidisciplinary surveillance is warranted.