Background: Histiocytosis X is a neoplastic disorder of Langerhans cells that usually occurs in children. Because histiocytosis X rarely occurs in adults, the diagnosis can frequently be missed by both clinicians and pathologists. Objective: The purpose of this investigation was to characterize the clinical and pathologic findings of histiocytosis X in adults. Methods: A retrospective study of four adults with histiocytosis X was undertaken. Paraffin-embedded biopsy specimens were stained with a panel of antibodies including S-100, vimen-tin, Ham-56, leukocyte common antigen, proliferating cell nuclear antigen (PCNA), UCHL-1, CD43, and Ki-1. Results: The predominant lesions were papules and pustules that usually involved the groin, axilla, and scalp. Histologically the infiltrate exhibited a periappendageal distribution. Strong positive staining for PCNA appeared to correlate with the clinical course. Conclusion: Histiocytosis X in adults has a predilection for skin sites rich in appendages and histologically shows a periappendageal infiltrate. PCNA staining of the histiocytes may indicate progressive disease and serve as a useful prognostic marker.
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