Background: Erdheim-Chester disease (ECD) is a rare xanthogranulomatous histiocytic disorder. Langerhans cell histiocytosis (LCH) is a proliferative disorder of histiocytes with a phenotype similar to dendritic Langerhans cells. Both are derived from myeloid stem cells in the bone marrow and, thus, can overlap. Objective: We report a rare case of hybrid LCH and ECD of the skin with systemic ECD. Methods: Pathologic examinations and human androgen-receptor gene assay were used to study this case. Results: A 34-year-old woman presented with recurrent ulcerative skin lesions on both thighs associated with polydipsia and polyuria since childhood. Radiography revealed osteosclerosis of bilateral distal tibias and soft tissue masses of bilateral chest walls and ankles. Pathologically, the chest wall lesions showed dense aggregates of lipid-laden histiocytes, which were CD68+/CD163+/S100 -/CD1a-. Combined with the clinical and radiographic findings, this xanthogranulomatous infiltrate was consistent with ECD. However, thigh skin showed discrete foci of a xanthogranulomatous infiltrate and S100+/CD1a+ Langerhans cells with eosinophils. In addition, Birbeck granules were found. Dissected tissues from both ECD and LCH were monoclonal, supporting their neoplastic nature. Limitations: Single case report is a limitation. Conclusion: ECD and LCH may have a close association with divergent differentiation from the same stem cells under different microenvironmental conditions.
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