An Overlap of Erdheim-Chester Disease and Rosai-Dorfman Disease With Isolated Bone Lesions Presenting a Diagnostic Challenge: A Case Report.

Some of the rarest diseases with cell accumulation in different organs are histiocytoses. Since these diagnoses are uncommon, other differential diagnoses must be excluded. We report a case of a 42-year-old man with episodes of knee pain and fever. Multiple sclerotic bone lesions were discovered by 18F-fluorodeoxyglucose positron emission tomography-computed tomography (FDG PET/CT). A bone biopsy was taken, and it showed a mixed infiltrate consisting of mature lymphocytes, plasma cells, and many histiocytes (CD68 + /CD163 + /S-100 -/+ /CD1a - /langerin - ), including some foamy macrophages and cells with emperipolesis. Histology showed characteristics for both Erdheim-Chester disease (ECD) and Rosai-Dorfman disease (RDD). Molecular analysis showed MAP2K1 mutation that can be seen with both diseases. Considering the clinical manifestations that resembled ECD and histologic RDD-like features, a diagnosis of overlap between ECD and RDD with isolated multifocal bone involvement was made. Treatment with pegylated interferon alfa (Peg-IFN-α) was started, and after three months clear clinical improvement was reported by the patient, although PET/CT demonstrated metabolically stable disease.