Primary Rosai-Dorfman disease of the femur.

We report a 19-year-old man with the rare occurrence of primary osseous Rosai-Dorfman disease (RDD). The patient presented with a painful, solitary, bone marrow-replacing lesion in the distal femur. A diagnosis of chronic osteomyelitis was initially made on tissue from a CT-guided needle biopsy of the lesion; however, the diagnosis of RDD was eventually made after histological and immunohistochemical analysis of material from a subsequent curettage. No lymphadenopathy or other sites of involvement were found on clinical evaluation and PET-CT. To our knowledge, this is the first report of solitary osseous RDD based on systemic staging with PET-CT. We review the clinical, imaging, and histological features of primary osseous RDD, including pitfalls in diagnosis.