Multifocal osseous and cutaneous Rosai-Dorfman-Destombes disease presenting as an aggressive intramedullary femoral lesion.

Rosai-Dorfman-Destombes disease (RDD) is a rare histiocytic disorder characterized by the accumulation of abnormal histiocytes in nodal and extranodal tissues. We report a 24-year-old male presenting with progressive right distal thigh pain for 1 month. MRI demonstrated a 3.5 cm intramedullary lesion in the distal right femoral metadiaphysis with minimal periosteal edema, raising concern for malignancy. Staging CT scans showed no evidence of metastatic disease. Biopsy of this lesion revealed the characteristic accumulation of CD1a-negative and S100-positive histiocytes with emperipolesis, consistent with RDD. Subsequently, PET/CT scan demonstrated multifocal osseous disease involving at least 4 sites, involving the distal right femur (SUVmax 9.0), left humerus (SUVmax 9.7), L4 vertebra, and sacrum, along with 2 cutaneous lesions and possible tonsillar and cervical lymph node involvement. The patient was initiated on the MEK inhibitor cobimetinib and follow-up PET/CT demonstrated complete metabolic resolution of the previously identified osseous disease after approximately 3 months. RDD should be considered in the differential diagnosis when imaging demonstrates concerning osseous lesions in young adults, as definitive diagnosis relies on histopathological evaluation.