An unusual case of Erdheim–Chester disease with features of Langerhans cell histiocytosis

Erdheim–Chester disease (ECD) and Langerhans cell histiocytosis (LCH) are both exceedingly rare histiocytic proliferations that can involve the skeletal system. We report on a case of ECD with some features suggestive of LCH. Radiographs demonstrated a large lytic lesion in the left femur, with multiple lesions of sclerosis involving both distal femurs and tibias. Both the lytic lesion and a sclerotic lesion were biopsied and demonstrated distinctive histologic features characteristic of ECD in the tibia and features of LCH in the femur. The clinical/radiologic and pathologic features that distinguish ECD and LCH as distinct entities are reviewed, and the underlying biological connection between them is discussed.