Giant cell tumor with secondary aneurysmal bone cyst: a unique presentation with an ossified extraosseous soft tissue mass.

The authors describe a case of giant cell tumor (GCT) with secondary aneurysmal bone cyst (ABC) in a 44-year-old man with chronic, intermittent knee pain. A unique feature is the presentation of GCT with an ossified extraosseous soft tissue mass. Radiograph demonstrates a multiloculated lytic lesion in the distal meta-epiphyseal region of the femur with an adjacent extraosseous soft tissue mass. The soft tissue mass was partially ossified along its margin and internal septa. MRI demonstrates a multiloculated lesion in the distal femur with multiple fluid-fluid levels and cortical penetration of the lesion. Both the intraosseous lesion and extraosseous soft tissue mass have similar MR signal characteristics. At surgery, the intraosseous component was found to be contiguous with the extraosseous soft tissue mass through a cortical perforation. To the best of our knowledge, this is the first case report of GCT with aneurysmal bone cyst initially presenting with an extraosseous soft tissue mass.