The radiographic findings and location of this lesion are typical of bizarre parosteal osteochondromatous proliferation (BPOP), also known as Nora's lesion. Identification of these entities on imaging as likely BPOP lesions is quite important, as histopathologically they may resemble a variety of more sinister lesions such as osteosarcoma, chondrosarcoma and fibrosarcoma.
BPOPs may show aggressive radiographic features and have aggressive behavior locally, but are benign and do not metastasize. Up to 50% may recur after resection.
As an aside, BPOP lesions are one of my favorite bone tumors, probably because they have "bizarre" in their name which makes them inherently interesting. They might be almost as cool as osteoid osteomas, the single coolest primary bone tumor.
Case images courtesy of George Matcuk, MD