Bizarre parosteal osteochondromatous proliferation

Bizarre parosteal osteochondromatous proliferations (BPOP), also known as Nora lesions, are benign exophytic osteochondral lesions which have an appearance similar to an osteochondroma and are typically seen in the hands and feet. On imaging, BPOPs are shown to be continuous with the underlying cortex, but usually without continuation of the medulla. 

Epidemiology

BPOPs are most often seen in young (20-30-year-old) patients. There is no recognized gender predilection.

Radiographic features

Plain radiograph

Typically seen as a well-marginated wide-based bony growth projecting into the soft tissues although often lacks the characteristic orientation away from the nearby physis seen with osteochondromas. The mineralizing exophytic lesion arises from the cortical bone with or without osteolysis, cortical flaring or a periosteal reaction. A lack of medullary involvement is characteristic of bizarre parosteal osteochondromatous proliferation, although radiographic features alone cannot reliably diagnose the lesion.

Treatment and prognosis

Bizarre parosteal osteochondromatous proliferations are benign lesions with no risk of distant metastasis but they may show marked local invasion and may recur after surgical excision.

History and etymology

Frederick E Nora (fl. 2020) is an American pathologist who first described bizarre parosteal osteochondromatous proliferations whilst working at the Mayo Clinic in 1983 ⁷. 

Differential diagnosis

Possible imaging differential considerations include:

• osteochondroma
• periosteal chondroma
• parosteal osteosarcoma
• periosteal osteosarcoma
• periosteal chondrosarcoma
• florid reactive periostitis
• turret exostosis
• subungual exostosis