Bizarre Parosteal Osteochondromatous Proliferation: Nora's Lesion

The purpose of this study was to review the imaging and anatomopathologic findings and to discuss the main differential diagnosis of bizarre parosteal osteochondromatous proliferation (BPOP) or Nora's lesion, a rare benign surface lesion of the bone. Histologically confirmed plain radiographs, ultrasound, CT and MRI images of four patients with BPOP were obtained and retrospectively reviewed. Three cases involving the hand and one involving the foot are reported.On plain radiographs, BPOP is a well-marginated, calcified or ossified mass arising directly from the cortical surface of the underlying bone. Ultrasound images show a low echoic peripheral cap around the lesion. CT images show the wide base of the lesion. On MRI, BPOP was of a low signal on T1, enhancing following gadolinium administration. Underlying bone and adjacent surrounding soft tissues were normal.