Twelve cases of bizarre parosteal osteochondromatous proliferation (BPOP), also known as Nora's lesion, are reported. Ten lesions were located in the small bones of the hands, and 2 were located in long bones (femur and proximal tibia). Patient age ranged from 12 to 63 years (average, 30.3 years). Radiography of the lesions in the hand bones showed calcific masses attached to the underlying cortex, without interruption of the latter. The long bone lesions revealed unusual findings. In the femur, BPOP presented with extensive cortical destruction and was suggestive of a malignant lesion. This presentation has not been described to date. In the tibia, the lesion was located in the soft tissue without cortical attachment. This type of BPOP probably represents an immature lesion that over time will mature to solid cortical attachment. On histologic examination, all lesions demonstrated 3 distinct components with variable degrees of representation: (1) hypercellular cartilage with calcification and ossification, with the calcified cartilage having a characteristic basophilic tinctorial quality; (2) cancellous bone undergoing maturation; and (3) spindle cell stroma without cytologic atypia. In 1 case with a long-standing history, the cartilaginous component was minimal. BPOP, together with florid reactive periostitis and turret exostosis, may represent different stages in the development of a posttraumatic proliferative process. BPOP apparently arises from the periosteal tissues through a process of cartilaginous metaplasia.
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