Bizarre parosteal osteochondromatous proliferation

Bizarre parosteal osteochondromatous proliferation (BPOP) (also known as a Nora lesion) is a benign exophytic osteochondral lesion which has an appearance similar to an osteochondroma, and is typically seen in the hands and feet. 

BPOPs are continuous with the underlying cortex, but usually without continuation of the medulla. 

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

Typically seen as well a 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 mineralising exophytic lesion arises from cortical bone with or without osteolysis, cortical flaring or a periosteal reaction. A lack of medullary involvement is characteristic of BPOP, although radiographic features alone cannot reliably diagnose the lesion.

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

Possible imaging differential considerations include:

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Article information

rID: 995
Section: Pathology
Synonyms or Alternate Spellings:
  • Nora lesion
  • Nora lesions
  • Bizarre parosteal osteochondromatous proliferation (BPOP)

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