title: Nora's lesion, a distinct radiological entity?
abstract: OBJECTIVE
  To describe the radiological findings of "Bizarre parosteal osteochondromatous proliferation"(BPOP)-otherwise known as Nora's lesion, to describe the natural evolution of BPOP and to assess radiologically if BPOP is indeed part of a spectrum of reactive lesions including florid reactive periostitis and turret exostosis.
  
  
  METHODS
  Four experienced musculoskeletal radiologists studied plain radiographs and other imaging documents of histologically-proven Nora's lesions, looking for soft-tissue changes, periosteal reaction/calcification and calcified/ossified pseudotumours, and compared those findings with findings on pathology reviewed by a peer group of pathologists.
  
  
  METHODS
  Twenty-four Nora's lesions originating from a series of 200 consecutive, histologically-verified bone (pseudo)tumours of the hand, seen by the "Netherlands Committee on Bone Tumours" for review and second opinion.
  
  
  RESULTS
  Nora's lesions have a recognised presentation on radiographs without specific MR characteristics. Natural evolution could be assessed retrospectively in four cases. Recurrent lesions were seen in seven cases and are difficult to differentiate from primary lesions.
  
  
  CONCLUSIONS
  Nora's lesion, defined as a "well-marginated mass of heterotopic mineralization arising from the periosteal aspect of an intact cortex, without medullary changes" has a distinct radiological presentation and is part of a spectrum of reactive lesions which includes florid reactive periostitis and turret exostosis. As it has a distinct radiological appearance, differential diagnosis of malignant lesions such as osteosarcoma and chondrosarcoma should be clear. It does not require immediate biopsy unless the natural evolution is unspecific.
authors.affiliation: Department of Radiology, Leiden University Medical Center, Albinusdreef 2, PO Box 9600, 2300, RC Leiden, The Netherlands.
authors.forename: E
authors.initials: E
authors.lastname: Dhondt
authors.forename: L
authors.initials: L
authors.lastname: Oudenhoven
authors.forename: S
authors.initials: S
authors.lastname: Khan
authors.forename: H M
authors.initials: HM
authors.lastname: Kroon
authors.forename: P C
authors.initials: PC
authors.lastname: Hogendoorn
authors.forename: A
authors.initials: A
authors.lastname: Nieborg
authors.forename: J L
authors.initials: JL
authors.lastname: Bloem
authors.forename: A
authors.initials: A
authors.lastname: De Schepper
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002114:Calcinosis / Q000000981:diagnostic imaging* / Q000473:pathology*; D002648:Child; D003937:Diagnosis, Differential; D018450:Disease Progression; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D015831:Osteochondroma / Q000000981:diagnostic imaging* / Q000473:pathology*; D010522:Periostitis / Q000000981:diagnostic imaging* / Q000473:pathology*; D011859:Radiography; D014463:Ultrasonography