title: Diagnostic criteria for pseudomalignant osteoblastoma.
abstract: OBJECTIVE
  Osteoblastoma is a rare bone tumour and differentiation from osteosarcoma is occasionally a diagnostic problem. The difficulty is further compounded when such a lesion microscopically shows cytological or architectural atypia.
  
  
  RESULTS
  A case of pseudomalignant osteoblastoma occurring in the left iliac crest of a 34-year-old Chinese woman is presented. Radiographic findings were non-diagnostic, but favoured a benign yet locally aggressive lesion. Histologically the diagnosis was based on the findings of classical osteoblastoma areas harbouring pseudomalignant osteoblasts in the absence of mitoses, but there were co-existing atypical architectural features also.
  
  
  CONCLUSIONS
  The significance and pathogenesis of the 'permeative pattern' are discussed. Difficulties in differentiating borderline lesions are emphasized.
authors.affiliation: Department of Clinical Pathology, Pamela Youde Nethersole Eastern Hospital, Chai Wan, Hong Kong.
authors.forename: F M
authors.initials: FM
authors.lastname: Cheung
authors.forename: W C
authors.initials: WC
authors.lastname: Wu
authors.forename: C K
authors.initials: CK
authors.lastname: Lam
authors.forename: Y K
authors.initials: YK
authors.lastname: Fu
mesh_terms: D000328:Adult; D001859:Bone Neoplasms / Q000175:diagnosis* / Q000473:pathology; D003937:Diagnosis, Differential; D005260:Female; D006801:Humans; D018215:Osteoblastoma / Q000175:diagnosis* / Q000473:pathology