Diagnostic criteria for pseudomalignant osteoblastoma

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.

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.

The significance and pathogenesis of the 'permeative pattern' are discussed. Difficulties in differentiating borderline lesions are emphasized.