[The x-ray morphology of parosteal osteosarcoma (POS)]

31 cases of parosteal osteosarcoma (POS) including both the differentiated and dedifferentiated variant were reviewed. Most patients were in the second and third decades of life. The tumours usually arose in the meta-diaphyseal (52%) and diaphyseal (33%) portion of long bones with a predilection for the knee (55%). Twenty-seven percent of patients showed involvement of the medullary cavity. 68% of lesions affected the bone cortex. A periosteal reaction was observed in 24% of patients. The characteristic but not pathognomic sign of a fine radiolucent line separating the tumour from the cortex was seen in only 48%. No characteristic radiographic features were evident that allowed a differentiation between the differentiated and the dedifferentiated POS. The presence or absence of medullary involvement was no a reliable diagnostic criterion. In all dedifferentiated POS, however, we observed a periosteal reaction.