A self limiting tumour.

Case history A 20 year old professional footballer presented with pain and swelling of the left thigh after a kick three weeks previously. He had sustained a number of injuries in the past including a previous calf haematoma because of the nature of his profession but none had been as severe. Clinical examination revealed a normal gait. There was definite but diVuse swelling of the extensor compartment of the left thigh that measured 1 cm more in circumference compared with the normal right side at the level of the mid-thigh. It was tender but no discrete mass could be felt. There was also slight limitation of knee flexion but extension was full and no hip abnormality was detected. The patient was otherwise well with no constitutional symptoms. The rest of the physical examination was normal. Plain radiographs of the left thigh were obtained. What abnormality does figure 1 show? What is the diVerential diagnosis?