Primary juxtacortical chondrosarcoma dedifferentiating after 20 years.

We present the clinical, radiographic and pathological features of a juxtacortical chondrosarcoma which underwent dedifferentiation to an osteosarcoma in a 47-year-old woman. The tumour, abutting the femoral diaphysis, had initially presented 20 years earlier. Local excision was performed at presentation and again 10 years later, but the tumour recurred on each occasion. Serial radiographs showed cortical saucerisation evolving to cortical buttressing and mounding. After 20 years a dramatic radiographic change heralded tumour dedifferentiation.