A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur, just above the knee. On examination, there is a firm, fixed mass with overlying skin warmth. Plain radiographs show a mixed lytic and sclerotic lesion at the distal femoral metaphysis with cortical destruction and a wide zone of transition. There is a prominent sunburst pattern of periosteal reaction. MRI reveals a large soft tissue mass with heterogeneous signal intensity and areas of necrosis. Which is the most likely diagnosis?