A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur just above the knee. He reports the pain is worse at night and does not respond well to NSAIDs. On examination, there is a firm, warm, tender mass over the distal femur with restricted knee flexion. Plain radiographs show a mixed lytic and sclerotic lesion in the metaphyseal region with cortical destruction and a sunburst pattern of periosteal reaction. What is the most likely diagnosis?