A 16-year-old boy from Mumbai presents with a 3-month history of progressive left knee pain and swelling. On examination, he has a firm, non-tender mass over the anteromedial aspect of the distal femur. Plain radiographs show a metaphyseal lesion with a "sunburst" (radiating spiculated) periosteal reaction and cortical destruction. MRI reveals a large soft tissue mass with mixed signal intensity and areas of necrosis. What is the most likely diagnosis?