A 14-year-old girl from Mumbai presents with a 2-month history of progressive pain and swelling over the proximal tibia. She reports night pain that wakes her from sleep. On examination, there is a firm, fixed mass over the proximal tibial metaphysis with overlying skin warmth. Radiographs reveal a mixed lytic and sclerotic lesion with cortical destruction, periosteal new bone formation in a sunburst pattern, and soft tissue extension. Serum alkaline phosphatase is 320 U/L (normal: 30–120) and LDH is 580 U/L (normal: 140–280). MRI shows a large intramedullary lesion with extraosseous soft tissue component. What is the most appropriate next step in management?