A 16-year-old boy presents with a 6-week history of progressive pain and swelling around the right knee. On examination, there is a firm, tender mass over the distal femur with overlying skin warmth. Plain radiographs show a mixed lytic-sclerotic lesion at the metaphysis with periosteal new bone formation (Codman's triangle). Serum alkaline phosphatase is elevated at 180 IU/L. What is the most appropriate next step in management?