A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the left femur. Plain radiographs show a mixed lytic-sclerotic lesion in the diaphysis with periosteal reaction and soft tissue mass. Laboratory investigations reveal elevated LDH (850 U/L) and normal alkaline phosphatase. A diagnosis of Ewing sarcoma is suspected. What is the most appropriate next step in management?