A 14-year-old boy presents to the orthopaedic clinic with a 3-month history of progressive pain and swelling over the left femur. He reports low-grade fever and night sweats. On examination, the left thigh is warm and tender with a palpable mass. Laboratory investigations show elevated ESR (78 mm/hr) and CRP (12 mg/dL). Plain radiographs reveal a mixed lytic and sclerotic lesion in the diaphysis of the femur with periosteal reaction. MRI shows a large soft tissue component. A biopsy reveals small round cells with scanty cytoplasm arranged in sheets, with positive immunohistochemistry for CD99 and FLI1. What is the most appropriate next step in management?