A 16-year-old girl presents with a 4-month history of pain and swelling in the right tibia. She denies trauma. On examination, there is a firm, tender swelling over the anterior tibia with overlying skin warmth. Laboratory findings show WBC 14,200/μL, ESR 55 mm/hr, and LDH 680 U/L (elevated). Plain radiographs show a lytic lesion in the diaphysis with a wide zone of transition and cortical destruction. MRI reveals a large intramedullary and extraosseous soft tissue component with heterogeneous signal intensity. What is the most appropriate next step in management?