A 14-year-old girl presents with a 6-week history of pain and swelling over the proximal tibia. She reports the pain is progressive and associated with a palpable mass. Blood investigations show alkaline phosphatase 450 U/L (normal <130) and LDH 520 U/L (normal <240). Plain radiographs reveal a metaphyseal lesion with mixed lytic and sclerotic areas, cortical breakthrough, and a prominent Codman triangle. CT chest is performed as part of staging. What is the most appropriate next step in management after confirming the diagnosis with biopsy?