A 16-year-old girl from Mumbai presents with a 2-month history of progressive pain in the left pelvis and hip. She reports night sweats and unintentional weight loss of 3 kg. Physical examination reveals a palpable mass in the left iliac region with restricted hip flexion. Laboratory tests show hemoglobin 9.8 g/dL, WBC 11,200/μL, ESR 72 mm/hr, and LDH 580 U/L. CT pelvis shows a large heterogeneous mass arising from the left ilium with cortical destruction and a large soft tissue component. Biopsy reveals small round blue cells with high nuclear-to-cytoplasmic ratio and brisk mitotic activity. Immunohistochemistry is positive for CD99 and FLI1. What is the most appropriate next step in management?