A 16-year-old girl from rural Maharashtra presents with a 6-week history of progressive pain and swelling of the right tibia. She denies trauma. On examination, there is a firm, fixed swelling over the proximal tibia with overlying skin erythema. Plain radiographs show a mixed lytic and sclerotic lesion in the diaphysis with an 'onion-skin' periosteal reaction. MRI reveals a large soft tissue component with marrow infiltration. A biopsy is performed, and histopathology shows sheets of small, round, uniform cells with high mitotic activity and scanty cytoplasm. Immunohistochemistry is positive for CD99 and FLI1. What is the next most important step in management?