A 16-year-old boy from Delhi presents with a 3-month history of progressive pain and swelling in the mid-shaft of the left femur. Physical examination reveals local warmth, tenderness, and a palpable mass. Plain radiographs show a mixed lytic-sclerotic lesion with periosteal reaction and cortical thickening in the diaphysis. Laboratory investigations reveal elevated ESR (78 mm/hr) and normal alkaline phosphatase. What is the most appropriate next step in management?