A 28-year-old woman presents with a 6-month history of progressive pain and swelling in the right knee. Plain radiographs reveal an eccentric, lytic lesion in the distal femur extending to the subarticular cortex. MRI shows a heterogeneous mass with hemosiderin deposits. Fine-needle aspiration cytology confirms multinucleated giant cells with stromal fibroblasts. What is the most appropriate next step in management?