A 14-year-old girl from Mumbai has been diagnosed with Ewing sarcoma of the right tibia (stage IIB, 8 cm lesion, no metastases). She has completed 12 weeks of neoadjuvant chemotherapy (doxorubicin, cisplatin, ifosfamide) with good radiological response (>90% tumor necrosis on MRI). What is the most appropriate next step in management?