A 16-year-old girl presents with a 2-month history of pain in the right tibia and a visible swelling. She denies trauma. Clinical examination reveals a firm, tender mass over the anterior tibia with warmth and mild erythema. Laboratory work shows WBC 11,200/μL, ESR 65 mm/hr, and CRP 8.5 mg/dL. Plain radiographs show a mixed lytic-sclerotic lesion in the diaphysis with 'onion-skin' periosteal reaction. CT chest is normal. Biopsy confirms Ewing sarcoma with EWSR1-FLI1 fusion gene. The patient is started on neoadjuvant chemotherapy. After 10 weeks, repeat imaging shows 90% reduction in tumour size and no new lesions. What is the significance of this radiological response?