A 14-year-old girl presents with a 2-month history of pain and swelling around the proximal tibia. She had a history of retinoblastoma treated with chemotherapy and external beam radiation at age 3 years. On examination, there is a firm, fixed mass over the proximal tibia. Plain radiographs show a mixed lytic and sclerotic lesion with cortical breakthrough and soft tissue extension. Serum alkaline phosphatase is 320 U/L and LDH is 580 U/L. MRI shows a large intramedullary lesion with marrow involvement extending into surrounding soft tissues. What is the most likely diagnosis?