A 14-year-old girl with a history of retinoblastoma (treated with chemotherapy and radiation 8 years ago) presents with progressive pain and swelling of the proximal humerus over the past 4 months. Radiographs show a mixed lytic and sclerotic lesion with cortical destruction and soft tissue mass. She undergoes staging investigations: chest CT shows multiple pulmonary nodules, and alkaline phosphatase is 580 U/L. Biopsy confirms osteosarcoma. Which of the following is the most important prognostic factor in this case?