Inclusion Criteria:

• Age 10 to 60 years.
• Histologically confirmed Ewing sarcoma.
• Newly diagnosed disease and scheduled to initiate first-line systemic therapy at Peking University People's Hospital.
• No prior systemic chemotherapy for Ewing sarcoma and no prior radiotherapy to the primary tumor.
• At least one evaluable lesion on baseline imaging; at least one measurable lesion according to RECIST version 1.1 is required for the primary outcome analysis.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.
• Estimated life expectancy greater than 3 months.
• Written informed consent provided by the participant or the participant's legally authorized representative.

Exclusion Criteria:

• History of another malignant tumor within the previous 3 years, except for an adequately treated malignancy with a negligible risk of recurrence, as determined by the investigator.
• New York Heart Association (NYHA) class III or IV heart failure.
• Uncontrolled concomitant illness, including uncontrolled diabetes mellitus or an active infection.
• Severe infection requiring intravenous antimicrobial treatment within 4 weeks before enrollment.
• Clinically significant bleeding or a clear bleeding tendency within 3 months before enrollment.
• Severe renal impairment, defined as an estimated glomerular filtration rate (eGFR) below 30 mL/min/1.73 m².
• Pregnancy or breastfeeding.
• Any condition that, in the investigator's judgment, may compromise participant safety, interfere with study assessments, or prevent compliance with the protocol.