Prospective Evaluation of Dynamic Serum ProGRP for Treatment Response Monitoring in Ewing Sarcoma

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.