Preoperative Hypofractionated RT + Immunotherapy & Surgery for Retroperitoneal Sarcoma (FUSION-02)

Inclusion Criteria: * Diagnosis \& Indication: Primary or recurrent abdominopelvic soft tissue sarcoma. Patients are deemed to have a high probability of R1/R2 resection with upfront surgery and are confirmed to require radiotherapy following Multidisciplinary Team (MDT) discussion. * Target Volume: In cases of multifocal disease, all lesions must be safely encompassable within a single radiotherapy target volume. * Age: Age ≥ 18 years. * Performance Status: Eastern Cooperative Oncology Group (ECOG) performance status 0-2; American Society of Anesthesiologists (ASA) score ≤ 2; patients must be expected to tolerate surgery and radiotherapy. * Pathology: Histologically confirmed soft tissue sarcoma. * Measurability: Presence of evaluable lesions. * Life Expectancy: Life expectancy \> 5 years. * Hematological \& Hepatic Function: Adequate organ function * Renal Function: Adequate renal function defined as Glomerular Filtration Rate (GFR) ≥ 50 ml/min (calculated by Cockcroft-Gault formula), and split renal function imaging confirms function in the contralateral kidney. * Contraception: Female patients of childbearing potential, or male patients with female partners of childbearing potential, must agree to use effective contraception during the entire treatment period and for 6 months after the last dose. * Consent: Signed informed consent form. * Follow-up: Capable of complying with follow-up requirements. Exclusion Criteria: * Significant Comorbidities: Patients with the foll…