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 following concurrent conditions: active hemorrhage, ulceration, intestinal perforation, intestinal obstruction, uncontrolled hypertension, cardiac insufficiency (NYHA Class III-IV), severe hepatic or renal dysfunction (Grade 4), mesenteric ischemia, or severe inflammatory bowel disease (IBD). * Excluded Histologies: Other pathological types, including Gastrointestinal Stromal Tumor (GIST), sarcomatoid carcinoma, sarcomas primarily treated with chemotherapy (e.g., rhabdomyosarcoma and PNET), desmoid fibromatosis, and benign tumors. * Unresectability: Tumor invasion into critical structures such as the liver, pancreatic head, or abdominal aorta, rendering the tumor deemed unresectable. * Prior Radiotherapy: History of prior radiotherapy to the same anatomical region. * Distant Metastasis: Presence of distant metastasis (M1) confirmed by comprehensive imaging/examination within 4 weeks prior to enrollment. * Contraindications to Surgery/Protocol: Presence of other severe medical comorbidities that contraindicate surgery or preclude participation in the study. * Planned Neoadjuvant Chemotherapy: Patients scheduled to receive (concurrent) neoadjuvant chemotherapy. * Recent Systemic Therapy: Receipt of chemotherapy or other systemic treatments within 4 weeks prior to enrollment.