• Participants must have a histologically confirmed STS of the retroperitoneal space or infra-peritoneal spaces of pelvis

• Participant must have radiologically measurable disease (RECIST 1.1), as confirmed by abdomino-pelvic CT or MRI

• Participant must have primary, locally recurrent, or metastatic disease requiring treatment of the retroperitoneal mass

• Tumor must be suitable for radiotherapy and surgery based on pre-treatment CT scan/MRI in multidisciplinary discussion with surgeon and radiation oncologist (anticipated macroscopically complete resection, R0/R1 resection)

• Age: 18 years or older

• ECOG performance status ≤2

• Absence of history of bowel obstruction, mesenteric ischemia, or severe chronic inflammatory bowel disease.

• Normal renal function (calculated creatinine clearance ≥50 mL/min)

• Normal bone marrow and hepatic function (white blood cell count ≥2·5 × 10⁹ cells per L, platelet count ≥80 × 10⁹ cells per L, and total bilirubin \<2 times upper limit of normal)

• Women of child-bearing potential must have a negative pregnancy test within 3 weeks prior to the first day of study treatment

• Patients capable of childbearing/reproductive potential should use adequate contraception

• Ability to understand and the willingness to sign a written informed consent document