Short Course Radiation Treatment for Patients With Primary or Locally Recurrent Retroperitoneal Sarcoma Prior to Surgery

Inclusion Criteria: * 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 Exclusion Criteria: * Sarcoma originating from bone structure, abdominal or gynecological viscera * Any of the following histological subtypes: gastrointestinal stromal tumor, rhabdomyosarcoma, primitive neuroectodermal tumor or other small round blue cell sarcoma, osteosarcoma, chondrosarcoma, aggressive fibromatosis, or sarcomatoid or metastatic carcinoma * Prior RT to the RPS * Prior abdominal or pelvic irradiation for other prior malignancy or other disease * Prior different invasive malignancy may be eligible per the discretion of the treating investigator and review by the Principal Investigator (PI) * Prior chemotherapy or immunotherapy within 6 weeks of start of RT * Pregnant women are excluded from this study because RT has the potential for teratogenic or abortifacient effects. * HIV-positive individuals on combination antiretroviral therapy are ineligible because of the potential for increased sensitivity to RT