To determine the safety of moderately hypofractionated radiation in the treatment of primary and locally recurrent RPS, based on the evaluation of acute radiation-related toxicity profile of each participant (30-day radiation toxicity)
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)
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