The investigators' primary outcome is overall complication rate after treatment with NA H-PBT and surgical resection based on National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 5.0.2 Secondary objectives include evaluating the acute toxicity of H-PBT prior to surgical resection, rate of progression between diagnosis and definitive resection, early post-operative complication rate after resection of RPS in patients who received NA H-PBT, and local recurrence-free survival at 1 and 2 years. A priori subset analyses will be conducted for patients with well-differentiated and dedifferentiated liposarcoma.~The investigators plan to accrue a minimum of 44 patients to evaluate the investigators' primary outcome. Treatment will be 5 doses of H-PBT including a simultaneous integrated boost to at-risk margins followed by surgical resection after 4-6 weeks. Patients will be followed in the post-operative setting according to standard of care surveillance for RPS.
The investigators' study titled PROTONS-RPS: a Phase II non-Randomized Open-label single-arm Trial Of Neoadjuvant Short-course hypofractionated proton beam therapy for non-metastatic RetroPeritoneal Sarcoma is a phase II trial evaluating the safety and efficacy of hypofractionated proton beam therapy (H-PBT) in the neoadjuvant (NA) setting for patients with non-metastatic retroperitoneal sarcoma (RPS) planned for surgical resection. This trial will include adult patients with resectable RPS.