• Adult age ≥ 18 presenting with first recurrent well-differentiated and/or dedifferentiated liposarcoma of retroperitoneal space or pelvis after previous macroscopically complete resection (R0/R1 resection)

• No distant metastasis on cross-sectional imaging of chest/abdomen/pelvis (CT and/or MRI) within 1 month to confirm the absence of metastatic disease

• Previous histologically proven well-differentiated or dedifferentiated liposarcoma histology only

• Sarcoma not originating from bone or abdominal or gynecological viscera

• Tumor confirmed to be resectable with likely R0/R1 resection, and all disease must be deemed to be treatable by RT (joint decision by surgeon and radiation oncologist at a sarcoma multidisciplinary team meeting)

• WHO performance status 0-2

• American Society of Anaesthesiologist (ASA) score 1-3

• No prior RT for the retroperitoneal liposarcoma

• Prior systemic therapy is allowed

⁃ No concurrent active malignancy (except for low risk skin malignancy, low risk prostate carcinoma, low risk breast carcinoma including in situ disease)

⁃ Women of childbearing potential must have a negative pregnancy test within 3 weeks prior to the first day of study treatment

⁃ Patient deemed able to comply with study requirements according to investigator evaluation

⁃ Signed, IRB-approved written informed consent