The aim of the study is to collect prospective data on the treatment outcomes in patients with first localized, resectable recurrent retroperitoneal well-differentiated and/or dedifferentiated liposarcoma undergoing curative intent treatment. Patients enrolled in this study will form a validation cohort of the TARPSWG recurrent RPS nomogram. The treatment decision (surgery alone, or preoperative RT +/- chemotherapy followed by surgery) is per the institutional multidisciplinary team recommendation.

This study is collecting participant data prospectively from hospital medical records. Participant details regarding diagnosis, treatments, outcomes, complications and survival status will be captured after patients are enrolled into the study and at specific time points though out the study. Participants will also be asked to complete quality of life questionnaires called the EORTC QLQ-C30 and the OLO-STO22 at 4 or 5 different time points which will take 15-20 minutes to complete. The overall duration of observation is from time of enrolment for 5 years.

Inclusion Criteria:

1. 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)
2. 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
3. Previous histologically proven well-differentiated or dedifferentiated liposarcoma histology only
4. Sarcoma not originating from bone or abdominal or gynecological viscera
5. 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)
6. WHO performance status 0-2
7. American Society of Anaesthesiologist (ASA) score 1-3
8. No prior RT for the retroperitoneal liposarcoma
9. Prior systemic therapy is allowed
10. No concurrent active malignancy (except for low risk skin malignancy, low risk prostate carcinoma, low risk breast carcinoma including in situ disease)
11. Women of childbearing potential must have a negative pregnancy test within 3 weeks prior to the first day of study treatment
12. Patient deemed able to comply with study requirements according to investigator evaluation
13. Signed, IRB-approved written informed consent

Exclusion Criteria:

1. Unresectable disease or likely R2 resection as assessed by the multidisciplinary sarcoma team
2. Extent of recurrence where preoperative RT to all visible disease is not deemed to be feasible
3. Contradiction for RT such as history of bowel obstruction or mesenteric ischemia or severe chronic inflammatory bowel disease
4. Myxoid liposarcoma histology
5. Pregnancy

Patients diagnosed with localized, resectable recurrent retroperitoneal liposarcoma attending and having treatment at the study site.