<h3>Purpose/Objective(s)</h3> Standard neoadjuvant radiotherapy (RT) offers a modest abdominal control benefit in retroperitoneal sarcoma (RPS), with dose escalation limited by adjacent organs at risk. The highest risk of positive margin and recurrence are surgically defined as the region abutting vessels and the posterior retroperitoneum. Techniques selectively escalating dose in this high-risk region through simultaneous integrated boost (SIB) have been employed to eradicate the increased microscopic disease burden. We hypothesized that neoadjuvant RT with SIB will improve abdominal control, compared to the standard technique (ST). <h3>Materials/Methods</h3> Patients with non-metastatic RPS, treated with definitive intent with either ST or SIB neoadjuvant RT followed by radical resection, were identified retrospectively. The ST was defined as 1.8-2Gy/fraction homogenously encompassing the treatment volume, whereas SIB treated the entire treatment volume (1.8-2Gy/fraction) with a boost (2-2.3Gy/fraction) to the high-risk region. Differences between the two groups were assessed with Chi-square or Mann-Whitney U test, when appropriate. Abdominopelvic control (APC) was defined from the date of diagnosis, estimated with Kaplan-Meier methods, and compared via log-rank test. Age, histology, grade, and T-stage were then included with RT modality in a cox-regression multivariable analysis (MVA). <h3>Results</h3> We identified 81 patients, treated between 1/2003 and 12/2021, who received either ST (n=54) or SIB (n=27) neoadjuvant RT followed by radical resection. The cohort had an average age of 62 (26-82 years), average tumor size of 16cm (1.4-40cm), and the majority were dedifferentiated liposarcoma (58%), cT4 (27.5%), and grade 3 (66%). The average RT dose was 50Gy (32.5-59.4Gy) vs 58.2Gy (57.4-63Gy) for ST and SIB groups, respectively (p < 0.0001). There were no significant differences in age, performance status, histology, or surgical margins between SIB and ST RT groups, but SIB patients had higher proportion of cT4 tumors (57% vs 11%, p < 0.0001). With an average follow-up of 45 months (5-188 months), the overall 5-year APC was 76%, with a significant improvement utilizing SIB (96% vs. 70%, p = 0.043), when compared to ST. On MVA, SIB remained an independent predictor for APC (HR 8.7, 95%CI 1.003-75.8, p = 0.05). <h3>Conclusion</h3> Neoadjuvant RT with SIB is associated with improved abdominopelvic control in retroperitoneal sarcoma, when compared to standard RT techniques. This is remarkable given the equivalent margin status in SIB and much higher proportion of T4 disease. Prospective studies are necessary to further validate these findings.