Dosimetric Feasibility of Hypofractionated Radiation Therapy with Simultaneously Integrated Central Boost for Retroperitoneal Sarcoma

Background: Retroperitoneal sarcomas (RPS) are rare malignancies that pose challenges due to their large size and proximity to organs-at-risk (OARs). Compared to conventional 5-week radiation therapy (RT), hypofractionated RT (HFRT) offers a shorter alternative with potential biologic benefit, and simultaneous integrated boost (SIB) can increase delivered dose. We evaluated the dosimetric feasibility of HFRT with a central tumor SIB for bulky RPS. Methods: Fifteen patients with RPS previously treated with preoperative conventionally fractionated RT were replanned using HFRT with a central SIB. Volumetric modulated arc therapy (VMAT) plans were created to deliver 42.75 Gy in 15 fractions to the planning target volume (PTV), 45 Gy to the internal gross tumor volume (iGTV), and 52.5 Gy to the SIB volume (1 cm contraction of iGTV). OAR planning objectives were prioritized during optimization. Dosimetric plan-generation feasibility was defined as generation of a treatment plan meeting prespecified target-coverage criteria; compliance with each OAR objective was assessed separately. Results: Mean tumor size was 18.3 cm (range 5.8–28.2 cm), with mean volumes for iGTV, SIB, and PTV of 1633.8 cc, 926.9 cc, and 3323.2 cc, respectively. All plans met the prespecified target coverage criteria for iGTV, SIB, and PTV. Ten of 15 plans met all target-coverage criteria and all prespecified OAR objectives; five met the target coverage criteria and non-bowel OAR objectives but exceeded at least one bowel objective by <1% of the specified constraint. All non-bowel OAR objectives were met except that the bilateral kidney objective was exceeded in 4 of 11 evaluable patients. The average homogeneity index for the iGTV and SIB were 0.21 (range 0.18–0.25) and 0.05 (range 0.04–0.06), respectively. The PTV 95% conformality index was 0.99 (range 0.97–1.0), indicating excellent target conformation with minimal normal tissue spillage. Conclusions: These findings support the dosimetric feasibility of generating hypofractionated radiation therapy plans with a central SIB for large retroperitoneal sarcomas. However, one-third of plans exceeded at least one bowel objective, and these planning results do not establish clinical deliverability, safety, or efficacy.