2164 - Re-Irradiation of Locally Recurrent Retroperitoneal Sarcomas Using CT-Adaptive Stereotactic Body Radiation Therapy

Purpose/Objective(s): For inoperable locally recurrent retroperitoneal sarcomas (LR-RPS), repeat radiation therapy (RT) may not be feasible due to the risk of ulceration or perforation of the stomach or duodenum/small bowel (D-SB). We performed a retrospective analysis of patients with unresectable LR-RPS treated with CT-based adaptive stereotactic body radiation therapy (CTA-SBRT) in the re-irradiation setting to determine the rate of toxicity and radiographic tumor progression, and to assess whether CTA-SBRT facilitates the delivery of an ablative prescribed dose to the high-dose planning target volume (HD-PTV) within dosing constraints.

Materials/Methods: A single-institution analysis was performed for patients with LR-RPS who received initial RT from 2009-2022 and were treated for a local recurrence with CTA-SBRT (35-50 Gy) between 4/2024 and 1/2025. Initial RT consisted of neoadjuvant (50.4 Gy/25-28) or post-operative RT (60 Gy/15). For each organ at risk (OAR), 50% repair was assumed. We then subtracted this value from the 5-fraction constraint to determine our new goal. Comparisons were made between each fraction of the scheduled plan (SP) and adaptive plan (AP) for HD-PTV coverage and OARs. Patient charts were reviewed for acute treatment-related toxicity and radiographic tumor response.

Results: A total of 20 fractions were delivered. The AP was chosen 100% of the time over the SP due to increased target coverage and/or decreased dose to OARs. For each fraction, there was an average decrease in dose to the D-SB of 21% (range 1.0-34.6%), and kidney of 1.0% (-5.5-13%). There was an average increase in dose to the stomach of 2.0% (-29.9-23.3%) and HD-PTV of 7.7% (2.7-34.6%). Interfractional differences for one patient are demonstrated in Table 1. Differences in D-SB and stomach dose are attributed to daily bowel filling and positioning. No acute toxicity was associated with CTA-SBRT during the mean 6.5-month follow-up period. Radiographic studies revealed either stable or partially responsive disease.

Conclusion: CTA-SBRT is a novel technique which allows delivery of ablative doses to LR-RPS despite prior use of RT. The dose of RT delivered to D-SB using CTA-SBRT was 21% less than with the SP. There was no acute toxicity or tumor progression during a mean follow-up period of 6.5 months.