ESTRO 2025 - Abstract Book

S1753

Clinical – Upper GI

ESTRO 2025

unpredictably, complicating treatment delivery. This study evaluates the effectiveness of different PRV margins in encompassing those organ movements, their benefit in reducing OARs dose and impact on PTV coverage.

Material/Methods: Ten consecutive patients with locally advanced or recurrent pancreatic carcinoma treated using daily adaptive RT on MR-Linac receiving 40 Gy in 5 fractions were identified. Daily adapted images were used for contouring OARs on each fraction for each patient, PRV margins 2mm, 3mm & 5mm were applied retrospectively to patients’ OARs on their initial planning scan to evaluate 1) whether they’ll encompass each OAR in every fraction and 2) their effects on OARs and PTV dose distributions. OARs lying outside 3 cm PTV radius were excluded. BED 10 to PTV D70% was used as indicator of treatment effectiveness. Results: Dose distributions for the stomach, duodenum, small bowel, and large bowel were evaluated across 48 treatment fractions (2 patients received 4 fractions instead of 5). Percentages of OARs at each adapted fraction contained within a 2mm, 3mm and 5mm PRV margins were: stomach 8.3%, 14.6% and 27.1%, respectively; duodenum 0%, 2.3% and 13.6%, respectively; large bowel was only contained in 2.9% of adapted fractions on each PRV margin and none of small bowel structure fell within its’ PRV margin (table1). Nonetheless, 2-3mm PRV margin was shown to notably reduce doses received by stomach and duodenum 0.1cc volumes without significantly compromising PTV BED dose, with 11.4%-16.9% relative BED 3 dose reduction for stomach and 10.9%-20.5% relative BED 3 reduction for duodenum. While 5mm PRV margin led to a significant reduction in PTV D70% BED dose, with median BED 10 values of 77.16Gy, 74.17Gy, and 64.9Gy for 2mm, 3mm, and 5mm PRV plans, respectively, compared to the baseline plan BED 10 of 81.3Gy (Figure1). Conclusion: This study underscores the limitations of PRV application for highly mobile OARs with significant inter-fraction OAR motion with most OARs falling outside the applied PRV margins, even when larger margins are used. While PRV margins provided dose mitigation benefits to the stomach and duodenum, larger margins compromised PTV coverage. We demonstrate that 2-3mm margins can serve as a potential effective dose mitigation strategy, when adaptive radiotherapy platforms are unavailable. References: Zhong J, Patel K, Switchenko J, Cassidy RJ, Hall WA, Gillespie T, et al. Outcomes for patients with locally advanced pancreatic adenocarcinoma treated with stereotactic body radiation therapy versus conventionally fractionated radiation. Cancer 2017;123:3486–93. https://doi.org/10.1002/cncr.30706 Brunner TB, Blanck O, Lewitzki V, Abbasi-Senger N, Momm F, Riesterer O, et al. Stereotactic body radiotherapy dose and its impact on local control and overall survival of patients for locally advanced intrahepatic and extrahepatic cholangiocarcinoma. Radiother Oncol 2019 ;132 :42–7. https ://doi.org/10.1016/j.radonc.2018.11.015 Herman JM et al. Cancer 2015;121:1128–37. https://doi.org/10.1002/cncr.29161 Keywords: Pancreatic SBRT, PRV margins, OARs BED dose

615

Digital Poster Adjuvant Radiotherapy in Patients with Gastric Signet-ring Cellular Carcinoma Undergone with Radical Surgery and Chemotherapy Xiaojun Liu, Shuping Li Radiation Oncology, Gansu Provincial Hospital, Lanzhou, China