S2179
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Figure 2. Mean dose–volume parameters for bladder and small bowel comparing scheduled (SCH) and adapted (ADP) plans. Conclusion: Daily ART with the Varian Ethos system for rectal cancer is clinically feasible and dosimetrically beneficial, providing significant improvement in target coverage while maintaining comparable OI sparing and manageable workflow times. These results support the implementation of ART to ensure optimal dose delivery while accounting for interfraction anatomical variability. Keywords: ART, Rectal Cancer, Dosimetric Analysis Daily Adaptive Radiotherapy in Prostate Cancer: Dosimetric and Workflow Evaluation from an Indian Tertiary Centre Using ETHOS with SGRT Naseem Imran Shaikh, Sheereen Fatima, Prasad Raj Dandekar, Purwa Prakash Jaiswal, Manish Ashok Bhosale, Mandar Mithilanath Rawool, Mohammed Dawood Shaikh, Omkar Chandrakant Awate, Ananda Ramchandra Jadhav, Sachin Kisan Rasal Radiation Oncology, Sir H N Reliance Foundation Hospital and Research Centre, Mumbai, India Purpose/Objective: To evaluate the clinical feasibility, dosimetric impact, and workflow efficiency of daily adaptive radiotherapy (ART) in prostate cancer using a surface-guided, ring- gantry Ethos system in a tertiary Indian institution Material/Methods: Nine patients with localized or locally advanced prostate cancer were treated with daily adaptive ART between August 2023 – October 2025, totaling 185 fractions (8 patients × 20 fractions, 1 patient × 25 fractions). Prescription schedules included 60 Gy/20 fractions ± 44 Gy/20 SIB and 68 Gy/25 + 50 Gy/25 SIB. For each fraction, a scheduled (reference) plan and an adapted (re-optimized) plan were compared using Digital Poster 4272 institutional dose guidance limits for the rectum, bladder, and bowel. Adaptation time, total session time, and 3D couch shift were recorded. Plan QA was performed using the 3 %/3 mm γ -index. Daily surface guidance (Vision RT SGRT) and re-verification CBCT ensured accurate setup prior to delivery. Paired Wilcoxon signed-rank tests were used to assess dosimetric differences between scheduled and adapted plans. Results: All 185 adapted fractions were successfully delivered. Median plan generation time was 5.8 min (IQR 5.0 – 6.3) and total on-couch time 24.9 min (IQR 23.1 – 26.7). Mean ± SD 3D couch shift was 0.09 ± 0.04 cm, confirming minimal setup variation. All plans passed
toward reduced small bowel dose in the adapted plans, while bladder parameters remained comparable. Across sessions, the mean reduction in contoured volume was 4.28% for PTV1 and 8.98% for PTV11, indicating progressive shrinkage of target volumes. The mean treatment time per fraction was 22 minutes, within clinically acceptable limits.
Figure 1. Mean dosimetric parameters for CTV1, CTV11, PTV1, and PTV11 in scheduled (SCH) and adapted (ADP) plans.
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