ESTRO 2025 - Abstract Book

S3292

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

Beam-on time differences between CW (starting gantry angle was 210) and LF were evaluated. Median population based prostate drift was estimated using a published data[1]. For each beam, a beam specific average displacement error (LF and CW) was introduced based on the elapsed time and the estimated prostate drift by shifting the beam’s RTDOSE matrix, followed by a dose summation. Approved plan and the LF, CW simulated doses were used to calculate dose volume histograms and dose constraint scorecards (PACE-B trial [2]) focusing on the CTV, rectum, bladder and urethra. Dose constraints between LF and CW approach were compared using Wilcoxon paired test using p<0.05 for statistically significant level.

Results:

A total of 120 plans were processed. Beam-on time was on average 9:19 (range 8:56-9:43) and 8:57 (r:8:39-9:31) min:sec for the LF and CW respectively, with average addition 0:20 (r: 0:15-0:29 m:s) for LF. The computed antero posterior prostate intra-fraction drift was 0.58 mm/min. CTV coverage loss was smaller using LF sequence (figure, p<0.01), while for bladder V37Gy, bladder and urethra D0.1cc no difference (p=0.09, p=0.87 and p= 0.65 respectively) were observed between LF and CW. All other OAR dose parameters were statistically significantly lower using the CW approach. Conclusion: Delivering lateral beams first during prostate SABR on the 1.5T MR-Linac instead of clockwise order does not increase the beam-on time substantially. Still lateral beams should be prioritized as it is less sensitive for antero posterior prostate drifting terms of target coverage loss.

Keywords: MRgRT, treatment planning, robustness

References: 1. Ballhausen H et al. Sci Data 2024 https://doi.org/10.1038/s41597-024-03365-2. 2. Tree AC et. al. Lancet 2022 https://doi.org/10.1016/S1470-2045(22)00517-4.