ESTRO 2025 - Abstract Book

S3207

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

ESTRO 2025

Results: Although the image quality and contrast of the spacer were often compromised by the overlapping bony structures, the spacer was robustly detected among 40% of the kV images, mostly around the anterior-posterior imaging angles. The prostate fiducials moved 1.0±0.6mm in the kV beam’s eye view during the treatment, compared to a larger movement of the spacer, 2.1±1.2mm. The relative motion between the fiducials and the spacer was 2.2±1.3mm. The motion patterns of the fiducial and spacer were significantly different (t-test, p <0.05) in 70% of the fractions. Rectum D1cc was 1Gy higher on average if reconstructed with traces of the spacer rather than fiducials. Conclusion: Simultaneous tracking of the fiducials and spacer is feasible with X-ray imaging for prostate SBRT. The amplitude and patterns of intrafractional motion are independent between the prostate and spacer. These findings could lead to a more sophisticated gating strategy that employs a structure-specific action threshold for ensuring accurate dose delivery, and a more accurate dose reconstruction for outcome research.

Keywords: prostate, intrafractional motion management

References: 1. Keall P, Nguyen D, O’Brien R, et al. Stereotactic prostate adaptive radiotherapy utilising kilovoltage intrafraction monitoring: the TROG 15.01 SPARK trial. BMC Cancer. 2017; 17:180 2. Neylon J, Ma T, Savjani R, et al. Quantifying Intrafraction Motion and the Impact of Gating for Magnetic Resonance Imaging-Guided Stereotactic Radiation Therapy for Prostate Cancer: Analysis of the Magnetic Resonance Imaging Arm From the MIRAGE Phase 3 Randomized Trial. Int J Radiat Oncol Biol Phys. 2024;118(5):1181-1191.