S2400
Physics - Quality assurance and auditing
ESTRO 2026
prostate has been applied widely and has been reported as non-inferior to standard fractionated prostate radiotherapy in clinical trials.1 2 The PACE-B trial protocol specifies intrafraction motion control is not mandated for beam-on times less than 180s and re-imaging should occur at approximately 180-240s to account for intrafraction motion. For SABR prostate treatment delivery times between 180-360s, di Franco et al. reported mean prostate displacements of -0.02 ± 0.1cm, -0.02 ± 0.12cm and -0.01 ± 0.16cm in the left/right, superior/inferior and anterior/posterior directions respectively3. These reported displacements suggest treatment delivery within 360s of imaging may be acceptable. The aim of this study is to assess intrafraction prostate displacements on CBCTs taken pre and post treatment delivery and determine a clinically achievable delivery time for SABR prostate on a linac with Surface Guided Radiation Therapy (SGRT) intrafraction monitoring. Material/Methods: A total of twenty-five post-treatment CBCT images were analyzed from five patients who received 36.25Gy in 5 fractions. All patients were treated using a 6 MVFFF beam and single 360° arc. Pre-treatment, post correction verification, and post-treatment CBCTs were acquired on each fraction. Translational and rotational displacements were retrospectively calculated on the post-treatment CBCTs to assess intrafraction prostate motion. The time intervals between pre-treatment and post-treatment CBCTs were recorded along with the treatment delivery time. Results: The median predicted delivery time from the treatment planning system was 81.8s (IQR = 78.3 to 84s). However, the actual median delivery time across all fractions was 94.5s (IQR = 88.4 to 130.7s). The variation from the predicted time was due to beam interruptions during treatment. The median time from the pre-treatment CBCT to the post-treatment CBCT was 383s (IQR = 334 to 411s). The median translational displacements of the prostate across all fractions in the left/right, superior/inferior and anterior/posterior directions were 0.20cm (IQR = 0.10 to 0.30cm), 0.10cm (IQR = 0.08 to 0.20cm) and 0.10cm (IQR = 0.10 to 0.23cm) respectively (Figure 1). Median rotational deviations yaw, pitch and roll were 0.25°, 0.35° and 0.40° respectively.
Conclusion: The use of strategies to limit MLC aperture variability enabled Inst.2 to produce less complex arcs, contributing to observed inter-institutional differences in PSQA outcomes alongside variations in QA software, phantoms, and procedures. While complexity metrics effectively quantify variability in plan optimisation and delivery, no consistent correlation with PSQA results was observed. This suggest that Inst.2 could potentially review its MLC aperture limits to allow greater plan modulation while maintaining high
deliverability. References:
1.Hernandez V, Hansen CR, Widesott L, et al. What is plan quality in radiotherapy? The importance of evaluating dose metrics, complexity, and robustness of treatment plans. Radiother Oncol. 2020;153:26-33. doi:10.1016/j.radonc.2020.09.038.2.Antoine M, Ralite F, Soustiel C, et al. Use of metrics to quantify IMRT and VMAT treatment plan complexity: A systematic review and perspectives. Phys Med. 2019;64:98-108. doi:10.1016/j.ejmp.2019.05.024.3.Hernandez V, Saez J, Pasler M, Jurado-Bruggeman D, Jornet N. Comparison of complexity metrics for multi-institutional evaluations of treatment plans in radiotherapy. Phys Imaging Radiat Oncol. 2018;5:37-43. Published 2018 Feb 22. doi:10.1016/j.phro.2018.02.002. Keywords: Plan Complexity, Multicentre, Patient- specific QA Digital Poster 4544 From Set-Up Verification to Beam-Off: Efficient Delivery of Prostate SABR with FFF Arcs Michael Roche, Orla McKivitt, Morgan Healy, Sean O'Cathail CUH/UCC Cancer Centre, Cork University Hospital, Cork, Ireland
Purpose/Objective: Stereotactic ABlative Radiotherapy (SABR) to the
Figure 1: Box and whisker plot for translation
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