S2274
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
a transperineal ultrasound (TPUS) system (Clarity, Elekta) for 174 SABR fractions (35 patients, 36.25 Gy / 5#), and 1078 for non-SABR (56 patients, 60 Gy / 20#) over the period September 2024 – June 2025. Prostate motion relative to the couch was assessed to determine:a) Prostate position at the start of treatment (for SABR and non-SABR)b) Prostate position at the end of treatment (for SABR and non- SABR)c) Proportion of treatment time prostate motion greater than 3 mm (for SABR only)For a) - c) assessing motion relative to the couch resulted in data independent of any couch position corrections applied during treatment. The proportion of treatment fractions where repositioning was required was also determined with tolerances of 5 mm (3 mm anterior) for non-SABR, and 3 mm in all directions for SABR. Results: Mean treatment time measured by monitoring was 299.8 ± 101.3 s and 235.4 ± 49.8 s and for SABR and non-SABR respectively. Drift motion (end position minus start position) was comparable for both fractionation regimes and most common in inferior and posterior directions, on average less than 1 mm for the whole patient cohort (figure 1). Repositioning was required most frequently for motion in the A/P direction (26.4% SABR, 10.4% non-SABR). For 70% of SABR fractions intrafraction prostate motion is less than 3 mm (30% greater than 3 mm). For 3.4% of SABR fractions motion was greater than 3 mm from the start position for the whole treatment fraction (figure 2).
Conclusion: Conclusion: Internal 3D target motion monitoring by x- ray fluoroscopy during IMRT lung cancer radiotherapy allowed reconstruction of the actually delivered LN target doses for a large patient cohort. Large motion- induced dose distortions at individual fractions tended to smear out when averaging the dose over several fractions. Keywords: Lung, DIBH, Motion monitoring, Dose reconstruction Digital Poster 4658 Assessing intrafraction prostate motion during stereotactic ablative radiotherapy using transperineal ultrasound Jack Aylward 1,2 , Daniel Williams 3 , Harley Stephens 4 , Beth Gledhill 1 , Chris Stepanek 1 , Miranda Francis 1 1 Department of Medical Physics and Bioengineering, Bristol Haematology & Oncology Centre, Bristol, United Kingdom. 2 Medical Physics, School of Applied Sciences, University of the West of England, Bristol, United Kingdom. 3 Department of Physics, University of Bristol, Bristol, United Kingdom. 4 Bristol Cancer Institute, Bristol Haematology & Oncology Centre, Bristol, United Kingdom Purpose/Objective: The magnitude of intrafraction prostate motion is well documented for conventional fractionation radiotherapy, however there is less data for stereotactic ablative radiotherapy (SABR) or the duration of significant intrafraction motion. This work aims to determine the magnitude and duration of intrafraction prostate motion for SABR to inform the ideal treatment time, treatment margins, and the need for intrafraction monitoring for this treatment technique. Material/Methods: Intrafraction prostate motion data was collected using
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