S2864
RTT - RTT education, training, and advanced practice
ESTRO 2026
without AlignRT by an average of 9 minutes (average treatment time: 21mins, range: 17–34mins). Across all 10 patients mean change in deltas during the time CBCTs were matched was ≤ 1mm, and ≤ 0.06mm while Arcs were delivered (Fig. 2).
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Monitoring lung SABR patient set-up on a ring- gantry Linac: AlignRt InBore in combination with CBCT Gail Anastasi 1 , Hannah Nayee 2 1 Radiotherapy Physics, Royal Surrey Hospital NHS Foundation Trust, Surrrey, United Kingdom. 2 Radiotherapy, Royal Surrey Hospital NHS Foundation Trust, Surrrey, United Kingdom Purpose/Objective: Our Satellite Centre received two ring-gantry upgrade installations, enhanced with AlignRT InBore Surface- Guided Radiotherapy (SGRT). There is limited literature on the use of ring-based lung SABR treatment delivery in conjunction with InBore SGRT. Therefore we audited our first 10 patients to ensure safe delivery. Aims:Assess the performance of InBore SGRT with ancillary equipment. Evaluate InBore SGRT monitoring compared to online CBCT imaging.Determine the reliability of InBore SGRT for continuous motion monitoring. Material/Methods: Patients were planned with two isocentres spaced 2.5mm apart in the superior-inferior direction, in order to avoid field automation and permit an interim CBCT scan. A suitable SGRT region of interest (ROI) was selected. Real-time delta SGRT shifts (Vert, Long, Lat, Yaw, Roll and Pitch) were noted at exhale at specific time points 1-5:1) Before 1st CBCT acquisition2) Before applying 1st CBCT moves3) After 1st Arc delivery4) Before applying interim CBCT moves5) After 2nd Arc deliveryThe deltas from the online CBCT match were also obtained. Results: The audit included 8 upper lobes and 2 middle lobes with one patient requiring abdominal compression. The SGRT ROI excluded the abdominal belt.A standard thorax ROI (Fig. 1a) may not be applied for SABR patients. For multiple lesions there were several ROIs created to correspond with tumour location. Due to the small InBore field-of-view, some patients required a smaller, more focused ROI. (Fig. 1b,c). This ensured the deltas remained in tolerance (Vert, Long, Lat: 0.2cm, Yaw, Roll, Pitch: 3o) for real-time monitoring.
The mean difference between AlignRT deltas and the CBCT match (soft-tissue) was ≤ 0.02cm. When AlignRT flagged a move, a soft tissue positional change was confirmed on CBCT. Conclusion: InBore SGRT for Lung SABR was adopted easily with decreased time on the treatment couch. The sub-mm change in deltas during Arc delivery makes us confident that we have a safe treatment approach. Moving forwards, one could consider omitting the interim CBCT and only acquiring it if AlignRT deltas are out of tolerance. Keywords: SGRT, ring-gantry, SABR Digital Poster Highlight 2253 Evaluating clinical triggers and replanning decisions in head and neck radiotherapy using synthetic CT Kok Ming Lee, Xue Jing Ong, Sze Yarn Sin Department of Radiation Oncology, National Cancer Centre Singapore, Singapore, Singapore Purpose/Objective: Adaptive replanning (Re-CT) in head and neck cancer (HNC) radiotherapy mitigates anatomical changes that can compromise target coverage or increase organ-at- risk (OAR) dose. In the absence of validated quantitative triggers, clinical decisions remain inconsistent, risking missed plan degradation and unnecessary replanning. This study examines anatomical predictors, dosimetric impact, and Re-CT decision appropriateness using deformable CBCT- derived synthetic CT (sCT). Material/Methods: Thirty-two patients with HNC treated with curative radiotherapy (Jan 2024–Sep 2025) were analysed; 17 underwent Re-CT due to anatomical changes. 32 mid-
Patients experienced shorter treatment appointments compared to patients treated on a TrueBeam Linac
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