S2921
RTT- RTT operational practice and workflow innovations
ESTRO 2026
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- treatment CBCTs (median fraction 16) were first rigidly registered to the planning CT, followed by deformable image registration to generate the sCT (Figure 1b). The original treatment plan was recalculated on sCT, and dose–volume histogram (DVH) parameters were compared with the original plan.To account for heterogeneous prescriptions, high-dose (PTV- HD:6000–7000 cGy) and low-dose targets (PTV- LD:5000–5600 cGy) were normalised to percentage of prescription dose (%Rx). Target coverage was evaluated using D95% and D99% (%Rx). Institutional planning guidelines define plan failure as D95% <100% Rx or D99% <93% Rx. OARs were assessed using absolute dose.Anatomical change was quantified by percentage weight loss and external contour separation change ( Δ CS) (Figure 1c). Clinical Re-CT decisions were classified against sCT dosimetry as appropriate, under-replanned (objective plan failure without Re-CT), or potentially over-replanned (Re-CT without plan failure).
Results: Re-CT patients demonstrated greater anatomical changes, including higher weight loss ( − 3.96 ± 2.75% vs. − 2.30 ± 2.61%, p=0.041) and larger Δ CS (10.71 ± 3.86 mm vs. 7.21 ± 2.17 mm, p=0.003) (Table 1).PTV-LD D99% was significantly lower in Re-CT patients compared with non-Re-CT group (80.7 ± 31.4%Rx vs 98.9 ± 2.1%Rx, p = 0.030). PTV-HD showed a downward trend but not statistically significant (PTV-HD D95% 99.0 ± 12.7%Rx vs 99.5 ± 5.4%Rx, p = 0.883; PTV-HD D99% 88.7 ± 27.1%Rx vs 94.9 ± 16.8%Rx, p = 0.438). OAR dose increases were small and not statistically significant (all p>0.17); parotid mean dose was modestly higher in the Re-CT group (p < 0.05).Clinical decision concordance with sCT dose was 68.75% (22/32): 8/17 Re-CTs were appropriate, 14/15 were correctly monitored without replanning, 1/32 (3.2%) was under-replanned with unrecognised target compromise, and 9/32 (28.13%) were potentially over- replanned without meeting plan failure criteria, despite noticeable contour change on CBCT.
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