S2865
RTT - RTT education, training, and advanced practice
ESTRO 2026
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.
Conclusion: Mid-treatment sCT offers objective dosimetric verification of anatomical changes. Δ CS of ≥ 10 mm may prompt early review, while weight loss is less consistently predictive. sCT-based decision support could reduce both overlooked failures and unnecessary replanning and warrants prospective validation. Keywords: Adaptive Radiotherapy, Synthetic CT, Dose References: 1. Hay, L. K., Paterson, C., McLoone, P., Miguel- Chumacero, E., Valentine, R., Currie, S., Grose, D., Schipani, S., Wilson, C., Nixon, I., James, A., & Duffton, A. (2020). Analysis of dose using CBCT and synthetic CT during head and neck radiotherapy: A single centre feasibility study. Technical innovations & patient support in radiation oncology, 14, 21–29. https://doi.org/10.1016/j.tipsro.2020.02.004 2. Allen, C., Yeo, A. U., Hardcastle, N., & Franich, R. D. (2023). Evaluating synthetic computed tomography images for adaptive radiotherapy decision making in head and neck cancer. Physics and imaging in radiation oncology, 27, 100478. https://doi.org/10.1016/j.phro.2023.100478 Digital Poster 2284 Resource burden in CBCT-Guided Radiotherapy: Identifying Priority Sites for Adaptive Implementation Vickie Kong 1,2 , Peter Chung 1,2 , Jeff Winter 1,2 , Winnie Li 1,2 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Canada. 2 Department of Radiation Oncology, University of Toronto, Toronto, Canada
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