S2703
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
Pearson’s correlation assessed directional relationships. Results: A total of 874 image sets were analyzed. Mean translational deviations (LR, SI, AP) for kV–kV were 0.22 ± 0.11, 0.24 ± 0.07, and 0.17 ± 0.08 mm, and for CBCT were 0.22 ± 0.11, 0.24 ± 0.08, and 0.19 ± 0.21 mm as shown in Figure 1, with no significant differences in systematic or random errors (p > 0.05). Significant differences were observed in rotational deviations for pitch and roll (p < 0.05): mean deviations for kV–kV were 0.64 ± 0.19° (pitch) and 0.58 ± 0.20° (roll), versus 0.57 ± 0.16° and 0.66 ± 0.15° for CBCT. Pearson correlation showed strong relationships in all directions except a weak correlation in the vertical axis (r = 0.2) as shown in Table 1. Conclusion: CBCT-based image guidance offers superior rotational setup accuracy compared to kV–kV imaging in H&N proton therapy. The volumetric data provided by CBCT improves alignment and accounts for anatomical changes, enhancing treatment precision. Routine CBCT verification could improve robustness and clinical outcomes in H&N proton therapy.
Conclusion: SGRT provides superior setup reproducibility and accuracy compared to conventional tattoo-based methods for breast radiotherapy. It offers a patient- centred, non-invasive alternative that maintains precision while eliminating the need for permanent skin markings, with potential to reduce physical and psychological burdens and improve patients’ quality of life. References: 1. Naidoo W, Leech M. Feasibility of surface guided radiotherapy for patient positioning in breast radiotherapy versus conventional tattoo-based setups- a systematic review. Technical Innovations & Patient Support in Radiation Oncology. 2022;22:39-49.2. Lastrucci A, Marrazzo L, Meattini I, Simontacchi G, Wandael Y, Talamonti C, et al. Advancing patient setup: A comprehensive scoping review of tattoo-less techniques in radiation therapy. Critical Reviews in Oncology/Hematology. 2024;204:104518 Keywords: SGRT, IGRT, Setup Reproducibility Digital Poster 797 Evaluating Image-Guided Precision: A Comparative Analysis of kV–kV and CBCT Setup Accuracy in Proton Therapy for Head and Neck Cancer Weerapat Wo-onsri, Thitirat Tienghongsakul, Nichakon Rakkiet, Phattharaphon Phattharakitbouaonkun Division of Radiation Oncology, Department of Radiology, King Chulalongkorn Memorial Hospital, The Thai Red Cross Society, Bangkok, Thailand Purpose/Objective: Accurate patient setup is crucial in proton therapy for head and neck (H&N) cancer to ensure precise dose delivery to the target while sparing organs at risk. This study aimed to compare setup variations and residual uncertainties between kV–kV orthogonal imaging and cone-beam CT (CBCT)–based image guidance in H&N proton therapy. Material/Methods: A retrospective analysis was performed on 35 H&N cancer patients immobilized with a head-and-shoulder mask, cushion, and knee support, treated with ProBeam proton therapy using IMPT (3 mm/3.5% robust optimization) between July 2022 and July 2025. For each fraction, setup verification was performed using both kV–kV orthogonal imaging and CBCT prior to beam delivery. Translational (LR, SI, AP) and rotational (Rotation, Pitch, Roll) corrections derived from both modalities were recorded. Systematic ( Σ ) and random ( σ ) errors were calculated for each axis. Paired t-tests were used to compare modalities, and
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