S2751
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
Purpose/Objective: Stereotactic Ablative Body Radiotherapy (SABR) for liver tumours requires high precision due to steep dose gradients and proximity of critical organs. End Expiration Breath Hold (EEBH) is most commonly usedto reduce target motion and irradiated volume [1] with Deep Inspiration Breath Hold (DIBH)utilised when targetslie close to the heart. Monitoring of breath-hold can be performed using respiratory surrogates such SGRT, and this work reviews setup uncertainties and experience when delivering DIBH
liver SABRusing SGRT. Material/Methods: Three consecutive patients who
received liver SABR were included, one with2 planning target volumes (PTVs). Imaging for each treatment fraction consisted ofi) AlignRT©SGRT setup in DIBH, ii) DIBH CBCT withcorrectionsapplied for internal anatomy, iii) AlignRT©reference capture in DIBH, iii) post-correction CBCT to confirm reference image suitability. For some fractions, additional CBCT scans were acquired.For each fraction, differences between the CBCT and SGRT readings were recorded in six degrees-of-freedom. The range of setup variation within each fraction was calculated by the difference between recorded maximum and minimum CBCT corrections.Number of CBCTs, re- setups and overall treatment time were alsorecorded. Results: Despite accurately aligned surface anatomy, internal anatomy was not always reproducible. Large displacements of internal anatomy were observed, with translational deviations up to 2.21 cm and rotational corrections reaching 3.8°. The largest translational changes occurred in the longitudinal direction, while rotational variation was widely distributed across all axes (Fig1).
Expected number of CBCTs per fraction is 2, however for this cohortthe mean number of CBCT images per treatment was 3.2. Every patientrequired more than the scheduled 45 minutes linac timeon at least one occasion(Fig2).
Conclusion: Internal liver position in DIBH can have a set up variationof up to 2.1 cm between CBCTs taken only minutes apart. For liver SABR, this amount of motion indicates that surface monitoringdoes not guarantee internal stabilityand confirms importance of post-correction CBCT to detect residual errors or breathing differences. Alongside the large amount of concomitant imaging, long treatment times, and over-utilisation ofmachine capacity, these findings suggest DIBH may not be suitable for all liver SABR patients without careful case-selection, further investigation and the use of real-time internal monitoring rather than surrogate surface-guidance. References:
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