ESTRO 2025 - Abstract Book

S4123

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

Conclusion: Residual deviations could be correlated with rotational corrections in pitch and roll, depending on anatomical site and fixation. Feasible thresholds could be determined: ≤1.0° pitch for spine and ≤2.0° pitch ≤1.5° roll for brain/head & neck, with only roll showing a significant correlation. For non-fixated sites, such thresholds were unachievable, recommending verification imaging after each 6DoF correction.

Keywords: 6dof couch, verification imaging

References: 1 Guckenberger M, Meyer J, Wilbert J, Baier K, Sauer O, Flentje M. Precision of image-guided radiotherapy (IGRT) in six degrees of freedom and limitations in clinical practice. Strahlenther Onkol. 2007;183(6):307-13.

2 MacroMedics, The Netherlands 3 Varian Medical Systems, USA

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Digital Poster Surface guided radiotherapy: rotational setup accuracy for prostate cancer patients Steven Floyd 1 , Philip Wheeler 2 , Dorothy Tan 1 , Clare Beswick 1 , Annie McKay 1 , Rebecca Lewis 1 , Harvinder Singh 1 , Matthew Williams 2 1 Radiotherapy Department, Velindre University NHS Trust, Cardiff, United Kingdom. 2 Radiotherapy Physics, Velindre University NHS Trust, Cardiff, United Kingdom Purpose/Objective: Surface Guided Radiotherapy (SGRT) enables inter-fraction patient positioning, as well as intra-fraction motion monitoring. Although other commercially available SGRT systems have published evidence to demonstrate their system accuracy when compared to tattoo based (TTB) alignment, there is less evidence to support its use for rotational setup with the Varian IDENTIFY system. The purpose of this study is to evaluate setup accuracy for rotational errors utilising Varian IDENTIFY SGRT, in comparison to TTB alignment, to establish the feasibility of transitioning to tattooless radiotherapy setup. Material/Methods: Prostate cancer patients were sequentially selected across March 2024 for this study. 18 patients were positioned with traditional 3-point TTB setup on Varian TrueBeam, and 18 patients positioned with IDENTIFY v3.0.3 (Varian Medical Systems, Palo Alto), on Varian Halcyon. The absolute residual rotational errors, according to automatic Cone Beam Computed Tomography (CBCT) registrations, were statistically compared for the two groups (Mann-Whitney U Test, p≤0.05). Both groups received daily online CBCT verification and data from fractions 1, 5, 10, 15 and 20 were evaluated. A setup threshold of +/-2° was utilised for the SGRT cohort. Where larger than expected rotational results were observed, these were investigated further to determine a root cause. This was performed by a review of the CBCT images and IDENTIFY SGRT Treatment Reports by a single, experienced user. Results: The pitch axis demonstrated the greatest error in both patient groups, with median values of 1.1° and 0.8° for the TTB and SGRT setups, respectively. In terms of roll and rotation, deviations were smaller and comparable for both techniques (Table 1). No statistically significant differences were found between TTB and SGRT setups for any of the rotational axes.