ESTRO 2024 - Abstract Book

S5587

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

ESTRO 2024

4. Sasaki M, Matsushita N, Fujimoto T, Nakata M, Ono Y, Yoshimura M, et al. New patient setup procedure using surface-guided imaging to reduce body touch and skin marks in whole-breast irradiation during the COVID-19 pandemic. Radiol Phys Technol 2023;16:422–9. https://doi.org/10.1007/s12194-023-00735-0. 5. Svestad JG, Heydari M, Mikalsen SG, Flote VG, Nordby F, Hellebust TP. Surface-guided positioning eliminates the need for skin markers in radiotherapy of right sided breast cancer: A single center randomized crossover trial. Radiotherapy and Oncology 2022;177:46–52. https://doi.org/10.1016/j.radonc.2022.10.017.

2590

Proffered Paper

Evaluation of contour to CBCT image registration for pelvic irradiation in MR-only radiotherapy

Ya Wang, Ina Nilo, Chiara Leonardi, Stephanie Tanadini-Lang, Matthias Guckenberger, Sophie Perryck, Riccardo Dal Bello

University Hospital Zürich, Radiation Oncology, Zurich, Switzerland

Purpose/Objective:

Magnetic Resonance (MR)-only radiotherapy aims to utilize the unique benefits of MR imaging for soft tissue contrast and functional imaging, while also limiting the additional dose and time of a planning computer tomography (CT) appointment (1). In MR-only radiotherapy, the dose calculation during treatment planning is performed on a synthetic CT (sCT) generated from the MR images, while the contours are done directly on the MR data. A few strategies exist for matching the MR image data to the daily cone-beam-computer tomography (CBCT) images. For example, a direct alignment of the MR image to the CBCT, or an alignment of the sCT to the CBCT. A direct MR to CBCT alignment is not generally available on the treatment machines, but it can be performed retrospectively offline. Previous studies have compared the spatial alignment of sCT to CBCT and MR to CBCT to the alignment of a conventional planning CT to CBCT, demonstrating no significant clinical difference. (2)(3)

This in-house study aims to examine the accuracy of contours to CBCT registrations compared to an MR to CBCT registration technique, with the goal of assessing its efficacy in the treatment of patients in the pelvic region.

Material/Methods:

Image registration data was analyzed for a cohort of 10 male patients treated for volumes in the pelvis using an MR only workflow. These patients received a planning MR (1.5T Siemens Sola) scan based on which a sCT was generated (syngo RT suite V60). Contours based on the MR images were delineated by a radiation oncologist (RO). Daily CBCT scans were acquired for each patient, with fractions 1, 7, 14, and the final fraction being analyzed retrospectively for long course treatments, and fractions 1, 3, and 5, for short 5-fraction stereotactic courses. A sCT to CBCT registration was completed by radiation therapists (RTT) during treatment and recorded. The contour to CBCT and MR to CBCT registrations were then retrospectively performed offline by an experienced RTT and RO respectively. Focus was placed on the clinical target volume in all cases. For the offline registrations, an auto-match was done with six degrees