Seminal vesicle intrafraction motion 67 Introduction In recent days, a growing number of patients with prostate cancer are being treated using online magnetic resonance imaging (MRI)-guided radiotherapy. While the first patients were treated with 20 fractions1,2, MRI-guided hypofractionated prostate radiotherapy for low- and intermediate-risk patients with five fractions has now successfully been performed.3,4 Previous studies on an MR-Linac have shown the significance of prostate intrafraction motion5 and its impact on the delivered dose distribution.6,7 While intrafraction motion studies of the prostate based on cine-MR imaging7,8 have been described extensively, literature on seminal vesicle intrafraction motion is sparse. Especially when moving to hypofractionated treatments for patients with intermediate to high-risk prostate cancer, there is a chance of possible tumour extension into the seminal vesicles.9 To ensure adequate coverage of the seminal vesicles, margins must be based on the observed intrafraction motion. Different studies assessed seminal vesicle intrafraction motion based on cone-beam computed tomography (CBCT), such as described by Sheng et al.10 However, these studies were mostly limited to using start- and end-scans, which provides no real-time motion. Gill et al.11 assessed seminal vesicle intrafraction translational motion for 11 patients based on 2D sagittal cine-MR imaging for 15min, followed by 12min coronal cine-MR imaging. However, to accurately determine intrafraction motion of the seminal vesicles, continuous imaging in multiple dimensions is required. In this study, we investigate seminal vesicle and prostate intrafraction motion based on 3D cine-MR, acquired during the beam-on period of MRI-guided prostate radiotherapy on a 1.5 T MR-Linac. To our knowledge, we are the first to report on six dimensions of freedom seminal vesicle intrafraction motion, acquired from 3D cine-MR under treatment conditions. This method may provide an opportunity to implement intrafraction plan adaption for patients treated with MRI-guided radiotherapy, by providing the complete intrafraction motion of the target(s) to a fast re-planning algorithm. In addition, we estimate seminal vesicles margins for our current workflow, based on the obtained results. Materials and methods Fifty low- and intermediate-risk prostate cancer patients were registered as part of an institutional review board approved registration and imaging study. Patients were treated on a 1.5 T MR-Linac (Elekta Unity12) and underwent hypofractionated prostate radiotherapy with five fractions of 7.25 Gy over a time span of 2.5 weeks between July 2019 and May 2020 at the University Medical Centre Utrecht. 4
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