MRI-guided radiotherapy for prostate cancer: the MOMENTUM study 111 Introduction Prostate motion within the pelvis is common due to the presence or absence of gas within the rectum, bowel movement, and filling of the urinary bladder. To account for uncertainties during dose delivery with external beam radiotherapy (EBRT), such as intrafraction motion, the prostate is irradiated with an uncertainty margin, also known as the Planning Target Volume (PTV). This margin is necessary for adequate dose delivery to the prostate. Unfortunately, the PTV margin also overlaps with the healthy bladder, rectum, and neurovascular structures, which may lead to post-treatment genitourinary (GU), gastrointestinal (GI), and/or erectile toxicity.1 Magnetic resonance imaging (MRI)-guided radiotherapy enables real-time visualisation of target volume and organs-at-risk (OARs) during EBRT.2 Currently, MRI-guided radiotherapy enables correction for interfraction motion and deformation by applying daily contour adaptation and subsequent online re-planning prior to dose delivery, without the use of fiducials or beacons. Furthermore, it allows for visualisation of intrafraction motion during dose delivery. Such imaging will enable beam pausing or treatment interruption in case there is substantial or unexpected intrafraction motion. This may reduce post-treatment toxicity while maintaining or improving tumour control.3 Currently, two commercial MRI-guided linear accelerator (MR-Linac) systems are available: the MRIdian (ViewRay Inc., Oakwood, U.S.A.) and the Unity MR-Linac (Elekta AB, Stockholm, Sweden). The first combines a 0.35 T (i.e. low-field) MR scanner with a 6 MV linear accelerator (a previous version used three Cobalt-60 heads) and the latter combines a 1.5 T (i.e. high-field) MR scanner with a 7 MV linear accelerator.4,5 Although several radiotherapy departments have already implemented MRI-guided radiotherapy as a standard treatment for low- and intermediate-risk localised prostate cancer, the theoretical advantages of MRI-guided radiotherapy have yet to be proven in clinical practice. Furthermore, clinical outcomes up to 12 months follow up (FU) have been reported for low-field MRI-guided radiotherapy6,7, but not yet for high-fieldMRI-guided radiotherapy. This is essential, as high-fieldMRIguided radiotherapy may induce different treatment-related challenges.8 The ‘Multi-OutcoMe EvaluatioN of radiation Therapy Using the MR-Linac’ study (The MOMENTUM study) was initiated to facilitate the evidence-based introduction of 1.5 T MRI-guided radiotherapy in daily practice.9 As a first step, we here report the 12-month toxicity, efficacy, and patient-reported outcomes (PROs) from the first prostate cancer patients enrolled in the MOMENTUM study, who were treated with 5 x 7.25 Gy on the Unity 1.5 T MR-Linac system. 6
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