Part I | Chapter 6 112 Materials and methods Patients This study was conducted within the MOMENTUM study, an international collaboration of early adopters of the 1.5 T MR-Linac system, which received approval by local Institutional Review Boards (IRB) of the participating institutions (NCT04075305).10 In MOMENTUM, all patients treated with radiotherapy on an MR-Linac in one of the participating institutions are eligible for participation. For the current analysis, we included all MOMENTUM participants treated for prostate cancer with 5 x 7.25 Gy on a 1.5 T MR-Linac between May 1st, 2019 and October 10th, 2021. Data acquisition Within MOMENTUM, patient baseline characteristics, physician-reported toxicity, and PROs were prospectively collected at baseline (before start of radiotherapy treatment) and at 3, 6, 12, and 24 months after the last radiotherapy fraction. Biochemical treatment response was evaluated by measuring prostate-specific antigen (PSA) level at baseline and during FU. Seventeen items of the Common Terminology Criteria for Adverse Events (CTCAE) version 5.011 were prospectively obtained from medical records. In case CTCAEs were recorded at multiple time points between two FU moments, the highest CTCAE grades were used: for 3 months, the highest CTCAE grades between the last fraction and 3 months FU; for 6 months, the highest CTCAE grades between 3 and 6 months FU; and for 12 months, the highest CTCAE grades between 6 and 12 months FU (i.e. cumulative incidence). All patients who signed informed consent for completing PRO questionnaires, received the European Organisation for Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire (QLQ)-C3012 and a subset of patients also the EORTC QLQ-PR2513. For each FU moment, a separate case report form (CRF) for PSA level, CTCAE, and PROs was filled out. Treatment All patients were treated in five fractions of 7.25 Gy with a two-day interval between fractions. Prior to the first fraction, a pre-treatment planning MR scan was acquired on which the target volume and OARs were delineated. Gross Tumour Volume (GTV), Clinical Target Volume (CTV), and PTV delineations were at the discretion of the treating physician and varied across institutions (Table S1 in Supplementary A). The Elekta Monaco® treatment planning system (Version 50.40.01, Elekta Inc., Stockholm, Sweden) was used to create intensity-modulated radiation therapy (IMRT) treatment plans, prescribing a dose of 36.25 Gy to the PTV. During each fraction, after positioning the patient on the treatment table, a daily online T2-weighted MR scan was acquired in treatment position. In case a so-called Adapt-to-Shape (ATS) workflow was applied, the contours from the pre-treatment planning MR or online MR from the first fraction (for fraction 2 to 5) were propagated onto the daily online MR.14 Afterwards, contours were manually adjusted if necessary.15 After approval of the daily contours, the treatment plan was recalculated and simultaneously a position verification (PV) MR scan was obtained. Adapt-to-Position (ATP) was applied in case of a substantial CTV shift, or
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