Part I | Chapter 6 120 four weeks at the time of filling out the QLQ-PR25 questionnaire, the percentage of patients who reported to have “quite a bit” to “very much” difficulty in getting or maintaining an erection increased significantly from 21.7% at baseline to 31.6% at 12 months FU. Previous reports on ED after SBRT treatment for prostate cancer showed a gradual decline in erectile function beyond 12 months FU up to 5 years after treatment. Therefore, longer FU and larger patient numbers are warranted to draw definitive conclusions regarding ED after MRI-guided radiotherapy.20 Currently, an advantage of MR-guidance is the absence of the need for fiducial marker placement.21 Theoretical advantages include intrafraction motion monitoring and correction for interfraction prostate motion (translation and rotation) in case of applying an ATS procedure, more accurate visualisation of the dominant intraprostatic lesion for focal boosting22, visualisation of neurovascular structures to allow sparing23, and the potential for MR biomarker-based adaptive treatment24. However, for MRI-guided radiotherapy to become a cost-effective alternative to conventional CTbased EBRT, brachytherapy, and/or prostatectomy, a substantial reduction in toxicity is needed.25 For this, comparative studies, preferably randomised controlled trials, are needed. The MIRAGE trial is the first randomised controlled trial comparing (low-field) MRI-guided radiotherapy with conventional CT-guided radiotherapy and is currently ongoing.26 An interim analysis showed promising results, including a significantly lower acute grade ³ 2 GU and GI toxicity in patients who received 5 x 8 Gy on an MR-Linac with 2 mm PTV margins compared to patients treated on a CT-guided linac with 4 mm PTV margins (incidence of grade 2 GU toxicity: 11 (22.4%) vs. 24 (47.1%), p = 0.01; incidence of grade ³ 2 GI toxicity: 0 (0%) vs. 7 (13.7%), p = 0.01).27 Furthermore, multiple prospective long-term registries are ongoing to collect FU data on toxicity and PROs in patients treated with MRI-guided radiotherapy as well as conventional EBRT, brachytherapy, prostatectomy, and active surveillance, which allow for comparison between the various treatments.9,28 Also, fast intrafraction MR scan acquisition, improved automatic contouring, and fast online and real-time adaptive re-planning during beam-on need to be implemented to enable further margin reduction to reduce toxicity, and to open up possibilities for extreme hypofractionation in two fractions.24,29-31 We acknowledge that our study suffers from some limitations. First, the rate of missing CTCAE data was substantial, which should be taken into account when comparing our results to literature. CTCAE data was prospectively registered, but not all radiation oncologists systematically documented the toxicity using the 17 predefined CTCAE items. Furthermore, not all patients had an in person appointment with their radiation oncologist at all FU moments and the COVID-19 pandemic even further reduced the number of in person appointments. Currently, efforts are being made to increase the CTCAE reporting rate, such as CTCAE registration using paper forms handed out to the physician as well as real-time remote symptom monitoring by a dedicated app.32 We expect that this will improve CTCAE registration. The gradual decline of data availability rates towards later FU moments, which is also present for PSA values and PROs, may be caused by a delay in data registration in the study database. Second, although the highest grade of CTCAE toxicity between 0 and 3 months was recorded for the 3 months FU time point, CTCAE registration was only standardised at 3 months FU. Therefore,
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