MRI-guided radiotherapy for prostate cancer: the MOMENTUM study 121 toxicity which settles before 3 months may have been missed if not documented in the medical records. This should be considered when comparing our toxicity outcomes at 3 months FU with other studies, such as the series reported by Bruynzeel et al.6 (standardised CTCAE registration at the last fraction, 6, and 12 weeks FU) and the PACE B trial18 (standardised CTCAE registration at 2, 4, 8, and 12 weeks FU). Third, the response rates for the PRO questionnaires were high during follow-up. However, a group of patients did not receive the QLQ-PR25 questionnaire, because they were simultaneously enrolled in another prostate-specific prospective registry (NCT04228211), for which the QLQ-PR25 was replaced with the Expanded Prostate Cancer Index Composite (EPIC)-26.28,33 The QLQ-PR25 and EPIC-26 are similar in terms of questions and domains, but not directly comparable. So, for these patients, PRO data was not lost, but they were not eligible for the QLQ-PR25 analyses. Finally, no PRO data is available on FU moments between baseline and 3 months FU (e.g. directly after the final treatment fraction or at 1 month post-treatment). A transient deterioration of PRO scores during and shortly after radiation therapy may therefore have been missed. Conclusion The results presented in the current study show that the treatment of localised prostate cancer with SBRT on a 1.5 T MR-Linac is effective and safe. A transient but significant increase in the cumulative incidence of physician-reported GU and GI toxicity was reported at 3 months FU and a significant increase in physician-reported ED rate was reported at 12 months FU. Compared to baseline, no relevant deterioration in patient-reported bowel and sexual activity domains was observed at 3, 6, and 12 months FU. However, there was a significant decline in urinary domain scores at 3, 6, and 12 months FU and sexual functioning domain scores at 6 and 12 months FU. These data are useful for counselling patients on expected outcomes after MRI-guided radiotherapy and can be used to inform study designs of future comparative-effectiveness studies. 6
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