MRI-guided radiotherapy for prostate cancer: the MOMENTUM study 119 Our findings are in line with the results of Bruynzeel et al.6, who reported the first early results in 101 prostate cancer patients who received 5 x 7.25 Gy on a low-field (0.35T) MR-Linac. Their patient group consisted of a higher-risk population (4.0% low-, 36.6% intermediate-, and 59.4% high-risk) and they employed a urethra-sparing technique. The QLQ-PR25 urinary and bowel domain scores were comparable to those observed in our study. Also, the cumulative incidence of grade ³ 2 GU and GI toxicity were 23.8% and 5.0% at 3 months FU, respectively, and were in the same range as the grade ³ 2 GU toxicity of 18.7% and GI toxicity of 1.7% in our study. In a subsequent paper by the same research group, the PROs in the same patient cohort up to 1 year after treatment were reported.7 Similar to the QLQ-PR25 results in our study, the effect sizes for the difference in PROs between baseline and 3, 6, and 12 months FU for both the urinary and bowel domain were small. The high rate of ADT use (83.2%), as a result of the predominantly high-risk patients included, caused a significant and clinically relevant negative effect on sexual activity. Because only 33% of patients completed the questions on sexual functioning, this domain was not analysed in their paper. In a meta-analysis by Jackson et al.17, in which the results of 32 stereotactic body radiation therapy (SBRT) studies (median dose per fraction: 7.25 Gy (range: 5-10 Gy) and median fraction number: 5 (range: 4-9)) were summarised, a cumulative incidence of early grade ³ 2 GU toxicity of 16.0% and GI toxicity of 6.2% were observed. Additionally, the cumulative incidence of late grade ³ 2 GU and GI toxicity was 13.0% and 5.4%, respectively. However, the results are not directly comparable to our results, as the timeframe of acute toxicity was not always £ 3 months in the studies included in the meta-analysis. Furthermore, late toxicity went beyond 12 months FU and toxicity was graded using both the CTCAE (19 studies) and Radiation Therapy Oncology Group (RTOG)/EORTC grading (13 studies) systems. More detailed information on acute toxicity after SBRT on a CT-guided linac is available from the PACE B trial.18 In the PACE B trial, the intervention arm consisted of patients with localised low- and intermediate-risk (NCCN) prostate cancer, who received 5 x 7.25 Gy with an additional secondary CTV dose target of 40 Gy on a CT-guided linac (n = 245 (59.0%) on a conventional linac and n = 170 (41.0%) on a CyberKnife system).18 Recommended CTV-to-PTV margins were 4-5mm (left-right, cranial-caudal, and anterior) and 3-5mm (posterior). None of the patients received ADT. The cumulative incidence of CTCAE grade ³ 2 GU and GI toxicity was 30.8% and 15.7% at 3 months FU, respectively, which is higher compared to our results. The lower toxicity that is reported in our study may be a result of more accurate dose delivery due to the ability to perform online MRI-guided ATP and ATS. Since ADT has a detrimental effect on sexual activity and functioning, we have limited our analysis of sexual activity and functioning to non-ADT patients only.19 We observed a significant decline in sexual functioning from baseline to 3 and 12 months FU. The effect sizes indicated a large and moderate effect, respectively, which emphasises the clinical relevance of the domain score decline. The significant increase in CTCAE erectile toxicity at 12 months FU supports this finding. To get a more detailed picture of sexual functioning of these patients, we looked at the individual questions of the QLQ-PR25. Of the non-ADT patients who reported to have been sexually active over the last 6
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