On-line daily plan optimization combined with a virtual couch shift procedure to address intrafraction motion in prostate magnetic resonance guided radiotherapy

Background and purpose: In daily adaptive magnetic resonance (MR)-guided radiotherapy, plans are adapted based on the patient's daily anatomy. During this adaptation phase, prostate intrafraction motion (IM) can occur. The aim of this study was to investigate the efficacy of always applying a subsequent virtual couch shift (VCS) to counter IM that occurred during the daily contour and plan adaption (CPa) procedure. Material and Methods: One hundred fifty patients with low and intermediate risk prostate cancer were treated with 5x7.25 Gy fractions on a 1.5 T MR-Linac. In each fraction, contour adaptation and dose re-optimization was performed using the session's first MR-scan. IM that occurred here was countered using two methods. One patient group had selective VCS (sVCS) applied if the CTV reached outside the PTV on a second MR acquired during plan optimization. The other group had always VCS (aVCS) applied for any prostate shift greater than 1 mm. Remaining IM during beam delivery was determined using 3D cine-MR. Results: Percentage of fractions where a VCS was applied was 28% (sVCS) vs 78% (aVCS). Always applying VCS significantly reduced influences of systematic prostate IM. Population random and systematic median values in all translations directions were lower for the aVCS than sVCS group, but not for the population random cranialcaudal direction. Conclusion: Applying VCS after daily CPa reduced impact of systematic prostate drift in especially the posterior and caudal translation direction. However, due to the continuous and stochastical nature of prostate IM, margin reduction below 4 mm requires fast intrafraction plan adaption methods.