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

Abstract

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.

Impact of intrafraction changes in delivered dose of the day for prostate cancer patients treated with stereotactic body radiotherapy via MR-Linac

“…However, a PTV margin reduction to <5 mm will likely result in an increased number of ATP plans after ATS on the MR-Linac, further increasing overall treatment time. The advantage of ATP after ATS is that it may help account for the additional motion, and this has been noted by [7] , where ATP after every ATS fraction resulted in a reduction in prostate drift in the posterior and inferior direction and can help decrease the bladder volume in the high dose region. However, with longer treatment times, there would be an increased likelihood of intrafraction motion.…”

“…There is an opportunity to explore PTV margin reduction based on these results or personalized PTV margins considering the extent of target coverage change and displacement varied between patients. However, PTV reduction to <4 mm should be implemented with caution due to intrafraction motion over the course of treatment adaptation [7] , [14] . It has been shown that over the course of a 10-minute cine imaging session reflective of treatment delivery time on a MR-Linac; using fiducial markers as a prostate tracking surrogate, translation trends of 1 mm posterior and 0.9 mm inferior occur [6] .…”

Fast and accurate deformable contour propagation for intra-fraction adaptive magnetic resonance-guided prostate radiotherapy

“…In such instances, we hypothesize that the large magnitude of the deformations together with the significantly different image features introduced by the gas pocket itself has led to the algorithm converging towards a local minimum and in turn causing a local misregistration. Our previous work on intra-fraction motion indicated that these rectal deformations are unpredictable and non-gradual [1] , [14] , [22] , [23] . Especially cases with large rectal deformations could benefit from an adaptive workflow and therefore warrant extra time to assure contours are accurate.…”

Daily online contouring and re-planning versus translation-only correction in neurovascular-sparing magnetic resonance-guided radiotherapy for localized prostate cancer

“…During the treatment plan calculation, an online 3D T2w position verification MR scan was made. The position verification scan was used to perform an additional ATP procedure directly before beam on to account for intratreatment patient motion during the ATS procedure [10] , [11] .

Fig.

…”