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. 2021 Jul 25:19:90-95.
doi: 10.1016/j.phro.2021.07.010. eCollection 2021 Jul.

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

Daan M de Muinck Keizer  1 , Jochem R N van der Voort van Zyp  1 , Eline N de Groot-van Breugel  1 , Bas W Raaymakers  1 , Jan J W Lagendijk  1 , Hans C J de Boer  1
Affiliations

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

Daan M de Muinck Keizer et al. Phys Imaging Radiat Oncol. .

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 cranial-caudal 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.

Keywords: Intrafraction motion; MR-Linac; MR-guided radiotherapy; Plan adaptation; Prostate cancer; Soft-tissue registration.

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Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
Prostate intrafraction translation graphs for all patients with respect to the daily pre-treatment scan. The horizontal lines provide the 95 percentile confidence intervals.
Fig. 2
Fig. 2
Prostate intrafraction rotation graphs for all patients with respect to the daily pre-treatment scan. The horizontal lines provide the 95 percentile confidence intervals.
Fig. 3
Fig. 3
The population systematic errors (Σ, provided on the left-hand side) and population random errors (σ, provided on the right-hand side) over time, for the tree main directions and both groups. The errors are provided with respect to the daily pre-treatment scan and incorporate applied ATP shifts.
See this image and copyright information in PMC

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