Gating has a negligible impact on dose delivered in MRI-guided online adaptive radiotherapy of prostate cancer.

Background and Purpose: MR-guided radiotherapy (MRgRT) allows real-time beam-gating to compensate for intra-fractional target position variations. This study investigates the dosimetric impact of beam-gating and the impact of PTV margin on prostate coverage for prostate cancer patients treated with online-adaptive MRgRT.
Materials and Methods: 20 consecutive prostate cancer patients were treated with online-adaptive MRgRT SBRT with 36.25 Gy in 5 fractions (PTV D _95%  ≥ 95% (N = 5) and PTV D _95%  ≥ 100% (N = 15)). Sagittal 2D cine MRIs were used for gating on the prostate with a 3 mm expansion as the gating window. We computed motion-compensated dose distributions for (i) all prostate positions during treatment (simulating non-gated treatments) and (ii) for prostate positions within the gating window (gated treatments). To evaluate the impact of PTV margin on prostate coverage, we simulated coverage with smaller margins than clinically applied both for gated and non-gated treatments. Motion-compensated fraction doses were accumulated and dose metrics were compared.
Results: We found a negligible dosimetric impact of beam-gating on prostate coverage (median of 0.00 Gy for both D _95% and D _mean ). For 18/20 patients, prostate coverage (D _95%  ≥ 100%) would have been ensured with a prostate-to-PTV margin of 3 mm, even without gating. The same was true for all but one fraction.
Conclusion: Beam-gating has negligible dosimetric impact in online-adaptive MRgRT of prostate cancer. Accounting for motion, the clinically used prostate-to-PTV margin could potentially be reduced from 5 mm to 3 mm for 18/20 patients.
Competing Interests: Conflict of interest statement Isak Wahlstedt’s Ph.D. studies are partly funded by Danish Comprehensive Cancer Center and ViewRay. Nicolaus Andratschke reports research grants, speaker honorary, and travel grants from Brainlab and ViewRay. Claus P. Behrens reports research contracts with Brainlab, Varian, and ViewRay outside of the submitted work. Ivan R Vogelius reports institutional research and teaching contracts with Brainlab, Varian, and ViewRay. Stefanie Ehrbar, Hubert S. Gabryś, Helena Garcia Schüler, Matthias Guckenberger, Abraham George Smith, Stephanie Tanadini-Lang, José D. Tascón-Vidarte, and Janita E. van Timmeren report no conflict of interest.
(Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)