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

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.

20 consecutive prostate cancer patients were treated with online-adaptive MRgRT SBRT with 36.25 Gy in 5 fractions (PTV D95% ≥ 95% (N=5) and PTV D95% ≥ 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.

We found a negligible dosimetric impact of beam-gating on prostate coverage (median of 0.00 Gy for both D95% and Dmean). For 18/20 patients, prostate coverage (D95%≥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.

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.

Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology. 2022 Mar 26 [Epub ahead of print]

Isak Wahlstedt, Nicolaus Andratschke, Claus P Behrens, Stefanie Ehrbar, Hubert S Gabryś, Helena Garcia Schüler, Matthias Guckenberger, Abraham George Smith, Stephanie Tanadini-Lang, José D Tascón-Vidarte, Ivan R Vogelius, Janita E van Timmeren

Department of Health Technology, Technical University of Denmark, Anker Engelunds Vej 1, Bygning 101A, 2800 Kongens Lyngby, Denmark; Department of Oncology, Centre for Cancer and Organ Diseases, Copenhagen University Hospital - Rigshospitalet, Blegdamsvej 9, 2100 Copenhagen, Denmark; Department of Oncology, Copenhagen University Hospital - Herlev and Gentofte, Borgmester Ib Juuls Vej 7, 2730 Herlev, Denmark. Electronic address: ., Department of Radiation Oncology, University Hospital Zurich, University of Zurich, Zurich, Switzerland., Department of Health Technology, Technical University of Denmark, Anker Engelunds Vej 1, Bygning 101A, 2800 Kongens Lyngby, Denmark; Department of Oncology, Copenhagen University Hospital - Herlev and Gentofte, Borgmester Ib Juuls Vej 7, 2730 Herlev, Denmark., Department of Computer Science, University of Copenhagen, Universitetsparken 1, 2100 Copenhagen, Denmark., Department of Oncology, Centre for Cancer and Organ Diseases, Copenhagen University Hospital - Rigshospitalet, Blegdamsvej 9, 2100 Copenhagen, Denmark; Department of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3B, 2200 Copenhagen, Denmark.

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