Gating has small impact on dose to OARs and PTV coverage in MR-guided daily ART of prostate cancer
OC-0041
Abstract
Gating has small impact on dose to OARs and PTV coverage in MR-guided daily ART of prostate cancer
Authors: Isak Wahlstedt1,2,3, Nicolaus Andratschke4, Claus P Behrens3, Stefanie Ehrbar4, Hubert S Gabryś4, Helena Garcia Schüler4, Matthias Guckenberger4, Abraham George Smith5, Stephanie Tanadini-Lang4, José David Tascón-Vidarte5, Ivan R Vogelius2,6, Janita E van Timmeren4
1Technical University of Denmark, Department of Health Technology, Kongens Lyngby, Denmark; 2Copenhagen University Hospital - Rigshospitalet, Department of Oncology, Copenhagen, Denmark; 3Copenhagen University Hospital - Herlev and Gentofte, Department of Oncology, Herlev, Denmark; 4University Hospital of Zurich, Department of Radiation Oncology, Zürich, Switzerland; 5University of Copenhagen, Department of Computer Science, Copenhagen, Denmark; 6University of Copenhagen, Department of Health and Medical Sciences, Copenhagen, Denmark
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Purpose or Objective
For patients treated with radiotherapy of the prostate, intrafraction motion could result in compromises in target coverage or increased dosage of nearby healthy tissue. Magnetic resonance guided radiotherapy (MRgRT) allows the beam to be interrupted (online beam-gating). This study investigates the dosimetric impact of beam-gating for prostate cancer patients treated with MRgRT, as well as the impact of planning target volume (PTV) size on prostate coverage.
Material and Methods
20 consecutive prostate cancer patients received MRgRT of 36.25 Gy or 34.40 Gy (PTV D95%≥ 100%) in 5 fractions with daily online plan adaptation. 2D cines acquired with 4 frames per second were used for gating on the prostate structure with a 3 mm expansion as the gating window. The prostate-to-PTV-margin was 3 mm posteriorly and 5 mm in all other directions.
2D cine images were extracted from the treatment planning software and used to track the center of mass in each frame using an in-house-developed Matlab script. The script then generated 2D weight maps of prostate positions in craniocaudal and anteroposterior directions with 1 mm resolution during beam-on (residual motion) and during both beam-on and beam-off (total motion). These weight maps were used to shift and sum the daily planned 3D dose distributions to obtain sagittal 2D motion-compensated dose distributions. We accumulated these to obtain daily motion-compensated dose accumulations (MCDA) for residual motion during the beam-on periods and, for comparison, MCDA in the simulated scenario of no gating but beam continuously on. We used deformable image registration with the hybrid algorithm in MIM.
Accumulated doses were evaluated based on D95% and Dmean to the prostate as well as for D2% to the bladder and rectum. To evaluate the impact of beam-gating on the prostate-to-PTV margin, we simulated coverage with smaller margins than clinically applied as follows. Four structures were generated on the simulation MRI per patient by increasing the prostate isotropically by 1 mm, 2 mm, 3 mm, and 4 mm and then subtracting the rectum structure. These structures simulate a prostate-to-PTV-margin of 4 mm, 3 mm, 2 mm, and 1 mm, respectively. We evaluated the coverage for all PTV margins for MCDA of residual motion (gating), total motion (without gating), and compared to accumulated planned static dose.
Results
Online beam-gating had little dosimetric effect on coverage and dose to organs at risk (Figure 1). All but two patients would have received D95%>100% with a 3 mm prostate-to-PTV margin instead of the clinical 5 mm margin (Figure 2). Patient 12 had the poorest prostate coverage in the initial plan and is indicated with a triangle in both figures.
Conclusion
For the patients in our study online-beam gating had little dosimetric effect. For 18 out of 20 patients, the clinically used prostate-to-PTV margin of 5 mm could have been reduced to 3 mm while still retaining 95% coverage even without the online gating.