Dosimetric Evaluation of Compressed Sensing Respiratory Gating in MRI-Guided Radiotherapy
J Charters1*, Y Abdulkadir1, D O'Connell1, Y Yang1, J Lamb1, (1) University of California, Los Angeles, Los Angeles, CA
Presentations
PO-GePV-M-190 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: A commercial 0.35T magnetic resonance imaging (MRI)-guided radiotherapy vendor recently introduced upgraded real-time imaging frame rate based on compressed-sensing techniques. Furthermore, changes to motion tracking algorithms were introduced. Compressed sensing can be used to increase image frame rate, but may also compromise image quality. In order to assess the impact of this upgrade on respiratory gating accuracy, we evaluated gated dose distributions pre- and post-upgrade using a motion phantom and radiochromic film.
Methods: Five motion traces were used to drive an MRI-compatible motion phantom, including two artificial waveforms with 4 and 6 second periods, and two free-breathing and one breath-hold breathing traces acquired from previously treated patients. A treatment plan was developed to deliver a 3 cm diameter spherical dose distribution typical of a stereotactic body radiotherapy plan. Gating was performed using 4 frames-per-second (fps) imaging pre-upgrade, and 8 fps post-upgrading using two different motion tracking algorithms, namely “small mobile targets” (SMT) and “large deformable targets” (LDT). Radiochromic film was placed in a moving insert within the phantom to measure dose. Triple-channel film dosimetry was performed and the planned and delivered dose distributions were compared using gamma with 3%/3mm criteria. One-tailed Wilcoxon signed-rank tests compared gamma indices between 4 fps and two algorithms at 8 fps.
Results: Gamma pass rates ranged from 63-93% over the 15 combinations of breathing trace and gating conditions examined. At a significance level of 0.05, median gamma indices for 4 fps and 8 fps SMT each exceeded that for 8 fps LDT. Median gamma indices between 4 fps and 8 fps SMT were not statistically distinguishable.
Conclusion: Respiratory gating at 8 fps provides dose distributions similar to that at 4 fps. The particular algorithm selected for motion tracking in the real-time images measurably influences gated dose distributions and should be chosen deliberately.
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