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Session: Multi-Disciplinary General ePoster Viewing [Return to Session]

Dosimetric Evaluation of Respiratory Gating On a 0.35 T Magnetic Resonance Guided Radiotherapy Linac

J Charters*, Y Abdulkadir, D O'Connell, Y Yang, J Lamb, University of California, Los Angeles, Los Angeles, CA


PO-GePV-M-230 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: A commercial 0.35T magnetic resonance imaging (MRI)-guided radiotherapy vendor recently introduced upgraded real-time imaging frame rates based on compressed sensing techniques. Furthermore, additional motion tracking algorithms were made available. Compressed sensing allows for increased image frame rates but may also compromise image quality. 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 waveforms (two artificial, two patient-derived free-breathing, and one breath-holding) were used to drive an MRI-compatible motion phantom. 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 on the “default” tracking algorithm and 8 fps post-upgrade using the “small mobile targets” (SMT) and “large deforming targets” (LDT) tracking algorithms. Radiochromic film was placed in a moving insert within the phantom to measure dose. The planned and delivered dose distributions were compared using the gamma index with 3%/3mm criteria. Dose area histograms (DAHs) were produced to calculate the dose to 95% (D95) of the sphere PTV and two simulated GTVs formed by contracting the PTV by 3 mm and 5 mm, respectively.

Results: Gamma pass rates ranged from 47-90% over the 15 combinations of breathing trace and gating conditions examined. D95 ranged from 85% to 103%. On average, the LDT algorithm yielded lower gamma values and lower D95 than the default and SMT algorithms.

Conclusion: Respiratory gating at 8 fps with the new tracking algorithms provides similar gating performance to the original algorithm with 4 fps, although the LDT algorithm had lower accuracy for our non-deformable target. This indicates that the choice of DIR algorithm should be chosen deliberately based on whether the target is rigid or deforming.

Funding Support, Disclosures, and Conflict of Interest: Drs. Lamb and Yang report consulting income from ViewRay, Inc., outside of this work.


Gating, Radiochromic Film, Dose Volume Histograms


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance

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