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Validation of Radixact Motion Tracking and Compensation for 3D Respiratory Motion

W Ferris1*, M Kissick2, W Culberson1, J Smilowitz1,3, (1) Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison (2) Accuray Inc., Madison, WI 53717 (3) Department of Human Oncology, School of Medicine and Public Health, University of Wisconsin - Madison


(Monday, 7/15/2019) 7:30 AM - 9:30 AM

Room: Stars at Night Ballroom 1

Purpose: To validate the use of Radixact motion tracking and compensation for 3D respiratory motion.

Methods: Accuray™ has proposed a motion compensation solution on the Radixact system to track and compensate for target motion using MLC and jaw positions. Motion is tracked using external LED markers and periodic kV snapshot images. Patient plans were delivered to a Phantom+™ phantom moving in 3D on a Hexamotion™ stage. The Phantom+ was modified to hold an Accuray “Ball-Cube� phantom with embedded fiducials. Plans with tight (3 mm) CTV-PTV margins were generated on patient datasets intended for treatment with motion compensation. Realistic motion was modeled using parameters from the patient’s 4DCT such as respiratory period and amplitude. The fidelity of the delivered dose distribution was compared to planned for three cases: no motion (P0M0), motion without tracking (P0M1) and motion with tracking and compensation (P1M1). The tracked and phantom trajectories were compared for the P1M1 case. Twelve patient cases with sites of lung, liver, and pancreas have been obtained.

Results: For a sample case, the modeled peak-to-peak amplitudes of motion in the IEC-X, -Y, and -Z directions were 8, 12, and 8 mm, respectively. The gamma pass rates (2%, 2mm, 0.2x Rx threshold) were 93.7% and 98.9% for the non-motion compensation plan (P0M1) and motion compensation plan (P1M1), respectively. The RMS error between the phantom positions and corresponding tracked position was 1.28 mm.

Conclusion: The motion compensation system effectively tracked the phantom motion for this patient plan. The dose distribution for the P1M1 case matched the planning distribution closer than the P0M1 case. The untracked case was found to be robust to tumor motion even without motion compensation as illustrated in the relatively high gamma pass rate. We expect to observe a larger impact of using motion compensation for more complex plans and motion.

Funding Support, Disclosures, and Conflict of Interest: Michael Kissick is employed by and has ownership interests in Accuray, Inc.


Image-guided Therapy, Organ Motion, Target Localization


TH- External beam- photons: Motion management (intrafraction)

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