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Session: SBRT Planning and Outcomes [Return to Session]

The Spatial Accuracy of Ring-Mounted Halcyon Versus TrueBeam Linac for Single-Isocenter/Multi-Target SBRT Treatment

R Mallory*, D Pokhrel, University of Kentucky, Lexington, KY


WE-C1030-IePD-F3-1 (Wednesday, 7/13/2022) 10:30 AM - 11:00 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 3

Purpose: SBRT treatment of oligometastatic lesions via single-isocenter/multi-target (SIMT) plan is more efficient than using multi-isocenter/multi-target SBRT. This study quantifies the spatial accuracy of two linac-based/image-guided SBRT systems for SIMT treatment pertaining to potential amplification of error as a function of target distance-to-isocenter.

Methods: We compare the Ring-Gantry Halcyon Linac equipped with fast iterative conebeam-CT (iCBCT) for image-guided SIMT treatment, and the SBRT-dedicated C-arm TrueBeam with standard CBCT. For both systems, Sun Nuclear’s MultiMet Winston-Lutz phantom with 6 metallic BBs distributed at different planes up to 7cm away from isocenter was aligned, imaged via CBCT, and couch corrections applied. To treat all 6 BBs, an Eclipse 10-field 3D-conformal plan using Field-in-Field (2×2 cm² MLC field to each BB) for different gantry, collimator, and couch (TrueBeam) positions was developed for both machines with 6MV-FFF beam, and the plan was delivered through ARIA once a week, and EPID images were analyzed via Sun Nuclear’s software for spatial positioning accuracy. On TrueBeam, the treatment plan was delivered twice: one with 3-DoF translational corrections and one with 6DoF perfect-pitch couch corrections.

Results: The average set up accuracy at isocenter was 0.55±0.37mm, 0.53±0.23mm, and 0.56±0.29mm and at 7cm distance-to-isocenter 0.59±0.30mm, 0.69±0.30mm, and 0.70±0.35mm for Halcyon, 3DoF TrueBeam and 6DoF TrueBeam, respectively-suggesting there are no clinically significant deviations of spatial uncertainty between machines correlating the geometric uncertainty and distance-to-isocenter. Weekly measurements demonstrated a reproducible positional accuracy of <1.0mm for off-axis targets up to 7cm from the isocenter. Therefore, no additional PTV-margin was suggested for the lesion within 7cm from isocenter.

Conclusion: This study confirms that Halcyon can deliver similar positional accuracy to SBRT-dedicated TrueBeam to off-axis targets up to 7cm from isocenter. These results are also important for benchmarking the spatial uncertainty of our extensively used TrueBeam Linac for SIMT HyperArc SRS/SRT treatment.

Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by Varian Medical Systems (Palo Alto, CA)


Quality Assurance, Stereotactic Radiosurgery, Image Guidance


Not Applicable / None Entered.

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