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Feasibility and Uncertainty Analysis for Using Halcyon for Stereotactic Radiotherapy

J Chapman*, B Cai , S Mutic , N Knutson , Washington University School of Medicine, St. Louis, MO


(Sunday, 7/14/2019) 1:00 PM - 2:00 PM

Room: Stars at Night Ballroom 1

Purpose: To evaluate the feasibility of using a ring gantry linear accelerator for stereotactic radiotherapy (SRT).

Methods: In this work, the feasibility of SRT using the three degree of freedom (3dof) Halcyon (Varian Medical Systems, Palo Alto, CA) system was investigated. Our dataset consisted of 275 total fractions of intracranial SRT for patients setup and shifted using optical surface tracking and a commercially available 6dof SRT system. Histograms of the data were produced using 0.1 cm translation bins and 0.5 degree rotation bins. A cumulative distribution function (CDF) was calculated from a Gaussian fit to the rotation histogram. Finally, from this CDF, a clinical tool was developed to calculate the number of patients requiring a resetup and rescan on a 3dof system.

Results: The rotation histogram was fit with an r-square of 0.987 and root-mean-square error of 7.5. For our fitted sigma of 0.91 degrees, our clinical tool showed that an isocentrically treated tumor with a maximum distance of 5 cm could be treated within a 0.2 cm margin, but would require 0.6% of patients to be resetup and rescanned using a 3dof system (10.4% using 0.1 cm margin). For a 2.5 degree sigma, this percentage increased to 18.0% (0.2 cm) and 32.3% (0.1 cm).

Conclusion: This work showed that as distance from isocenter to the tumor edge increases, rotation uncertainty must rapidly decrease for the tumor to remain within a 0.1 to 0.2 cm margin using 3dof systems. For 3dof systems, treating with 5 fraction SRT using 0.2 cm margins seems feasible as most patient setups would be within the treatment margin and not require a rescan. However, 6dof corrections are preferable for SRT and are required for single fraction radiosurgery. In the near future, this work will be extended to three dimensions and composite rotations.

Funding Support, Disclosures, and Conflict of Interest: This research is supported by funding from Varian Medical Systems (VMS). Dr. Sasa Mutic receives consulting income from VMS for work outside of this project. This activity has been reviewed by Washington University's (WU's) Institutional Conflict of Interest Committee (ICOIC) in accordance with WU's Institutional Conflict of Interest (ICOI) Policy.


Modeling, Setup Errors, Stereotactic Radiosurgery


TH- External beam- photons: intracranial stereotactic/SBRT

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