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Clinical Experience Using the Navphan Phantom QA System for Independent Verification of SRS Patient Positioning Systems

D Ouellette1*, J McKenna2, (1) Weill Cornell Medical College, Brooklyn, NY, (2) New York Presbyterian Hospital, New York, NY


PO-BPC-Virtual-29 (Saturday, 4/17/2021)   [Eastern Time (GMT-4)]

Purpose: The purpose of this study is three-fold: 1) To verify the setup accuracy and repeatability of the NavPhan system; 2) To cross-verify the imaging isocenter assigned by IsoCal with the isocenter of the ExacTrac positioning system. 3) To determine the frequency of shifts above our clinical threshold at extreme couch angles.

Methods: The Navphan software was used to produce a virtual spherical phantom with dicom point <0,0,0> centered at the origin with fiducial rotation parameters (?=0°: f=45°; voxels = 0.125mm³). A treatment plan was created that contained fields and DRR’s at the four primary gantry angles and two fields with extreme table kicks (G=0°; T = 90° & 270°). IsoCal verification was performed daily prior to measurements. The ExacTrac system was calibrated using a Winston Lutz calibration procedure. ExacTrac was used to automatically register the 8-fiducials embedded in the phantom and a 6 degree-of-freedom couch was used for setup (tol = 0.7mm, 0.5°). Portal images were taken and fused using the correlated points workflow in Aria software. The phantom was independently setup 11 times over two days. ExacTrac residual data was analyzed to determine setup reproducibility. ExacTrac and Aria residuals were compared for a subset of the data (N=5).

Results: Phantom setup and localization showed good repeatability with respect to ExacTrac, with an average displacement of 0.17±0.17mm (N=11). The IsoCal isocenter and ExacTrac isocenter agree within 0.25±0.16mm (N=5),. The pitch and yaw of Varian agreed with ExacTrac within 0.05°±0.28° and 0.06°±0.20° respectively. ExacTrac detected that phantom positioning was outside of tolerance after 32% of extreme table kicks (N=22). After applying the required shifts, ExacTrac and IsoCal agreed within 0.13±0.08mm.

Conclusion: The Navphan can be used as a reliable commissioning or QA tool to verify agreement between multiple imaging systems, verify geometric setup accuracy, and test clinically relevant treatment parameters.


Not Applicable / None Entered.


Not Applicable / None Entered.

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