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Optimizing Accuracy of the BrainLab SRS Mask System On a Varian TrueBeam Linac

B Rasmussen

B Rasmussen*, K Chu, Marquette General Hospital, Marquette, MI

SU-E-T-420 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Compare the geometric accuracy of the BrainLab cone SRS system with paper target overlays against the Varian TrueBeam CBCT image guidance system as well as to optimize CT protocols for planning.

Methods: A head phantom with rigid geometric inserts (mounted in the BrainLab headring) was used to test the localization accuracy of the BrainLab system and the TrueBeam CBCT system. A 2 mm diameter BB (to act as a target) was placed on one of the surfaces inside the phantom. Planning CT scans were acquired with both helical and sequential techniques with a range of slice thicknesses. Localization was performed using the BrainLab CT/X-ray localizer box and subsequent planning was performed in iPlan. Printed target position overlays were then used to set up the phantom on the TrueBeam. A CBCT was acquired, 3D/3D matching was performed, and the differences in position between the paper printouts and 3D/3D match were recorded. Position of laser intersection on BB target was used as the standard for accuracy assessment.

Results: The BrainLab localization system was found to align the phantom to within approximately 1.0 mm in any one direction of the target BB center. CBCT image guidance with 3D/3D matching was able to improve alignment to within approximately 0.5 mm in any one direction. The observed difference is partially attributed to a 0.1° mechanical misalignment of the couch mount and thus in the BrainLab localizer box.

Conclusion: CBCT guidance was found to be more accurate than BrainLab localizer alone. Oversampled, helical scanning for planning is recommended because of the smooth table motion, quicker scan time, and no loss of localization accuracy compared to sequential scanning. Quality assurance testing of the CBCT system accuracy should be performed after verifying the lasers with the standard Winston-Lutz test and just prior to each treatment.

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