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Off-Axis Winston-Lutz Analysis Performed On Elekta Versa HD Linear Accelerators

R Pappafotis*, L Rankine , J Fenoli , R McGurk , The University of North Carolina at Chapel Hill, Chapel Hill, NC


(Wednesday, 7/17/2019) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 4

Purpose: Multiple target, single-isocenter techniques for linac-based radiosurgery have increased in popularity because of their potential to shorten treatment planning and delivery times. Previously published data detailed the accuracy and feasibility of this single-isocenter technique on Varian linear accelerators. We present results for an off-axis Winston Lutz test performed on two Elekta Versa HD linear accelerators.

Methods: A Winston-Lutz test was performed using a 0.8mm ball-bearing (BB) phantom positioned at isocenter and at six other off-axis distances: shifted 2, 4, and 7cm in both the gantry-target (GT) and left-right (LR) directions. The BB phantom was localized using lasers, and its position was fine-tuned to the kV isocenter using kV-CBCT. At the four cardinal gantry angles, 3x3cm fields at collimator 0 and 180 degrees were centered on the BB and delivered to the EPID for analysis.

Results: Considering GT and LR BB shift directions on both linacs, the maximum kV to MV isocenter deviations at BB positions of 0, 2, 4, and 7cm off isocenter were 0.32, 0.43, 0.29, and 0.5mm, respectively. Under the same conditions, the maximum virtual MV startshot radii were 0.59, 0.51, 0.5, 0.57mm. Neither metric displayed a trend towards the presence of greater targeting uncertainty when moving up to 7 cm off isocenter.

Conclusion: Results from two of our institution’s Elekta Versa HD linear accelerators show that there is minimal effect on radiation field targeting accuracy for targets up to 7cm off isocenter when considering displacements in the GT or LR directions. Given these results, a 1mm isotropic expansion of the target would be sufficient to mitigate the errors in targeting accuracy for planning volumes up to 7cm off-isocenter with no couch rotations. Clinical implementation of single-isocenter SRS should be accompanied by more stringent daily QA, similar to the tests described here.


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