Program Information
A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images
M Zhang , Q Fan*, Y Lei , S Li , S Zhou , University of Nebraska Medical Center, Omaha, NE
Presentations
SU-I-GPD-T-215 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: With usually a millimeter-level PTV margin, SBRT/SRS poses a stringent requirement on the isocentricity of the Linac. This requirement is partly fulfilled by routine isocenter QA to verify the size and location of the isocenter. Currently the popular QA methods such as spoke shot are developed before SBRT/SRS becomes popular and when IGRT is largely absent. In this work, we propose a new isocenter QA approach based on EPID imaging to provide the community with a superior alternative.
Methods: A Brainlab ball bearing (BB) was aligned with laser and portal images were acquired with a 2×2 cm² open field and a 15 mm cone on a TrueBeam STx machine. The gantry, collimator, and table were rotated separately at evenly distributed angles to acquire a series of images. The location and diameter of the isocenters were determined using the relative shifts of the BB and the field edges between images of different angles. The reproducibility and robustness of this approach were evaluated by repeating measurements 10 times independently and simulating clinical scenarios of asymmetric jaws and misalignment of BB phantom, respectively.
Results: The isocenter diameter derived from open-field EPID images was roughly 0.16 mm, 0.18 mm, 0.50 mm for the collimator, table, and gantry, respectively. For the collimator and gantry, images taken with the cone gave considerably smaller isocenter diameter. Results remained almost the same in the presence of BB misalignment and asymmetric jaws error, and between independent measurements. Isocenter location and diameter derived from images taken at a limited number of angles (< 11) were adequately accurate to represent those derived from images of fully sampled angles.
Conclusion: An EPID based isocenter QA approach is proposed and demonstrated to be accurate, robust, and reproducible. This approach provides a superior alternative to conventional isocenter QA methods with no additional cost.
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