A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images

Purpose: Withusually a millimeter-level PTV margin, stereotactic radiosurgery (SRS) andstereotactic body radiation therapy (SBRT) pose a stringent requirement on theisocentricity of the Linac. This requirement is partly fulfilled by routineisocenter quality assurance (QA) test to verify the size and location of theisocenter. The current common QA methods such as spoke shot were developed beforeSBRT/SRS became popular and when IGRT was largely absent and hencehave their limitations. In this work, we describe an isocenter QA approachbased on portal imaging to provide the community with a superior alternative. Methods: The proposed approachutilizes a BrainLab ball bearing (BB) phantom in conjunction with an electronic portal imaging devices (EPID) imager.The BB phantom was first aligned with a calibrated room laser system. Portalimages were then acquired using 6 MV beam with a 2 × 2 cm2 open field and a 15 mm cone on a Varian TrueBeam STx machine. The gantry,collimator, and table were rotated separately at selected angles to acquire aseries of portal images in order to determine the isocenter of each rotatingsystem. The location and diameter of these isocenters were determined bycalculating therelative displacement of either BB or open field edge between the acquired EPID images. The demonstrationof the reproducibility and robustness of this EPID-based approach was carried out by repeating measurements 10 timesindependently for each rotating system and simulating clinical scenarios ofasymmetric jaws and misalignment of BB phantom, respectively. Results: Forour TrueBeam STx machine, the isocenter diameter derived from open-field EPIDimages was roughly 0.15 mm, 0.18 mm, 0.49 mm for the collimator, table, andgantry, respectively. For the collimator and gantry, images taken with the conegave considerably smaller isocenter diameter. Results remained almost unchangeddespite the presence of simulated BB misalignment and asymmetric jaws error,and between independent measurements. Isocenter location and diameter derivedfrom images obtained at a limited number of angles (≤11) were adequatelyaccurate to represent those derived from images of densely sampled angles. Conclusions: An EPID-based isocenter QA approach is describedand demonstrated to be accurate, robust, and reproducible. This approachprovides a superior alternative to conventional isocenter QA methods with no additionalcost. It can be implemented with convenience for any linear accelerator withan EPID imager.