Purpose: The myQA SRS is a novel 2D digital detector array for patient specific quality assurance (QA) and commissioning of stereotactic radiosurgery (SRS). The myQA SRS system includes the detector array, 3D phantom, and various inserts for film and ion chamber measurements. Additionally, a Winston-Lutz insert is being developed by IBA for myQA SRS. The aim of this study is to investigate the feasibility of using this novel Winston-Lutz insert to evaluate Linac isocenter accuracy.
Methods: The Winston-Lutz insert is made from water equivalent acrylonitrile butadiene styrene (230mmx155mmx40mm) with a ceramic ball 5mm in diameter. Winston-Lutz test was performed on one Varian TrueBeam and one TrueBeam sTX with 20mmx20mm MLC field size. During the Winston-Lutz test, the insert was placed inside the 3D phantom and initially aligned to the isocenter by taking two orthogonal MV images. Eight images were acquired for each test with Couch=0° (Gantry=270°, 0°, 90°, 180°) and Gantry=0° (Couch=270°, 315°, 45°, 90°). Measurements were performed with 2.5MV and 6MV to evaluate the imaging quality. The maximum distance between the ceramic ball and isocenter was analyzed with IBA myQA machine and SNC machine software and compared to Winston-Lutz test with Brainlab phantom.
Results: For Winston-Lutz test on the sTX with IBA phantom, the maximum 2D and 3D deviations are 0.77mm and 0.89mm, respectively. Large differences were observed for the treatment couch. Compared to 6MV, 2.5MV provided better contrast and made the ceramic ball detection much easier during the QA process. At the meantime, the maximum 2D and 3D deviations from IBA phantom are equivalent to the Brainlab phantom when analyzed with both IBA and SNC software.
Conclusion: A novel Winston-Lutz insert was developed and integrated with myQA SRS system. This new Winston-Lutz phantom provided equivalent results compared to the Brainlab phantom and streamlined isocenter accuracy check for SRS.
Funding Support, Disclosures, and Conflict of Interest: This study is supported by IBA Dosimetry