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Sensitivity and concordance study of an open source EPID-based linear accelerator test suite
Lana C Critchfield1, Cory S Knill2, Ellie Durussel3
1Department of Radiation Oncology, University of Michigan, Wyoming, Michigan, USA.
Background:
Electronic portal imaging devices (EPIDs) enhance linear accelerator (linac) quality assurance (QA) efficiency but are not universally used beyond patient-specific IMRT QA. A multi-institutional Consortium seeks to standardize linac QA utilizing an EPID-based test suite.
Purpose:
This study evaluates the sensitivity of a test suite to intentional linac and plan errors and the concordance with linac fluctuations over time.
Methods:
Baseline and error-introduced tests were performed on a Varian TrueBeam and Clinac. Evaluated parameters included enhanced dynamic wedge factors (EDW) (6MV), central axis dose (6MV, 6MV FFF, 16MV), focal spot alignment and beam symmetry (6MV, 16MV), dose rate-gantry speed and multi-leaf collimator (MLC) leaf speed (6MV). Errors included varying EDW angles (10°-60°), output changes, beam steering for focal spot and symmetry inaccuracies, and meterset adjustments of control points for dose rate-gantry speed and MLC leaf speed tests. Measurements were performed with an EPID, analyzed via test suite software, and compared to an ion chamber (IC) or array. Concordance analysis was performed on a single linac over 5 months to assess the test suite's ability to quantify deviations during routine operations. Measurement reproducibility was assessed.
Results:
Results were reported as differences between EPID and an IC or array. For each EDW, wedge factor differences were <0.5% (TrueBeam) and <1.5% (Clinac). Central axis dose variations stayed within ±0.8%. For focal spot, EPID underestimated the radial direction and overestimated the transverse direction. Symmetry measurements showed strong linearity (r > 0.99), though EPID measurements underestimated changes in symmetry relative to baseline. Maximum differences for dose rate-gantry speed and MLC leaf speed were <0.4%. In the concordance study, output differences averaged 0.3% ± 0.3% (6MV), 0.5% ± 0.3% (6MV FFF), and 0.3% ± 0.2% (16MV). Systematic differences in symmetry were observed between EPID and a 2D-array with standard deviations <0.2%.
Conclusions:
Comparisons with traditional detectors showed the EPID test suite can detect errors and supports commissioning and clinical integration.
