On-board imagers are quite suitable for both geometric and dosimetric testing of linac system involved in AT. Electronic format of the acquired data lends itself to benchmarking, transparency, as well as longitudinal use of AT data. While the tests were performed on a specific model of a linear accelerator, the proposed approach can be extended to other linacs.

Potential errors due to human factors were decreased for the ATP compared to ATP so it is possible that a largely automated linac ATP can mitigate many error occurrences. Manufacturers should be careful when designing an EPID-based ATP to address errors in the EPID pixel sensitivity map which can potentially lead to a significant impact on patients' treatment.

Purpose: Quality assurance (QA) of complex linear accelerators is critical and highly time consuming. Varian’s Machine Performance Check (MPC) uses IsoCal phantom to test geometric and dosimetric aspects of the TrueBeam systems in <5min. In this study we independently tested the accuracy and robustness of the MPC tools. Methods: MPC is automated for simultaneous image‐acquisition, using kV‐and‐MV onboard‐imagers (EPIDs), while delivering kV‐and‐MV beams in a set routine of varying gantry, collimator and couch angles. MPC software‐tools analyze the images to test: i) beam‐output and uniformity, ii) positional accuracy of isocenter, EPIDs, collimating jaws (CJs), MLC leaves and couch and iii) rotational accuracy of gantry, collimator and couch. 6MV‐beam dose‐output and uniformity were tested using ionization‐chamber (IC) and ICarray. Winston‐Lutz‐Tests (WLT) were performed to measure isocenter‐offsets caused by gantry, collimator and couch rotations. Positional accuracy of EPIDs was evaluated using radio‐opaque markers of the IsoCal phantom. Furthermore, to test the robustness of the MPC tools we purposefully miscalibrated a non‐clinical TrueBeam by introducing errors in beam‐output, energy, symmetry, gantry angle, couch translations, CJs and MLC leaves positions. Results: 6MV‐output and uniformity were within ±0.6% for most measurements with a maximum deviation of ±1.0%. Average isocenter‐offset caused by gantry and collimator rotations was 0.316±0.011mm agreeing with IsoLock (0.274mm) and WLT (0.41mm). Average rotation‐induced couch‐shift from MPC was 0.378±0.032mm agreeing with WLT (0.35mm). MV‐and‐kV imager‐offsets measured by MPC were within ±0.15mm. MPC predicted all machine miscalibrations within acceptable clinical tolerance. MPC detected the output miscalibrations within ±0.61% while the MLC and couch positions were within ±0.06mm and ±0.14mm, respectively. Gantry angle miscalibrations were detected within ±0.1°. Conclusions: MPC is a useful tool for QA of TrueBeam systems and its automation makes it highly efficient for testing both geometric and dosimetric aspects of the machine. This is very important for hypo‐fractionated SBRT treatments. Received support from Varian Medical Systems, Palo Alto, CA 94304‐1038.

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