Inaccurate Positioning Might Introduce Significant Calibration Errorsto a Detector-Array QA Device for Flatten Filter Free Beams

Purpose: This study investigated the calibration error caused by misalignment of the beam with the MapCheck2 due to inaccurate positioning. We hypothesized the calibration for the Flatten-Filter-free (FFF) beam is more vulnerable to the positioning error than the conventional flattened beam. Materials and Methods: A MapCheck2 was calibrated for the 10MV conventional and FFF beams with perfect alignment and with an introduced 1-cmpositioning error. The effect of positioning error was modeled as a detector independent multiplication factor to predict the calibration error for misalignment up to 1 cm. The calibrated sensitivities of both beams were compared to evaluate their dependence on the beam type. Results: The 1-cm positioning error lead to 0.39% and 5.24% local calibration error for the conventional and FFF beams, respectively. After propagating to the edges of the MapCheck2, the calibration errors became 6.5% and 57.7%. The propagation error increased almost linearly with the positioning error. The percentage difference of sensitivities between the conventional and FFF beams was small (0.11±0.49%) without positioning error but was significantly larger (-32.2±17.8%) with1-cmmisalignment. Conclusion: The positioning error is not handled by the current commercial calibration algorithm. The calibration errors for the FFF beam are ~9 times greater than that for the conventional beam with identical positioning error, and a small 1mm positioning error might lead up to 8% calibration error. Since the sensitivities depend weakly on the beam type, it is recommended to cross-check the calibrated sensitivities between the conventional and FFF beams to detect potential calibration errors.