Beam energy metrics for the acceptance and quality assurance of Halcyon linear accelerator

Purpose: Establish and compare two metrics for monitoring beam energy changes in the Halcyon platform and evaluate the accuracy of these metrics across multiple Halcyon linacs. Method: The first energy metric is derived from the diagonal normalized flatness (F_DN), which is defined as the ratio of the average measurements at a fixed off-axis equal distance along the open profiles in two diagonals to the measurement at the central axis with an ionization chamber array (ICA). The second energy metric comes from the area ratio (AR) of the quad wedge (QW) profiles measured with the QW on the top of the ICA. Beam energy is changed by adjusting the magnetron current in a non-clinical Halcyon. With D_10cm measured in water at each beam energy, the relationships between F_DN or AR energy metrics to D_10cm in water is established with linear regression across six energy settings. The coefficients from these regressions allow D_10cm(F_DN) calculation from F_DN using open profiles and D_10cm(QW) calculation from AR using QW profiles. Results: Five Halcyon linacs from five institutions were used to evaluate the accuracy of the D_10cm(F_DN) and the D_10cm(QW) energy metrics by comparing to the D_10cm values computed from the treatment planning system (TPS) and D_10cm measured in water. For the five linacs, the D_10cm(F_DN) reported by the ICA based on F_DN from open profiles agreed with that calculated by TPS within –0.29 ± 0.23% and 0.61% maximum discrepancy; the D_10cm(QW) reported by the QW profiles agreed with that calculated by TPS within –0.82 ± 1.27% and –2.43% maximum discrepancy. Conclusion: The F_DN-based energy metric D_10cm(F_DN) can be used for acceptance testing of beam energy, and also for the verification of energy in periodic quality assurance (QA) processes.