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.