Purpose: Monthly energy verification quality assurance (QA) evaluates the stability of the energy spectrum delivered by a Linac. Modeling the QA setup may allow physicists to determine whether failures during QA are due to changes in Linac operation or measurement setup.
Methods: Electron energy verification records measured with an IBA Matrixx detector array on an Elekta Versa Linac were collected from the years 2018-2020. Electron energies included 6MeV, 9MeV, 12MeV, and 15MeV. Matrixx assesses the electron energy using eight ion chambers shielded by different thicknesses of high-Z inserts to simulate different effective depths. The signal measured in each detector is normalized by an unshielded chamber on the central axis to obtain an effective percent depth dose (PDDₑ) curve. Measurements were compared to the reference measurements acquired during commissioning with a ±2% tolerance applied to each chamber. The detector array and high-Z inserts were simulated in Pinnacle Treatment Planning System (TPS). The dose to each chamber was calculated with the reference conditions used during energy verification QA: 100 monitor units and a 20x20cm² field. For each electron energy, the PDDₑ was compared between the Pinnacle simulation and those obtained during each energy verification measurement.
Results: From 2018-2020, 42 measurements were acquired with one failure for both 6MeV and 15MeV beams; 9MeV and 12MeV beams resulted in no failures. Differences between measurements and reference ranged from 0.0%-2.9% (mean: 0.37%), 0.0%-1.8% (mean: 0.19%), 0.0%-1.2% (mean: 0.29%), and 0.0%-2.1% (mean: 0.62%) across chambers for 6MeV, 9MeV, 12MeV, and 15MeV beams, respectively. Simulated doses differed from reference measurements on average by 3.16%, 2.65%, 1.53%, and 1.04% for 6MeV, 9MeV, 12MeV, and 15MeV beams, respectively.
Conclusion: Simulating the energy verification measurement setup and adjusting the energy in the TPS may allow for an assessment of beam stability relative to commissioning and changes in the energy spectrum.