Decomposing a Beam Profile Constancy into Energy and Symmetry Components and Its Evaluation Using an Ionization Chamber Array

Abstract

Purpose: We have proposed a new formulation that can decompose the profile constancy defined in the AAPM TG 142 into energy and symmetry constancies by measuring beam profiles using an IC profiler (ICP). Methods: Measured profiles were laterally inverted to calculate averaged profiles in the lateral direction, thereby cancelling asymmetric components. Validation tests were performed by comparing the proposed calculation and measured results under various experimental conditions. Calculated profile constancies were further compared to decomposed energy and symmetry constancies. Results: The energy constancy calculated from the averaged beam profile by lateral inversion and the measured PDD(10) constancy agreed within 0.1% when only symmetries were varied. The calculated energy and symmetry constancies, and the measured results agreed within 0.2% when both energies and symmetries were varied. Conclusion: The linac beam profile constancy has been decomposed into energy and symmetry terms. The proposed formulation has been validated by comparing the calculations and the direct measurements using the ICP. We have shown that QA/QC for profile constancy tests can be efficiently performed using the proposed formulation.