Beam Matching Assessment of Two Medical Linear Accelerators Using Flatness and Symmetry Profile Measurements from Routine Quality Assurance

Beam matching of linear accelerators ensures interchangeable treatment delivery across machines and supports clinical flexibility. This study evaluates beam matching of two Elekta Synergy Platform linacs using routine quality-assurance (QA) flatness and symmetry profiles collected in 2024–2025. Routine profile scans were acquired with a PTW Semiflex ionization chamber (0.125 cm³) in a 3D water phantom at SSD 90 cm, 10 cm depth, and a 10×10 cm² field. Flatness and symmetry were calculated for in-plane and cross-plane profiles across periodic QA sessions and compared between Linac 2 and Linac 3 using descriptive statistics to assess inter-machine differences and compliance with QA tolerances. Mean flatness (in-plane) was 103.91% (SD 0.27) for Linac 3 and 104.26% (SD 0.64) for Linac 2; cross-plane flatness means were 104.89% (SD 0.23) and 105.04% (SD 0.19), respectively. Symmetry means were closely matched: in-plane 100.67% vs 100.55% (mean Δ ≈ 0.11%) and cross-plane 100.52% vs 100.48% (mean Δ ≈ 0.03%). Typical inter-machine absolute differences across metrics ranged 0.23–0.45%. One isolated in-plane flatness excursion (~2.04%) was observed and flagged for follow-up. Longitudinal routine QA shows the two Elekta Synergy linacs are well matched for routine 3DCRT and many IMRT/VMAT workflows, with low inter-machine variability. Continued periodic verification and prompt investigation of outliers are recommended, and stricter criteria should apply for stereotactic treatments Keywords: Beam Matched Linacs, Routine QA, Flatness Profile, Symmetry Profile