Author: Ashley Elizabeth Bonner, Michael G. Snyder 👨🔬
Affiliation: Corewell Health William Beaumont University Hospital, Department of Radiation Oncology, Corewell Health William Beaumont University Hospital 🌍
Purpose: Develop and evaluate metrics for assessing beam matching of profiles across Elekta linacs in a multisite clinical setting.
Methods: Four Elekta linear accelerators (three VersaHD and one Agility) across multiple clinic sites were evaluated. One VersaHD (VRO2) was designated as the reference machine to assess the performance of the other three units. For 6 MV, crossline and inline scans for a 30x30cm2 field size at a 10 cm depth were obtained from previous annual beam scans. In accordance with definitions in TG-45, flatness and symmetry for each profile were compared to VRO2, with a target deviation of within 1% for each parameter. To conduct a more thorough evaluation, a detailed point-by-point comparison of scanning data was performed for each scan. Subsequently, the point values were normalized to the maximum dose within the usable field (30x30cm2) and the percent difference from VRO2 was evaluated. The goal was to ensure that the normalized point comparison values for each machine had a mean difference of less than 0.5% when compared to VRO2 reference.
Results: The flatness for the VRO2 crossline scan was 1.41%, with the other machines showing values within 1% of this (1.52%, 1.62%, and 1.20%). The inline scans exhibited similar results: 1.37%, 1.48%, 1.59%, and 1.35%. For symmetry, the VRO2 crossline scan had a value of 100%, while the other machines showed values of 100.53%, 100.24%, and 100.51%. The inline scans were all within 1%, with values of 100%, 100.57%, 100%, and 100.57%. The normalized point-by-point percent difference comparison yielded a mean value of less than 0.20% for all machines and scans, with the max difference being under 0.65%.
Conclusion: The data confirms the Elekta linacs’ 6 MV 30x30cm2 profiles at 10 cm depth are matched, with flatness/symmetry within 1% and normalized point comparison means within 0.5% of the standard.