Characterization of Ethos therapy systems for adaptive radiation therapy: A multi‐machine comparison

PURPOSE: The recently released Ethos therapy system (Varian Medical Systems) allows for online CBCT‐guided adaptive radiation therapy (RT). The clinical introduction of multiple systems requires machine characterization and machine variation quantification to allow patient interchangeability between systems. Despite several clinical introductions, limited vendor‐independent information on machine performance is available. Our aim was to determine the relevant dosimetric and mechanical characteristics of individual machines and to quantify machine variations. METHODS: Six Ethos treatment machines, equipped with a 6‐MV FFF beam including dual‐layer MLC and kV‐CBCT system, were recently introduced clinically after extensive machine characterization and pre‐configured beam model verification. Point doses and profiles were measured and compared to vendor‐provided reference data and dose calculations. Also, dose calculations were verified based on point measurements for non‐standard fields and dose distributions for optimized treatment plans. Agreements between dose profiles (dose distributions) were quantified using 1D (3D) γ‐analysis. Additionally, we quantified leaf transmission, dosimetric leaf gap (DLG) and couch attenuation, determined isocenter accuracy and kV‐MV isocenter coincidence and verified the kV‐CBCT system. Machine variations were quantified for all dosimetric and mechanical characteristics. RESULTS: For all machines, distinct agreements were found between measurements and vendor‐provided reference data as well as measurements and dose calculations. Mean γ(1%/1mm) values for all profiles were below 0.30. All profiles, point measurements and dose distributions matched well among the six machines. Minimal machine variations were found in terms of DLG (0.05 mm), leaf transmission (0.001%), isocenter accuracy (0.08 mm), kV‐MV isocenter coincidence (0.15 mm), couch attenuation (0.69%), and CBCT imaging dose (0.29 mGy). CONCLUSIONS: This study demonstrates excellent agreement between individual Ethos therapy systems and vendor‐provided reference data as well as a pre‐configured beam model. Furthermore, our results show good consistency among all machines and provide valuable insights on relevant machine characteristics. The systematically obtained results provide benchmark data for future clinical introduction of Ethos therapy systems.