C series linacs

Measurements for this study were primarily performed on a single Varian Trilogy C‐series linac with Millennium 120 MLC software version 8.1 immediately before it was decommissioned. A reduced version of the experiment was performed on a Varian TrueBeam linac with Millennium 120 MLC running software version 2.7 Maintenance Release 4. During all dynamic deliveries used in this study log files were recorded, and the MLC positions from these were assessed. For the Varian C‐series linacs, log files are recorded every 50 ms.

To test the short‐term reproducibility of the IsoCal system, a physicist performed the IsoCal calibration three times on the same day on the same linac. For each calibration, the phantom and collimator plate were set up independently from the previous setup, the OBI application and IsoCal software were restarted, and the imaging arms were retracted and reextended to ensure that there was no unintentional linkage between the calibrations. This reproducibility test was done on five different Varian C‐series linacs to look for machine dependencies. The IsoCal calibrations are stored as offset vectors (X, Y) in an XML file for the kV and MV imager panel locations. We compared the shape of the IsoCal correction curves versus gantry angle, as well as the extrema of the corrections in each direction for the three independent calibrations of each linac, and we also compared those offsets across five linacs (Mi, i=1,…,5).