In reference to which MV Iso option to use, “Field center” option will calculate centroid of your 10×10 field and use that as radiation isocenter. “Image center” option uses the calibration of your MV imager to establish radiation ISO. VRT trainers instructed that it is up to us to decide, but usually “image center” is used if you have ISOCAL, which is apparently something on TrueBeams. Since we do not have ISOCAL, I am planning on using the “field center” option, but will probably use MLC to define my 10×10, as I trust MLC tolerance more than jaw tolerance.
Hope this helps.