Figure 2.

Varian TrueBeam linear accelerator with electronic portal imaging system and reference systems employed in this work. The linac reference system is centered at the isocenter and fixed with respect to the linac room. The EPID reference system rotated with the gantry. Its origin was on the upper left corner of the imager. The reference system employed for comparing the marker with the MLC leaves positions (collimator reference system), as well as the gantry reference system are not represented. The gantry reference system was centered at the isocenter and rotates with the gantry, around the Z≡Z^′ axis.