Development of a LINAC head model for the CyberKnife VSI‐System using EGSnrc Monte Carlo system

Introduction In order to understand the interaction processes of photons and electrons of the CyberKnife VSI‐System, a modeling of the LINAC head must take place. Here, a Monte Carlo simulation can help. By comparing the measured data with the simulation data, the agreement can be checked. Materials and methods For the Monte Carlo simulations, the toolkit EGSnrc with the user codes BEAMnrc and DOSXZYnrc was used. The CyberKnife VSI‐System has two collimation systems to define the field size of the beam. On the one hand, it has 12 circular collimators and, on the other, an IRIS‐aperture. The average energy, final source width, dose profiles, and output factors in a voxel‐based water phantom were determined and compared to the measured data. Results The average kinetic energy of the electron beam for the CyberKnife VSI LINAC head is 6.9 MeV, with a final source width of 0.25 cm in x‐direction and 0.23 cm in y‐direction. All simulated dose profiles for both collimation systems were able to achieve a global gamma criterion of 1%/1 mm to the measured data. For the output factors, the deviation from simulated to measured data is