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 < 1% from a field size of 12.5 mm for the circular collimators and from a field size of 10 mm for the IRIS-aperture.

Conclusion: The beam characteristics of the CyberKnife VSI LINAC head could be exactly simulated with Monte Carlo simulation. Thus, in the future, this model can be used as a basis for electronic patient-specific QA or to determine scattering processes of the LINAC head.