Geant4-based Monte Carlo Simulation of the Leksell Gamma Knife®

The Monte Carlo code Geant4 has been used to simulate the Leksell Gamma Knifereg and to verify its treatment planning system Leksell GammaPlanreg (LGP). By means of this radio surgical technique, intracranial lesions can be treated in a single session with high precision and critical brain structures can be protected. Radiation from 201
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Co sources comes through a collimation system to the target area, focusing on the isocenter, where the maximum dose amount is released. Monte Carlo simulations were employed to calculate the dose distribution in a spherical water phantom, the same we used for the validation measurements. In order to simplify the code, we simulated just one single source, rotating the phantom at 201 angular positions, the same of the sources in the device. We validated the simulation by measuring the dose distribution with radiochromic films HS, for all the available collimators (4, 8, 14, 18 mm). Good agreement was found between experimental data and results of Monte Carlo simulations. Next we used the application to verify the dose calculated by LGP. Comparisons between Geant4 and LGP outputs in one and two dimensions were performed at different planes (axial, coronal and sagittal) and depths. The outcomes show that LGP calculates correctly the dose distribution in a homogeneous phantom. Different results were found when different density materials are also included, because LGP assumes the phantom only composed by water. In conclusion, the obtained results encourage to use Monte Carlo Geant4 simulations to investigate problems concerning the stereotactic radiosurgery, also in the perspective of the simulation of a whole treatment session.