Exploration of Monte Carlo Simulation from a Case Study of Particle Transport in the Gamma Knife Perfexion™ Machine

Introduction: Gamma Knife Perfexion™ delivers 192 Cobalt-60 sources to the focal point (isocenter), and the patient is fixed using a stereotactic frame. In conformal techniques, the width of the penumbra resulting in an out-of-field dose of normal tissue adjacent to the tumor must be accurately determined. The purpose of this study was to calculate the penumbra widths of a single beam and 192 beams for different collimator sizes of the Gamma Knife Perfexion™ using the BEAMNRC/DOSXYZNRC Monte Carlo simulation code and compare the results with EBT3 film dosimetry data. Material and Methods: To investigate the physical penumbra width (80-20%), the single beam and 192 beam profiles were obtained using the DOSXYZNRC code and EBT3 films located at the isocenter point in a spherical solid water phantom with a diameter of 160 mm.  Results: The results showed that the Gamma Passing Rate (GPR) value for all collimator sizes has a value above 97%. The single-beam penumbra widths obtained from simulation data for 4, 8, and 16 mm collimator sizes along the X-axis were 0.75, 0.77, and 0.87 mm, respectively. The data for 192 beams obtained from the simulation were 2.60, 4.80, and 8.70 mm along the X-axis. Conclusion: The differences between measured and simulated penumbra widths are in an acceptable range. However, for more precise measurement in the penumbra region with a high dose gradient, a Monte Carlo simulation is recommended.