Monte Carlo stimulation of the angular, radial and energy distribution of a medical linear accelerator using MCNP4C code

Introduction: Previous investigations have shown that Monte Carlo methods are suitable for the simulation of the transportations photon beam in medical linear accelerators. The simulated beams could be used for the measurement of the dose distribution in phantoms and patients’ body. Angular, energy and radial distributions are the most important data of photon beams and can be used for clinical applications of simulation. The phase-space data of the Elekta SL75/25 Linac was simulated for MCNP4C code. In this investigation, we have presented detailed data of the angular, energy and radial distributions at four scoring plate perpendicular to the central axis of the photon beam. Materials and Methods: Firstly, we simulated the linac head geometry and verified its' accuracy. The mean energy and radial intensity distribution of the linac electron beam was determined by the comparison of the simulated and measured percentage depth doses (PDDs) and beam profile curves. Four scoring plates located 0.1cm under the target, the primary collimator, the flattening filter and the secondary collimator were simulated as concentric circles starting at 0.1cm radius and with 0.1cm intervals up to 3.5cm. Thereafter, bremsstrahlung radiations were simulated and transported in the linac head down to the water phantom and the angular, energy and radial distributions at the four scoring plate were scored. Results: Comparison of our results with the previous reports from the EGS4 code indicated that the simulated photon spectra resulted from the MCNP4C code is a little more at high energies. The mean energy was 2.18MeV having being in good agreement with previous investigations. At all the scoring planes, the beam becomes softer as we get away from the central axis of the beam. The maximum and minimum variation happens for the target and the secondary collimators with a value of 1.24MeV and 0.13MeV respectively. The head structures of the linac, altogether, caused a beam hardening of 0.61MeV, but among all the components of the head structures, the flattening filter has most effect in this regard. The radial distributions of the primary collimator and flattening filter indicate a difference of 0.6% and 0.05% from that of the central bin respectively. The photon fluence is reduced to 50.04% after crossing the flattening filter with the majority of them being at low energies. Conclusion: Higher energy noted from the use of MCNP4C Code is due to the energy physics cut-off card used, causing a shift in the mean energy of the primary electrons toward the higher energies and consequently increasing the relevant photon fluence. The primary collimators and the flattening filter have the most effect on the fluence uniformity and the energy fluence uniformity respectively. Passing the radiation through the head structures causes the beam hardening with the flattening having the most effect.