Determination of the initial electron beam parameters in the Monte Carlo linac simulation of the photon beam using MCNP4C code

Aim: For Monte Carlo simulations and patient dose distributions, it is important to accurately determine the parameters of the initial electron incident on target. These parameters, such as mean energy, energy distribution and intensity distributions, have traditionally determined by a trial and error process. In this study we purpose an easy, efficient and accurate method for determination of the initial beam parameters. Materials and method: An Elekta linac head structure for 6MV photon beam was precisely simulated based the data provided by the manufacturer using MCNP4C Monte Carlo code. First we have set initial electron source as a plane surface source with fix mean energy without any energy and radial distributions and have gotten PDD curve. For comparison of the distribution and adoption of this PDD curve with the PDD curve that was obtained experimental measurement, K-S test and one sample T-test of the difference them were done. The mean energy was changed by o.2MV step until the adoption of simulated PDD curve with the measured PDD curve was obtained. Then source radius with and without distribution was changed from 0.05 to 0.3mm until the best adoption between simulated and measured doseprofile was gained. After that energy distribution was changed from 3% to 9% until the best adoption of the PPD curve was gained (according the previous tests). Results: Depth-dose curve were considerably affected by the mean energy and energy distribution of the initial electron beam. These effects were depended on electron cut off and the production the bremsstrahlung on each step. The PDD curve was unaffected by the source radius and radial intensity of electron beam but dose-profile sensitive to them. The simulated PDD had difference smaller than 1% with experimental measurement in each depth. Conclusion: Simulating the radiation source of a photon beam, the initial electron beam, should be tuned accurately. This method can produce absorbed dose within acceptable precision with minimum time consuming and Monte Carlo simulation.

کلیدواژه ها:

Benchmarking ، Monte Carlo ، MCNP4C code ، Initial electron