Iranian Journal of Medical Physics (Jun 2005)
A Monte Carlo Simulation of Photon Beam Generated by a Linear Accelerator
Abstract
ntroduction: Monte Carlo simulation is the most accurate method of simulating radiation transport and predicting doses at different points of interest in radiotherapy. A great advantage of the Monte Carlo method compared to the deterministic methods is the ability to deal accurately with any complex geometry. Its disadvantage is the extremely long computing time required to obtain a dose distribution with good statistical accuracy. Materials and Methods: The MCNP-4C Monte Carlo code was used to simulate a 9 MV photon beam from a Neptun 10PC linear accelerator. The accelerator was modeled as a complete unit consisting of a target, exit window, initial collimator, primary collimator, flattening filter, monitor chamber and secondary collimator. The geometrical details and the composition of each component was either obtained from the manufacturer or was directly measured. The simulation of the source was performed in a two step process. Initially, the electron source was defined. Secondly, the bremsstrahlung energy spectra and the fluence distribution at the scoring planes were used to define the photon source. The simulated electron beam energy followed a Gaussian distribution, with FWHM equal to 12% in nominal energy. The used intensity distribution of the electron beam also followed a Gaussian distribution with a FWHM equal to 0.34 cm. To compute the photon beam data a 50 × 50 × 40 cm 3 water phantom located at SSD = 100 cm was simulated. The depth dose and the dose profile curves were calculated for four different field sizes (5×5, 10×10, 20×20 and 30×30 cm 2 ) and compared against the measured values. The low-energy cut-off for the photons and electrons was 10 and 500 KeV, respectively. The measurements were carried out by using a Scanditronix dose scanning system and a 0.12 cm 3 RK ionization chamber. Results: To verify the simulated model, the calculated Monte Carlo dose data were compared against the corresponding measured values. The energy spectra and the angular distribution of the x-ray beam generated by the Neptun 10PC linac was examined. The result showed an efficiency of about 73% for the production of bermsstrahlung photon by the target. The agreement between the calculated and the measured depth dose and the dose profile was generally better than 2% for all the fields. Discussion and Conclusion: The simulation of the Neptun 10PC linac performed in this work is capable of computing the depth dose data and the beam profiles in water phantom for all the predefined fields including 5×5, 10×10, 20×20 and 30×30 cm 2 . Therefore, it can be concluded that MCNP-4C is a suitable tool for the dose calculation in radiotherapy. The simulated linac machine and the resulting data can be used to predict the dose distribution in all complex fields.
