A Monte Carlo Simulation of Photon Beam Generated by a Linear Accelerator

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.