Monte Carlo simulation of a medical accelerator: application on a heterogeneous phantom

The objective of this study is to seek an accurate and efficient method to calculate the dose distribution for small fields in high gradient heterogeneity, typical for Intensity Modulated Radiation Therapy (IMRT) technique on head and neck regions. This motivates a Monte Carlo (MC) simulation of the photon beam for the two nominal potential energies of 25 and 6 MV delivered by a medical linear electron accelerator (Linac) used at the Centre Antoine Lacassagne. These investigations were checked by means of an ionization chamber (IC). Some first adjustments on parameters given by the manufacturer for the 25 and the 6 MV data have been applied to optimize the adjustment between the IC and the MC simulation on the depth-dose and the dose profile distributions. The good agreement between the MC calculated and the measured data are only obtained when the mean energies of the electron beams are respectively 15 MeV and 5.2 MeV and the corresponding spot size diameter 2 and 3 mm. Once the validation of the MC simulation of the Linac is overcome, these results permit us in a second part to check the calculation data given by a treatment planning system (TPS) on a heterogeneous phantom. The result shows some discrepancies up to 7% between TPS and MC simulation. Those differences come from a bad approximation of the material density by the TPS. These encouraging results of the MC simulation will permit us afterwards to check the dose deposition given by the TPS on IMRT treatment. (authors)