Simulation of a 6 MV Elekta Precise Linac photon beam using GATE/GEANT4

The GEANT4-based GATE Monte Carlo (MC) platform was initially focused on PET and SPECT simulations. The new release v6.0 (February 2010) proposes new tools dedicated for radiation therapy simulations. In this work, we investigated some part of this extension and proposed a general methodology for Linac simulations. Details of the modeling of a 6 MV photon beam delivered by an Elekta Precise Linac, with radiation fields ranging from 5 x 5 to 30 x 30 cm2 at the isocenter are presented. Comparisons were performed with measurements in water. The simulations were performed in two stages: first, the patient-independent part was simulated and a phase space (PhS) was built above the secondary collimator. Then, a multiple source model (MSM) derived from the PhS was proposed to simulate the photon fluence interacting with the patient-dependent part. The selective bremsstrahlung splitting (SBS) variance reduction technique proposed in GATE was used in order to speed up the accelerator head simulation. Further investigations showed that the SBS can be safely used without biasing the simulations. Additional comparisons with full simulations performed on the EGEE grid, in a single stage from the electron source to the water phantom, allowed the evaluation of the MSM. The proposed MSM allowed for calculating depth dose and transverse profiles in 48 hours on a single 2.8 GHz CPU, with a statistical uncertainty of 0.8% for a 10 x 10 cm2 radiation field, using voxels of 5 x 5 x 5 mm3. Good agreement between simulations and measurements in water was observed, with dose differences of about 1% and 2% for depth doses and dose profiles, respectively. Additional gamma index comparisons were performed; more than 90% of the points for all simulations passed the 3%/3 mm gamma criterion. To our knowledge, this feasibility study is the first one illustrating the potential of GATE for external radiotherapy applications.