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Objective: The aim of this study was to build a multiple source model for a 6 MV Photon beam from the Varian Clinac iX linear accelerator (at the Department of Radiology, Ramathibodi Hospital).
Methods: Beam Data Processer (BEAMDP) was used to analyze the phase space dara from the EGSnrc (Electron-Gamma Shower National Research Council of Canada)/BEAMnrc code and obtain the parameters for beam representation. A DOSXYZn code was used to calculate dose distribution in a 3D rectangular voxel. The simulation compose of two unknown parameters, energy and full width at half maximum (FWHM). These parameters were determined by using an iterative process and a comparison with the percentage depth dose and the beam profile form the measurement. The five source model were comprised of a point sub-source for target, circular planar sub-source for primary collimator and flattening filter, rectangular planar sub source for election, and positron. The characteristics of beams such as relative intensity, field planar influence distribution and energy distribution from original phase space were derived by using the BEAMDP code.
Result: The initial parameter found an energy of 6.5 MeV and radial intensity distribution with a width of 0.6 mm FWHM. Most of the depth dose and beam profile between our source model and the measurement showed good agreement within +2% for field size of 5 x 5, 10 x 10, 15 x 15 and 20 x 20 cm2 except of the 30 x 30 cm2 field size at 100 cm SSD. The relative point dose of the source model in homogenous phantom deviated with +1% and +2% between the original phase space and the measurement respectively. The source model was shown to reduce disk space requirement and CPU time, while the statistical uncertainty of dose calculation was the same as the full Monte Carlo simulation.
Conclusion: Our source model accurately simulates the dosimetric characteristics of a photon beam for all field size, except for 30 x 30 cm2.
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