Dosimetric comparison of medical linear accelerator using FLUKA Monte Carlo method

Monte Carlo modeling of a linear accelerator is the first and most important step in Monte Carlo dose calculations in radiotherapy. We developed a 6 MV photon beam model for Varian ClinacR series for dose calculation purposes using FLUKA Monte Carlo code. PDDs and beam profiles for 5x5 cm2, 10x10 cm2, and 20x20 cm2 field sizes were calculated and compared with measured data. The beam profiles were calculated in three depths of dmax, 5 cm and 10 cm. The linac head components, including the target, primary collimator, absorber, flattening filter, ionization chamber, and secondary collimators and jaws, were simulated based as per the information provided by manufacturer. FLAIR (FLUKA Advanced Interface) is used to visualize and edit the geometry, use of user defined functions and debugging is done using this interface. First, an initial Monte Carlo simulation of the accelerator head was performed to produce the phase space (PS) file for different field sizes and energies of the primary electron beam. EMF-CUT and DELTA RAY FLUKA cards are used for better agreement. In all simulations, the energy cut-offs were 0.7 MeV for electrons and 0.01 MeV for photons. Experimental measurements are taken using for 100 cm source-to-surface (550) in 50x50x50 cm3 PTW water phantom using 0.6 cc cylindrical ionization chamber. All measurements were performed with a 6 MV linear accelerator photon beam using the maximum available dose rate, i.e. 400 MU min-1. Experimental and calculated dose values were in good agreement both in PDD and in profiles. For PDD, especially for depths less than 20 cm good agreements are seen. But slight disagreement is seen at dose build-up point. The results in the penumbra of the radiation field have 5-10% disagreements. (author)