Introducing a Method to Derive a Multiple Virtual Point Source Model of Linac for Photon Beam Dose Calculation Using Monte Carlo Method

In this study a new simple but a very effective method is introduced for the beam modeling of the invariant part of a medical linear accelerator. In this method¡ instead of segmentation of scoring plane and analysis of phase space file¡ the mirror image of a virtual point source¡ energy and angular distributions and dependencies between them are derived¡ directly. Then¡ the method was used for the beam modeling of a 6MeV photon beam of the Siemens ONCOR Impression accelerator¡ where the TALLYX capability of MCNP4C was used. Consequently a multiple point source model with angular dependent photon energy spectra was obtained. Then¡ the percentage depth dose curves and the lateral dose distributions in water phantom were calculated using the present model for three field sizes including 4cm×4cm¡ 10cm×10cm and 40cm×40cm¡ and the results were compared to those of full Monte Carlo simulations. The results showed that the agreement between them the for all the field sizes are excellent. Benefits of this method as compared with the phase space file analysis method including the ease of application and the errors removal caused by the spatial segmentation of the phase space data.