On axis and off axis bremsstrahlung energy spectra calculated by EGS4 Monte-Carlo code

For accurate 3D treatment planning new models of dose calculation are being developed which require the knowledge of the energy spectra of photons incident on the surface of the patients. In this study the EGS4 Monte-Carlo code system with the PRESTA electron transport algorithm was used in order to compute the photon energy spectra at on axis and off-axis points for a number of linacs manufactured by G.E. The different parts of the accelerator head were modeled by simple geometrical volumes and the photon beams were produced by simulating the bombarding of the target with monoenergetic electron beams assumed to have a spatial distribution. The analysis of the calculated spectra with and without flattening filter showed us the photon beam quality variation at off-axis points. To verify the validity of the calculated spectra we introduced them as input to a dose spread arrays convolution program in order to calculate TMR as a function of depth of the total and primary dose distribution in a waterlike fantom. The calculated depth dose distributions were compared with experimental data. The theoretical and experimental results are consistent and allow us to consider that the photon energy spectra can be determined with acceptable accuracy using Monte Carlo simulations even with a simplified accelerator head geometry