Fluence and energy espectra for photon beams: A Monte Carlo study

Much of the algorithms used to calculate dose distributions for photon beams generated by clinical electron linacs (Zhou et al. 2010, Sikora and Alber 2009, Tillikainen 2009) consider three particle sources: a photon source corresponding to Bremsstrahlung in the target (primary source), a second photon source that takes into account those photons produced by scattering into the primary collimator, the flattening filter and the secondary collimation system (scattering source) and an contamination electron source that considers those electrons produced in the various components of the linac head and the air space between it and the patient (contamination electron source). Most of the published works in which the behavior of the particles produced in linac heads is characterized (Chaney et al. 1994, Sempau et al. 2001, SheikhBagueri and Rogers 2002, Fix et al. 2005, Hirshi et al. 2007, Grevillot et al. 2011) describe the sources in terms of their characteristics at the phantom entrance. However, the calculation models take into account that these particles originate at different positions with specific values of position, movement direction and energy. In this work we aim at describing the source characteristics at the particular positions where particles originate.