ACCURATE MONTE-CARLO CALCULATIONS OF THE COMBINED ATTENUATION AND BUILDUP FACTORS, FOR ENERGIES (20-1500 KEV) AND DISTANCES (0-10 CM) RELEVANT IN BRACHYTHERAPY

The combined build-up and attenuation factor, B exp(-mu-r), of point isotropic photon sources in a water medium has been calculated using the Monte Carlo method, for energies (20-1500 keV) and distances (1-10 cm) relevant in brachytherapy. For the transport of photons and electrons, up-to-date and self-consistent total, partial and differential cross sections were used. The influence of coherent (Rayleigh) and incoherent (Compton) scattering, as well as the effects of the source and medium geometries on the calculations, were investigated in detail and it was found that these effects can lead to significant deviations from published data, especially at low energies and/or large distances from the sources. Our results can be used for any mono- or multi-energetic photon source in the energy range 20-1500 keV with uncertainties of the order of 2-3%, and they may influence treatment planning especially in the case of organs at risk which are usually near the edge of the body.