Microdosimetric event distributions in sub-cellular volumes with a general purpose Monte Carlo code

The general purpose Monte Carlo code PENELOPE is used to calculate microdosimetric quantities including dose-weighted lineal energy spectra, which can be used to predict relative biological effect (RBE), for binary radiation therapies that utilise the photoabsorption of X-ray of high-Z materials. Spectra are calculated for Gd homogenously distributed at a concentration of 10 mg/ml in water and irradiated by 70 keV monoenergetic photons, around 20 keV above the k-edge of Gd (50.239 keV), which has been shown to give optimal dose enhancement, and for a metallic Gd surface in close proximity (within 2 μm) to a sensitive component of the nucleosome, modelled as a sphere of water of 1 μm diameter, for 60 and 70 keV monoenergetic X-rays. X-ray interactions with homogenously distributed Gd lead to a greater population of high lineal energy electrons than in liquid water, probably due to the creation of short range Auger electrons and photoelectrons, whereas interactions with Gd outside of the sensitive volume are longer ranged increasing the population of low lineal energy electrons. The data does not support increased therapeutic advantage through increased RBE in the case of Gd bearing contrast systems where little cellular absorption of Gd occurs. Homogenously distributed Gd leads to higher lineal energies than pure water, probably due to the creation of short range, high LET Auger and photoelectrons, whereas photoelectrons that originate in Gd that are outside the sensitive volume tend to have relatively higher energies and long ranges increasing the population of low LET electrons.