Evaluation of the relative biological effectiveness of the Auger electrons produced during gadolinium neutron capture therapy using microdosimetric approach

Determination of the relative biological effectiveness (RBE) of Auger electrons is a challenging task in radiobiology. In this study, we have estimated the RBE of internal conversion (IC) and Auger electrons released during Gadolinium neutron capture reaction (GNCR) by means of biological weighting functions (BWFs) with microdosimetric approach. Regarding the different distribution of Gadolinium (Gd) relative to the DNA as a target, the microdosimetric parameters of the Gd electrons were calculated using the Geant4 Monte Carlo toolkit and ROOT software. Assuming Gd infiltration into the cells and uniform distribution inside the Cell, the lineal energy distribution of Gd electrons in DNA was used instead of the lineal energy distribution of external radiations in the micrometer-diameter targets, which has been conventionally used in the mentioned methods. The results show that the calculated RBE values of Gd electrons using BWFs (2.68) for the case where Gd distributed at the center of the DNA are approximately equivalent to the RBE value of the therapeutic neutrons, which were measured in the literatures with the same biological conditions. According to the results, although the changes of the RBE of Gd electrons to the different distribution of Gd relative to the DNA are small, the amount of biological dose of the Gd electrons in the DNA is strongly dependent on the Gd distribution. In the case where Gd distributed at the center of DNA, the mean biological dose of Gd electrons in DNA during one GNCR (227.8 kGy.Eq) is large enough for occurring double-strand breaks (DSB) of the DNA. If we have accurate information about the spatial distribution of Gd or Auger-electron emitters inside the cell, by comparing to the results obtained in this study, we can have a better estimation of the RBE of the Gd electrons or in general Auger electrons.