The dose calculation in the software uses a Monte Carlo module (developed through a partnership with the CNES). This algorithm can be used either in a forward process or a reverse one. In the first case, the software manages the transport of electrons and photons (including secondary particles) from 1keV to 10 MeV, in the 3D model. Creation of secondary photons and electrons are taken into account. Any type of energy spectrum and source geometry can be defined. Sensitive volumes (SV) are selected by the user and FASTRAD computes the deposited energy inside the SVs. The reverse Monte Carlo module is dedicated to the dose calculation due to an isotropic irradiation of electrons in a complex and multi-scale geometry, and as a result, the forward algorithm can lead to large computational times. The principle of the reverse method is to use: