An innovative approach using X-rays interactions with heavy elements seems to open a promising way of treatment for resistant cancers, such as high-grade gliomas. The use of gold nanoparticles to treat mice bearing subcutaneous tumors led to encouraging results. However, the physical processes and biological impact of the photon activation of nanoparticles are not well understood. The aim of this study was to evaluate, at the cellular level, the dose enhancement in presence of nanoparticles and the properties of the secondary electrons production using the Monte Carlo transport code PENELOPE. An increase of electron production up to a factor 1000 is observed in presence of a AuNP. An auto-absorption of low energy electrons for NP with larger diameters is also observed. From the comparison of the calculated dose enhancement factor (DEF) in the cellular geometry with the measured SER4Gy, encouraging results are obtained.