The application of nanoparticles (NPs) in radiotherapy is an increasingly attractive technique to improve clinical outcomes. The internalisation of NPs within the tumour cells enables an increased radiation dose to critical cellular structures. The purpose of this study is to investigate, by means of Geant4 simulations, the dose enhancement within a cell population irradiated with a 150 kVp photon field in the presence of a varying concentration of tantalum pentoxide (Ta2O5) NP aggregates, experimentally observed to form shells within tumour cells. This scenario is compared to the more traditionally simulated homogeneous solution of NP material in water with the same weight fraction of Ta2O5, as well as to a cell population without NPs present. The production of secondary electrons is enhanced by increased photoelectric effect interactions within the high-Z material and this is examined in terms of their kinetic energy spectra and linear energy transfer (LET) with various NP distributions compared to water. Our results indicate that the shell formation scenario limits the dose enhancement at 150 kVp. The underlying mechanism for this limit is discussed.