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Due to distinct photoelectric absorption properties and physiochemical features, high-atomicelement nanomaterials are frequently used as radiosensitizer for radiotherapy, such as gold nanoparticles. which in-turn can increase radiation dose to tumours while minimizing exposure to normal tissues. The aim of this study is to investigate the dose enhancement due to the presence of nanogold particles mixed with breast tissue using a real radiotherapy plan with two photon energies, with and without flattening filter. A fully validated linac head was used to model a complete field-in-field (FIF) radiotherapy breast plan using 4 and 6 MV with a flattening filter (FF) and flattening filter-free (FFF) using BEAMnrc/EGSnrc MC code. The dose distributions were simulated in a female ICRP voxel computational phantom. The dose enhancement ratio (DER) and flattening filter-free enhancement ratio (FFFER) were calculated for a range of nanogold concentrations. The results showed that the DER increased with increasing nanogold concentration. The maximum DER was achieved when FFF was used and the increase was higher for 4 MV than 6 MV. The FFFER was higher for the 4 MV beam than 6 MV when 40 mg/ml nanogold was used. The combination of higher nanogold concentrations, lower photon energies, and FFF resulted in a higher dose enhancement. The implementation of nanogold particles in breast tissue enhanced the dose deposition, especially with FFF, as presented in this study. The results would improve nanogold-enhanced breast radiotherapy and dose delivery with an increase in patient throughput. [ABSTRACT FROM AUTHOR]