Introduction
Gold nanoparticles (GNPs) are known to enhance radiosensitization in cancer treatment, particularly in their immediate vicinity. This study examines the influence of the GNP size and dose rate on secondary electron production during the delivery of FLASH electron beams.

Methods
Using Geant4-DNA Monte Carlo simulation, GNPs with varying diameters (5, 10, 50, and 100 nm) were placed at the center of a 10 µm diameter water sphere. The Geant4 Livermore model was applied to the GNPs, while the Geant4 DNA-physics and -chemistry models were used for the water sphere. Electrons, with an energy of 1 MeV and ultra-high dose rates (UHDRs) of 60, 100, and 150 Gy/s, were directed from the GNPs' surfaces into the water sphere. Simulations continued until electron interactions with the GNPs and water sphere reached predefined dose values for each UHDR. The yield enhancement factor (YEF), defined as the ratio of secondary electrons with and without the GNP, was analyzed in relation to the GNP size and dose rate.

Results
YEFs within the water sphere increased with larger GNP diameters across all UHDRs. For example, at 60 Gy/s, YEFs rose from 1.022 for 5 nm GNPs to 1.081 for 100 nm GNPs. However, YEFs declined as the UHDR increased from 60 Gy/s to 150 Gy/s. Notably, at a UHDR of 60 Gy/s, the YEF near the 100 nm GNP center (2.75) was higher compared to the 5 nm GNP (1.73).

Conclusions
These preliminary findings suggest that larger GNPs combined with lower UHDRs yield higher secondary electron enhancement factors, providing valuable insights for optimizing GNP-enhanced FLASH radiotherapy.