In silico Monte Carlo with novel particle tagging: Assessing gold radiosensitivity in voxelized scenario of brachytherapy

Radiotherapy is widely acknowledged as one of the most effective treatments for solid and metastatic tumors. The sensitivity of tissues or cells to radiation is typically estimated using survival curves derived from laboratory experiments with in vitro cell culture models. However, some radioresistant cancer cells can pose significant treatment challenges. High atomic number (Z) nanoparticles, known as radiosensitizing agents, can induce substantial radiosensitization when positioned near therapeutic targets, resulting in a dose increase unattainable by conventional methods. This effect is associated with the emission of secondary electrons by high atomic number materials, effectively transforming them into secondary radiation sources. This study utilized a high-resolution voxelized computational model of an in vitro culture medium to investigate the insertion and impact of gold particles. Monte Carlo code (MCNP6.2) was used to model and simulate a brachytherapy scenario with a High Dose Rate (HDR) ¹⁹²Ir source in a matrix of stop positions. The radiosensitivity of gold particles was evaluated using the dose enhancement factor (DEF), calculated based on the concentration of gold nanoparticle (AuNP) clusters in the culture medium (mg-AuNP/g) and the energy dependence in the in vitro samples. Simulations demonstrated a proportional relationship between DEF and concentration, enabling the creation of a predictive equation for DEF values, which was validated against published data. Additionally, energy was found to significantly influence DEF values. The DEF versus energy curve obtained exhibits similarities to the curve of the Sensitizer Enhancement Ratio (SER) versus dose, though the two differ in their determinants. SER is determined through various radiobiological mathematical models based on the biological effect derived from the fractional survival curve as a function of dose response. Furthermore, a special TAG tally was employed to identify the types of secondary electron (photoelectrons, Auger electrons and knock-on electrons and Compton scattering) production in the medium, their physical contributions, and their behavior at different energy sources of 100 keV, ¹⁹²Ir and 1 MeV.