Technical Note: A simulation study on the feasibility of radiotherapy dose enhancement with calcium tungstate and hafnium oxide nano- and microparticles.

Purpose: To assess the radiotherapy dose enhancement (RDE) potential of calcium tungstate (CaWO ) and hafnium oxide (HfO ) nano- and microparticles (NPs). A Monte Carlo simulation study was conducted to gauge their respective RDE potentials relative to that of the broadly studied gold (Au) NP. The study was warranted due to the promising clinical and preclinical studies involving both CaWO and HfO NPs as RDE agents in the treatment of various types of cancers. The study provides a baseline RDE to which future experimental RDE trends can be compared to.

Methods: All three materials were investigated in silico with the software Penetration and Energy Loss of Positrons and Electrons (PENELOPE 2014) developed by Francesc Salvat and distributed in the United States by the Radiation Safety Information Computational Center (RSICC) at Oak Ridge National Laboratory. The work utilizes the extensively studied Au NP as the "gold standard" for a baseline. The key metric used in the evaluation of the materials was the local dose enhancement factor (DEF ). An additional metric used, termed the relative enhancement ratio (RER), evaluates material performance at the same mass concentrations.

Results: The results of the study indicate that Au has the strongest RDE potential using the DEF metric. HfO and CaWO both underperformed relative to Au with lower DEF of 2-3 × and 4-100 ×, respectively.

Conclusions: The computational investigation predicts the RDE performance ranking to be: Au > HfO > CaWO .