Experimental benchmarking of Monte Carlo simulations for radiotherapy dosimetry using monochromatic X-ray beams in the presence of metal-based compounds

The local dose deposition obtained in X-ray radiotherapy can be increased by the presence of metal-based compounds in the irradiated tissues. This finding is strongly enhanced if the radiation energy is chosen in the kiloelectronvolt energy range, due to the proximity to the absorption edge. In this study, we present a MC application developed with the toolkit GEANT4 to investigate the dosimetric distribution of a uniform monochromatic X-ray beam, and benchmark it against experimental measurements. Two validation studies were performed, using a commercial PTW RW3 water-equivalent slab phantom for radiotherapy, and a custom-made PMMA phantom conceived to assess the influence of high atomic number compounds on the dose profile, such as iodine and gadolinium at different concentrations. An agreement within 9% among simulations and experimental data was found for the monochromatic energies considered, which were in the range of 30-140 keV; the agreement was better than 5% for depths < 60 mm. A dose enhancement was observed in the calculations, corresponding to the regions containing the contrast agents. Dose enhancement factors (DEFs) were calculated, and the highest values were found for energies higher than the corresponding K-edges of iodine and gadolinium. The in-silico results are in line with the empirical findings, which suggest that GEANT4 can be satisfactorily used as a tool for the calculation of the percentage depth dose (PDD) at the energies considered in this study in the presence of contrast agents.