Session Item

In recent years there has been an increasing interest in the combination of high atomic number compounds and radiotherapy, due to the enhancement in dose deposition resulting from the boosted production of secondary electrons. This effect is particularly evident when using energies in the proximity of the absorption edge of the metal used. In this study, we used Monte Carlo simulations to evaluate the contribution of gold-containing compounds to the depth dose profiles in combination with X-rays in the kiloelectronvolt energy range. Different concentration of up to 30 mg/ml of gold were used to analyse the contribution to the Percentage Depth Dose (PDD). Some results obtained in-silico through GEANT4 were compared with the dose measured with Gafchromicâ films deposited by monochromatic X-rays produced at the European Synchrotron Radiation Facility.

The developed Geant4 application simulated the irradiation of a 2x2 cm² field beam impinging on an custom-made phantom with dimensions of 50x50x88 mm³. The General Particle Source (GPS) class allowed the characterisation of the source energy, position and size. The phantom has been voxelised using the Geant4 mesh. We studied different energies ranging from 30 to 140 keV. The G4EmStandardPhysics_option4 physics libraries included in Geant4 version 10.5 have been used as they are currently the most accurate for this kind of applications. The same set up has been used experimentally. The solid water phantom employed was made of slabs with different width (typically 0.5-1 cm in width), and one of these slabs allowed the insertion of a PTW chamber to take the dosimetry measurement. A second set of measurements has been carried out using the Gafchromicâ films EBT3, widely used in clinics (sensitivity dose range 0.1 – 20 Gy).

The PDD profiles obtained in-silico with Geant4 are in good agreement with the experimental data (Fig 1). In this particular case the phantom was filled with gold at a concentration of 1.8 mg/ml. Even if the concentration was very low the dose enhancement compared to water can still be observed, and it shows a maximum at 45 keV, where the increase in dose deposition is about 65% at lower depths and 25% at higher depths. The dose enhancement factors (DEF) were computed as the ratio between the dose recorded when the phantom filled with the high-Z solution and the dose recorded when they were filled with water. The highest DEF is found at an energy of 45 keV.

The effects of the introduction of gold compounds at different concentrations in the phantom has been analysed. The calculations clearly highlight a dose enhancement effect and match well with the experimental results. DEF have been evaluated and they are both photon energy and concentration dependent.