ESTRO 2020 Abstract book

S804 ESTRO 2020

PO-1422 Monte Carlo study of dose deposition in kilovoltage X-ray radiotherapy using gold as dose enhancer J. SPIGA 1 , P. Pellicioli 2 , J. Duffy 1 , S. Manger 1 , A. Bravin 2 1 University of Warwick, Physics, Coventry, United Kingdom ; 2 European Synchrotron Radiation Facility, Biomedical beamline, Grenoble, France Purpose or Objective 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. Material and Methods The developed Geant4 application simulated the irradiation of a 2x2 cm 2 field beam impinging on an custom-made phantom with dimensions of 50x50x88 mm 3 . 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). Results 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. Conclusion 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.

which good agreement between calculations and measurements was not obtained. Then new plans can be prepared that are more likely to be implemented correctly. It is important to consider the stability of the machine working when changing the Gamma criterion. ASC tool could be helpful to achieve better verification results. PO-1421 Investigating dose calculation accuracy around gas-filled tissue expanders using 3D-printed phantoms T. Kairn 1 , M. Lathouras 1 , S. Sylvander 1 , S. Crowe 1 1 Royal Brisbane and Women's Hospital, Radiation Oncology- Cancer care Services, Herston, Australia Purpose or Objective Gas-filled temporary tissue expanders are increasingly being used during breast cancer treatments, due to improved patient comfort and convenience, compared to older saline-filled systems. However the implantation of a silicone bladder containing a large volume of gas and a metallic gas reservoir creates obvious challenges for radiotherapy treatment planning, especially regarding dose calculation accuracy. In this study, 3D-printed phantoms were designed and fabricated specifically for use in evaluating the accuracy of dose calculations for two different types of dynamic photon radiotherapy treatments of breast cancer. Material and Methods Dose calculations provided by the Varian Eclipse treatment planning system, for volumetric modulated arc therapy (VMAT) treatments, the Tomotherapy Hi-Art treatment planning system, for helical tomotherapy (HT) treatments, and the Tomotherapy Planned Adaptive dose calculation and review system, for HT adaptive radiotherapy (HT- ART), were evaluated using radiochromic film measurements made in and around two purpose-designed 3D printed phantoms. One of these phantoms was designed to be as anatomically-realistic as possible, including overlying breast tissue, bolus, and contralateral breast tissue, for placement on a humanoid thorax phantom, with a sample tissue expander. The other phantom was designed as a simple rectilinear box, with sides with approximately anatomically-realistic thicknesses and densities and with a simple CO2 canister used to model the reservoir, for use in providing a clear and unequivocal comparison of the HT and HT-ART dose calculations. Both phantoms were printed using PLA using an Ultimaker 2plus 3D-printer. Results Measurements showed that the metal reservoir increased the uncertainty of downstream dose calculations, for all three systems, although the HT-ART system was least affected by this component. By contrast, the VMAT and HT treatment planning systems provided more accurate dose calculations throughout the gas volume within the implant, and therefore provided more accurate dose-volume histograms (DVHs) when the gas volume was included in the target (PTV). Generally, all three systems provided comparatively accurate calculations of dose in the thin layer of tissue surrounding the implant, in regions not affected by the reservoir, as well as accurate calculations of skin dose in areas covered by bolus. Conclusion The use of gas-filled temporary tissue expanders during breast radiotherapy treatments leads to unavoidable uncertainties in treatment planning dose calculations, including potential under-dosing downstream of the metal reservoir and inaccurate dose calculations within the gas volume. Although a PTV that includes the implant may be needed for plan optimisation, the creation of a separate PTV that excludes the implant is advisable, so that DVH evaluation refers only to tissue, where the accuracy of the dose calculation is truly important.

Poster: Physics track: Radiation protection, secondary tumour induction and low dose

PO-1423 Ionizing radiation dose-effect indicator for cancer risks in medical radiation personnel S. Shahid 1 , K. Masood 2 , M. Riaz 3 , S. Abubakar 4 , S. Ali 5 1 National University of Computer and Emerging Sciences NUCES- FAST Lahore Pakistan, Sciences & Humanities, Lahore, Pakistan ; 2 Institute of Nuclear Medicine &