ESTRO 38 Abstract book

S944 ESTRO 38

well as Monte Carlo calculations. AXB calculations were in good agreement with profile and absolute dose measurements, highly better than AAA calculations. EP-1751 Topical skin agent application-thickness influence on surface dose in external radiation therapy K. Miko 1 , J. Kobayashi 1 , Y. Ono 1 , T. Tanino 2 , N. Uchida 2 1 Tottori University Hospital, Radiology, Yonago, Japan ; 2 Tottori University Hospital, Radiology- Department of Pathophysiological Therapeutic Science, Yonago, Japan Purpose or Objective Radiation dermatitis during radiotherapy sometimes affects patients’ quality of life or continuity of treatment. Topical skin agents such as ointments, lotions or creams may be prescribed. However, there remain concerns of a bolus effect due to the applied thickness or of beam scattering due to trace-metal components, possibly increasing irradiating surface doses. The purpose of this study was to investigate the effects of varied application thicknesses of topical agents, and their base ingredients or trace-metals on the surface dose, experimentally. Material and Methods The calibration curves in the range of 0 cGy to 500 cGy of 6, 10 MV-X-ray were acquired using Gafchromic dosimetry Film (EBT 3).Next, the films were placed on the surface of a water-equivalent phantom, then polyethylene wrap sheet. Various topical agents were applied on the sheet in varying thickness ranging from 0 mm to 5.00 mm. 100 MU of 6 MV or 10 MV X-ray was exposed to the film with SSD 100 cm, 10 × 10 cm 2 field size. The topical agents were simple creams or ointments, with ingredients such as silver, zinc, sodium and iodine. Each entrance surface dose was calculated by conventional Gafchromic dosimetry film method. Results The surface-absorbed dose increased with increasing thicknesses of simple cream. Simple regression analysis revealed a strong correlation between thickness of topical agent and surface dose (y = -0.088 x 2 +0.868 x +1.000, R 2 = 0.996). The surface dose with cream containing trace- metal was greater than that with simple cream (p<0.05). The difference between the dose with simple cream and the dose with trace-metal cream became greater with a decrease of application thickness. The surface dose difference between simple cream and trace-metal cream was greater in cases of 6 MV-X ray than that in 10 MV-X ray, especially when the application thickness was thin. Even at a thickness of 0.27 mm, which is similar to clinical use, elevations of surface dose were observed.

of optimum depth dose indexes, the effects of the depth dose measurement uncertainties on the HVL determination and the sensitivity of this method to safely predict changes in the dose calibration factors are some of the areas that require more investigation. These topics will be studied in a second step in this project. EP-1750 Evaluation of Acuros XB in the presence of metallic elements R. Chipana 1 , T. Younes 1,2 , L. Vieillevigne 1,2 1 Institut Claudius Regaud- Institut Universitaire du Cancer de Toulouse, Department of Medical Physics, Toulouse, France ; 2 Centre de Recherche en Cancérologie de Toulouse- UMR 1037 INSERM- Université Toulouse 3- ERL5294 CNRS- Oncopole, Department of Medical Physics, Toulouse, France Purpose or Objective The aim of this work was to evaluate Acuros XB (AXB) in presence of metallic elements, such as hip prosthesis and dental implants. Comparisons against Monte Carlo (MC), Anisotropic Analytical Algorithm (AAA) calculations and measurements were undertaken. Material and Methods In order to challenge AXB algorithm, radiation photon beams were selected to transit through metallic elements. Firstly, AXB calculations were performed in simple virtual phantoms constituted of water, Titanium and stainless steel and were compared with AAA and with GATE (Geant4 Application for Tomography Emission) platform based Monte Carlo as the benchmark. Secondly, measurements were undertaken in water tank phantom containing hip prosthesis (in Titanium, and in stainless steel) and dental implants (in stainless steel alloy). The PTW PinPoint 3D T31016 ion chamber was used to measure profiles and absolute doses at different depths beyond the metallic implants. The measured doses were then compared to the predicted ones by AXB and AAA. AXB and MC dose calculations were reported to dose-to-medium. Results In the virtual phantoms, AXB calculations were in good agreement with GATE. AAA failed predicting the backscatter in front of water/Titanium or water/stainless steel and exhibited differences up to 11%. Beyond 2 mm after the metallic heterogeneity, AXB differed by 1% and 4% for Titanium and stainless steel respectively, whereas AAA differed by 3% and 12%. The absolute dose measurements (around 2 cm after the metallic implants) for 6 and 10 MV photons beams indicated maximum differences of 1.5% and 5.8% for AXB and AAA, respectively. The PinPoint 3D dose profiles presented good agreement (<1% on average) compared with AXB calculations, in contrary to AAA calculations, as shown in the figure 1.

Fig.1: Comparison of dose profiles: measurements with the PinPoint 3D and calculations with AXB and AAA at around 2 cm beyond the titanium hip prosthesis for 6 MV and 10 MV photon beams. Conclusion Even when the beams transited through the metallic implants, AXB has been shown to handle perturbations as