ESTRO 2021 Abstract Book

S1307

ESTRO 2021

PO-1582 Spencer-Attix stopping power ratios for flattening filter and flattening filter free photon beams A. Delbaere 1,2 , T. Younes 1,2 , L. Simon 1,2 , C. Khamphan 3 , L. Vieillevigne 1,2 1 Centre de Recherches en Cancérologie de Toulouse, UMR1037 INSERM - Université Toulouse 3 – ERL5294 CNRS, Toulouse, France; 2 Institut Claudius Regaud—Institut Universitaire du Cancer de Toulouse, Department of Medical Physics, Toulouse, France; 3 Institut du Cancer – Avignon Provence, Department of Medical Physics, Avignon, France Purpose or Objective As mentioned in the IAEA TRS-483 Code of Practice, few studies 1,2 reported Spencer-Attix (SA) water-to-air stopping power ratios for flattening filter free (FFF) beams and compared them with flattening filter (WFF) beams for the reference field. The aim of this work was to provide for the Varian TrueBeam STx another set of SA stopping power ratios for 6 WFF, 6 FFF, 10 WFF and 10 FFF beams using Monte Carlo method and to confront them with those published 1,2 . The impact of reducing the field size on the SA stopping power ratios was also evaluated. Materials and Methods All simulations were carried out using GATE(v9.0)/Geant4 (v10.6.patch01) Monte Carlo code with the emstandard option 3 physics list. The Varian TrueBeam STx phase-space files (PSFs) of the 6 WFF, 6 FFF, 10 WFF and 10 FFF were used as radiation sources and to generate PSFs for the 10x10 cm 2 reference field size and 1x1 cm 2 field size. Restricted and unrestricted stopping powers for each energy of spectrum were extracted using the G4EmCalculator class in Geant4. Total electron (+positron) fluence spectra differential in energy calculated under reference conditions were used to determine SA stopping power ratios. All quantities were computed according to the new ICRU Report 90 recommendations related to the mean excitation energies and densities for water and air. Results For the reference field, our values of SA stopping power ratios for both 6 and 10 MV (WFF and FFF) beams were in agreement with the ones published 1,2 (<1%). A value of 1.123 was obtained for 6 WFF and 1.125 for 6 FFF. For the 1x1 cm 2 , the SA stopping power ratios were close to those obtained for the reference field with a maximum deviation of 0.3%. Conclusion Conclusion: The SA stopping power ratios calculated in this work were in agreement with those reported by other authors 1,2 . Further studies on the impact of physics models implemented in Geant4 are ongoing. 1. Dalaryd M, Knöös T and Ceberg C 2014 Combining tissue-phantom ratios to provide a beam-quality specifier for flattening filter free photon beams Med. Phys. 41 : 111716 2. Czarnecki D, Poppe B and Zink K 2017 Monte Carlo-based investigations on the impact of removing the flattening filter on beam quality specifiers for photon beam dosimetry Med. Phys. 44 2569–80 PO-1583 3D In-vivo dosimetry for preclinical dose verification of small animal irradiation: a phantom study F. Biltekin 1 , G. Ozyigit 1 1 Hacettepe University, Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey Purpose or Objective To evaluate the feasibility of electronic portal imaging device (EPID) based 3D in-vivo dosimetry system for preclinical treatment verification of linear accelerator based therapies in small animal irradiation using fully 3D printed mouse phantom. Materials and Methods In the present study, the workflow can be divided into three steps; i) modelling and 3D printing of mouse phantom, ii) creating treatment plans for various anatomical sites and iii) treatment verification with 3D in- vivo dosimetry. In the first part, CT dataset of a mouse previously scanned for radiobiological experiment was transferred to RayStation treatment planning system (TPS) to create 3D model of external body. This structure set was exported to 3DSlicer software with SlicerRT extension in DICOM format and external body was saved as .obj file. Then, Meshmixer software was used to make the necessary arrangements and to save the file in .stl file format for 3D printing. Tissue equivalent mouse phantom were printed in Makerbot Replicator Z18 3D- printer using optimal printing parameters (60% infill percentage and sunglass infill pattern) as illustrated in Figure 1a-b. In the second part, target volumes for various anatomical sites of therapeutic interest including whole brain and total lung were defined using fused real CT dataset (Figure 1c-d). After that, four different treatment plans, 3D-CRT with lateral opposed fields and VMAT technique with single arc (Arc angle: from 175º to 185º) for whole brain, AP-PA treatment fields and VMAT technique with single arc (Arc angle: from 175º to 185º) for total lung irradiation, were created using 6 MV photon energy in Elekta Versa HD linear accelerator. In the last part, measurements were performed for all created treatment plans to simulate preclinical irradiation in mouse phantom using EPID-based iViewDose v.1.0.1 software working in conjunction with the existing EPID panel. During evaluation, EPID-reconstructed and calculated dose distribution were analyzed using 3D γ analysis method. As an evaluation criterion, 3 mm DTA and 3% DD were used. Pass-fail criteria of the treatment planning is based on the mean γ value, the maximum 1% γ value and the percentage of points with γ ≤ 1 within the 50% isodose surface of the planned maximum dose.