Abstract Book

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ESTRO 37

thermoluminescent dosimeters were used to measure the dose distributions. They were located in different configurations in the proximity of the interphases to evaluate the influence of the heterogeneities within the phantom. Results The results obtained using a 4 mm diameter collimator show that from 5 mm of the 139-cc air cavity, the obtained dose is overestimated in a 5.7% the value reported by the plan. However, for an inhomogeneity of 2.5 cm thickness of aluminum is possible to obtain up to a 14.7% less dose than the planned value and in a lesser extent, for an inhomogeneity of 8 mm thickness of bone, an underestimation of 3% was found. Conclusion Although the presented results correspond to extreme cases, they suggest that it is important the planning algorithms for this type of treatments take into account the tissue differences in the intracranial zone to calculate the supplied dose to the target with greater precision. EP-1846 Influence of beam spoiler and air gap on dose distribution in build-up region for X6 MV open field E. Dabrowska 1,2 , A. Zawadzka 1 , P. Kukolowicz 1 , P. Kowalczyk 3 , R. Podgorski 3 , M. Wojasiński 3 , T. Ciach 3 , R. Graczyk 4 , T. Zawistowski 5 1 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Department of Medical Physics, Warsaw, Poland 2 University of Warsaw, Department of Biomedical Physics, Warsaw, Poland 3 Warsaw University of Technology, Department of Biotechnology and Bioprocess Engineering, Warsaw, Poland 4 Polish Academy of Sciences, Space Research Centre, Warsaw, Poland 5 CreoTech, Instruments S.A., Piaseczno, Poland Purpose or Objective Based on CBCT images we noticed that there are several different problems with reproducibility of bolus positioning (E. Dąbrowska, et al., Investigation of reproducibility of bolus position based on kV CBCT imaging, Radiotherapy & Oncology, Journal of the European Society for Radiotherapy and Oncology, Volume 123, Supplement 1, 2017), especially air gap occurrence. The aim of this study was to determine the influence of air gaps between bolus and phantom on dose distribution in build-up region for X6 MV. The novelty of the work is using low density boluses fabricated with 3D printer. Material and Methods Depth dose measurements were perform for X6 MV 10x10 cm 2 open field, generated with Varian CL 2300 C/D. The measurements were carried out with the Markus plane parallel ionization chamber (PTW, 0.055 cm 3 volume), connected to Unidos (PTW) electrometer. As a 1 cm thick beam spoilers we used 3D-printed thermoplastic polyurethane (TPU) cuboids filled with 5%, 10%, 15% and 20% honeycomb structure (thickness of 0.5 g/cm 2 , 0.17 g/cm 2 , 0.22 g/cm 2 ). To generate air gap we used homemade styrofoam frames (fig. 1). Measurements were performed at a physical depth of 1, 5, 10 and 15 mm for all beam spoilers without air gap and with 10, 20, 30 and 40 mm air gaps, at constant 90 cm SSD.

Results The results of measurements were presented on graphs (fig. 2, 3). Based on fig. 2 we can see that beam spoilers increased the dose in build-up region. The bigger is spoiler filling the larger is surface dose. The same dependence we can see in fig. 3, irrespective of air gap size. Regardless of spoiler percentage filling with honeycomb structure, the larger air gap size the smaller dose on 1 mm depth.

Conclusion Influence of beam spoiler filling and air gap on dose distribution in build-up region for X6 MV was investigated. Beam spoiler increased the dose in build-up region. The larger thickness of beam spoiler (larger filling) the larger influence on build-up dose. Occurrence of air gaps lowers the surface dose. However, the influence of air gap in the range of 10 - 40 mm is not very pronounced. For bolus with density of 0.22 g/cm 2 , regardless of air gap size the dose on 1 mm depth was always larger than 95% of maximum dose. The issue will be investigated for wedged and dynamic fields. EP-1847 Dosimetry of radiotherapy machines with intermediate non-equilibrium field sizes L. Mirzakhanian 1 , R. Bassalow 2 , C. Huntzinger 2 , J. Seuntjens 1 1 McGill University, Medical Physics, Montreal, Canada 2 RefleXion Medical, 25821 Industrial Blvd- Hayward- 9454, CA, USA Purpose or Objective The purpose of this study is to introduce a methodology for the calibration of upcoming radiotherapy machines with intermediate non-equilibrium field sizes. An intermediate non-equilibrium field size refers to a field for which the lateral charged particle equilibrium condition of the machine specific reference field ( msr ) is