ESTRO 36 Abstract Book

S756 ESTRO 36 2017 _______________________________________________________________________________________________

– 63.1%) for VMAT and 21.5 % (0.7%- 60.1%) for B-VMAT. For V10, the VMAT medium value was 23.2% (0%- 37.2%) and the B-VMAT medium value was 8.9% (0%- 24.4%). Conclusion B-VMAT for mediastinal tumors is clearly superior to usual VMAT for breast doses, mainly the low doses, and equivalent in the rest of dosimetric parameters. Although the inclusion of more patients is needed, our preliminary results show B-VMAT like a great technical advance in mediastinal radiotherapy. Electronic Poster: Physics track: Basic dosimetry and phantom and detector development EP-1433 Photoneutron Flux Measurement via NAA in a Radiotherapy Bunker with an 18 MV Linear Accelerator T. Gulumser 1 , Y. Ceçen 1 , A.H. Yeşil 1 1 Akdeniz University- School of Medicine, Department of Radiation Oncology, Antalya, Turkey Purpose or Objective In cancer treatment, high energy X-rays are used which are produced by linear accelerators (LINACs). If the energy of these beams is over 8 MeV, photonuclear reactions occur between the bremsstrahlung photons and the metallic parts of the LINAC. As a result of these interactions, neutrons are also produced as secondary radiation products (γ,n) which are called photoneutrons. The study aims to map the photoneutron flux distribution within the LINAC bunker via neutron activation analysis (NAA) using indium-cadmium foils. Material and Methods The radiotherapy bunker hosts a Philips SLI-25 LINAC which is used for experimental studies. The measurements are taken at the highest energy of the LINAC which corresponds to 18 MeV bremsstrahlung photons. Indium and cadmium foils were used at 91 different points within the bunker. Neutron activation was performed by irradiating the room with 10000 monitor units (MU) at different gantry angles. The field was 40x40 cm 2 open. The activated indium foils are then counted in a High Purity Germanium (HPGe) detector system. Since indium has a high absorption cross section for thermal and epithermal neutrons, bare indium foil irradiation results in flux information of that region. However cadmium has high absorption cross section in the epithermal and fast region. If one filters the indium foils by cadmium coatings, the difference in the count yields thermal fluxes which are of interest for the doses to the patients in radiotherapy. Results Result of the analysis shows that the maximum neutron flux in the room occurs at just above of the LINAC head towards to gun direciton. This is expected since most of the neutrons are produced when the electron beam hits the tungsten target and then the primary collimation occurs. Both the target and the primary collimator are located at the top of the gantry head. The maximum thermal neutron flux obtained is 3x10 5 neutrons/cm 2 .second which is higher than a standard americium-beryllium (Am-Be) neutron source. At the isocenter plane (SSD=100 cm), the fluxes were 5.4x10 4 at the center, 1.5x10 4 at 2.5 m away and 9.9x10 3 n/cm 2 .s at the room wall which is 3.8 m away from isocenter. The flux at the maze entrance was measured nearly six in a ten thousand less (81 n/cm 2 .s). Conclusion The neutron flux distribution within the bunker was measured with detail using 91 points. Neutron flux distribution within the bunker found and the graph was plotted. Thus neutron flux can found any desired point in the room by iterations. The flux decreases as we move

away the isocenter which is compatible with the literature. The magnitude of the neutron fluxes shows that there is a significant amount of neutron dose within the room. The corresponding neutron dose to the patient however is only 0.1-0.3 % of the total dose. However, neutrons have a high RBE and this unwanted dose is not calculated with the TPS. The future work would be to compare the results with the Monte Carlo simulations. EP-1434 Comparison of small-field output factor measurements C. Oliver 1 , V. Takau 1 , D. Butler 1 , I. Williams 1 1 ARPANSA, Radiotherapy, Yallambie, Australia Purpose or Objective The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) held a comparison in April 2016 whereby participants came to ARPANSA and measured the output factor of a 5 mm cone . The goal of the comparison was to compare the consistency of the small-field output factor measured by independent medical physicists with The participants measured the output factor of the 5 mm cone using a 6 MV photon beam at a source to surface distance of 95 cm and depth in water of 5 cm. ARPANSA provided a 3D scanning water tank for detector positioning but all detectors were brought by participants. The participant was asked to measure the output factor as accurately as possible. All post measurement analysis, correction factor determination and uncertainty calculations were supplied by the participant. Results their own apparatus. Material and Methods Fifteen groups travelled to ARPANSA and a total of thirty independent measurements of the output factor were made. The most popular method of measurement was with film but measurements were also made with ionisation chambers, semiconductor detectors, diamond detectors and a scintillation detector. A large volume ionisation chamber measuring dose area product was also used in the comparison. The standard deviation of all the measurements was 5.6 % with the maximum variation between two results being 42 %. Conclusion This exercise gave an indication of the consistency of the small-field dosimetry being performed in Australia at the present time. There is no currently accepted protocol for these measurements and a wide range of detectors are being used with correction factors being applied from a variety of sources. The dissemination of the small-field methods and techniques currently being used will aid the consistency of these measurements. EP-1435 Evaluation of single material and multi- material patient-specific, 3D-printed radiotherapy phantoms D. Craft 1 , E. Burgett 2 , R. Howell 1 1 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA 2 Idaho State Univeresity, Department of Nuclear Engineering, Pocatello Idaho, USA