ESTRO 36 Abstract Book

S765 ESTRO 36 _______________________________________________________________________________________________

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

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 Purpose or Objective Anthropomorphic phantoms are used in a variety of ways in radiation therapy for both research and quality assurance purposes. Most anthropomorphic phantoms are of generalized patients, but 3D printing technology can be used to fabricate patient-realistic phantoms for special QA and verification procedures. Most 3D printers, however, can only print in one or two materials at a time, so true patient heterogeneity is limited. In this study, we examined two different patient specific, 3D printed phantoms created based on the same patient to determine the accuracy of single and multi-material phantoms. Material and Methods The phantoms used in this study were designed from the clinical CT data for a post-mastectomy patient treated at our institution. The CT data was trimmed to remove the patient’s head and arms to preserve anonymity and simplify printing. Phantom 1 was designed by converting the trimmed CT data into a 3D model with a CT threshold of >-500 Hounsfield units (HU). This model was sliced into 2.5-cm-thick sagittal slices and printed one slice at a time. All slices were printed with polylactic acid (PLA) representing all body tissues, but with air cavities and lower density regions like the lungs left open. Sagittal slices were chosen for their superior fit with each other, and minimal material warping relative to axial slices. Phantom 2 was designed by converting the CT data into three separate 3D models with a CT threshold of <-147 HU for air cavities, -147 to 320 HU for soft tissue, and >320 HU for bone. The models were sliced into 1-cm-thick axial slices, and printed. The slices were printed from the soft tissue model using a custom formulated high impact polystyrene (HIPS) with the air and bone models left open. After printing, the open bone model sections were filled with a liquid resin polymer with an equivalent density to bone. The phantoms were evaluated for their materials and overall accuracy to the original patient CT. Blocks of PLA, HIPS, and the bone resin material were all imaged to determine their average HU. The phantoms were also each imaged and registered with each other and the original patient CT to determine the consistency and accuracy of each phantom. Results The materials used and their properties are summarized in Table 1. Phantom 1 was fabricated from PLA, which isn’t particularly tissue equivalent, but did print relatively consistently. The bone resin and HIPS of phantom 2 more accurately reflect tissue heterogeneity, but have more variations in their printed consistency.

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