ESTRO 36 Abstract Book

S771 ESTRO 36 _______________________________________________________________________________________________

Results Results showed that LYSO was the best candidate and so detector blocks of this material were manufactured and used to examine the behaviour of a full insert ring. The experimental setup consisted on two 4x4 LYSO arrays coupled to the PDPC. Each crystal of the array was 7 x 7 x 20 millimeters, fitting perfectly the size of the PDPC dies. The detector blocks were placed facing each other and separated by 30 mm with a Na-22 source in the middle and rotating at a 30 degrees steps to imitate the geometry of a ring of the PET insert. The outcome was used to create a GATE simulation that explores the behaviour of this insert in the presence of strong magnetic fields. Such simulation involves the design of a ring device that moves along the patient axis and the calculation of the detection efficiency varying the geometry and speed of the apparatus. Conclusion Our findings show an adequate coupling of the PDPC with the LYSO arrays and the simulation corroborates an appropriate functionality of the detection system inside an MRI machine. The next study phase involves the construction of a prototype for real testing. EP-1446 Can parallel plate ion chambers be used for PDD measurements in FFF beams? S. Vargas Castrillón 1 , F. Cutanda 2 1 Lanarkshire Beatson West of Scotland Cancer Centre, Radiotherapy Physics, Airdrie, United Kingdom 2 Western General Hospital, Oncology Physics, Edinburgh, United Kingdom Purpose or Objective The IAEA TRS-398 code of practice for radiation dosimetry recommends measuring photon percentage depth-dose (PDD) curves with plane-parallel ion chambers. This code of practice was published before flattening-filter free beams became widely used in clinical practice. This choice of detector for PDD measurement should be re-assessed for FFF beams, as the effect of recombination and polarity factors, among others, could lead to differences. The purpose of this work is to assess the use of plane parallel ion chambers for PDD measurements in FFF beams, and to compare them with other chamber types. Material and Methods Beams from Varian TrueBeam linacs, 6 FFF and 10 FFF were used for this study. Depth dose curves (SSD=100cm) were acquired with a PTW 31010 Semiflex, two small volume chambers (Scanditronix-Wellhofer CC04 and PTW 31016 Pin Point 3D), PTW 34001 Roos, Scanditronix- Wellhofer Roos and NACP 02 parallel plate chambers. Measurements were carried out both in a PTW MP3 and an IBA Bluephantom2 water tanks with PTW Tandem, PTW Unidos E and CCU electrometers. PDD scans with plane parallel ion chambers were acquired and corrected for ion recombination. In order to apply this correction, recombination factors were measured with the two voltage technique for different depths and field sizes. The effect of polarity was evaluated using both polarities for measurements. Measurements with different detectors were carried out for a set of field sizes, ranging from 5x5 to 40x40 cmxcm and SSD 100 cm SSD. Results It was found that parallel plate chambers show the closest agreement between PDD curves acquired with different polarities, being the differences below 0.1% for all depths and 40x40 cmxcm.PDDs for one single polarity and different ion chambers have been corrected for recombination and compared. The largest difference in PDD among different ion chambers, once corrected for recombination, has been found for the Scanditronix Roos chamber at 350 mm deep (excluding build up) for all field sizes, which would amount to: 0.7% for 6 FFF and 0.6% for

Conclusion The application of different sensitometric curves and correction methods caused errors in detected failed points and passing rates in radiochromic film dosimetry. This can lead misinterpret detected dose errors in quality assurance. It is significant to apply the optimal sensitometric curve with appropriate correction methods, especially in modulated radiation fields for reliable and accurate film dosimetry. EP-1445 Performance evaluation of scintillators for SiPM PET/MRI Brain Imaging N. Campos Rivera 1 , B. Seitz 1 1 University of Glasgow, School of Physics & Astronomy, Glasgow, United Kingdom Purpose or Objective The combination of PET and MRI has shown a great potential to study the processes and progression of diseases, such as cancer and Alzheimer’s, as well as to control and observe novel treatments response. In the last decade, whole-body hybrid systems have been manufactured, such as the Biograph mMR by Siemens. However, this apparatus tend to be very expensive and to have limited performance on the neurological field. Thus, a brain-dedicated portable PET insert with MRI compatibility seems to be worth to investigate Material and Methods Recently, a SiPM with electronics integrated on cell level has been developed: the Philips Digital Photon Counting (PDPC). This device delivers a digital signal of the detected photon counts as well as their timestamp, making it a potential candidate for PET applications. During this work, the performance of several scintillation materials (GAGG, LYSO, LUAG and BGO) coupled with the PDPC was studied. Each PDPC module has 64 channels of 3.2 x 3.8 millimeters, comprising 3200 cells of 59.4 x 64 micrometers and each scintillator crystals is 3 x 3 x 30 millimeters, polished on all faces with five of them covered with a white reflective coating. Analysis on the intrinsic performance of the SiPM (IV curves, temperature dependence, crosstalk) and energy and time resolution for every scintillator was carried out.