ESTRO 2020 Abstract Book

S702 ESTRO 2020

10x10 cm 2 field and delivering 100 MU (setup in Figure 1). The number of counts revealed by the scintillator was correlated with the dose value measured by a 0.125 cc ion chamber. With the same calibration experimental setup, the detector was then characterised in terms of signal to noise ratio (SNR), response dependence from B orientation, reproducibility and dose linearity. SNR was evaluated delivering 5 times 100 MU with a 10x10 cm 2 field and repeating the measurements using a bare fiber to estimate the noise.B orientation was evaluated placing the detector at 3 orientations (0°,90° and 270°) respect to the B force lines and delivering 3 times 100 MU per configuration. Dose linearity was investigated delivering 10,20,50,60,100,200,500 and 1000MU and calculating the R 2 coefficient. Field size dependence were also investigated by measuring the output factor (OF) under the same experimental conditions. The results were compared with those measured using the 0.125cc ion chamber and a 0.004 mm 3 synthetic diamond and those calculated using a Montecarlo (MC) Treatment Planning System (resolution grid=0.1 mm, 2.4 million of histories).

Conclusion The detector can be effectively used for small fields dosimetry in quality controls of MRgRT. Further studies, regarding the elaboration of 2D arrays and the integration into MR surface coils are currently under development to use this detector as in-vivo dosimeter during online MRgART PO-1330 On the Implementation and Validation of 3D Computational Pediatric Phantoms in Commercial TPS A. Gupta 1 , Y. Qiao 1 , S. Shrestha 1 , C. Owens 1 , C. Lee 2 , C. Ditty 3 , S. Smith 1 , R. Weathers 1 , R.M. Howell 1 1 University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA ; 2 National Cancer Institute, Division of Cancer Epidemiology & Genetics, Bethesda, USA ; 3 RaySearch Laboratories, Physics, Cincinnati, USA Purpose or Objective Our group developed an in-house computational phantom that can be scaled to any age from infant to adult. The age-based scaling accounts for non-uniform growth of different body regions (in three dimensions [3D]). Our phantom was originally modeled in FORTRAN and has been used for more than three decades for many late effects studies of pediatric cohorts treated with conventional radiation therapy (RT). However, the phantom cannot be used for cohorts treated with modern RT with complex beam arrangements designed using computed tomography (CT) based treatment planning. Here, we aimed to (1) develop a 3D model of the phantom in Digital Imaging and Communications in Medicine (DICOM) format, (2) scale the phantoms’ body regions and organs to different pediatric ages, and (3) convert the phantom from FORTRAN to DICOM format across the full age range. Material and Methods We developed a script to generate the 3D model of the phantom in DICOM format using RayStation treatment

Results The mean SNR was 116.3±2.4. The detector repeatability was within 1%, its response variation based on the orientation respect to the B was below 2% (-1.6% at 90° and -0.8% at 270°). Figure 2 shows the detector raw response in function of the time obtained during calibration, the integral response obtained to increasing of MUs and the OF results. The detector has a temporal resolution of 5 Hz and it shows linear response (R 2 =1) in the dose range investigated. All the OF measured with the scintillator are in accordance within 1% with those measured using the other detectors and calculated by MC simulation. The maximum variation was 0.8%, observed for 12.5 and 6.6 cm 2 fields