ESTRO 2020 Abstract book

S750 ESTRO 2020

(MC) Treatment Planning System (resolution grid=0.1 mm, 2.4 million of histories).

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

PO-1329 Characterisation of a scintillator for small fields and in-vivo dosimetry in MR guided Radiotherapy D. Cusumano 1 , L. Placidi 1 , E. D'Agostino 2 , L. Boldrini 1 , V. Valentini 1 , M. De Spirito 1 , L. Azario 1 1 Fondazione Policlinico Universitario A.Gemelli IRCCS, Dipartimento di Diagnostica per immagini- Radioterapia Oncologica ed Ematologia, Roma, Italy ; 2 DoseVue NV BioVille, DoseVue, Diepenbeek, Belgium Purpose or Objective One of the criticalities in online adaptive magnetic resonance guided radiotherapy (MRgART) is represented by the lack of in-vivo dosimetry systems able to experimentally verify the adapted dose distribution before to deliver it on the patient. The aim of this study was to characterise a new detector consisting in a semi-sphere of 0.1cc volume containing inorganic scintillator and coupled to an optical fiber. The feasibility of using such detector for small fields measurements and in vivo application during online MRgART was evaluated in presence of low magnetic field (B). Material and Methods The detector characterisation was carried out in presence of 0.35 T B using a 6 MV FFF MR-Linac (Dose Rate=600 MU/min) and a water tank. A preliminary dose calibration was performed by exposing the detector orthogonally to a 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