ESTRO 2020 Abstract Book

S701 ESTRO 2020

PO-1328 Ionization quenching correction for 3D scintillator detectors for spot scanning proton therapy F. Alsanea 1 , S. Beddar 1 1 U.T. M.D. Anderson Cancer Center, Radiation Physics, Houston- TX, USA Purpose or Objective The ionization quenching phenomenon in scintillators must be corrected to obtain accurate dosimetry in particle therapy. The purpose of this study was to develop a methodology for correcting camera projection measurements of a 3D scintillator detector exposed to The 3D scintillator detector consists of a liquid scintillator filled tank (20 cm x 20 cm x 20 cm) and three cameras. The scintillation light is collected by a three camera system, each consisting of an objective lens and a scientific complementary metal–oxide–semiconductor (sCMOS) camera. We have exposed our detector to four different proton beam energies produced by the synchrotron at M.D. Anderson Cancer Center Proton Therapy Center (85.6, 100.9, 144.9, and 161.6 MeV). We used Monte Carlo simulations to obtain the dose and linear energy transfer (LET) for these beam energies. Only one axial projection was used to generate integrated depth dose curves (the same quenching correction would apply for all three cameras). We compared the light output to the dose calculated by the Monte Carlo simulation and applied Birks’ scintillation model to fit the measured light. Optical artefact corrections were used to correct for refraction at the air-scintillator interface, and image perspective resulting in sub-millimeter accuracy in calculating physical distances within the tank. However, these corrections did not account for the non-orthogonal integration of data off the central axis of the image. Therefore, we compared the light output to an integrated Monte Carlo dose and LET along the non-orthogonal path to use Birks’ quenching correction model. Results After accounting for the non-orthogonal integration of the data, the Monte Carlo assigned values of fluence-averaged LET for each pixel on the camera projection correlated well with the light output independent of the delivered proton beam energy. In particular, the correction reduced the dose error at the Bragg peak region from 15% to 3% for low energy proton beams. Overall, the doses at the Bragg peak region using the Birks' model were less than ±3% of the Monte Carlo dose after implementing the non- orthogonal path corrections. Conclusion We have improved the application of Birks’ model quenching corrections in 3D scintillators by numerically projecting the dose and LET 3D grid to camera projections. In our future work, we will obtain the light distribution in 3D from the three camera projections to simplify the application of quenching correction models in 3D scintillator detectors. proton pencil beams. Material and Methods

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