ESTRO 2020 Abstract book

S135 ESTRO 2020

gene expression patterns, paralleled by enrichment of myeloid immune cell gene signatures. Anti-VEGF-A treatment reduces mesenchymal gene expression and attenuates radiation-induced "mesenchymalization". In normal brain tissue, RTX induces expression of necrosis- and inflammation-associated gene sets which is also attenuated by anti-VEGF-A blockade. Conclusion We provide evidence that anti-VEGF-A treatment may offer particular benefits in combination with RTX when applied to GBM of mesenchymal subtype by modulating subtype plasticity. Furthermore, anti-VEGF-A treatment may be neuroprotective – not only – in reirradiation settings. These observations deserve further clinical evaluation. PH-0238 Optimisation of a Prompt-Gamma Slit Imaging System for Particle Beam Range Verification A. Vella 1 , F. Van den Heuvel 1 1 University of Oxford, Oncology, Oxford, United Kingdom Purpose or Objective In-vivo range verification is crucial in hadron therapy and can be achieved using Prompt-Gamma (PG) secondary radiation. The spatial pattern of the induced PG emission is strongly correlated with the absorbed dose. We aim to optimise via Monte Carlo (MC) simulations a PG slit camera by characterising an Energy-Dispersive Pixelated (EDP) imaging detector and by exploring the slit sizes. The EDP detectors can replace bulky scintillators for a more compact device with energy-dispersive and imaging capabilities. Current PG devices are manufactured using thick tungsten slit collimators (F. Hueso-González, 2018). A reduced thickness would decrease manufacture costs and make the device suitable for gantry fitting. Material and Methods The PG imaging system comprises a slit collimator and an EDP detector placed behind the collimator. The detector can simultaneously capture energy-spectra and images of the gamma-ray emission. A range of different thicknesses and slit apertures is explored for optimising the PG system. The slit was modelled with/without the knife-edge. The PG EDP camera is characterised through FLUKA/PENELOPE (F. Salvat, 2018) (A. Ferrari, 2018) MC simulations to gather PG energy-spectra and spatial distributions. The PG energy-spectra are generated by a 150MeV proton beam within a 200mm diameter-200mm wide PMMA phantom in FLUKA. Pixelated images simulated in PENELOPE are captured at the detector through the slit (40 to 100mm thick, 10-20-30mm aperture) with/without the knife-edge. Finally, Image Quality Metrics (IQM) are measured to assess the collimator performances and find the optimal configuration. Proton range (PR) was calculated and compared with the initial emission in FLUKA. Results Ideally, the thickness should be enough to prevent leakage through the collimator and absorb gamma-rays. The current PG device has a relatively thick tungsten slit collimator (120mm) and a 12mm gap. But the thickness could be reduced (~50-60mm) with the same aperture (A. Vella, 2017) (fig.1). The knife-edge collimator overall performances significantly worsen (SNR~30%, CNR~8%, FWHM~40%) compared to the slit. From FLUKA/PENELOPE simulations, the PR within the phantom is approximately 120-130mm. The 60mm thick/10mm aperture slit achieved the optimum, while the knife-edge underestimates the PR (fig. 2). Poster Highlights: Poster highlights 8 PH: Planning and validation of ion beams

Conclusion The PG EDP detector can simultaneously capture energy- spectra and images of the PG emission of the particle beam. The performances of the slit collimator with/without knife-edges were assessed through MC simulations to optimise the current PG imaging system. The aperture should be 20mm to preserve the spatial resolution. The PG slit camera can be equipped with a reduced collimator thickness (~50-60mm), CNR/SNR are not significantly affected, and manufacturing costs reduced. Finally, there is no benefit of using the knife- edge collimator. PH-0239 Time-resolved dosimetry for validation of 4D dose calculation in PBS proton beam radiotherapy N. Kostiukhina 1 , H. Palmans 2 , M. Stock 2 , D. Georg 1 , B. Knäusl 1 1 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria ; 2 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria Purpose or Objective Validation of 4D dose calculations (4D-DC) is an essential step in the evaluation of motion mitigation techniques in pencil beam scanned (PBS) particle therapy. In this study, the distortion of dose distributions due to the interplay between beam and organ motion was simulated and validated using a recently proposed 4D dosimetry approach [1]. Material and Methods