ESTRO 2020 Abstract book

S801 ESTRO 2020

have higher normal tissue tolerance while remaining the same tumor control compared to broad beam irradiation. For its clinical use, sources of high brilliance such as synchrotrons seem to provide the most promising properties. However due to their size and cost alternative compact photon sources are being investigated for their usability in MRT. Therefore, a special collimator with divergent slits has been manufactured, that is able to shape photons from a conventional X-ray source to microbeams. The purpose of this work is to develop an algorithm for the dose calculation of MRT at the Xenx x-ray irradiator (Xstrahl Ltd, UK), which is a preclinical radiation device for small animal research. Material and Methods The dose calculation is based on a hybrid approach that combines Monte Carlo simulation and convolution based dose calculation [1].The method, which was originally developed for synchrotron radiation, was adjusted for dose calculation in the divergent field of an x-ray tube. Calculations were benchmarked against full Monte Carlo (MC) simulations in Geant4 and film dosimetry at the XenX. In all calculations the x-ray source was modelled as an anode surface emitting photons isotopically in all directions from an Gaussian shaped focal spot according to the x-ray tube specifications. The energy spectrum of the Xenx x-ray tube was simulated with Monte Carlo. The dose distribution was scored and measured in a PMMA phantom. Results Figures 1 and 2 show the results of both calculation techniques for the peaks and valleys. The dose curves were normalized to the maximum value of the peaks. It can be seen that Hybrid and full MC show agreement in the valley dose for all depth. In the peak region the deviation from the maximum is less than 5% for doses in up to 60 mm depth, while it increases to over 10% for lower doses deeper in the phantom. Conclusion In the peak region the hybrid algorithm shows excellent agreement with Monte Carlo for high doses. The discrepancy for lower doses will be further investigated, although it is of minor importance for small animal treatment. More importantly, this data will be verified by experiments before using this algorithm for treatment planning.

References [1] Donzelli, Mattia, et al. "Hybrid dose calculation: a dose calculation algorithm for microbeam radiation therapy." Physics in Medicine & Biology 63.4 (2018): 045013. PO-1417 A GPU Monte Carlo to support clinical routine in a compact spot scanning proton therapy system J. Gajewski 1 , A. Schiavi 2 , N. Krah 3 , V. Patera 2 , G. Vilches- Freixas 4 , J. Martens 4 , M. Unipan 4 , A. Rucinski 1 , B. Nijsten 4 , G. Bosmans 4 , I. Rinaldi 4 1 Institute of Nuclear Physics, Pan, Krakow, Poland ; 2 University of Rome, Sapienza, Rome, Italy ; 3 University of Lyon/CNRS CREATIS UMR5220- Centre Léon Bérard, Creatis UMR5220, Lyon, France ; 4 Maastricht Radiation Oncology MAASTRO clinic, Radiation Oncology, Maastricht, The Netherlands Purpose or Objective The purpose of this work is to implement an independent fast Monte Carlo (MC) dose calculation tool, Fred, to complement our clinical Treatment Planning System (TPS). Current efforts concentrate on making the MC code compatible with our proton delivery system as well as