ESTRO 2021 Abstract Book

S390

ESTRO 2021

Conclusion A scintillator dosimeter was developed and successfully used during pre-clinical proton FLASH studies. The measured spot duration and dose rates were in excellent agreement with log files.

OC-0507 Microbeams excellent tumour control and high normal tissue tolerance: limitations and perspectives V. Djonov 1 , C. Fernandez 1 , V. Trappetti 2 , J. Fazzari 1 , O. Martin 1 1 Institute of Anatomy, Medicine, University of Berne, Berne, Switzerland; 2 Institute of Anatomy , Medicine, University of Berne, Berne, Switzerland Purpose or Objective Microbeam Radiation Therapy (MRT) is an innovative radiotherapeutic approach. A collimator subdivides the homogeneous radiation field into an array of co-planar, high-dose beams that are tens of micrometres wide and spaced a few hundred micrometres apart. MRT significantly improved local tumour control with minimal normal tissue toxicity in different preclinical models. The biggest challenge in translating MRT to the clinic, are the high peak doses of around 300-600 Gy delivered by ultra-fast dose-rates, currently only possible at synchrotron facilities. Therefore, to advance MRT clinical translation, we have explored the therapeutic potential of temporal fractionation and combined treatment strategies with clinically relevant doses. Materials and Methods At the European Synchrotron Research Facility in Grenoble mice harbouring melanomas in their ears or subcutaneous human glioblastoma xenografts received either (i) a single MRT session of 400 Gy peak dose, (ii) three MRT sessions of 133 Gy peak dose delivered over 3 days in different anatomical planes, or (iii) low dose MRT of 150 Gy prior to the therapeutic treatment in combination with either gold nanoparticles (Au-NP) or the cytostatic drug cisplatin. Results Temporally fractionated MRT (3 x 133 Gy) ablated 50% of mouse melanomas, preventing organ metastases and tumour recurrence for 18 months post-treatment. In the rest of the animals, the median survival increased by 2.5-fold compared to the single MRT session and by 4.1-fold compared to untreated mice. MRT at 150 Gy combined with Au-NP increased the median survival by more than 2-fold compared to the single MRT session, and 6.6-fold compared to untreated mice. For the glioblastoma model, MRT at 150 Gy combined with cisplatin treatment reduced the tumour volume by 6 fold compared to the cisplatin alone and 60 fold compared to the untreated mice. Conclusion Temporally fractionated MRT and low dose MRT combined with Au-NP or cisplatin increased the efficacy of