EP-2198: Implementation of very high energy electron grid therapy: Monte Carlo study of source definition

International audiencePurpose or ObjectiveThe use of very high-energy (70-300 MeV) electron (VHEE) beams for radiation therapy has recently started to be explored [1,2]. The main advantages over photons include the fact that small diameter VHEE beams can be scanned, thereby producing finer resolution intensitymodulated treatment than photon beams, a sharper lateral penumbra in the first centimeters and a reduced sensitivity to tissue heterogeneity. Along this line, the combination of VHEE with the benefits of Spatially Fractionated Radiotherapy (a significant increase in normal tissue tolerance [3]) has been recently proposed [4]. This novel approach, called VHEE grid therapy, is to be implemented at the future French Platform forResearch and Applications with Electrons (PRAE). This facility [5] will deliver 70 MeV electron beams in a first phase, reaching 140 MeV in the second one. The purpose of this work was to define the most adequate source and beamline parameters for pre-clinical studies.Material and MethodsMonte Carlo simulations (GATE version 7.1) were used to assess the dose distributions resulting from various possible configurations. The influence of technically feasible beam parameters (beam sizes, beam energies) achievable at PRAE facility, as well as that of various airgaps, were characterized (see Figure). Depth-dose curves and beam width were used as figures of merit. In addition, the influence of center-to-center (ctc) distance between the pencil beams was evaluated to study the variation in terms of peak-to-valley dose ratio (PVDR), arelevant dosimetric parameter for normal tissue sparing.Our main targets are neurological, i.e., targets that can be immobilized against cardio-respiratory cycles.ResultsOur results show the feasibility of implementing our strategy at PRAE. If sub-millimetric beams would be requested at all depths in the rat head in order to exploit dose-volume effects, high energies (>= 140 MeV) and low air-gap (<= 15 cm) would be needed. This would be associated with very high PVDR values over the rat brain, thus potential high tissue sparing could be expected.However, energies around 70 MeV could be used to treat tumors up to 1 cm depth (center of rat head, approximately). Experimental dosimetry measurements are foreseen to validate our calculations.ConclusionThe present Monte-Carlo study show the potential of the VHEE Grid-Therapy approach to increase high normal tissue tolerance with technically feasible beam parameters achievable at PRAE. This support the interest of performing pre-clinical experiments to evaluate its therapeutic benefit to treat brain cancer