CERN-based Experiments and Monte Carlo Simulations on Focused VHEE (Very High Energy Electron) Beams For Radiotherapy

Very High Energy Electrons (VHEEs) are a promising alternative to conventional radiotherapy due to their highly penetratingnature and their applicability as a modality for FLASH (ultra-high dose rate) radiotherapy. The dose distributions due to VHEE need to be optimised; one option is through the use of quadrupole magnets to focus the beam, reducing the dose to healthy tissue and allowing for targeted dose delivery at conventional or FLASH dose rates. This paper presents the first targeted area of focused VHEE dose delivery in air and a water phantom, obtained using six quadrupoles at fixed field strengths and adjusted by changing only the strength of one quadrupole and the beam charge at the CLEAR (CERN Linear Electron Accelerator forResearch) facility for beam energies >200 MeV. Experimental results have been verified with Monte Carlo modelling. This targeted dose delivery method in the form of a ’Spread-Out Electron Peak’ (SOEP), analogous to the Spread-Out Bragg Peak(SOBP) for protons, could potentially be used in future (FLASH) VHEE treatments. As a step towards this, the effect of the vacuum beampipe aperture size and spacings between quadrupoles were explored and optimised in Monte Carlo simulations.