Proton radiotherapy is becoming more widely used due to the highly favourable dose distribution in the form of the Bragg peak, with minimal dose delivered downstream from the target in the beam direction. In addition to conventional radiotherapy, FLASH radiotherapy in which ultra-high dose rates are used to deliver radiotherapy in a manner that reduces normal tissue toxicity is currently being investigated 4 with the first patient treated in Lausanne in 2019 5. The aim of radiotherapy is to deliver a lethal dose to the tumour whilst sparing healthy tissue as much as possible since it was first invented the field has advanced greatly and highly conformal photon radiotherapy techniques such as intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) are now routinely used in cancer treatment 3. Radiotherapy is used in the treatment of around half of all cancer cases worldwide 1, and 27% of cases in the UK 2. We have investigated both symmetric and asymmetric focusing as well as focusing with a range of beam energies. The dose distributions of unfocused VHEE produce high entrance and exit doses compared to other radiotherapy modalities unless focusing is employed, and in this case the entrance dose is considerably improved over existing radiations. VHEEs are less sensitive to inhomogeneities than proton and photon beams, have a deep dose reach and could potentially be used to deliver FLASH radiotherapy. Very high energy electron (VHEE) beams are an exciting prospect in external beam radiotherapy. This has a significantly reduced entrance dose compared to a proton-based spread-out Bragg peak (SOBP). We also present a weighted sum of focused electron beams to form a spread-out electron peak (SOEP) over a target region. We show that the focal point is readily modified by linearly changing the quadrupole magnet strength only. This paper presents the first demonstration of deeply penetrating dose delivery using focused very high energy electron (VHEE) beams using quadrupole magnets in Monte Carlo simulations.
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