High dose-rate radiotherapy, known as FLASH, has been shown to increase the differential response between healthy and tumour tissue. Moreover, Very High Energy Electrons (VHEEs) provide more favourable dose distributions than conventional radiotherapy electron and photon beams. Plane-parallel ionisation chambers are the recommended secondary standard systems for clinical reference dosimetry of electrons, therefore chamber response to these high energy and high dose-per-pulse beams must be well understood. Graphite calorimetry, the UK primary standard, has been employed to measure the dose delivered from a 200 MeV pulsed electron beam. This was compared to the charge measurements of a plane-parallel ionisation chamber to determine the absolute collection efficiency and infer the ion recombination factor. The dose-per-pulse measured by the calorimeter ranged between 0.03 Gy/pulse and 5.26 Gy/pulse, corresponding to collection efficiencies between 97% and 4%, respectively. Multiple recombination models currently available have been compared with experimental results. This work is directly applicable to the development of standard dosimetry protocols for VHEE radiotherapy, FLASH radiotherapy and other high dose-rate modalities. However, the use of secondary standard ionisation chambers for the dosimetry of high dose-per-pulse VHEEs has been shown to require large corrections for charge collection inefficiency.

/pdf/the-challenge-of-ionisation-chamber-dosimetry-in-ultra-short-3mmrpakvch.pdf

The challenge of ionisation chamber dosimetry in ultra-short pulsed high dose-rate Very High Energy Electron beams

https://re.public.polimi.it/bitstream/11311/1160246/1/2020%20The%20European%20Joint%20Research%20Project%20UHDpulse%20%e2%80%93%20Metrology%20for%20advanced.pdf

The European Joint Research Project UHDpulse - Metrology for advanced radiotherapy using particle beams with ultra-high pulse dose rates.

Active plasma lensing is a compact technology for strong focusing of charged particle beams, which has gained considerable interest for use in novel accelerator schemes. While providing kT/m focusing gradients, active plasma lenses can have aberrations caused by a radially nonuniform plasma temperature profile, leading to degradation of the beam quality. We present the first direct measurement of this aberration, consistent with theory, and show that it can be fully suppressed by changing from a light gas species (helium) to a heavier gas species (argon). Based on this result, we demonstrate emittance preservation for an electron beam focused by an argon-filled active plasma lens.

Emittance Preservation in an Aberration-Free Active Plasma Lens.

Very high energy electron (VHEE) beams have been proposed as an alternative radiotherapy modality to megavoltage photons; they penetrate deeply without significant scattering in inhomogeneous tissue because of their high relativistic inertia. However, the depth dose distribution of a single, collimated VHEE beam is quasi-uniform, which can lead to healthy tissue being overexposed. This can be largely overcome by focusing the VHEE beam to a small spot. Here, we present experiments to demonstrate focusing as a means of concentrating dose into small volumetric elements inside a target. We find good agreement between measured dose distributions and Monte Carlo simulations. Focused radiation beams could be used to precisely target tumours or hypoxic regions of a tumour, which would enhance the efficacy of radiotherapy. The development of new accelerator technologies may provide future compact systems for delivering these focused beams to tumours, a concept that can also be extended to X-rays and hadrons. Very high energy electrons (VHEE) penetrate deeply in tissues and can provide an alternative to photon irradiation for tumour treatment. Using VHEE beams at CERN Linear Electron Accelerator for Research (CLEAR) focused into a water phantom, the authors demonstrate on-axis dose enhancement at a depth of 5–6 cm, proving that such beams can produce dose concentration to small volume elements, hence limiting the effect on adjacent healthy tissues if used for radiotherapy.

/pdf/an-experimental-study-of-focused-very-high-energy-electron-58j07fhs5l.pdf

An experimental study of focused very high energy electron beams for radiotherapy

This paper presents the first plasmid DNA irradiations carried out with Very High Energy Electrons (VHEE) over 100–200 MeV at the CLEAR user facility at CERN to determine the Relative Biological Effectiveness (RBE) of VHEE DNA damage yields were measured in dry and aqueous environments to determine that ~ 99% of total DNA breaks were caused by indirect effects, consistent with other published measurements for protons and photons Double-Strand Break (DSB) yield was used as the biological endpoint for RBE calculation, with values found to be consistent with established radiotherapy modalities Similarities in physical damage between VHEE and conventional modalities gives confidence that biological effects of VHEE will also be similar—key for clinical implementation Damage yields were used as a baseline for track structure simulations of VHEE plasmid irradiation using GEANT4-DNA Current models for DSB yield have shown reasonable agreement with experimental values The growing interest in FLASH radiotherapy motivated a study into DSB yield variation with dose rate following VHEE irradiation No significant variations were observed between conventional and FLASH dose rate irradiations, indicating that no FLASH effect is seen under these conditions

/pdf/evaluating-very-high-energy-electron-rbe-from-nanodosimetric-3dawhzulzd.pdf

Evaluating Very High Energy Electron RBE from nanodosimetric pBR322 plasmid DNA damage

The conversion of the CALIFES beamline of CTF3 into the “CERN Linear Electron Accelerator for Research” (CLEAR) facility was approved in December 2016. The primary focus for CLEAR is general accelerator R&D and component studies for existing and possible future accelerator applications. This includes studies for high gradient acceleration methods, e.g. for CLIC and plasma technology, and prototyping and validation of accelerator components, e.g. for the HL-LHC upgrade. The facility also provides irradiation test capabilities for characterisation of electronic components and for medical applications. A description of the facility with details on the achievable beam parameters, and the status and plans are presented.

/pdf/the-clear-user-facility-at-cern-44xi6nwdei.pdf

The CLEAR user facility at CERN

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The CLEAR user facility at CERN