Evaluating very high energy electron RBE from nanodosimetric pBR322 plasmid DNA damage

Abstract

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.

Figure 1

(a) Single-strand break yields and (b) double-strand break yields for 100–200 MeV electron beam irradiation of dry pBR322 plasmid DNA based on McMahon data fitting.

Figure 2

(a) Single-strand break yields and (b) double-strand break yields for 100–200 MeV electron beam irradiation of wet pBR322 plasmid DNA based on data fitting to the McMahon fit. Plasmids irradiated at Conventional (~ 0.5 Gy/s) and FLASH (> 10⁸ Gy/s) dose rates.

Figure 3

RBE values for (a) VHEE and protons with dry plasmid DSB yield as the biological endpoint and (b) VHEE, low-energy electrons and protons with wet plasmid DSB yield (Small, Vysin, CLEAR) or cell survival fraction (Herskind, Spadinger, Zackrisson) as the biological endpoint. Experimental data taken from references^–.

Figure 4

Double-strand break yields for experimental and computational plasmid DNA irradiation with 100–200 MeV electrons. Geant4-DNA simulation performed with half-cylinder DNA geometry with a 10 bp separation defined for DSB induction and damage determined by an energy threshold of 8.22 eV. Standard errors calculated based on 10³ repeats.

Figure 5

Schematic of the CLEAR beamline and two experimental areas, Figure produced by Kyrre Sjobak and reproduced here with kind permission from Kyrre Sjobak.

Figure 6

(a) Schematic (produced by author K.L.S.) and (b) photograph (taken by author K.L.S.) of experimental set-up for irradiation of dry plasmid samples on glass microscope slides. EBT-XD film placed behind samples to show sample coverage by beam.

Figure 7

(a) Schematic (produced by author K.L.S. and (b) photograph (taken by author K.L.S.) of experimental set-up for irradiation of wet plasmid samples in Eppendorf tubes. EBT-XD film placed behind samples to determine dose delivered to samples.

Figure 8

(a) Cropped UV image of agarose gel following electrophoresis containing wet plasmids irradiated by 100 MeV electrons. Labelled bands indicate presence of SC, OC and L plasmid forms. Uncropped image shown in SF5. (b) SC, OC and L plasmid proportions as a function of dose following integration of band intensities, fitted to Eqs. (2), (3) and (4). Error bars calculated based on five gel repeats.