# Extension of the Discrete Electron Transport Capabilities of the Geant4-DNA Toolkit to MeV Energies
## 1 Introduction  
Monte Carlo (MC) track-structure (TS) simulations are an established tool for quantitative analysis of radiation effects at the cellular and DNA levels [1]. The strength of MCTS simulations rests on their ability to model (charged) particle interactions in a discrete manner according to the corresponding single-scattering cross sections. Thus, the MCTS approach is capable of revealing, at the highest level of detail, the spatial pattern of energy deposition in the medium. Another distinct advantage of MCTS simulations, that follows from their molecular-scale resolution, is the potential to go beyond the physical stage and simulate the subsequent radiolysis of water, which underlines the so-called indirect radiation action in cells, as well as the initial DNA damage along with its degree of complexity [2, 3, 4, 5]. Currently, there exist more than thirty MCTS codes (reviewedin [6; 7; 8]) which have been developed specifically for radiobiological applications, including micro- and nano-dosimetry. These codes differ mainly on the interaction cross sections used for simulating the transport of charged particles (mainly electrons) in a liquid water medium, although they may further differ regarding the details of the modeling of the physico-chemical reactions and biological damage, if available [9; 10; 11; 12; 13]. However, except for the Geant4-DNA toolkit [8; 14; 15; 16; 17], all currently available MCTS codes are not publicly available, they are the propriety of the investigator (or research group), which limits their usage by the broader community of researchers in the field. Therefore, as far as we know, there is no systematic intercomparison of MCTS codes (apart from [10] which is limited to only two codes). Because the amount of input data and computing time increases rapidly with particle energy, MCTS simulations are not currently used in clinical practice where energies are usually in the MeV (or MeV/amu) range. In clinical applications, MC simulations by the condensed-history approach are the preferred choice because they are much faster than MCTS simulations and are also easily extended to different transport media. However, conventional MC simulations using the condensed-history technique are not suitable for nanometer (or molecular) scale resolution, due to the inherent limitations of the condensed-history technique [8; 18]. Although MCTS simulations are traditionally applied to the transport of low-energy particles, e.g., electrons with energies below 10-100 keV, there is an increasing interest in extending their application to higher energies, in order to cover the full radiotherapeutic domain, including some emerging modalities like FLASH [19; 20; 21; 22; 23]. This interest mainly comes from the need to link the physical stage of radiation action (e.g., via micro- or nano-dosimetry) to the radiobiological effects and provide the ground for developing biophysical or mechanistically inspired dose-response models [24; 25]. Among other things, such models are potentially useful for the development of robust radiobiological-based optimization of treatment planning systems [26; 27]. Apart from conventional electron beam radiotherapy (EBRT), which uses electron beams in the 6-18 MV energy range [28], energetic electrons (\(\sim\)MeV) are also of interest for risk estimates from space radiation exposure of crewed missions [29; 30; 31; 32; 33; 34]. These include studies of the Van Allen outer belt, which consists of electrons up to about 10 MeV, as well as the energetic delta rays produced by the GeV/amu ions of the galactic cosmic ray (GCR) spectrum [35; 36; 37]. In addition, emerging treatment technologies, like FLASH radiotherapy, which use high energy electrons as a primary beam, require MCTS simulations up to several MeV, as a means to explore the radiobiological FLASH effect which relates to the spatial distribution of radiolytic species [38; 39; 40].