# Characterization of Ultra-High-Dose Rate Electron Beams with ElectronFlash [PERSON]
## 1 Introduction  
FLASH radiotherapy (FLASH-RT) is a promising cancer irradiation treatment, currently under investigation, which reduces the toxicity on the healthy tissues while keeping the same efficiency in the tumor cure as in conventional treatments.  
In the early 1960s, some experiments demonstrated that a very-high-dose rate in ultra-short radiation pulses increased cell survival, that is, it spares healthy tissues, compared toconventional (CONV) treatments [1; 2; 3; 4]. After three decades, this phenomenon was again investigated and confirmed by [PERSON] and their team [5]. In FLASH RT, the entire dose is delivered in a very short time. The definition of the FLASH regime demands the specification of different inter-dependent temporal parameters such as pulse repetition frequency (PRF), pulse number (\(n_{p}\)), pulse width (\(t_{p}\)), dose per pulse (\(D_{p}\)), in-peak dose rate (\(\widetilde{D}_{p}\)), average dose rate (\(\widetilde{D}\)), and the total irradiation time (\(t_{i}\)).  
Typically, the FLASH effect was observed with an irradiation time of \(t_{i}<100\) ms [6], an average dose rate \(\widetilde{D}>100\) Gy/s [7], an in-peak dose-rate of \(D_{p}>10^{6}\), and a pulse repetition frequency PRF \(>100\) Hz [8; 9]. It must be noted that these are not threshold values, and to the best of our current knowledge, there is no well-defined limit for the instantaneous dose rate, but only an empirical demonstration that sparing normal tissue can actually be achieved. In addition, the FLASH electron pulse usually provides a dose \(>1\) Gy, which is three orders of magnitude greater than a conventional electron pulse (<1 mGy/pulse). These parameters are shown in Figure 1.  
After [PERSON]'s experiments, many multidisciplinary teams published articles that confirmed the beneficial FLASH effect in healthy tissues [10]. Even though many studies have been carried out in recent years, the mechanisms underlying the FLASH effect remain largely unknown.  
In order to investigate the FLASH effect, several efforts were made to develop new devices able to irradiate in an ultra-high-dose rate regime, such as the Oriatron eRT6 [11] and the Mobetron [12].  
Furthermore, some clinical linacs were modified and successfully employed to deliver electron beams with an average dose rate of approximately 5000 Gy/s, so as to be compatible with the threshold values which define the FLASH effect (>40 Gy/s). However, because of their dosimetric and geometric properties, they are only suitable for small-animal experiments [13; 14; 15].  
A new linac dedicated to FLASH irradiation was installed at the Institut Curie in Orsay (France) to replace the Kinetron [16] previously used by [PERSON]'s team. The accelerating structure, described in some detail in Section 2.1, is called ElectronFlash (EF) [17; 18]. It operates at 5 and 7 MeV, and it can irradiate in conventional and FLASH modalities by changing the pulse width, the pulse repetition frequency, and the number of pulses. The flexibility of the machine allows the investigation of the FLASH effect on small animals or cell samples, with uniform dose profiles. The characterization of the electron beam in terms of doses was necessary to calibrate the data obtained in the radiobiological and pre-clinical experiments performed at Institut Curie.  
Figure 1: Typical time structure of electron beam parameters.