# Transformative Technology for FLASH Radiation Therapy
## 7 High-Current Electron Linear Accelerator for X-ray FLASH-RT Developed by RadiaBeam and UCLA
### Very-High-Energy Electron (VHEE) Flash-RT  
Very-high-energy electron (VHEE) radiotherapy is a key area of opportunity to apply technology developed for the particle physics community to the new field of FLASH-RT. Direct use of an electron beam for radiotherapy provides one of the most readily scalable paths to achieve FLASH dose rates, as apparent in the existing body of experimental evidence for FLASH that has been predominantly performed with direct electron beams [144; 145; 146; 147]. Existing facilities have been modified for FLASH radiotherapy experiments [148; 137; 146] and commercial systems such as the Oriatron eRT6 from PMB-Alcen [149], Kinetron from CGRMeV [148], Novac7 from Sordina [150], and the Mobetron from IntraOp [151] have been employed for FLASH capability at sub-10 MeV electron energies. While clinical and pre-clinical commercial devices proceed with development to provide ultra-high-dose-rate direct electron therapy [152; 153; 154; 155], treatment of superficial tumors in human patients has already begun [2]. The key technological advance where active research is needed concerns the development of medical accelerators that can reach the high beam energies required for the treatment of deep-seated tumors throughout the body. The electron beam energy determines the penetration depth, with a 100 MeV beam reaching a depth of about 40 cm, sufficient to cover almost all deep-seated tumors [156; 157]. VHEE therapy has yet to be realized in a clinical setting, because the existing equipment lacks the capability to reach these beam energies. The maximum energy obtained for direct electron FLASH-RT thus far has been only 20 MeV, using a modified Varian Clinac [144; 145]. The size and power requirements to simply extend this structure to produce 100 MeV beams would be prohibitive for clinical use. The high beam energies used for VHEE treatments will also impose additional shielding requirements on the treatment facility. Treatment-planning studies using Monte Carlo simulations indicate that a dose rate of approximately \(2\times 10^{4}\) Gy/s can be achieved per milliamp of average beam current over a 10 cm by 10 cm field size. Current RF-driven linear accelerator research programs aimed at meeting the demand for VHEE capability have primarily focused on advanced, normal-conducting, high-gradient accelerator RF technology. A CHUV-CERN collaboration to build the deep electron FLASH therapy (DEFT) facility plans to combine an X-band linac design developed through CLIC research with an S-band photoinjector [158]. Designs for a VHEE system at the PRAE accelerator utilize an S-band linac in addition to an S-band photoinjector, prioritizing linac performance reliability [159]. The implementation of a photoinjector with a medical linac has yet to be demonstrated, but the strategy of utilizing a photoinjector has already been proposed as an opportunity to harness the speed and flexibility of laser-based beam-shaping techniques [141]. While the proposed DEFT facility will occupy a length of around ten meters, an R&D effort currently underway at the SLAC National Accelerator Laboratory seeks to reach an even more compact VHEE system using a cryogenic X-band accelerator to achieve VHEE beam energies in only one meter. This approach harnesses the enhanced efficiency and higher gradient obtainable in a distributed coupling linac, combined with the improved scaling at cryogenic temperatures [142] to reach the target gradient of 100 MeV/ m, already demonstrated in previous experiments at SLAC using a cryogenic X-band structure at the X-band test area (XTA), see Figure 16[143]. A key aspect of the proposed VHEE system is the reliance on a commercial RF source, limiting the initial peak power to roughly 6 MW, but ensuring the final product can be widely adopted for commercial use.