# Transformative Technology for FLASH Radiation Therapy
## 7 High-Current Electron Linear Accelerator for X-ray FLASH-RT Developed by RadiaBeam and UCLA  
### X-ray FLASH-RT  
An attractive tool for delivering FLASH-RT could be a FLASH-capable X-ray system. More than 80% of all radiotherapy is delivered with X-rays [130]. They are the most versatile form of radiation therapy and the most cost-effective. Unfortunately, the physical process for generating X-rays is not very efficient [131]; therefore, a high-power accelerator is needed for X-ray FLASH-RT [132]. Furthermore, one would still like to achieve as much conformality as possible. Conformality, combined with the healthy-tissue-sparing FLASH effect, promises to dramatically improve patient outcomes [5].  
Considering the inevitable reduction in effective dose rate with the intensity modulation and transmission through small apertures, a linac that can deliver 100 Gy in one second or faster is challenging but not impossible. Conventional 6 MV medical linacs produce a flattening filter-free dose rate of around 0.2 Gy/s at one meter from the X-ray target--three orders of magnitude too low. However, they are on the low end of the spectrum of linac powers [133]. A typical medical linac has a beam power on the order of 1 kW. In comparison, industrial accelerators for sterilizing food and medical products can achieve beam powers of several hundred kW [134].  
Figure 12: Measurement geometry, bremsstrahlung field flatness, and pulse-to-pulse repeatability [121]. ([PERSON]; et al., Sci. Rep. 2021, 11, 17104, [[https://doi.org/10.3389/fonc.2019.01563](https://doi.org/10.3389/fonc.2019.01563)]([https://doi.org/10.3389/fonc.2019.01563](https://doi.org/10.3389/fonc.2019.01563)) (accessed on 9 September 2022), reproduced with permission under the terms of the Creative Commons Attribution License (CC BY), [[http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)]([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/))) (accessed on 9 September 2022).  
Another factor that allows for the improvement in the dose rate is increasing the beam energy. The conversion efficiency from electron beam power to X-ray power scales approximately with E\({}^{3}\), so a small increase in energy can make a big difference in X-ray intensity. The higher X-ray energy also allows greater penetration. However, there are two major downsides to higher photon energies: larger lateral penumbra (a measure of the fuzziness at the edge of the beam) and greater neutron production (which causes activation and an unwanted dose to the patient and the environment). Photon energies up to 20 MV are commonly used in RT. We consider 10-18 MV to be optimal for achieving a high dose rate while limiting the negative factors.  
One could also consider reducing the distance from the source to the target. However, this can only be done to a certain point without sacrificing useability. Achieving good conformality requires placing one or more collimators between the beam source and the patient. Along with the pure physical limitations on fitting the equipment around the patient, this limits the source-to-surface distance (SSD) to 80 cm at the smallest.