# Transformative Technology for FLASH Radiation Therapy
## 7 High-Current Electron Linear Accelerator for X-ray FLASH-RT Developed by RadiaBeam and UCLA
### New Technology for X-ray FLASH-RT  
RadiaBeam and UCLA are working on a solution for X-ray FLASH therapy that takes advantage of a single linac based on already-demonstrated technology and an innovative yet straightforward method for intensity modulation [135]. The major innovation of the proposed project is the development of the rotational direct aperture optimization with a decoupled (ROAD) multi-leaf collimator (MLC) ring [136]. Intensity modulation has been the key driver in improved patient outcomes in RT over the past three decades, but there has been no solution to do this in the short time required by FLASH. With ROAD, the linac pulses are timed to align with a counter-rotating ring of 75 pre-shaped MLC apertures. As the linac and MLC rings rotate in opposite directions at 60 rpm, 150 modulated beams are delivered in 1 s, each delivering up to 0.7 Gy to the tumor. ROAD can achieve physical dose conformality superior to state-of-the-art VMAT plans free from the MLC mechanical limitation, yet with the added benefit of the FLASH effect. Figure 13 shows a model of the proposed ROAD-FLASH system [137]. The linac (see Table 4 for a summary of parameters) consists of a 1.3 A, 140 kV electron gun, prebuncher [138], and two traveling wave linac sections powered by a commercially available 20 MW L-band klystron with 167 us pulses at 150 Hz, to bring an 8.14 mA average current electron beam to 18 MeV. Assuming a dose conversion factor of 2000 Gy/min/mA at 18 MeV, such a linac will be able to provide an uncollimated dose rate of 271 Gy/s at 1 m (8.14 mA \(\times\) 2\(\cdot\)10\({}^{3}\)/60 Gy/s/mA), which is equivalent to a \(\sim\)100 Gy/s collimated dose at 80 cm, assuming a \(\sim\)25% dose delivery efficiency [139]. The beam is transported through  
Figure 13: Rendering of the ROAD FLASH-RT system. A segment of the decoupled MLC ring is shown in the figure inset with three MLC modules. The linac is triggered to produce the beam when the target is aligned with the MLC to produce a VMAT-like treatment. a rotary vacuum joint into a rotating magnetic gantry that brings the beam to a rotating X-ray target directed at the patient. Figure 14 shows the proposed FLASH delivery using the ROAD method. There are a total of 75 MLC modules on a ring that is separate from the X-ray source. The X-ray pulses are triggered when the source is sequentially aligned with the MLC apertures. Counter-rotating the MLC ring allows more aperture shapes to be programmed for a further improved physical dose conformity. \begin{table}
\begin{tabular}{l c c} \hline \hline
**System** & **ROAD** & **Conventional [18]** \\ \hline Energy [MeV] & 18 & 6 \\ Pulse length [us] & 167 & 4 \\ Rep rate [Hz] & 150 & 250 \\ Duty cycle & 2.5\% & 0.1\% \\ Injected current [A] & 1.3 & 0.5 \\ Transmission & 25\% & 25\% \\ Peak current [A] & 0.325 & 0.125 \\ Average current [mA] & 8.14 & 0.125 \\ Dose rate factor [Gy/min/mA at 1 m] & 2000 & 120 \\
**Dose rate, uncollimated, at 1 m** & **271** & **0.25** \\
**[Gy/s]** & 25\% & 25\% \\ Dose delivery efficiency & **106.0** & **0.10** \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters of ROAD high-current electron linear accelerator for X-ray FLASH therapy. Figure 14: FLASH-RT delivery using ROAD. The figure shows four beams with the corresponding pre-shaped MLC in the upper-left corner of respective beams. Cumulative radiation dose to a brain-tumor patient is shown in the center. With an increasing understanding of the underlying FLASH mechanism, it is necessary to further quantify the FLASH effect at the treatment-planning stage as part of the inverse optimization goal. The feasibility has been demonstrated for the simultaneous dose and dose rate optimization (SDDRO) with protons [140] and X-ray ROAD FLASH [136]. In the latter, the hypothesized oxygen depletion effect was parameterized into the planning system to show a larger FLASH effect for normal tissue sparing with larger individual pulses. The information will guide the design of high-output and FLASH-ready linear accelerators.