# Transformative Technology for FLASH Radiation Therapy
## 7 High-Current Electron Linear Accelerator for X-ray FLASH-RT Developed by RadiaBeam and UCLA
### Fast 3D High-Speed Beam Scanner for Hadron FLASH-RT  
Proton therapy, and hadron therapy in general, allows potentially far greater dose-shaping control than conventional photon therapy or VHEE through the energy-dependent Bragg peak, which determines the depth at which the peak dose will be delivered. Proton cyclotron facilities for cancer therapy routinely alter the proton beam energy used in treatment by passing the beam through a so-called range shifter, a physical barrier of material, typically plastic, which reduces the beam energy according to the thickness of the plate. While the strategy is a reliable and robust method for changing the beam energy, the process of switching between range-shifter settings is time-consuming, on the order of a second [175] when compared to the desired time scale of a total FLASH treatment that is a few hundred milliseconds, and also degrades the lateral penumbra of the beam. Synchrotron facilities, used for both proton and carbon cancer treatment, can change the beam energy by adjusting the acceleration cycling settings, avoiding the mechanical motion of a range shifter, but face challenges to achieving the FLASH dose rate. The demand for high-speed changes to the beam energy presents a tantalizing opportunity to apply accelerator technology in which RF-driven energy modulation could accomplish the same objective as the range shifter with changes on the sub-\(\upmu\)s scale. This research thrust has already gained traction in a program at the SLAC National Accelerator Laboratory to develop a 3D high-speed beam scanner for hadron therapy. The objectives of this project are to design and demonstrate the component technology needed to modulate the beam energy and transverse steering, sufficient to cover a 4 L volume at a FLASH dose rate. The energy modulator design builds on research concepts developed at SLAC for high-energy physics applications, taking the high gradient capability of a distributed coupling S-band structure and using it to reach a +/-30 MeV beam energy, equivalent to a range of 15 cm in treatment depth, in a one-meter structure [32]. SLAC's hadron-scanning program tackles not only the challenge of RF-driven energy modulation but also transverse steering. Unlike conventional photon-based radiotherapy, VHEE and proton therapy allow for pencil-beam scanning, which takes advantage of the Lorentz force to steer the charged particle trajectories. Thus far, transverse pencil-beam scanning for protons has been routinely accomplished using electromagnets, which allow the beam to cover a large treatment field on the order of 30 cm by 40 cm at the patient isocenter [175; 176]. This technique offers valuable flexibility in terms of coverage area with minimal beam distortion and, while not as fast as an RF-driven process, is compatible with the timescale of the FLASH dose delivery. Changes to the beam position can be accomplished on the scale of a few hundred microseconds [175]. Varian has already announced FLASH capability with their existing proton therapy equipment and has actively invested in FLASH therapy research through the FlashForward(tm) Consortium [177]. These developments suggest that proton therapy will likely be one of the leading modalities for FLASH treatment in the near future. The ultrafast 3D beam shaping for hadron therapy championed by the SLAC-led collaboration on RF-driven beam manipulation offers an opportunity for the accelerator technology of the high-energy physics community to revolutionize the speed and flexibility of proton therapy equipment. SLAC's proton deflector cavity, shown in Figure 19, is a prime example, taking inspiration from CERN's crab cavity research for beam steering [178] and optimizing a new cavity design for sub-relativistic protons [179]. The initial design for the SLAC 3D-scanning system utilizes a few-cell deflector structure, with cavities oriented orthogonally for the full range of transverse motion. The angular kick provided by the RF-driven deflector is enhanced by a set of static permanent magnet quadrupoles (PMQ).