Purpose: To investigate the vital FLASH parameters and associated, needed accelerator and delivery system development for FLASH systems.
Methods: Based on existing FLASH data, different accelerator and ionizing radiation types, pertaining to the relevant Ultra High Dose Rate (UHDR) needs, were reviewed. Calculations were performed for the candidate technologies.
Results: Only a few beams reproduced the Flash Effect (FE) in vivo. Electron-based observational data indicate that it is likely that the t/DRave and D(pulse)/DR(pulse) are critical parameters for reducing the radiation toxicity in normal tissues. Also, irradiation times shorter than 0.5 s (DRave greater than several tens of Gy/s) and pulse doses greater than 0.2 Gy are required to achieve the FE. Proton DR(ave) values greater than 80 Gy/s are observed to elicit the FE. Pencil Beam Scanning delivery has been shown to produce the FE, but the role of scanning speed and spot size on the FE needs further investigation. These minimum FE-inducing requirements must be considered when obtaining or designing UHDR systems.
Conclusion: For proton FLASH, cyclotrons and synchro-cyclotrons are notably limited in machine output for beam energies below their maximum extraction energy, due to the need for energy absorbers. These absorbers not only reduce beam energy, but also beam current due to proton loss. Therefore, cyclotrons and synchro-cyclotrons may require placing the energy absorber proximal to the patient. Linear proton accelerators are attractive for conformal proton UHDR deliveries. Laser accelerators also show promise for proton UHDR, either as full energy accelerators or as linac injectors to provide high-output, stronger energy proton sources, reducing the space-charge limitation. Electron FLASH is attractive from a size and cost perspective, but current systems are limited to shallow targets. Transmission electron UHDR can also be considered using very high energy electron beams, most likely based on laser accelerators.
Funding Support, Disclosures, and Conflict of Interest: Jonathan Farr holds a senior management position at ADAM SA, Meyrin, Switzerland and is a shareholder in Advanced Oncotherapy (AVO), plc, London, UK.