Simulation study on the dosimetric characteristics of focused electron beams with different energies for FLASH radiotherapy

Objective
Focused high-energy electron beams are a promising option for FLASH radiotherapy of deep-seated tumors because beam focusing may improve dose concentration at depth while reducing unnecessary irradiation of surrounding normal tissues. This study aimed to investigate the dosimetric characteristics of focused electron beams for FLASH radiotherapy and to provide insight into electron beam energy selection.
Approach
An electron beam focusing system was developed, with the beam transport line optimized using the Transport code. The dosimetric performance of 1 GeV, 500 MeV, and 200 MeV electron beams passing through this system was systematically simulated in a water phantom using the Monte-Carlo code FLUKA. The dose distributions of beams with different energies were quantitatively compared under the same optical configuration. 
Main results
Among the investigated beam energies, the 1 GeV electron beam exhibited the strongest focusing capability and achieved the minimum full width at half maximum (FWHM) at the focal point, offering unique advantages for spatially fractionated radiotherapy (SFRT). The 500 MeV beam showed only a limited increase in focal size and FWHM compared with the 1 GeV beam, whereas the 200 MeV beam exhibited relatively poor focusing performance in deep regions. Consequently, the 500 MeV electron beam may represent a viable substitute for the 1 GeV beam in specific clinical or engineering environments, as it provides a favorable balance between dosimetric performance and implementation cost.. 
Significance
The systematic analysis of the dosimetric characteristics of electron beams at different energies provided in this study can serve as a valuable reference for related research and for the future clinical implementation of FLASH radiotherapy based on high energy electron beams.