A novel 3D proton pencil beam scanning scheme and key physical design of the corresponding rapid cycling synchrotron for FLASH delivery

<i>Objective.</i>numerous preclinical and clinical experiments of proton FLASH radiotherapy have been conducted, however, the time requirement of approximately several hundred milliseconds has far limited the application of FLASH to large target volumes with three-dimensional (3D) pencil beam scanning (PBS). Another concern is dose control since the dose feedback system is not sufficiently rapid for FLASH delivery. To address these issues, we proposed a novel 3D proton PBS scheme that has the potential to realize FLASH delivery. The corresponding technical conceptual design is also presented.<i>Approach</i>. Our proposed scheme utilizes variable energy proton beam, as well as the corresponding rapid cycling synchrotron (RCS). The scanning layers are parallel to the beam direction in our scheme. We use square waves to manipulate beam in longitudinal phase space to separate bunch and use kicker to fast extract the separated bunch to irradiate a single scanning spot. The separated bunch generation process is repeated for each scanning spot and this process is completed inside the ring before extraction. This can form a feedforward to control the number of particles delivered to each scanning spot. Besides, the energy of the separated bunch extracted at different time is different, so we scan obliquely within the scanning layer.<i>Main results</i>. By integrating these techniques, arbitrary voxel irradiated by proton beam can receive FLASH dose rate on the temporal scale of hundreds of milliseconds, making FLASH delivery of proton PBS for a 1 l tumor feasible. The total irradiation time can be much greater than the order of hundreds of milliseconds since distant layers can be considered independent under the condition that scanning layers parallel to the beam.<i>Significance</i>. Proton FLASH radiation therapy has received considerable attention from the radiotherapy community in recent years. We provided a scheme that has the potential to solve the problem of irradiation time limit and dose feedback time limit in 3D PBS proton FLASH for large target volume (1 l), which to some extent may put proton FLASH delivery research forward.