A novel treatment planning method for proton FLASH radiotherapy: Combining FLASH with VMAT

In this work, we propose a new treatment planning method for multi-beam FLASH radiotherapy (Stitching FLASH) that combines FLASH delivered by proton beams, with volumetric modulated arc therapy (VMAT). The Stitching FLASH method requires the target to be divided into different sections. For sections close to an Organ-at-risk (OAR), proton FLASH beams were used to deliver radiation dose. For the remaining target volume, VMAT was used to achieve optimal target coverage. To validate the Stitching FLASH method, we used matRad, a treatment planning toolkit developed by DKFZ.

We applied it first, as proof of concept, to irradiate three spherical phantoms surrounded by several OARs and second, to irradiate a cervical spinal cord tumor, which we then compared to the clinical VMAT plan. In our phantoms study, we managed to fulfil the minimum dose and dose-rate conditions for protecting the OARs with FLASH, while obtaining excellent target coverage and superb dose conformity and homogeneity.

For the spinal cord tumor irradiation, we were able to significantly increase the Clinical Target Volume (CTV) coverage and minimum dose, while keeping the doses in the OARs below the clinical thresholds using the FLASH protecting effect. With the novel FLASH treatment planning method proposed in this work, we have demonstrated that it is possible to protect critical OARs, while maintaining excellent target coverage and dose homogeneity, as well as conformity. This shows that the Stitching FLASH method is promising for proton FLASH radiotherapy