A Novel Ultrahigh-Dose-Rate Proton Therapy Technology: Spot-Scanning Proton Arc Therapy + FLASH (SPLASH)

Purpose To take full advantage of FLASH dose rate(40Gy/s) and high dose conformity, we introduce a novel optimization and delivery technique, the spot-scanning proton arc therapy(SPArc)+FLASH (SPLASH). Materials and methods SPLASH framework was implemented in an open-source proton planning platform (MatRad, Department of Medical Physics in Radiation Oncology, German Cancer Research Center-DKFZ). It optimizes with (1) the clinical dose-volume constraint based on dose distribution and (2) the dose-average dose rate by minimizing the monitor uint (MU) constraint on spot weight and accelerator's beam current sequentially, enabling the first dynamic arc therapy with voxel-based FLASH dose rate. This new optimization framework is able to minimize the overall cost function value combined with plan quality and voxel-based dose rate constraints. Three representative cases(brain, liver and prostate cancer) were used for testing purposes. Dose-volume histogram (DVH), dose rate volume histogram (DRVH), and dose rate map were compared among intensity-modulated proton radiotherapy (IMPT), SPArc, and SPLASH. Results SPLASH/SPArc could offer superior plan quality than IMPT in terms of dose conformity. The DRVH results indicated SPLASH could significantly improve V40Gy/s in the target and region of interest(ROI) for all tested cases compared with SPArc and IMPT. The optimal beam current per spot is simultaneously generated, which is within the existing proton machine specifications in the research version(<200nA). Conclusion SPLASH offers the first voxel-based ultra-dose-rate and high-dose conformity treatment using proton beam therapy. Such a technique has the potential to fit the needs of a broad range of disease sites and simplify clinical workflow without applying a patient-specific ridge filter, which has never been demonstrated before.