Purpose: The flash dose rate on the order of 40 Gy/s has the potential to spare normal tissue with iso-effective tumor growth delay. Meanwhile, the spot-scanning proton arc therapy(SPArc), demonstrated its superior dosimetric conformity and plan robustness compared to the conventional intensity modulated proton therapy(IMPT). To take the full advantage of flash dose rate and the high dose conformity, we introduce a novel optimization and delivery technique, SPArc FLASH (SPLASH). Methods: SPLASH framework was implemented in an open-source proton planning platform (matRad). It optimizes the spot moniter unit(MU) weighting and cyclotron beam current per spot simultaneously. More specifically, it simultaneously optimized with the clinical dose-volume constraint based on dose distribution and optimized the dose-average dose rate by minimizing the monitor unit constraint on spot weight and maximum cyclotron beam current to minimize the cost function value combined by dose and dose rate. A intracranial target (volume = 20.8cc) case was used for testing purpose. Prescription is 50 Gy in 25 fractions. Dose-volume histogram, dose averaged dose rate histogram and dose rate map per spot were compared between SPArc and SPLASH. Results: The dose-averaged dose rate histogram indicated that flash dose rate (40Gy/s) could not be achieved using the original SPArc with the current clinical beam current configuration. With a same plan quality, SPLASH improved dose-averaged dose rate of Vr>40Gy/s in PTV, optic chiasm, optic nerve and brainstem from 0%, 0%, 0%, 0% in SPArc plan to 100%,100%,100%,75%, respectively. The optimal cyclotron beam current per spot is simultaneously generated. Conclusion: The SPLASH offers simultaneously optimizing dose distribution and dose rate. It could generate a treatment plan with a superior conformal dose distribution and ultra-high dose rate, which has never been demonstrated before.
Liu G, Zhao L, Li X, Zhang S, Dai S, Lu X, et al [Ding X] A novel ultra-high dose rate proton therapy technology: spot-scanning proton arc therapy FLASH (SPLASH). Med Phys. 2022 June;49(6):E521.