Optimized Pencil-Beam Scanning Enables Flash-Compatible Whole-Brain Proton Therapy at Clinical Fractionation
Abstract
Purpose
To systematically evaluate if optimized spot-delivery patterns in pencil-beam scanning can reach FLASH-compatible dose rates (>40Gy/s) at clinically relevant fractions of 4Gy and to identify key machine and planning parameters for whole-brain irradiation (WBI).
Methods
A representative 4Gyx5 WBI transmission proton plan on a Varian ProBeam system was utilized to evaluate modified scanning strategies that (1) segment the target into multiple longitudinal sections, (2) deliver contiguous blocks of spots in a circular pattern within each section, and (3) combine the two strategies with ROI prioritization to enhance dose rate in critical structures (brainstem). A standard zig-zag pattern served as a baseline. Per fraction volumetric average dose rate (ADR, d*=10%) was calculated in MatRad for different numbers of sections, block sizes, spot times (1–2ms), spacing (5–7mm), and scanning speeds (10–3mm/ms). Results were generalized on three spherical targets of different dimensions.
Results
For WBI, optimized sectioned scanning with optimal delivery parameters (1ms spot time, 10mm/ms scanning speed, 7mm spacing) increases the mean ADR from 62Gy/s (zig-zag) to 169Gy/s. ROI prioritization enhanced brainstem ADR from 48Gy/s (zig-zag) to 220Gy/s while maintaining whole-brain ADR of 155Gy/s. The baseline plan with conventional delivery parameters has a mean ADR of 17Gy/s. Best delivery parameters were transferable across target sizes. Sensitivity analysis of delivery parameters identified spot spacing as dominant for WBI, followed by scanning velocity and spot time. Section count emerged as the primary optimization lever (2.1x improvement), with geometry-dependent optima (2, 6, 30, and 36 for the three spheres and WBI, respectively), indicating that ROI prioritization required fewer sections, whereas block size 1 and optimal spacing consistently yielded the best results across all geometries.
Conclusion
This clinically feasible approach enables FLASH-compatible dose rates across large brain volumes while maintaining conventional fractionation, potentially reducing neurotoxicity associated with WBI.