Dynamic Range Modulation via Integrated Universal Range Shifter and Ridge Filter: Enabling Simplified Delivery for FLASH-Enabled IMPT

Objective: In proton FLASH therapy, using a transmission beam with hypofractionated doses can minimize energy layer switching interruptions to preserve FLASH effect. However, intensity-modulated proton therapy (IMPT) struggles to achieve high FLASH dose rates. This study proposes a novel system that combines universal range shifters (URS) and universal ridge filters (URF) to enable FLASH effect in IMPT. Approach: Water equivalent thickness (WET) optimization was first performed to determine the URS thickness relative to tumor depth, followed by URF modulation to broaden the monoenergetic Bragg peak for longitudinal target coverage. Plan optimization incorporated a spot sparsification algorithm. The proposed system using 218 MeV beams was evaluated in three anatomical sites (brain, liver, and lung tumors) for 10 cases each. Furthermore, this study evaluated the impact of hardware switching speeds and two delivery strategies: (1) bi-URS, constraining URS transitions to binary states, and (2) bi-URF, restricting URF modulation to binary configurations. Main Results: The bi-URS and TB plans achieved mean D98% of 98.30% ± 0.80% versus 98.60% ± 0.90% in brain cases, and 98.40% ± 0.50% versus 97.20% ± 1.00% in liver cases, respectively. Compared to bi-URF, the bi-URS modality significantly reduced delivery time (e.g., brain: 0.305 ± 0.09 s vs. 1.031 ± 0.096 s, p < 0.001). And it demonstrated superior target conformity index (CI) (brain: 0.660 ± 0.020 and liver: 0.653 ± 0.022) compared to TB plans (brain: 0.357 ± 0.082, p < 0.001 and liver: 0.479 ± 0.100, p = 0.012) while maintaining high FLASH average dose rate (ADR) coverage (V40Gy/s of 87.60 ± 15.96% for brain and 91.21 ± 14.42% for liver cases) with URF 20-ms switching speed. Furthermore, it effectively spared the Dmax of hippocampus (bi-URS: 19.00 ± 5.80 Gy, TB: 37.71 ± 17.71 Gy, p=0.041) in brain cases and the residual liver V5 Gy in liver cases (bi-URS: 27.79 ± 10.9%, TB: 68.64 ± 18.72% , p=0.002). However, in lung cases, the CI of the bi-URS modality, while superior to that of the TB modality, remained below the clinical expectation of 0.6. Significance: The combined URS and URF delivery strategy (bi-URS) improved dose conformity of the target, and maintained high FLASH effect coverage in brain and liver cases, demonstrating its potential as a Bragg-peak PBS approach for FLASH radiotherapy.