Novel Bragg peak FLASH radiotherapy: treatment of base-of-skull tumors within existing clinical guidelines.

BACKGROUND AND PURPOSE: Proton therapy is commonly delivered for base-of-skull (BOS) tumors to minimize risk of normal-tissue injury. Novel Bragg peak FLASH may improve protection for organs at risk (OARs). We investigated if proton pencil-beam scanning (PBS) Bragg peak FLASH could achieve comparable dosimetry at ultra-high dose rates to conventional-rate (CONV)-intensity-modulated proton therapy (IMPT) for potential clinical use. MATERIALS AND METHODS: PBS Bragg peak FLASH and CONV-IMPT plans were optimized for 10 consecutive patients with BOS tumor in an in-house treatment-planning system and hypofractionation regimen (30 GyE in five fractions) to assess dosimetric quality while reflecting clinical practice, parameters, and hypothetical fraction doses for FLASH effect. FLASH dose-rate coverage (V 40Gy/s) and dose-rate volume histograms (DRVHs) were quantified across OARs. FLASH (≥40 Gy/s) was achieved with a minimum MU per spot of 300-500 and a minimum spot time (MST) of 0.5 ms. FLASH dose-rate coverage, OAR sparing, and target coverage were averaged across all cases. RESULTS: FLASH generated higher CTV D max than CONV-IMPT (114% vs. 108%). Dose metrics for OARs were comparable between modalities (p > 0.05). Average dose rate (ADR) DRVHs of OARs indicated 74.1% V 40Gy/s FLASH dose-rate coverage without dose threshold, and 94.4% V 40Gy/s FLASH dose-rate coverage with 1-Gy dose threshold. CONCLUSION: PBS Bragg peak FLASH can deliver conformal target dosimetric coverage, ultra-high dose rates, and comparable OAR dosimetry to CONV-IMPT, suggesting that this novel technique is feasible for BOS tumor FLASH radiotherapy. The study supports future research to explore FLASH's biological and clinical benefits in BOS tumors and OAR sparing.