Proton Bragg peak flash stereotactic radiosurgery for recurrent glioblastoma reirradiation.

BACKGROUND AND PURPOSE: Stereotactic Radiosurgery with robotic image-guided systems is promising for recurrent glioblastoma (rGBM), but long delivery times may increase morbidity and reduce accuracy. Proton single-energy Bragg-peak FLASH-RT (SEBP-FLASH) may address these limits by delivering each field in <1 s, with potential for normal-tissue sparing through the FLASH effect. MATERIALS AND METHODS: The SEBP-FLASH technique was used to reoptimize treatment for rGBM patients treated with robotic image-guided systems. Patient-specific aperture devices were incorporated into the SEBP-FLASH technique for sharper penumbra. The gross tumor volume (GTV) coverage, conformity index (CI), dose gradient index (GI50 and GI30), and dose-volume histogram (DVHs), were evaluated. Additionally, ultra-high dose-rate ratios in critical organs-at-risk were assessed. RESULTS: SEBP-FLASH successfully met all clinical objectives. Both SEBP-FLASH and robotic techniques showed comparable performance, with GI50 and GI30 values of 2.9 ± 0.5 and 6.2 ± 1.8 for SEBP-FLASH, versus 2.7 ± 0.2 and 5.5 ± 0.5 for robotic (p-values at 0.085 and 0.189). SEBP-FLASH demonstrated superior conformality, achieving a CI of 1.4 ± 0.2 compared to CK's 1.9 ± 0.3 (p-value 0.001). All other metrics were similar across both techniques. For the brain, the FLASH dose rate coverage (V40 Gy/s) exceeded 80% under a 5 Gy dose threshold and minimum monitor unit of 400. CONCLUSIONS: This study evaluated the dose-volume characteristics of SEBP-FLASH and robotic image-guided systems and found broadly comparable dose distributions, suggesting that SEBP-FLASH may be a feasible option for rGBM reirradiation. SEBP-FLASH also showed potential advantages in conformity, normal tissue sparing, and reduced on-table time.