FLASH Stereotactic Body Radiation Therapy for Spine Tumors Using a Single-Energy Proton Pristine Bragg Peak Delivery Technique

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Purpose To investigate the dosimetric performance and dose rate of Bragg peak FLASH (BP-FLASH) for spinal cord stereotactic body radiation therapy (SBRT). Methods and Materials Ten consecutive patients with spinal tumors treated with conventional intensity modulated proton therapy (CONV-IMPT) SBRT (40 Gy in 5 fractions) were selected for this study. These patients were reoptimized using an in-house FLASH algorithm and a single-energy Bragg peak approach. The dose distributions and dose metrics for target coverage and critical organs-at-risk (OARs) were compared. BP-FLASH plans dose rates were calculated using an average-dose-rate. The FLASH ratios (V 40Gy/s ) were assessed with dose thresholds at 0.2, 2, and 5 Gy. The doses and dose rates for the 10 patients were averaged, and a t test was performed comparing CONV-IMPT and BP-FLASH. Results Dosimetric analysis revealed that the BP-FLASH plans deliver a similar dose as CONV-IMPT plans to critical OARs. However, in BP-FLASH, the clinical target volume received a higher maximum dose than CONV-IMPT (115.1% versus 108.9%, P = .001). No notable differences were observed in the maximum doses to the spinal cord ( P = .122) or esophagus ( P = .327). FLASH dose rates for all the OARs exceeded 80% with 2 Gy dose threshold. When increased to 5 Gy, V 40Gy/s increased to >95% for composite plan doses. Conclusions BP-FLASH SBRT is a promising treatment for challenging spinal cord cancers, which achieved ultra-high-dose rates for FLASH effect and maintained the same dosimetry quality as the CONV-IMPT plans.

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