B. Selvaraj1, X. Zhao1, H. Lin1,2, J. Shen3, C. Cheng4, A. Bookbinder1, H. Wu5, A. Zhai1, I. J. Choi1,6, A. M. Chhabra1, S. Hasan1, C. B. Simone II1,6, Y. Yamada6, and M. Kang1; 1New York Proton Center, New York, NY, 2Memorial Sloan Kettering Cancer Center, New York, NY, 3Department of Radiation Oncology, Mayo Clinic, Phoenix, AZ, 4Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, 5Department of Radiation Oncology, the Affiliated Cancer Hospital of Zhengzhou University & Henan Cancer Hospital, Zhengzhou, China, 6Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY
Purpose/Objective(s): The spinal cord is a very sensitive organ-at-risk (OAR) for quality-of-life-limiting radiation-induced myelopathies. It is challenging to treat a complex torus-shaped target containing the spinal cord, even with the most advanced cutting-edge proton pencil beam scanning (PBS). We hypothesize that a novel Bragg peak FLASH (BP-FLASH) radiotherapy (RT) can deliver conformal FLASH RT to spinal cancers using clinical 40Gy in 5 fraction stereotactic body radiation therapy (SBRT) regimen. Materials/
Methods: Ten SBRT cases of spinal metastasis (with 7 thoracic- and 3 cervical-spinal cord patients), previously treated with conventional multiple-energy intensity-modulated proton therapy (CONV-IMPT), were selected for this study. Clinical target volume (CTV) ranged from 22.1cc to 106.0cc. All patients were treated using 40Gy in 5-fraction SBRT regimen. An in-house FLASH platform used to optimize BP-FLASH plans, with similar beam arrangement to CONV-IMPT. The dose distributions and dose metrics for target coverage and critical OARs were compared between the two techniques. The dose rate of the BP-FLASH plans was computed based on the conservative method of average-dose-rate (ADR) and evaluated voxel-based dose rate coverage (V40Gy/s) for FLASH-sparing effect assessment with dose thresholds at 2, and 5Gy, respectively. Results: The 10 patients averaged doses and dose rates were compared between CONV-IMPT and BP-FLASH, using a t-test, revealing a statistically significant difference. In BP-FLASH, the CTV received a higher maximum dose compared to CONV-IMPT (115.1% versus 108.9%, p=0.001). No notable differences were observed in the Dmax for the spinal cord (27.21 ± 2 Gy vs. 26.2 ± 2 Gy, p=0.122) and esophagus (27.98± 5 Gy vs. 30.0± 5 Gy, p=0.327). Additionally, CONV-IMPT exhibited lower total lung V5 and V20 values compared to BP-FLASH, with respective percentages of 2.9% vs. 5.3% (p-value 0.041) and 1.4% vs. 2.3% (p-value 0.033). However, the metrics for both techniques stayed within safe thresholds. In the evaluation of FLASH dose rates, the V40Gy/s for all the OARs exceeded 80% with 2Gy dose threshold. When increased to 5Gy, V40Gy/s climbed over 95%. Conclusion: BP-FLASH plans are capable of delivering highly conformal doses to targets, effectively sparing the spinal cord, and achieving outcomes comparable to those of multi-energy CONV-IMPT. All OARs received high FLASH ratios at a dose threshold of 5Gy, suggesting enhanced FLASH-sparing capabilities. This innovative technique shows great promise in reducing side effects on normal tissues during spinal cancer SBRT treatments. Keywords: Stereotactic Body Radiation Therapy, Spine radiation therapy, Proton, FLASH radiotherapy
