2040 - Feasibility Study of Bragg Peak Proton FLASH Radiotherapy in Pediatric Brain Tumors
Presenter(s)
S. Hiltner1, K. Canikligil2, W. Vaughan3, B. A. Morris4, K. A. Bradley5, B. Y. Durkee5, H. Lin6, M. Kang6,7, S. Lazarev8, S. L. Wolden9, C. B. Simone II6, and C. Cheng5; 1Department of Physics, Rutgers University, Piscataway, NJ, 2Department of Cell Biology and Neuroscience, Rutgers University, Piscataway, NJ, 3Department of Biology, University of Wisconsin-Madison, Madison, WI, 4Department of Human Oncology, University of Wisconsin Hospitals and Clinics, Madison, WI, 5Department of Human Oncology, University of Wisconsin, Madison, WI, 6New York Proton Center, New York, NY, 7University of Wisconsin, Madison, WI, 8Icahn School of Medicine at Mount Sinai, New York, NY, 9Memorial Sloan Kettering Cancer Center, New York, NY
Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) and hypofractionation deliver higher biologically effective doses (BEDs) in fewer sessions, improving local control and emerging as a promising approach for pediatric patients. The ultra-high dose rates of FLASH therapy can offer an additional normal tissue-sparing effect, with the potential to improve organ-at-risk (OAR) protection. This study aims to investigate the dosimetric characteristics of Bragg peak FLASH treatment planning for hypofractionated pediatric brain tumor therapy, demonstrating its feasibility in treating pediatric cancer patients.
Materials/Methods: Seven consecutive pediatric patients with brain cancer with a single CTV who had previously received proton pencil beam scanning (PBS) treatment were included in this study. Bragg peak FLASH treatment planning utilized a single-energy layer and was optimized using an in-house treatment planning system (TPS) commissioned with a cyclotron FLASH beam model. Treatment plans were developed to deliver a hypofractionated regimen of 24 Gy in three fractions using the Bragg peak FLASH technique, with minimal MU constraints applied to balance plan quality and FLASH ratios. Conventional dose-rate PBS (CONV-PBS) plans, optimized with multiple-energy layers, served as the reference standard for comparison. Dosimetric parameters, including target coverage and OAR dose constraints, were evaluated. The voxel-based dose rate was calculated by considering PBS spot scanning and delivery time using the average dose rate metric. A dose rate histogram was generated to assess the FLASH ratio for critical OARs.
Results: Both FLASH and CONV-PBS plans were normalized so that 95% of the CTV received 24 Gy. The doses to major OARs, including the brain, brainstem, optic nerves, and optic chiasm, were comparable between CONV-PBS and Bragg peak proton FLASH plans (all p-values > 0.05). Specifically, the D0.03cc doses for the optic nerves, optic chiasm, and brainstem were 0.4 Gy, 0.9 Gy, and 22.1 Gy, respectively, for CONV-PBS plans, and 0.6 Gy, 1.1 Gy, and 22.8 Gy for Bragg peak plans. The mean dose and V12Gy for the brain were 2.1 Gy and 7.2% for CONV-PBS, and 2.5 Gy and 7.4% for Bragg peak plans. Using a minimum MU of 400, corresponding to a maximum beam current of 170 nA at the isocenter, the FLASH ratio for the brain and brainstem were 83.5% and 89.0%, respectively, with a field dose threshold of 2 Gy. When applying a field dose threshold of 5 Gy, the FLASH ratios increased to nearly 100%.
Conclusion: To our knowledge, this is a first dosimetric report of FLASH in pediatric patients. The Bragg peak FLASH technique achieved dosimetry comparable to CONV-PBS for pediatric brain cancers, maintaining high standards. It offers the potential for enhanced normal tissue protection, improved quality of life, and dose escalation for better tumor control. Further research is needed to fully understand its biological effects and clinical benefits.