2167 - Stereotactic Radiosurgery for Multiple Brain Metastases Using Proton Bragg Peak Conformal FLASH Therapy
Presenter(s)
N. Lynch1, C. Cheng1, X. Zhao2, Z. Wei3, H. Lin4, B. A. Morris5, B. Durkee1, C. B. Simone II4, and M. Kang4,6; 1Department of Human Oncology, University of Wisconsin, Madison, WI, 2Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD, 3Department of Mechanical Engineering, University of Wisconsin, Madison, WI, 4New York Proton Center, New York, NY, 5Department of Human Oncology, University of Wisconsin Hospitals and Clinics, Madison, WI, 6University of Wisconsin, Madison, WI
Purpose/Objective(s): Stereotactic radiosurgery (SRS) is a primary technique for treating brain metastases; however, it can be associated with loss of neurocognitive function and necrosis, especially when treating multiple lesions. FLASH radiotherapy has demonstrated superior normal tissue sparing, with emerging evidence indicating its potential for improved neurocognitive preservation. However, its clinical feasibility for treating multiple brain metastases has not, to date, been assessed. This study aims to establish a proof-of-concept for applying this novel modality in brain metastases treatment.
Materials/Methods: We conducted a retrospective study comparing standard intensity-modulated proton therapy (IMPT) with the single-energy Bragg peak (SEBP) FLASH method for delivering a single 18 Gy fraction in four patients. The SEBP plan used a single-energy layer from the cyclotron, along with a range shifter and range compensator, to achieve Bragg peak distal tracking and enable ultra-high dose rate (UHDR) delivery. By incorporating multi-field optimization and using the same beam arrangement as conventional multi-energy IMPT techniques, SEBP achieved highly conformal dose delivery to the target. A dose rate volume histogram (DRVH) was used to evaluate the UHDR ratio (V40Gy/s), assessing critical organs at risk (OARs), including normal brain tissue and the brainstem. Dosimetric parameters were compared between IMPT and SEBP FLASH to determine treatment feasibility and potential benefits.
Results: The 3D dose distribution was comparable between the two techniques, with the CTV maximum dose being 117% for IMPT and 115% for SEBP. The IMPT plan demonstrated slightly better dosimetric performance for OARs, with all metrics remaining within clinical constraints. The maximum doses to the brainstem were 11.4 Gy for IMPT and 13.9 Gy for SEBP, whereas the V12Gy for normal brain tissue was 3.2% for IMPT and 5.5% for SEBP. UHDR evaluation was performed on a per-field basis, as the gantry rotation time (>10 seconds) between fields was significantly longer than the total beam delivery time of ~100 ms. With a 2 Gy dose threshold, the UHDR ratio (V40Gy/s) for the brain and brainstem were 51% and 53%, respectively. Increasing the dose threshold to 5 Gy improved the V40Gy/s ratio to 70.9% and 71.0%, indicating a higher proportion of the high-dose region achieving FLASH dose rates.
Conclusion: SEBP FLASH delivery offers a highly conformal dose for targeting multiple brain metastases with SRS, with dosimetric parameters meeting clinical recommendations and achieving dose rates that can allow for a FLASH effect. This approach shows promise in balancing effective tumor control with enhanced normal tissue protection, potentially reducing toxicities.