FLASH Stereotactic radiosurgery for brain metastases using proton Bragg peak tracking can achieve IMPT equivalent dosimetry.

Abstract

Stereotactic radiosurgery (SRS) is a primary technique for treating brain metastasis, however it can be associated with loss of neurocognitive function and necrosis. FLASH radiotherapy has demonstrated superior normal tissue sparing, however its clinical feasibility for treating multiple brain metastases has not yet been assessed. This study aims to establish a proof of concept for applying this novel modality in brain metastases treatment. We implemented a single-energy Bragg peak (SEBP) tracking technique into an in house treatment planning platform. SEBP uses 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. Standard of care IMPT was compared with the SEBP-FLASH method for delivering a single 18 Gy(RBE) fraction in eight patients with multiple brain metastases. Dosimetric parameters were compared between IMPT and SEBP-FLASH and dose rate volume histograms (DRVH) were used to evaluate the UHDR ratio (V40Gy/s) to determine treatment feasibility and potential benefits. The 3D dose distributions were comparable between the two techniques, with each producing conformal dose distributions while remaining within clinical constraints. UHDR evaluation of SEBP-FLASH plans demonstrated that all critical organs at risk (OARs), including normal brain tissue and the brainstem achieved (>97%) FLASH dose rate coverage at a dose threshold to 5 Gy(RBE). SEBP-FLASH delivery provides conformal dose distributions suitable for stereotactic treatment of multiple brain metastases, with dosimetric parameters meeting clinical recommendations and achieving the dose rates required for the FLASH effect. This approach shows promise in balancing effective tumor control with enhanced normal tissue protection, potentially reducing toxicities.