Distinct Metabolomic and Proteomic Signatures of Early Brain Damage Triggered by the Entrance Plateau Versus the Spread-Out Bragg Peak of Proton Radiation

Proton therapy spares normal tissues better than photon therapy, potentially reducing toxicity while maintaining tumor control. However, challenges remain due to variations in proton dose distribution, particularly at the distal edge of the spread-out Bragg peak (SOBP); for organs at risk, such variation is crucial. We evaluated the biological effects by comparing two positions of the proton profile, the entrance plateau (EP) and SOBP, in a murine model and investigated distinct metabolomic and proteomic signatures. Mice exposed to the EP beam segment (LETd = 0.8 keV/µm) exhibited less weight reduction than their SOBP-irradiated counterparts (LETd = 2.6 keV/µm). Two hours post-irradiation, the SOBP caused more severe DNA damage in the hippocampus and thalamus. Hematoxylin and eosin staining revealed eosinophil aggregation in both groups, with more surviving neurons in the EP group. Metabolomic profiles differed more between the EP and SOBP groups at 2 h than at 3 days post-irradiation. Relative to EP, SOBP irradiation at 2 h increased fructose-1,6-bisphosphate (FBP), dihydroxyacetone phosphate (DHAP), and inosine but decreased prostaglandin F2α; subsequently, proteomic analysis at day 3 showed that calcium signaling, NF-κB, and endocytosis pathways were enriched in the SOBP group. Combined multi-omics analysis further demonstrated that SOBP irradiation significantly activated the pentose phosphate pathway, purine metabolism, and phospholipase D signaling, while concurrently suppressing arachidonic acid metabolism. Our findings underscore the need for early detection of proton-induced brain toxicity and demonstrate that the higher-LET SOBP segment causes more severe damage than the EP. Targeting these dysregulated multi-omics pathways may offer a promising strategy for mitigating radiation-induced brain injury during proton therapy.