Ultra-High Dose Rate Helium Ion Beams: Minimizing Brain Tissue Damage while Preserving Tumor Control

Ultra-high dose rate radiotherapy with electrons and protons has shown potential for cancer treatment by effectively targeting tumors while sparing healthy tissues (FLASH effect). This study aimed to investigate the potential FLASH sparing effect of ultra-high-dose rate helium ion irradiation, focusing on acute brain injury and subcutaneous tumor response in a preclinical in vivo setting. Raster-scanned helium ion beams were used to compare the effects of standard dose rate (SDR at 0.2 Gy/s) and FLASH (at 141 Gy/s) radiotherapy on healthy brain tissue. Irradiation-induced brain injury was studied in C57BL/6 mice via DNA damage response, using nuclear γH2AX as a marker for double-strand breaks (DSB). The integrity of neurovascular and immune compartments was assessed through CD31 ⁺ microvascular density and activation of microglia/macrophages. Iba1+ ramified and CD68 ⁺ phagocytic microglia/macrophages were quantified, along with the expression of inducible nitric oxide synthetase (iNOS). Tumor response to SDR (0.2 Gy/s) and FLASH (250 Gy/s) radiotherapy was evaluated in A549 carcinoma model, using tumor volume and Kaplan-Meier survival as endpoints. The results showed that helium FLASH radiotherapy significantly reduced acute brain tissue injury compared to SDR, evidenced by lower levels of DSB and preserved neurovascular endothelium. Additionally, FLASH radiotherapy reduced neuroinflammatory signals compared to SDR, as indicated by fewer CD68+ iNOS+ microglia/macrophages. FLASH radiotherapy achieved tumor control comparable to that of SDR radiotherapy. This study is the first to report the FLASH sparing effect of raster scanning helium ion radiotherapy in vivo, highlighting its potential for neuroprotection and effective tumor control.