Extracellular DNA-mediated Differential Bystander Effects Induced by FLASH or Conventional Proton Radiation

A cutting-edge advancement known as FLASH radiotherapy, administered at an ultra-high dose rate of ≥40 Gy/s, has garnered considerable attention for its ability to spare normal tissues while retaining its efficacy in targeting tumors. However, the lower toxicity in normal tissues does not unequivocally guarantee the absence of potential effects on bystander tissues. In this study, normal human lung epithelial BEAS-2B and HSAEC1-KT cells were subjected to conventional (100 MeV/u, 1.7 Gy/min) and FLASH (100 MeV/u, 40 Gy/s) proton irradiation. We found that the conditioned culture medium and extracellular DNA (ecDNA) from conventional proton irradiation demonstrate higher efficacy in prompting bystander cell damage, reflected in increased γH2AX foci, reduced cell viability, and heightened apoptotic fractions. Additionally, ecDNA predominantly activated NF-κB signaling pathways in bystander cells and promoted the production of inflammatory factors and reactive oxygen species (ROS). These findings provide evidence that FLASH irradiation may exhibit a reduced impact on damaging bystander cells, contrasting with conventional irradiation, which induces comparatively higher levels of damage in these bystander cells.