Fig. 1

Generation of reactive oxygen species (ROS) in response to ionizing radiation. Ionizing radiation induces damage of cellular structures in two primary ways: direct damage to DNA and generation of free radical-containing reactive molecules. Free radicals are generated through the interactions between ionizing radiation and small oxygen containing molecules (including water). These interactions commonly form hydroxide and generate free electrons. Free electrons can then interact with intracellular oxygen to form superoxide. Free radicals that are generated by ionizing radiation can react with DNA, lipid membranes, and proteins causing damage and/or dysfunction to various cellular structures. The cell has mechanisms designed to mitigate and manage damage from free radicals. Hydroxide ions are reduced by the enzyme glutathione peroxidase and superoxide ions are reduced to hydrogen peroxide by superoxide dismutase. Hydrogen peroxide generated by superoxide dismutase is used by catalase to generate water. Significant damage to cellular structures occurs when ionizing radiation-induced generation of radicals out-paces the cell’s ability to clear these reactive molecules