Control of cancer by irradiation therapy alone or in conjunction with combination chemotherapy is often limited by organ specific toxicity. Ionizing irradiation toxicity is initiated by damage to normal tissue near the tumor target and within the transit volume of radiotherapy beams. Irradiation-induced cellular, tissue, and organ damage is mediated by acute effects, which can be dose limiting. A latent period follows recovery from the acute reaction, then chronic irradiation fibrosis (late effects) pose a second cause of organ failure. We have developed the technology for radioprotective gene therapy using the transgene for the antioxidant manganese superoxide dismutase, delivered to specific target organs (lung, esophagus, oral cavity, oropharynx, and bladder) using gene transfer vectors including plasmid/liposomes (PL) and adenovirus. Irradiation protection by MnSOD transgene overexpression at the cellular level has been demonstrated to be localized to the mitochondrial membrane. Using MnSOD transgene constructs lacking the mitochondrial localization leader sequence, and in other experiments attaching this localization signal to otherwise non-radioprotective cytoplasmic Cu/ZnSOD, mitochondrial localization has been demonstrated to be critical to protection. Organ specific injection of MnSOD-PL prior to irradiation demonstrates transgene expression for 48-72 hours, and an associated decrease in ionizing irradiation-induced expression of inflammatory cytokine mRNA and protein. Significant reduction of organ specific tissue injury has been demonstrated in several organ systems in rodent models. Application of MnSOD-PL gene therapy in the setting of fractionated chemo-radiotherapy is being tested in clinical trials for prevention of esophagitis during treatment of non-small cell carcinoma of the lung, and in prevention of mucositis during combination therapy of carcinomas of the head and neck. Encouraging results in pre-clinical models suggest that radioprotective gene therapy may facilitate dose escalation protocols to allow increases in the therapeutic ratio of cancer radiotherapy.