Recent advancement in the radiotherapy technology has allowed conformal delivery of high doses of ionizing radiation precisely to the tumors while sparing large volume of the normal tissues, which have led to better clinical responses. Despite this technological advancement many advanced tumors often recur and they do so within the previously irradiated regions. How could tumors recur after receiving such high ablative doses of radiation? In this review, we outlined how radiation can elicit anti-tumor responses by introducing some of the cytokines that can be induced by ionizing radiation. We then discuss how tumor hypoxia, a major limiting factor responsible for failure of radiotherapy, may also negatively impact the anti-tumor responses. In addition, we highlight how there may be other populations of immune cells including regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs) that can be recruited to tumors interfering with the anti-tumor immunity. Finally, the impact of irradiation on tumor hypoxia and the immune responses according to different radiotherapy regimen is also delineated. It is indeed an exciting time to see that radiotherapy is being combined with immunotherapy in the clinic and we hope that this review can add an excitement to the field.
Anoxia ; Cytokines ; Hope ; Immune System ; Immunotherapy ; Macrophages ; Radiation, Ionizing ; Radiotherapy ; T-Lymphocytes, Regulatory

Tumor hypoxia, a common feature occurring in nearly all human solid tumors is a major contributing factor for failures of anticancer therapies. Because ionizing radiation depends heavily on the presence of molecular oxygen to produce cytotoxic effect, the negative impact of tumor hypoxia had long been recognized. In this review, we will highlight some of the past attempts to overcome tumor hypoxia including hypoxic radiosensitizers and hypoxia-selective cytotoxin. Although they were (still are) a very clever idea, they lacked clinical efficacy largely because of ‘reoxygenation’ phenomenon occurring in the conventional low dose hyperfractionation radiotherapy prevented proper activation of these compounds. Recent meta-analysis and imaging studies do however indicate that there may be a significant clinical benefit in lowering the locoregional failures by using these compounds. Latest technological advancement in radiotherapy has allowed to deliver high doses of radiation conformally to the tumor volume. Although this technology has brought superb clinical responses for many types of cancer, recent modeling studies have predicted that tumor hypoxia is even more serious because ‘reoxygenation’ is low thereby leaving a large portion of hypoxic tumor cells behind. Wouldn’t it be then reasonable to combine hypoxic radiosensitizers and/or hypoxia-selective cytotoxin with the latest radiotherapy? We will provide some preclinical and clinical evidence to support this idea hoping to revamp an enthusiasm for hypoxic radiosensitizers or hypoxia-selective cytotoxins as an adjunct therapy for radiotherapy.
Anoxia* ; Cytotoxins ; Hope ; Humans ; Oxygen ; Radiation, Ionizing ; Radiotherapy* ; Treatment Outcome ; Tumor Burden

FLASH radiotherapy (FLASH RT) is a technique to deliver ultra-high dose rate in a fraction of a second. Evidence from experimental animal models suggest that FLASH RT spares various normal tissues including the lung, gastrointestinal track, and brain from radiation-induced toxicity (a phenomenon known as FLASH effect), which is otherwise commonly observed with conventional dose rate RT. However, it is not simply the ultra-high dose rate alone that brings the FLASH effect. Multiple parameters such as instantaneous dose rate, pulse size, pulse repetition frequency, and the total duration of exposure all need to be carefully optimized simultaneously. Furthermore it is critical to validate FLASH effects in an in vivo experimental model system. The exact molecular mechanism responsible for this FLASH effect is not yet understood although a number of hypotheses have been proposed including oxygen depletion and less reactive oxygen species (ROS) production by FLASH RT, and enhanced ability of normal tissues to handle ROS and labile iron pool compared to tumors. In this review, we briefly overview the process of ionization event and history of radiotherapy and fractionation of ionizing radiation. We also highlight some of the latest FLASH RT reviews and results with a special interest to neurocognitive protection in rodent model with whole brain irradiation. Lastly we discuss some of the issues remain to be answered with FLASH RT including undefined molecular mechanism, lack of standardized parameters, low penetration depth for electron beam, and tumor hypoxia still being a major hurdle for local control. Nevertheless, researchers are close to having all answers to the issues that we have raised, hence we believe that advancement of FLASH RT will be made more quickly than one can anticipate.