Mechanisms of the FLASH effect: current insights and advances

Abstract

Radiotherapy is a fundamental tool in cancer treatment, utilized in over 60% of cancer patients during their treatment course. While conventional radiotherapy is effective, it has limitations, including prolonged treatment durations, which extend patient discomfort, and toxicity to surrounding healthy tissues. FLASH radiotherapy (FLASH-RT), an innovative approach using ultra-high-dose-rate irradiation, has shown potential in selectively sparing normal tissues while maintaining unaltered tumor control. However, the precise mechanisms underlying this "FLASH effect" remain unclear. This mini-review explores key hypotheses, including oxygen depletion, radical-radical interactions, mitochondrial preservation, differential DNA damage repair, and immune modulation. Oxygen levels significantly affect tissue response to radiation by promoting radical recombination, preserving mitochondrial function, and differentially activating DNA repair pathways in normal versus tumor tissues. However, the extent to which oxygen depletion contributes to the FLASH effect remains debated. Additionally, FLASH-RT may modulate the immune response, reducing inflammation and preserving immune cell function. To further enhance its therapeutic potential, FLASH-RT is increasingly being combined with complementary strategies such as radioprotectors, immunomodulators, and nanotechnology platforms. These combinations aim to amplify tumor control while further reducing normal tissue toxicity, potentially overcoming current limitations. Despite promising preclinical evidence, the exact mechanisms and clinical applicability of FLASH-RT require further investigation. Addressing these gaps is crucial for optimizing FLASH-RT and translating its potential into improved therapeutic outcomes for cancer patients. Continued research is essential to harness the full benefits of the FLASH effect, offering a paradigm shift in radiation oncology.

Keywords: cancer cells; cancer metabolism; cell death; flash; radiotherapy; ultra-high-dose rate irradiation.

FIGURE 1

Comparative mechanisms of CONV-RT and FLASH-RT in cancer versus healthy cells: schematic integration of experimental evidence and current hypotheses.The figure illustrates the impact of CONV and FLASH irradiation modalities on DNA repair and damage, mitochondrial injury, cell cycle arrest, senescence, autophagy, apoptosis, and immune responses in both healthy and cancer cells. Moreover, the scheme also highlights the potential for FLASH-RT to selectively target cancer cells while sparing healthy tissues, thereby reducing collateral damage and improving therapeutic outcomes. CONV-RT delivers radiation at a standard dose rate, typically in the range of a few Gy per minute, while FLASH-RT administers radiation at an UHDR, often exceeding 40 Gy per second. However, oxygen depletion alone may not fully account for the observed biological benefits. Additional mechanisms, such as ROS modulation, immune responses, and metabolic alterations, likely may play significant roles. The figure emphasizes the critical role of ROS in mediating the differential effects of FLASH-RT. Tumor cells, which typically exhibit elevated levels of endogenous ROS, experience oxidative damage due to radical accumulation. In contrast, normal tissues contain robust antioxidant reserves that rapidly neutralize ROS, reducing the formation of harmful peroxidized compounds, such as peroxyl radicals and organic peroxides. This protects normal cells from oxidative damage to proteins, lipids, and DNA. Following UHDR irradiation, normal cells exhibit lower ROS levels compared to CONV-RT, which helps preserve mitochondrial integrity, oxidative metabolism, and ATP production. As a result, cellular energy is maintained, and the release of cytochrome c, a key promoter of apoptotic cell death, is reduced, favoring the survival of healthy cells. In contrast, CONV-RT increases mtROS production, leading to mitochondrial damage, fission, and a heightened risk of apoptosis or necrosis. In tumors, UHDR irradiation induces mtROS accumulation, causing mitochondrial damage and enhancing the apoptotic response, ultimately aiding tumor control. Unrepaired DNA damage, particularly DSBs, may plays a crucial role in the cellular response to IR. The differential DNA repair mechanisms between tumor and normal cells, including the activation of the cGAS-STING pathway, may contribute to the FLASH effect. This pathway promotes immunogenic cell death and stimulates innate immune responses, potentially enhancing tumor immunogenicity while safeguarding normal tissues. In summary, both CONV-RT and FLASH-RT effectively damage cancer cells, but FLASH-RT may offer a more rapid and potent effect due to its UHDR. Importantly, FLASH-RT demonstrates a potential protective effect on healthy cells, minimizing radiation-induced damage compared to CONV-RT, which can cause significant harm to surrounding normal tissues.