Comparison of Biological Effects of γ-Radiation of Low and Ultra-High Dose Rate on Lymphocytes and Cultured Human Malignant Lymphoma Cells

The influence of beam parameters on FLASH effect

Ultra-high dose rate radiotherapy (FLASH-RT) is typically defined as an external beam radiotherapy that utilizes a dose rate of 40 Gy/s or higher, compared with conventional dose rate radiotherapy (≤0.1 Gy/s). The primary advantage of FLASH-RT lies in its ability to minimize damage to organs at risk surrounding the cancer while preserving the anti-tumor effect. This phenomenon, known as the FLASH effect, has been widely studied in various bodily systems. However, recent publication of negative research findings related to FLASH-RT warrant a reassessment of whether this definition is accurate. Therefore, this review aims to critically examine how various beam parameters impact the manifestation of the FLASH effect. Following extensive literature review, we propose that an average dose rate of 40 Gy/s to be the lowest dose that triggers the FLASH effect. Beyond this threshold, different organs, including the brain, lungs, intestine, and skin, required varying minimum single total doses to trigger FLASH effects, with a trend of enhanced FLASH-RT protective effects as the single total doses increased. Moreover, single or multiple pulses and the characteristic parameters of the pulse structure, including single pulse dosage, pulse width, pulse interval, pulse frequency, and total irradiation time, were found to also impact the FLASH effect.

Key changes in the future clinical application of ultra-high dose rate radiotherapy

Ultra-high dose rate radiotherapy (FLASH-RT) is an external beam radiotherapy strategy that uses an extremely high dose rate (≥40 Gy/s). Compared with conventional dose rate radiotherapy (≤0.1 Gy/s), the main advantage of FLASH-RT is that it can reduce damage of organs at risk surrounding the cancer and retain the anti-tumor effect. An important feature of FLASH-RT is that an extremely high dose rate leads to an extremely short treatment time; therefore, in clinical applications, the steps of radiotherapy may need to be adjusted. In this review, we discuss the selection of indications, simulations, target delineation, selection of radiotherapy technologies, and treatment plan evaluation for FLASH-RT to provide a theoretical basis for future research.

Mechanisms of FLASH effect

FLASH radiotherapy (FLASH-RT) is a novel radiotherapy technology defined as ultra-high dose rate (≥ 40 Gy/s) radiotherapy. The biological effects of FLASH-RT include two aspects: first, compared with conventional dose rate radiotherapy, FLASH-RT can reduce radiation-induced damage in healthy tissue, and second, FLASH-RT can retain antitumor effectiveness. Current research shows that mechanisms of the biological effects of FLASH-RT are related to oxygen. However, due to the short time of FLASH-RT, evidences related to the mechanisms are indirect, and the exact mechanisms of the biological effects of FLASH-RT are not completely clear and some are even contradictory. This review focuses on the mechanisms of the biological effects of FLASH-RT and proposes future research directions.

Association of Cancer Stem Cell Radio-Resistance Under Ultra-High Dose Rate FLASH Irradiation With Lysosome-Mediated Autophagy

Cancer stem cell (CSC) is thought to be the major cause of radio-resistance and relapse post radiotherapy (RT). Recently ultra-high dose rate “FLASH-RT” evokes great interest for its decreasing normal tissue damages while maintaining tumor responses compared with conventional dose rate RT. However, the killing effect and mechanism of FLASH irradiation (FLASH-IR) on CSC and normal cancer cell are still unclear. Presently the radiation induced death profile of CSC and normal cancer cell were studied. Cells were irradiated with FLASH-IR (∼10⁹ Gy/s) at the dose of 6–9 Gy via laser-accelerated nanosecond particles. Then the ratio of apoptosis, pyroptosis and necrosis were determined. The results showed that FLASH-IR can induce apoptosis, pyroptosis and necrosis in both CSC and normal cancer cell with different ratios. And CSC was more resistant to radiation than normal cancer cell under FLASH-IR. Further experiments tracing lysosome and autophagy showed that CSCs had higher levels of lysosome and autophagy. Taken together, our results suggested that the radio-resistance of CSC may associate with the increase of lysosome-mediated autophagy, and the decrease of apoptosis, necrosis and pyroptosis. To our limited knowledge, this is the first report shedding light on the killing effects and death pathways of CSC and normal cancer cell under FLASH-IR. By clarifying the death pathways of CSC and normal cancer cell under FLASH-IR, it may help us improve the understanding of the radio-resistance of CSC and thus help to optimize the future clinical FLASH treatment plan.