Ultra-high dose-rate (FLASH) radiotherapy is a new mode of radiotherapy that can control tumors while significantly reducing damage to surrounding normal tissues. Since its official naming in 2014, FLASH radiotherapy research has continued to grow, and in 2019 it entered a period of exponential development. Studies have shown that electrons, photons, and protons can all induce the FLASH effect, but the proton beam can overcome the physical limitations of shallow incident depth and high penetration dose, which is considered to be the most effective solution. This article summarizes previous research results of FLASH proton therapy from the research at the cell, animal and clinical levels, and summarizes the hypothesis of the FLASH effect mechanism, aiming to provide reference for promoting the research and clinical transformation of FLASH proton therapy.

Objective:To compare the relative proportion of direct action (ray particles directly destroy biological molecules such as DNA and indirect action (radical-mediated oxidative damage) in the damage caused by X-ray and proton irradiation of lung cancer cells.Methods:Unirradiated human lung adenocarcinoma A549 cells and human large cell lung cancer NCI-H460 cells were cultured in media containing 0, 0.125, 0.25, 0.5, 0.75 mol/L dimethyl sulfoxide (DMSO) for 1 h to obtain plating efficiency (PE) values, thereby determining whether DMSO affected cell survival. Following pretreatment with each DMSO concentration, cells were exposed to X-ray irradiation at physical doses of 2, 4, 6, 8 Gy and proton irradiation at equivalent doses of 2, 4, 6, 8 GyE, respectively. Survival fractions (SF) and maximum protection (MP) values were calculated to evaluate the effects of varying DMSO concentrations on post-irradiation cell survival and to quantify the contribution of indirect radiation damage mechanisms (higher MP indicates greater indirect effect contribution). PE, SF, and MP values were determined using clonogenic assays. Comparisons among multiple groups were performed using one-way ANOVA followed by Tukey's multiple comparison, and comparisons between irradiation groups were analyzed using independent samples t-tests. Results:The PE of unirradiated cells treated with varying DMSO concentrations showed no statistically significant differences. Following pretreatment at different DMSO concentrations and subsequent irradiation with X-rays or protons, the protective effect of DMSO reached saturation at 0.5 mol/L. At this concentration, comparison of the average MP values across 4 radiation doses revealed: In A549 cells, the MP value was 54.21%±1.73% for X-ray irradiation group and 39.69%±0.72% for proton irradiation group ( t=16.82, P<0.001); in NCI-H460 cells, the MP value was 52.04%±1.00% for X-ray irradiation group and 41.31%±0.70% for proton irradiation group ( t=10.19, P<0.001). Conclusions:Under biologically equivalent doses, proton irradiation demonstrates greater reliance on direct effects in lung cancer cells killing compared with X-ray irradiation.

Ultra-high dose-rate (FLASH) radiotherapy is a new mode of radiotherapy that can control tumors while significantly reducing damage to surrounding normal tissues. Since its official naming in 2014, FLASH radiotherapy research has continued to grow, and in 2019 it entered a period of exponential development. Studies have shown that electrons, photons, and protons can all induce the FLASH effect, but the proton beam can overcome the physical limitations of shallow incident depth and high penetration dose, which is considered to be the most effective solution. This article summarizes previous research results of FLASH proton therapy from the research at the cell, animal and clinical levels, and summarizes the hypothesis of the FLASH effect mechanism, aiming to provide reference for promoting the research and clinical transformation of FLASH proton therapy.

Objective:To compare the relative proportion of direct action (ray particles directly destroy biological molecules such as DNA and indirect action (radical-mediated oxidative damage) in the damage caused by X-ray and proton irradiation of lung cancer cells.Methods:Unirradiated human lung adenocarcinoma A549 cells and human large cell lung cancer NCI-H460 cells were cultured in media containing 0, 0.125, 0.25, 0.5, 0.75 mol/L dimethyl sulfoxide (DMSO) for 1 h to obtain plating efficiency (PE) values, thereby determining whether DMSO affected cell survival. Following pretreatment with each DMSO concentration, cells were exposed to X-ray irradiation at physical doses of 2, 4, 6, 8 Gy and proton irradiation at equivalent doses of 2, 4, 6, 8 GyE, respectively. Survival fractions (SF) and maximum protection (MP) values were calculated to evaluate the effects of varying DMSO concentrations on post-irradiation cell survival and to quantify the contribution of indirect radiation damage mechanisms (higher MP indicates greater indirect effect contribution). PE, SF, and MP values were determined using clonogenic assays. Comparisons among multiple groups were performed using one-way ANOVA followed by Tukey's multiple comparison, and comparisons between irradiation groups were analyzed using independent samples t-tests. Results:The PE of unirradiated cells treated with varying DMSO concentrations showed no statistically significant differences. Following pretreatment at different DMSO concentrations and subsequent irradiation with X-rays or protons, the protective effect of DMSO reached saturation at 0.5 mol/L. At this concentration, comparison of the average MP values across 4 radiation doses revealed: In A549 cells, the MP value was 54.21%±1.73% for X-ray irradiation group and 39.69%±0.72% for proton irradiation group ( t=16.82, P<0.001); in NCI-H460 cells, the MP value was 52.04%±1.00% for X-ray irradiation group and 41.31%±0.70% for proton irradiation group ( t=10.19, P<0.001). Conclusions:Under biologically equivalent doses, proton irradiation demonstrates greater reliance on direct effects in lung cancer cells killing compared with X-ray irradiation.