DNA strand break induction of aqueous plasmid DNA exposed to 30 MeV protons at ultra-high dose rate

Effect of FLASH proton therapy on primary bronchial epithelial cell organoids

Purpose

The effects of conventional (CONV) and FLASH proton therapy on primary bronchial epithelial cell (PBEC) organoids from individuals with chronic obstructive pulmonary disease (COPD) were investigated. The primary objective was to compare the effect of FLASH and CONV on COPD PBEC organoids with a focus on DNA damage, organoid formation, and gene expression.

Methods

PBECs were obtained from six COPD donors, cultured as three-dimensional (3D) organoids and exposed to 2 and 8 Gy CONV and FLASH proton radiation at the Holland Proton Therapy Center. DNA damage was assessed by γH2AX staining. Organoid formation capacity was assessed by counting the organoids formed after reseeding irradiated cells at 24 h and 7 days. Bulk RNA sequencing (RNAseq) and qPCR analyses were performed to identify pathways and differences in the radiation response.

Results

γH2AX foci analysis showed a significant dose-dependent increase in DNA damage at 1 h for both CONV and FLASH treatments, without differences between the two modalities. Organoid formation assays revealed a dose-dependent decrease in organoid formation capacity at 24 h for both treatments. At 7 days, 2 Gy FLASH-treated samples showed significantly reduced organoid formation compared to 2 Gy CONV (p = 0.008). RNAseq identified CONV and FLASH-induced changes in expression of DNA-damage response and apoptosis pathway genes. A dose-dependent upregulation of MDM2, GDF15, DDB2, BAX, P21, AEN and a decrease in MKi67 expression was confirmed by qPCR analysis.

Conclusion

No significant differences were found in DNA damage or gene expression profiles between CONV and FLASH. The organoid formation assay showed a prolonged detrimental effect in the FLASH-treated organoids, suggesting a more complex interaction of FLASH with lung epithelial cells. The results of this study contribute to the advancement of robust in vitro human lung models for investigating the mechanisms of action of FLASH, potentially facilitating the treatment of NSCLC patients with proton FLASH therapy.

Impact of Ultra-High-Dose-Rate Irradiation on DNA: Single-Strand Breaks and Base Damage

Studying different types of DNA damage induced by ultra-high-dose-rate (UHDR) irradiation is essential for understanding the mechanism underlying the FLASH effect. pBR322 plasmid DNA was irradiated using an electron FLASH beam. The content of each subtype of plasmid DNA was measured via gel electrophoresis, and the extent of DNA double-strand breaks (DSBs) and single-strand breaks (SSBs) under UHDR and conventional-dose-rate irradiation (CONV) was quantitatively compared. Furthermore, by adding the endonucleases Nth and Fpg, the extent of base damage in the UHDR and CONV group was quantitatively analyzed. In addition, the effects of different plasmid concentrations on the damage degree were studied. The induction rates of SSBs (×10-3 SSB/Gy/molecule) under UHDR and CONV were 21.7 ± 0.4 and 25.8 ± 0.3, respectively. When treated with the Fpg and Nth enzymes, the base damage induction rates (×10-3 SSB/Gy/molecule) under UHDR and CONV irradiation were 43.3 ± 2.0 and 58.4 ± 4.5, respectively. Additionally, UHDR irradiation consistently reduced SSBs and base damage at both high and low plasmid concentrations, although the absolute level of DNA damage was still influenced by the plasmid concentration. UHDR has a significant effect on reducing SSBs and base damage when compared to CONV across plasmid concentrations.

Demonstration of ultra-high dose rate electron irradiation at FLASHlab@PITZ

Objective.The photo injector test facility at DESY in Zeuthen (PITZ) is building up an R&D platform, known as FLASHlab@PITZ, for systematically studying the FLASH effect in cancer treatment with its high-brightness electron beams, which can provide a uniquely large dose parameter range for radiation experiments. In this paper, we demonstrate the capabilities by experiments with a reduced parameter range on a startup beamline and study the potential performance of the full beamline by simulations.Approach.To measure the dose, Gafchromic films are installed both in front of and after the samples; Monte Carlo simulations are conducted to predict the dose distribution during beam preparation and help understand the dose distribution inside the sample. Plasmid DNA is irradiated under various doses at conventional and ultra-high dose rate (UHDR) to study the DNA damage by radiations. Start-to-end simulations are performed to verify the performance of the full beamline.Main results.On the startup beamline, reproducible irradiation has been established with optimized electron beams and the delivered dose distributions have been measured with Gafchromic films and compared to FLUKA simulations. The functionality of this setup has been further demonstrated in biochemical experiments at conventional dose rate of 0.05 Gy s-1and UHDR of several 105 Gy s-1and a varying dose up to 60 Gy, with the UHDR experiments finished within a single RF pulse (less than 1 millisecond); the observed conformation yields of the irradiated plasmid DNA revealed its dose-dependent radiation damage. The upgrade to the full FLASHlab@PITZ beamline is justified by simulations with homogeneous radiation fields generated by both pencil beam scanning and scattering beams.Significance.With the demonstration of UHDR irradiation and the simulated performance of the new beamline, FLASHlab@PITZ will serve as a powerful platform for studying the FLASH effects in cancer treatment.

FLASH radiotherapy: mechanisms, nanotherapeutic strategy and future development

Ultra-high dose-rate (FLASH) radiotherapy serves as an ideal procedure to treat tumors efficiently without harming normal tissues and has demonstrated satisfactory antitumor effects in multiple animal tumor models. However, the biological mechanisms of FLASH radiotherapy have not yet been fully elucidated, and the small number of devices delivering FLASH dose rate has limited its wide application. This review summarizes the possible biological mechanisms and antitumor effects of FLASH radiotherapy, its application in nanotherapeutic strategy, as well as its challenges and future development. Furthermore, some valuable guidance for promoting the progress of FLASH radiotherapy in nanotherapeutic strategies are provided.

The oxygen puzzle in FLASH radiotherapy: A comprehensive review and experimental outlook

FLASH radiotherapy is attracting increasing interest because it maintains tumor control while inflicting less damage to normal tissues compared to conventional radiotherapy. This sparing effect, the so-called FLASH effect, is achieved when radiation is delivered at ultra-high dose rates (≥40 Gy/s). Although the FLASH effect has already been demonstrated in several preclinical models, a complete mechanistic description explaining why tumors and normal tissues respond differently is still missing. None of the current hypotheses fully explains the experimental evidence. A common point between many of these is the role of oxygen, which is described as a major factor, either through transient hypoxia in the form of dissolved molecules, or reactive oxygen species (ROS). Therefore, this review focuses on both forms of this molecule, retracing old and more recent theories, while proposing new mechanisms that could provide a complete description of the FLASH effect based on preclinical and experimental evidence. In addition, this manuscript describes a set of experiments designed to provide the FLASH community with new tools for exploring the post-irradiation fate of ROS and their potential biological implications.

Current views on mechanisms of the FLASH effect in cancer radiotherapy

FLASH radiotherapy (FLASH-RT) is a new modality of radiotherapy that delivers doses with ultra-high dose rates. The FLASH effect was defined as the ability of FLASH-RT to suppress tumor growth while sparing normal tissues. Although the FLASH effect has been proven to be valid in various models by different modalities of irradiation and clinical trials of FLASH-RT have achieved promising initial success, the exact underlying mechanism is still unclear. This article summarizes mainstream hypotheses of the FLASH effect at physicochemical and biological levels, including oxygen depletion and free radical reactions, nuclear and mitochondria damage, as well as immune response. These hypotheses contribute reasonable explanations to the FLASH effect and are interconnected according to the chronological order of the organism's response to ionizing radiation. By collating the existing consensus, evidence and hypotheses, this article provides a comprehensive overview of potential mechanisms of the FLASH effect and practical guidance for future investigation in the field of FLASH-RT.

VHEE FLASH sparing effect measured at CLEAR, CERN with DNA damage of pBR322 plasmid as a biological endpoint

Ultra-high dose rate (UHDR) irradiation has been shown to have a sparing effect on healthy tissue, an effect known as 'FLASH'. This effect has been studied across several radiation modalities, including photons, protons and clinical energy electrons, however, very little data is available for the effect of FLASH with Very High Energy Electrons (VHEE). pBR322 plasmid DNA was used as a biological model to measure DNA damage in response to Very High Energy Electron (VHEE) irradiation at conventional (0.08 Gy/s), intermediate (96 Gy/s) and ultra-high dose rates (UHDR, (2 × 109 Gy/s) at the CERN Linear Electron Accelerator (CLEAR) user facility. UHDRs were used to determine if the biological FLASH effect could be measured in the plasmid model, within a hydroxyl scavenging environment. Two different concentrations of the hydroxyl radical scavenger Tris were used in the plasmid environment to alter the proportions of indirect damage, and to replicate a cellular scavenging capacity. Indirect damage refers to the interaction of ionising radiation with molecules and species to generate reactive species which can then attack DNA. UHDR irradiated plasmid was shown to have significantly reduced amounts of damage in comparison to conventionally irradiated, where single strand breaks (SSBs) was used as the biological endpoint. This was the case for both hydroxyl scavenging capacities. A reduced electron energy within the VHEE range was also determined to increase the DNA damage to pBR322 plasmid. Results indicate that the pBR322 plasmid model can be successfully used to explore and test the effect of UHDR regimes on DNA damage. This is the first study to report FLASH sparing with VHEE, with induced damage to pBR322 plasmid DNA as the biological endpoint. UHDR irradiated plasmid had reduced amounts of DNA single-strand breaks (SSBs) in comparison with conventional dose rates. The magnitude of the FLASH sparing was a 27% reduction in SSB frequency in a 10 mM Tris environment and a 16% reduction in a 100 mM Tris environment.

FLASH Radiotherapy: Mechanisms of Biological Effects and the Therapeutic Potential in Cancer

Radiotherapy is an important treatment for many unresectable advanced malignant tumors, and radiotherapy-associated inflammatory reactions to radiation and other toxic side effects are significant reasons which reduce the quality of life and survival of patients. FLASH-radiotherapy (FLASH-RT), a prominent topic in recent radiation therapy research, is an ultra-high dose rate treatment known for significantly reducing therapy time while effectively targeting tumors. This approach minimizes radiation side effects on at-risk organs and maximally protects surrounding healthy tissues. Despite decades of preclinical exploration and some notable achievements, the mechanisms behind FLASH effects remain debated. Standardization is still required for the type of FLASH-RT rays and dose patterns. This review addresses the current state of FLASH-RT research, summarizing the biological mechanisms behind the FLASH effect. Additionally, it examines the impact of FLASH-RT on immune cells, cytokines, and the tumor immune microenvironment. Lastly, this review will discuss beam characteristics, potential clinical applications, and the relevance and applicability of FLASH-RT in treating advanced cancers.

FLASH radiotherapy: A new milestone in the field of cancer radiotherapy

Radiotherapy plays a pivotal role in the control and eradication of tumors, but it can also induce radiation injury to surrounding normal tissues while targeting tumor cells. In recent years, FLASH-Radiotherapy (FLASH-RT) has emerged as a cutting-edge research focus in the field of radiation therapy. By delivering high radiation doses to the treatment target in an ultra-short time, FLASH-RT produces the FLASH effect, which reduces the toxicity to normal tissues while achieving comparable tumor control efficacy to conventional radiotherapy. This review provides a brief overview of the development history of FLASH-RT and its impact on tumor control. Additionally, it focuses on introducing the protective effects and molecular mechanisms of this technology on various normal tissues, as well as exploring its synergistic effects when combined with other tumor therapies. Importantly, this review discusses the challenges faced in translating FLASH-RT into clinical practice and outlines its promising future applications.

Dose Rate Effects on Hydrated Electrons, Hydrogen Peroxide, and a OH Radical Molecular Probe Under Clinical Energy Protons

We report the dose rate dependence of radiation chemical yields (G value) of water radiolysis products under clinical energy protons (230 MeV) to understand mechanisms of the FLASH radiotherapy performed at ultra-high dose rate (>40 Gy/s). The G value of 7-hydoroxy-coumarin-3-carboxylic acid (7OH-C3CA) produced by reactions of coumarin-3-carboxylic acid (C3CA) with OH radicals and oxygen is evaluated by fluorescence method. Also, those of hydrated electrons and hydrogen peroxide are derived by absorption method using Saltzman and Ghomley techniques, respectively. Both G values of 7OH-C3CA and hydrated electrons decrease with increasing dose rate. The relative evolution of 7OH-C3CA is -39 ± 2% between 0.1 and 50 Gy/s. This value is higher than that of hydrated electrons, measured at -21 ± 4%. The G value of hydrogen peroxide in ultra-pure water also decreases with increasing dose rate. In comparison to these findings, we represent the increase of the G value of hydrogen peroxide with increasing dose rate in the mixture solution of MeOH and NaNO3, which act as scavengers of OH radicals and hydrated electrons, respectively, that decompose hydrogen peroxide. This finding indicates that a complex track structure can be expected with increasing dose rate and the reduction of OH radicals by forming hydrogen peroxide would be related to the sparing effect of healthy tissues.

Possible mechanisms and simulation modeling of FLASH radiotherapy

FLASH radiotherapy (FLASH-RT) has great potential to improve patient outcomes. It delivers radiation doses at an ultra-high dose rate (UHDR: ≥ 40 Gy/s) in a single instant or a few pulses. Much higher irradiation doses can be administered to tumors with FLASH-RT than with conventional dose rate (0.01-0.40 Gy/s) radiotherapy. UHDR irradiation can suppress toxicity in normal tissues while sustaining antitumor efficiency, which is referred to as the FLASH effect. However, the mechanisms underlying the effects of the FLASH remain unclear. To clarify these mechanisms, the development of simulation models that can contribute to treatment planning for FLASH-RT is still underway. Previous studies indicated that transient oxygen depletion or augmented reactions between secondary reactive species produced by irradiation may be involved in this process. To discuss the possible mechanisms of the FLASH effect and its clinical potential, we summarized the physicochemical, chemical, and biological perspectives as well as the development of simulation modeling for FLASH-RT.

FLASH Radiotherapy: Expectations, Challenges, and Current Knowledge

Major strides have been made in the development of FLASH radiotherapy (FLASH RT) in the last ten years, but there are still many obstacles to overcome for transfer to the clinic to become a reality. Although preclinical and first-in-human clinical evidence suggests that ultra-high dose rates (UHDRs) induce a sparing effect in normal tissue without modifying the therapeutic effect on the tumor, successful clinical translation of FLASH-RT depends on a better understanding of the biological mechanisms underpinning the sparing effect. Suitable in vitro studies are required to fully understand the radiobiological mechanisms associated with UHDRs. From a technical point of view, it is also crucial to develop optimal technologies in terms of beam irradiation parameters for producing FLASH conditions. This review provides an overview of the research progress of FLASH RT and discusses the potential challenges to be faced before its clinical application. We critically summarize the preclinical evidence and in vitro studies on DNA damage following UHDR irradiation. We also highlight the ongoing developments of technologies for delivering FLASH-compliant beams, with a focus on laser-driven plasma accelerators suitable for performing basic radiobiological research on the UHDR effects.

Examining the Occurrence of the FLASH Effect in Animal Models: A Systematic Review and Meta-Analysis of Ultra-High Dose Rate Proton or Carbon Ion Irradiation

Purpose: This systematic review and meta-analysis sought to assess whether ultra-high dose rate (UHDR) ion irradiations can induce the FLASH effect in animal models. Methods: A comprehensive search of the Web of Science, PubMed, and EMBASE databases was conducted from inception until March 20, 2023, to identify studies involving irradiated animals subjected to proton or carbon ion beams at varying dose rates. The research content should include various indicators that can reflect the effect and safety of radiation, such as survival, normal tissue toxicity, inflammatory response, tumor volume, etc Results: Compared to conventional dose rate (CONV) ion irradiations, UHDR ion irradiations can significantly improve mouse survival (HR 0.48, 95% CI 0.29 to 0.78, I2 = 0%) and maintain comparable tumor control. There was no significant impact of different dose rates on the survival of zebrafish embryos (SMD 0.11, 95% CI -0.31 to 0.53, I2 = 85%). Subgroup analysis showed that radiation dose was an important factor affecting the survival of zebrafish embryos. Achieving normal tissue sparing may require higher radiation dose under UHD.In mouse and zebrafish embryo models, normal tissue sparing did not always occur after UHDR ion irradiations. In addition, only a limited number of cytokines (CXCL1, IL-6, GM-CSF, G-CSF, HMGB1, and TGF-β) and immune cells (microglia and myeloid cells) showed differences at different dose rates. Conclusions: UHDR ion irradiation can achieve FLASH effect, but the reproducibility of normal tissue sparing remains a challenge. Compared to CONV irradiation, UHDR ion irradiations demonstrated equivalent or even superior tumor control.

Modeling for predicting survival fraction of cells after ultra-high dose rate irradiation

Objective. FLASH radiotherapy (FLASH-RT) with ultra-high dose rate (UHDR) irradiation (i.e. > 40 Gy s-1) spares the function of normal tissues while preserving antitumor efficacy, known as the FLASH effect. The biological effects after conventional dose rate-radiotherapy (CONV-RT) with ≤0.1 Gy s-1have been well modeled by considering microdosimetry and DNA repair processes, meanwhile modeling of radiosensitivities under UHDR irradiation is insufficient. Here, we developed anintegrated microdosimetric-kinetic(IMK)model for UHDR-irradiationenabling the prediction of surviving fraction after UHDR irradiation.Approach.TheIMK model for UHDR-irradiationconsiders the initial DNA damage yields by the modification of indirect effects under UHDR compared to CONV dose rate. The developed model is based on the linear-quadratic (LQ) nature with the dose and dose square coefficients, considering the reduction of DNA damage yields as a function of dose rate.Main results.The estimate by the developed model could successfully reproduce thein vitroexperimental dose-response curve for various cell line types and dose rates.Significance.The developed model would be useful for predicting the biological effects under the UHDR irradiation.

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.

Induction of DNA strand breaks and oxidative base damages in plasmid DNA by ultra-high dose rate proton irradiation

PURPOSE

Radiation cancer therapy with ultra-high dose rate (UHDR) exposure, so-called FLASH radiotherapy, appears to reduce normal tissue damage without compromising tumor response to therapy. The aim of this study was to clarify whether a 59.5 MeV proton beam at an UHDR of 48.6 Gy/s could effectively reduce the DNA damage of pBR322 plasmid DNA in solution compared to the conventional dose rate (CONV) of 0.057 Gy/s.

MATERIALS AND METHODS

A simple system, consisting of pBR322 plasmid DNA in 1× Tris-EDTA buffer, was initially employed for proton beam exposure. We then used formamidopyrimidine-DNA glycosylase (Fpg) enzymes. which convert oxidative base damages of oxidized purines to DNA strand breaks, to quantify DNA single strand breaks (SSBs) and double strand breaks (DSBs) by agarose gel electrophoresis.

RESULTS

Our findings showed that the SSB induction rate (SSB per plasmid DNA/Gy) at UHDR and the induction of Fpg enzyme sensitive sites (ESS) were significantly reduced in UHDR compared to CONV. However, there was no significant difference in DSB induction and non-DSB cluster damages.

CONCLUSIONS

UHDR of a 59.5 MeV proton beam could reduce non-clustered, non-DSB damages, such as SSB and sparsely distributed ESS. However, this effect may not be significant in reducing lethal DNA damage that becomes apparent only in acute radiation effects of mammalian cells and in vivo studies.

Nanoscopic biodosimetry using plasmid DNA in radiotherapy with metallic nanoparticles

Nanoscopic lesions (complex damages), are the most lethal lesions for the cells. As nanoparticles have become increasingly popular in radiation therapy and the importance of analyzing nanoscopic dose enhancement has increased, a reliable tool for nanodosimetry has become indispensable. In this regard, the DNA plasmid is a widely used tool as a nanodosimetry probe in radiobiology and nano-radiosensitization studies. This approach is helpful for unraveling the radiosensitization role of nanoparticles in terms of physical and physicochemical effects and for quantifying radiation-induced biological damage. This review discusses the potential of using plasmid DNA assays for assessing the relative effects of nano-radiosensitizers, which can provide a theoretical basis for the development of nanoscopic biodosimetry and nanoparticle-based radiotherapy.

Clustered DNA Damage Patterns after Proton Therapy Beam Irradiation Using Plasmid DNA

Modeling ionizing radiation interaction with biological matter is a major scientific challenge, especially for protons that are nowadays widely used in cancer treatment. That presupposes a sound understanding of the mechanisms that take place from the early events of the induction of DNA damage. Herein, we present results of irradiation-induced complex DNA damage measurements using plasmid pBR322 along a typical Proton Treatment Plan at the MedAustron proton and carbon beam therapy facility (energy 137-198 MeV and Linear Energy Transfer (LET) range 1-9 keV/μm), by means of Agarose Gel Electrophoresis and DNA fragmentation using Atomic Force Microscopy (AFM). The induction rate Mbp^-1 Gy^-1 for each type of damage, single strand breaks (SSBs), double-strand breaks (DSBs), base lesions and non-DSB clusters was measured after irradiations in solutions with varying scavenging capacity containing 2-amino-2-(hydroxymethyl)propane-1,3-diol (Tris) and coumarin-3-carboxylic acid (C3CA) as scavengers. Our combined results reveal the determining role of LET and Reactive Oxygen Species (ROS) in DNA fragmentation. Furthermore, AFM used to measure apparent DNA lengths provided us with insights into the role of increasing LET in the induction of highly complex DNA damage.

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.

Radiation Chemical Yields of 7-Hydroxy-Coumarin-3-Carboxylic Acid for Proton- and Carbon-Ion Beams at Ultra-High Dose Rates: Potential Roles in FLASH Effects

It has been observed that healthy tissues are spared at ultra-high dose rate (UHDR: >40 Gy/s), so called FLASH effect. To elucidate the mechanism of FLASH effect, we evaluate changes in radiation chemical yield (G value) of 7-hydroxy-coumarin-3-carboxylic acid (7OH-C3CA), which is formed by the reaction of hydroxyl radicals with coumarin-3-carboxylic acid (C3CA), under carbon ions (140 MeV/u) and protons (27.5 and 55 MeV) in a wide-dose-rate range up to 100 Gy/s. The relative G value, which is the G value at each dose rate normalized by that at the conventional dose (CONV: 0.1 Gy/s >), 140 MeV/u carbon-ion beam is almost equivalent to 27.5 and 55 MeV proton beams. This finding implies that UHDR irradiations using carbon-ion beams have a potential to spare healthy tissues. Furthermore, we evaluate the G value of 7OH-C3CA under the de-oxygenated condition to investigate roles of oxygen to the generation of 7OH-C3CA effect. The G value of 7OH-C3CA under the de-oxygenated condition is lower than that under the oxygenated condition. The G value of 7OH-C3CA under the de-oxygenated condition is higher than those under UHDR irradiations. By direct measurements of the oxygen concentration during 55 MeV proton irradiations, the oxygen concentration drops by 0.1%/Gy, which is independent of the dose rate. When the oxygen concentration directly affects to yields of 7OH-C3CA, the rate of decrease in the oxygen concentration may be correlated with that of decrease in the G value of 7OH-C3CA. However, the reduction rate of G value under UHDR is significantly higher than the oxygen consumption. This finding implied that the influence of the reaction between water radiolysis species formed by neighborhood tracks could be strongly related to the mechanisms of UHDR effect.