Ultrafast Tracking of Oxygen Dynamics during Proton FLASH

Mechanisms of Action in FLASH Radiotherapy: A Comprehensive Review of Physicochemical and Biological Processes on Cancerous and Normal Cells

The advent of FLASH radiotherapy (FLASH-RT) has brought forth a paradigm shift in cancer treatment, showcasing remarkable normal cell sparing effects with ultra-high dose rates (>40 Gy/s). This review delves into the multifaceted mechanisms underpinning the efficacy of FLASH effect, examining both physicochemical and biological hypotheses in cell biophysics. The physicochemical process encompasses oxygen depletion, reactive oxygen species, and free radical recombination. In parallel, the biological process explores the FLASH effect on the immune system and on blood vessels in treatment sites such as the brain, lung, gastrointestinal tract, skin, and subcutaneous tissue. This review investigated the selective targeting of cancer cells and the modulation of the tumor microenvironment through FLASH-RT. Examining these mechanisms, we explore the implications and challenges of integrating FLASH-RT into cancer treatment. The potential to spare normal cells, boost the immune response, and modify the tumor vasculature offers new therapeutic strategies. Despite progress in understanding FLASH-RT, this review highlights knowledge gaps, emphasizing the need for further research to optimize its clinical applications. The synthesis of physicochemical and biological insights serves as a comprehensive resource for cell biology, molecular biology, and biophysics researchers and clinicians navigating the evolution of FLASH-RT in cancer therapy.

Detection of FLASH-radiotherapy tissue sparing in a 3D-spheroid model using DNA damage response markers

PURPOSE

The oxygen depletion hypothesis has been proposed as a rationale to explain the observed phenomenon of FLASH-radiotherapy (FLASH-RT) sparing normal tissues while simultaneously maintaining tumor control. In this study we examined the distribution of DNA Damage Response (DDR) markers in irradiated 3D multicellular spheroids to explore the relationship between FLASH-RT protection and radiolytic-oxygen-consumption (ROC) in tissues.

METHODS

Studies were performed using a Varian Truebeam linear accelerator delivering 10 MeV electrons with an average dose rate above 50 Gy/s. Irradiations were carried out on 3D spheroids maintained under a range of O2 and temperature conditions to control O2 consumption and create gradients representative of in vivo tissues.

RESULTS

Staining for pDNA-PK (Ser2056) produced a linear radiation dose response whereas γH2AX (Ser139) showed saturation with increasing dose. Using the pDNA-PK staining, radiation response was then characterised for FLASH compared to standard-dose-rates as a function of depth into the spheroids. At 4 °C, chosen to minimize the development of metabolic oxygen gradients within the tissues, FLASH protection could be observed at all distances under oxygen conditions of 0.3-1 % O2. Whereas at 37 °C a FLASH-protective effect was limited to the outer cell layers of tissues, an effect only observed at 3 % O2. Modelling of changes in the pDNA-PK-based oxygen enhancement ratio (OER) yielded a tissue ROC g0-value estimate of 0.73 ± 0.25 µM/Gy with a km of 5.4 µM at FLASH dose rates.

CONCLUSIONS

DNA damage response markers are sensitive to the effects of transient oxygen depletion during FLASH radiotherapy. Findings support the rationale that well-oxygenated tissues would benefit more from FLASH-dose-rate protection relative to poorly-oxygenated tissues.

Intracellular Oxygen Transient Quantification in Vivo During Ultra-High Dose Rate FLASH Radiation Therapy

PURPOSE

Large, rapid extracellular oxygen transients (ΔpO2) have been measured in vivo during ultra-high dose rate radiation therapy; however, it has been unclear if they match intracellular oxygen levels. Here, the endogenously produced protoporphyrin IX (PpIX) delayed fluorescence signal was measured as an intracellular in-vivo oxygen sensor to quantify these transients, with direct comparison to extracellular pO2. Intracellular ΔpO2 is closer to the cellular DNA, the site of major radiobiological damage, and therefore should help elucidate radiochemical mechanisms of the FLASH effect and potentially be translated to human tissue measurement.

METHODS AND MATERIALS

PpIX was induced in mouse skin through intraperitoneal injection of 250 mg/kg of aminolevulinic acid. The animals were also administered a 50 µL intradermal injection of 10 µM oxyphor G4 (PdG4) for phosphorescence lifetime pO2 measurement. Paired oxygen transients were quantified in leg or flank tissues while delivering 10 MeV electrons in 3 µs pulses at 360 Hz for a total dose of 10 to 28 Gy.

RESULTS

Transient reductions in pO2 were quantifiable in both PpIX delayed fluorescence and oxyphor phosphorescence, corresponding to intracellular and extracellular pO2 values, respectively. Reponses were quantified for 10, 22, and 28 Gy doses, with ΔpO2 found to be proportional to the dose on average. The ΔpO2 values were dependent on initial pO2 in a logistic function. The average and standard deviations in ΔpO2 per dose were 0.56 ± 0.18 mm Hg/Gy and 0.43 ± 0.06 mm Hg/Gy for PpIX and oxyphor, respectively, for initial pO2 > 20 mm Hg. Although there was large variability in the individual animal measurements of ΔpO2, the average values demonstrated a direct and proportional correlation between intracellular and extracellular pO2 changes, following a linear 1:1 relationship.

CONCLUSIONS

A fundamentally new approach to measuring intracellular oxygen depletion in living tissue showed that ΔpO2 transients seen during ultra-high dose rate radiation therapy matched those quantified using extracellular oxygen measurement. This approach could be translated to humans to quantify intracellular ΔpO2. The measurement of these transients could potentially allow the estimation of intracellular reactive oxygen species production.

FLASH Radiotherapy: What Can FLASH's Ultra High Dose Rate Offer to the Treatment of Patients With Sarcoma?

FLASH is an emerging treatment paradigm in radiotherapy (RT) that utilizes ultra-high dose rates (UHDR; >40 Gy)/s) of radiation delivery. Developing advances in technology support the delivery of UHDR using electron and proton systems, as well as some ion beam units (eg, carbon ions), while methods to achieve UHDR with photons are under investigation. The major advantage of FLASH RT is its ability to increase the therapeutic index for RT by shifting the dose response curve for normal tissue toxicity to higher doses. Numerous preclinical studies have been conducted to date on FLASH RT for murine sarcomas, alongside the investigation of its effects on relevant normal tissues of skin, muscle, and bone. The tumor control achieved by FLASH RT of sarcoma models is indistinguishable from that attained by treatment with standard RT to the same total dose. FLASH's high dose rates are able to mitigate the severity or incidence of RT side effects on normal tissues as evaluated by endpoints ranging from functional sparing to histological damage. Large animal studies and clinical trials of canine patients show evidence of skin sparing by FLASH vs. standard RT, but also caution against delivery of high single doses with FLASH that exceed those safely applied with standard RT. Also, a human clinical trial has shown that FLASH RT can be delivered safely to bone metastasis. Thus, data to date support continued investigations of clinical translation of FLASH RT for the treatment of patients with sarcoma. Toward this purpose, hypofractionated irradiation schemes are being investigated for FLASH effects on sarcoma and relevant normal tissues.

Direct Measurements of FLASH-Induced Changes in Intracellular Oxygenation

PURPOSE

The goal of our study was to characterize the dynamics of intracellular oxygen during application of radiation at conventional (CONV) and FLASH dose rates and obtain evidence for or against the oxygen depletion hypothesis as a mechanism of the FLASH effect.

METHODS AND MATERIALS

The measurements were performed by the phosphorescence quenching method using probe Oxyphor PtG4, which was delivered into the cellular cytosol by electroporation.

RESULTS

Intracellular radiochemical oxygen depletion (ROD) g-value for a dose rate of 100 Gy/s in the normoxic range was found to be 0.58 ± 0.03 μM/Gy. Intracellular ROD g-values for FLASH and CONV dose rates in the normoxic range were found to be nearly equal. As in solution-based studies, intracellular ROD was found to exhibit strong dependence on oxygen concentration in the range of 0 to ∼40 μM [O2].

CONCLUSIONS

Depletion of oxygen in cells in vitro by a clinical dose of proton radiation delivered as FLASH is unable to produce a transient state of hypoxia and, therefore, unable to induce radioprotection. The difference between ROD g-values for FLASH and CONV dose rates, detected previously in solutions-based experiments, disappears when measurements are conducted inside cells. Understanding this phenomenon should provide additional insight into the role of oxygen in FLASH radiation therapy and help to decipher the mechanism of the FLASH effect.

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.

Acute Hypoxia Does Not Alter Tumor Sensitivity to FLASH Radiation Therapy

PURPOSE

Tumor hypoxia is a major cause of treatment resistance, especially to radiation therapy at conventional dose rate (CONV), and we wanted to assess whether hypoxia does alter tumor sensitivity to FLASH.

METHODS AND MATERIALS

We engrafted several tumor types (glioblastoma [GBM], head and neck cancer, and lung adenocarcinoma) subcutaneously in mice to provide a reliable and rigorous way to modulate oxygen supply via vascular clamping or carbogen breathing. We irradiated tumors using a single 20-Gy fraction at either CONV or FLASH, measured oxygen tension, monitored tumor growth, and sampled tumors for bulk RNAseq and pimonidazole analysis. Next, we inhibited glycolysis with trametinib in GBM tumors to enhance FLASH efficacy.

RESULTS

Using various subcutaneous tumor models, and in contrast to CONV, FLASH retained antitumor efficacy under acute hypoxia. These findings show that in addition to normal tissue sparing, FLASH could overcome hypoxia-mediated tumor resistance. Follow-up molecular analysis using RNAseq profiling uncovered a FLASH-specific profile in human GBM that involved cell-cycle arrest, decreased ribosomal biogenesis, and a switch from oxidative phosphorylation to glycolysis. Glycolysis inhibition by trametinib enhanced FLASH efficacy in both normal and clamped conditions.

CONCLUSIONS

These data provide new and specific insights showing the efficacy of FLASH in a radiation-resistant context, proving an additional benefit of FLASH over CONV.

Proton and Electron Ultrahigh-Dose-Rate Isodose Irradiations Produce Differences in Reactive Oxygen Species Yields

Purpose:

Investigations into ultra-high dose rate (UHDR) radiotherapy have dramatically risen because of the observed normal tissue sparing FLASH effect without sacrificing tumor control. The purpose of this study was to provide a direct beamline comparison of protons and electrons to determine where UHDR to conventional dose rates (CDR) changes affect the resultant radiochemistry.

Methods and Materials:

We used well characterized assays of reactive oxygen species (ROS) and oxygen consumption to assess the radiolysis in protein solutions. Three optical reporters related to ROS (CellROX Deep Red, reflects highly reactive radicals; Amplex Red reflects H2O2; and Oxyphor reflects partial pressure loss (ΔpO2)). A Varian ProBeam proton cyclotron and a converted Varian Trilogy electron linac were used for irradiation at both their CDR and UHDR capable level, to assess the assay change per unit dose.

Results:

For both protons and electrons an expected reduction in ROS was noted going from CDR to UHDR, and results interpreted as a reduction in the ratio of UHDR/CDR yield. The CellROX assay showed no difference between beamlines, each showing ~80% reduction in ROS from CDR to UHDR. The Amplex assay showed the largest inter-beamline difference, with ~5% loss using protons vs ~69% loss with electrons, in protein solution. The Oxyphor assay of ΔpO2 showed a small difference in CDR to UHDR with a 23% loss with protons and 43% loss with electrons.

Conclusion:

Interpretation of ROS assays and oxygen consumption is notoriously challenging. These assays might be interpreted by their most activating species’ lifetime. The assay for highly reactive OH●, appeared independent of beamline, whereas the assays for the longer lived H2O2 species and ΔpO2 assay showed differences between beamlines via the UHDR/CDR ratio. This work can be used for FLASH hypothesis testing by comparing these assays to isodose biological FLASH effects in vivo.

FLASH Radiotherapy: A FLASHing Idea to Preserve Neurocognitive Function

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.

Radiation-Chemical Oxygen Depletion Depends on Chemical Environment and Dose Rate: Implications for the FLASH Effect

PURPOSE

FLASH (dose rates >40 Gy/s) radiation therapy protects normal tissues from radiation damage, compared with conventional radiation therapy (∼Gy/m). Radiation-chemical oxygen depletion (ROD) occurs when oxygen reacts with radiation-induced free radicals, so a possible mechanism for FLASH involves radioprotection by the decreased oxygen as ROD occurs. High ROD rates would favor this mechanism, but prior studies have reported low ROD values (∼0.35 µM/Gy) in chemical environments such as water and protein/nutrient solutions. We proposed that intracellular ROD might be much larger, possibly promoted by its strongly reducing chemical environment.

METHODS AND MATERIALS

ROD was measured, using precision polarographic sensors, from ∼100 µM to zero in solutions containing intracellular reducing agents ± glycerol (1M), to simulate intracellular reducing and hydroxyl-radical-scavenging capacity. Cs irradiators and a research proton beamline allowed dose rates from 0.0085 to 100 Gy/s.

RESULTS

Reducing agents significantly altered ROD values. Most greatly increased ROD but some (eg, ascorbate) actually decreased ROD and additionally imposed an oxygen dependence of ROD at low oxygen concentrations. The highest values of ROD were found at low dose rates, but these montonically decreased with increasing dose rate.

CONCLUSIONS

ROD was greatly augmented by some intracellular reducing agents but others (eg, ascorbate) effectively reversed this effect. Ascorbate had its greatest effect at low oxygen concentrations. ROD decreased with increasing dose rate in most cases.

Mean dose rate in ultra-high dose rate electron irradiation is a significant predictor for O2consumption and H2O2yield

Objective. The objective of this study was to investigate the impact of mean and instantaneous dose rates on the production of reactive oxygen species (ROS) during ultra-high dose rate (UHDR) radiotherapy. The study aimed to determine whether either dose rate type plays a role in driving the FLASH effect, a phenomenon where UHDR radiotherapy reduces damage to normal tissues while maintaining tumor control.Approach. Assays of hydrogen peroxide (H2O2) production and oxygen consumption (ΔpO2) were conducted using UHDR electron irradiation. Aqueous solutions of 4% albumin were utilized as the experimental medium. The study compared the effects of varying mean dose rates and instantaneous dose rates on ROS yields. Instantaneous dose rate was varied by changing the source-to-surface distance (SSD), resulting in instantaneous dose rates ranging from 102to 106Gy s-1. Mean dose rate was manipulated by altering the pulse frequency of the linear accelerator (linac) and by changing the SSD, ranging from 0.14 to 1500 Gy s-1.Main results. The study found that both ΔH2O2and ΔpO2decreased as the mean dose rate increased. Multivariate analysis indicated that instantaneous dose rates also contributed to this effect. The variation in ΔpO2was dependent on the initial oxygen concentration in the solution. Based on the analysis of dose rate variation, the study estimated that 7.51 moles of H2O2were produced for every mole of O2consumed.Significance. The results highlight the significance of mean dose rate as a predictor of ROS production during UHDR radiotherapy. As the mean dose rate increased, there was a decrease in oxygen consumption and in H2O2production. These findings have implications for understanding the FLASH effect and its potential optimization. The study sheds light on the role of dose rate parameters and their impact on radiochemical outcomes, contributing to the advancement of UHDR radiotherapy techniques.

Review of optical reporters of radiation effects in vivo: tools to quantify improvements in radiation delivery technique

Significance

Radiation damage studies are used to optimize radiotherapy treatment techniques. Although biological indicators of damage are the best assays of effect, they are highly variable due to biological heterogeneity. The free radical radiochemistry can be assayed with optical reporters, allowing for high precision titration of techniques.

Aim

We examine the optical reporters of radiochemistry to highlight those with the best potential for translational use in vivo, as surrogates for biological damage assays, to inform on mechanisms.

Approach

A survey of the radical chemistry effects from reactive oxygen species (ROS) and oxygen itself was completed to link to DNA or biological damage. Optical reporters of ROS include fluorescent, phosphorescent, and bioluminescent molecules that have a variety of activation pathways, and each was reviewed for its in vivo translation potential.

Results

There are molecular reporters of ROS having potential to report within living systems, including derivatives of luminol, 2'7'-dichlorofluorescein diacetate, Amplex Red, and fluorescein. None have unique specificity to singular ROS species. Macromolecular engineered reporters unique to specific ROS are emerging. The ability to directly measure oxygen via reporters, such as Oxyphor and protoporphyrin IX, is an opportunity to quantify the consumption of oxygen during ROS generation, and this translates from in vitro to in vivo use. Emerging techniques, such as ion particle beams, spatial fractionation, and ultra-high dose rate FLASH radiotherapy, provide the motivation for these studies.

Conclusions

In vivo optical reporters of radiochemistry are quantitatively useful for comparing radiotherapy techniques, although their use comes at the cost of the unknown connection to the mechanisms of radiobiological damage. Still their lower measurement uncertainty, compared with biological response assay, makes them an invaluable tool. Linkage to DNA damage and biological damage is needed, and measures such as oxygen consumption serve as useful surrogate measures that translate to in vivo use.

Modeling ultra-high dose rate electron and proton FLASH effect with the physicochemical approach

Objective. A physicochemical model built on the radiochemical kinetic theory was recently proposed in (Labarbeet al2020) to explain the FLASH effect. We performed extensive simulations to scrutinize its applicability for oxygen depletion studies and FLASH-related experiments involving both proton and electron beams.Approach. Using the dose and beam delivery parameters for each FLASH experiment, we numerically solved the radiochemical rate equations comprised of a set of coupled nonlinear ordinary differential equations to obtain the area under the curve (AUC) of radical concentrations.Main results. The modeled differences in AUC induced by ultra-high dose rates appeared to correlate well with the FLASH effect. (i) For the whole brain irradiation of mice performed in (Montay-Gruelet al2017), the threshold dose rate values for memory preservation coincided with those at which AUC started to decrease much less rapidly. (ii) For the proton pencil beam scanning FLASH of (Cunninghamet al2021), we found linear correlations between radicals' AUC and the biological endpoints: TGF-β1, leg contracture and plasma level of cytokine IL-6. (iii) Compatible with the findings of the proton FLASH experiment in (Kimet al2021), we found that radicals' AUC at the entrance and mid-Spread-Out Bragg peak regions were highly similar. In addition, our model also predicted ratios of oxygen depletionG-values between normal and UHDR irradiation similar to those observed in (Caoet al2021) and (El Khatibet al2022).Significance. Collectively, our results suggest that the normal tissue sparing conferred by UHDR irradiation may be due to the lower degree of exposure to peroxyl and superoxide radicals. We also found that the differential effect of dose rate on the radicals' AUC was less pronounced at lower initial oxygen levels, a trait that appears to align with the FLASH differential effect on normal versus tumor tissues.

Comet Assay Profiling of FLASH-Induced Damage: Mechanistic Insights into the Effects of FLASH Irradiation

Numerous studies have demonstrated the normal tissue-sparing effects of ultra-high dose rate 'FLASH' irradiation in vivo, with an associated reduction in damage burden being reported in vitro. Towards this, two key radiochemical mechanisms have been proposed: radical-radical recombination (RRR) and transient oxygen depletion (TOD), with both being proposed to lead to reduced levels of induced damage. Previously, we reported that FLASH induces lower levels of DNA strand break damage in whole-blood peripheral blood lymphocytes (WB-PBL) ex vivo, but our study failed to distinguish the mechanism(s) involved. A potential outcome of RRR is the formation of crosslink damage (particularly, if any organic radicals recombine), whilst a possible outcome of TOD is a more anoxic profile of induced damage resulting from FLASH. Therefore, the aim of the current study was to profile FLASH-induced damage via the Comet assay, assessing any DNA crosslink formation as a putative marker of RRR and/or anoxic DNA damage formation as an indicative marker of TOD, to determine the extent to which either mechanism contributes to the "FLASH effect". Following FLASH irradiation, we see no evidence of any crosslink formation; however, FLASH irradiation induces a more anoxic profile of induced damage, supporting the TOD mechanism. Furthermore, treatment of WB-PBLs pre-irradiation with BSO abrogates the reduced strand break damage burden mediated by FLASH exposures. In summary, we do not see any experimental evidence to support the RRR mechanism contributing to the reduced damage burden induced by FLASH. However, the observation of a greater anoxic profile of damage following FLASH irradiation, together with the BSO abrogation of the reduced strand break damage burden mediated by FLASH, lends further support to TOD being a driver of the reduced damage burden plus a change in the damage profile mediated by FLASH.

Proton FLASH effects on mouse skin at different oxygen tensions

Objective. Irradiation at FLASH dose rates (>40 Gy s-1) has received great attention due to its reported normal tissue sparing effect. The FLASH effect was originally observed in electron irradiations but has since been shown to also occur with both photon and proton beams. Several mechanisms have been proposed to explain the tissue sparing at high dose rates, including effects involving oxygen, such as depletion of oxygen within the irradiated cells. In this study, we investigated the protective role of FLASH proton irradiation on the skin when varying the oxygen concentration.Approach. Our double scattering proton system provided a 1.2 × 1.6 cm2elliptical field at a dose rate of ∼130 Gy s-1. The conventional dose rate was ∼0.4 Gy s-1. The legs of the FVB/N mice were marked with two tattooed dots and fixed in a holder for exposure. To alter the skin oxygen concentration, the mice were breathing pure oxygen or had their legs tied to restrict blood flow. The distance between the two dots was measured to analyze skin contraction over time.Main results. FLASH irradiation mitigated skin contraction by 15% compared to conventional dose rate irradiation. The epidermis thickness and collagen deposition at 75 d following 25 to 30 Gy exposure suggested a long-term protective function in the skin from FLASH irradiation. Providing the mice with oxygen or reducing the skin oxygen concentration removed the dose-rate-dependent difference in response.Significance. FLASH proton irradiation decreased skin contraction, epidermis thickness and collagen deposition compared to standard dose rate irradiations. The observed oxygen-dependence of the FLASH effect is consistent with, but not conclusive of, fast oxygen depletion during the exposure.

Hypoxia signaling in cancer: Implications for therapeutic interventions

Hypoxia is a persistent physiological feature of many different solid tumors and a key driver of malignancy, and in recent years, it has been recognized as an important target for cancer therapy. Hypoxia occurs in the majority of solid tumors due to a poor vascular oxygen supply that is not sufficient to meet the needs of rapidly proliferating cancer cells. A hypoxic tumor microenvironment (TME) can reduce the effectiveness of other tumor therapies, such as radiotherapy, chemotherapy, and immunotherapy. In this review, we discuss the critical role of hypoxia in tumor development, including tumor metabolism, tumor immunity, and tumor angiogenesis. The treatment methods for hypoxic TME are summarized, including hypoxia-targeted therapy and improving oxygenation by alleviating tumor hypoxia itself. Hyperoxia therapy can be used to improve tissue oxygen partial pressure and relieve tumor hypoxia. We focus on the underlying mechanisms of hyperoxia and their impact on current cancer therapies and discuss the prospects of hyperoxia therapy in cancer treatment.

A phenomenological model of proton FLASH oxygen depletion effects depending on tissue vasculature and oxygen supply

Introduction

Radiation-induced oxygen depletion in tissue is assumed as a contributor to the FLASH sparing effects. In this study, we simulated the heterogeneous oxygen depletion in the tissue surrounding the vessels and calculated the proton FLASH effective-dose-modifying factor (FEDMF), which could be used for biology-based treatment planning.

Methods

The dose and dose-weighted linear energy transfer (LET) of a small animal proton irradiator was simulated with Monte Carlo simulation. We deployed a parabolic partial differential equation to account for the generalized radiation oxygen depletion, tissue oxygen diffusion, and metabolic processes to investigate oxygen distribution in 1D, 2D, and 3D solution space. Dose and dose rates, particle LET, vasculature spacing, and blood oxygen supplies were considered. Using a similar framework for the hypoxic reduction factor (HRF) developed previously, the FEDMF was derived as the ratio of the cumulative normoxic-equivalent dose (CNED) between CONV and UHDR deliveries.

Results

Dynamic equilibrium between oxygen diffusion and tissue metabolism can result in tissue hypoxia. The hypoxic region displayed enhanced radio-resistance and resulted in lower CNED under UHDR deliveries. In 1D solution, comparing 15 Gy proton dose delivered at CONV 0.5 and UHDR 125 Gy/s, 61.5% of the tissue exhibited ≥20% FEDMF at 175 μm vasculature spacing and 18.9 μM boundary condition. This percentage reduced to 34.5% and 0% for 8 and 2 Gy deliveries, respectively. Similar trends were observed in the 3D solution space. The FLASH versus CONV differential effect remained at larger vasculature spacings. A higher FLASH dose rate showed an increased region with ≥20% FEDMF. A higher LET near the proton Bragg peak region did not appear to alter the FLASH effect.

Conclusion

We developed 1D, 2D, and 3D oxygen depletion simulation process to obtain the dynamic HRF and derive the proton FEDMF related to the dose delivery parameters and the local tissue vasculature information. The phenomenological model can be used to simulate or predict FLASH effects based on tissue vasculature and oxygen concentration data obtained from other experiments.

Real-time optical oximetry during FLASH radiotherapy using a phosphorescent nanoprobe

The rapid depletion of oxygen during irradiation at ultra-high dose rate calls for tissue oximeters capable of high temporal resolution. This study demonstrates a water-soluble phosphorescent nanoprobe and fiber-coupled instrument, which together are used to measure the kinetics of oxygen depletion at 200 Hz during irradiation of in vitro solutions.

oChanges in Radical Levels as a Cause for the FLASH effect: Impact of beam structure parameters at ultra-high dose rates on oxygen depletion in water

The influence of different average and bunch dose rates in electron beams on the FLASH effect was investigated. The present study measures O2 content in water at different beam pulse patterns and finds strong correlation with biological data, strengthening the hypothesis of radical-related mechanisms as a reason for the FLASH effect.

Oxygen Monitoring in Model Solutions and In Vivo in Mice During Proton Irradiation at conventional and FLASH Dose Rates

FLASH is a high-dose-rate form of radiation therapy that has the reported ability, compared with conventional dose rates, to spare normal tissues while being equipotent in tumor control, thereby increasing the therapeutic ratio. The mechanism underlying this normal tissue sparing effect is currently unknown, however one possibility is radiochemical oxygen depletion (ROD) during dose delivery in tissue at FLASH dose rates. In order to investigate this possibility, we used the phosphorescence quenching method to measure oxygen partial pressure before, during and after proton radiation delivery in model solutions and in normal muscle and sarcoma tumors in mice, at both conventional (Conv) (∼0.5 Gy/s) and FLASH (∼100 Gy/s) dose rates. Radiation dosimetry was determined by Advanced Markus Chamber and EBT-XL film. For solutions contained in sealed glass vials, phosphorescent probe Oxyphor PtG4 (1 μM) was dissolved in a buffer (10 mM HEPES) containing glycerol (1 M), glucose (5 mM) and glutathione (5 mM), designed to mimic the reducing and free radical-scavenging nature of the intracellular environment. In vivo oxygen measurements were performed 24 h after injection of PtG4 into the interstitial space of either normal thigh muscle or intra-muscular sarcoma tumors in mice. The "g-value" for ROD is reported in mmHg/Gy, which represents a slight modification of the more standard chemical definition (μM/Gy). In solutions, proton irradiation at conventional dose rates resulted in a g-value for ROD of up to 0.55 mmHg/Gy, consistent with earlier studies using X or gamma rays. At FLASH dose rates, the g-value for ROD was ∼25% lower, 0.37 mmHg/Gy. pO2 levels were stable after each dose delivery. For normal muscle in vivo, oxygen depletion during irradiation was counterbalanced by resupply from the vasculature. This process was fast enough to maintain tissue pO2 virtually unchanged at Conv dose rates. However, during FLASH irradiation there was a stepwise decrease in pO2 (g-value ∼0.28 mmHg/Gy), followed by a rebound to the initial level after ∼8 s. The g-values were smaller and recovery times longer in tumor tissue when compared to muscle and may be related to the lower initial endogenous pO2 levels in the former. Considering that the FLASH effect is seen in vivo even at doses as low as 10 Gy, it is difficult to reconcile the amount of protection seen by oxygen depletion alone. However, the phosphorescence probe in our experiments was confined to the extracellular space, and it remains possible that intracellular oxygen depletion was greater than observed herein. In cell-mimicking solutions the oxygen depletion g-vales were indeed significantly higher than observed in vivo.