A physicochemical model of reaction kinetics supports peroxyl radical recombination as the main determinant of the FLASH effect

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

Radiation cancer therapy with ultra-high dose rate exposure, so called FLASH radiotherapy, appears to reduce normal tissue damage without compromising tumor response. The aim of this study was to clarify whether FLASH exposure of proton beam would be effective in reducing the DNA strand break induction. We applied a simple model system, pBR322 plasmid DNA in aqueous 1 × TE solution, where DNA single strand breaks (SSBs) and double strand breaks (DSBs) can be precisely quantified by gel electrophoresis. Plasmid DNA were exposed to 27.5 MeV protons in the conventional dose rate of 0.05 Gy/s (CONV) and ultra-high dose rate of 40 Gy/s (FLASH). With both dose rate, the kinetics of the SSB and DSB induction were proportional to absorbed dose. The SSB induction of FLASH was significantly less than CONV, which were 8.79 ± 0.14 (10-3 SSB per Gy per molecule) and 10.8 ± 0.68 (10-3 SSB per Gy per molecule), respectively. The DSB induction of FLASH was also slightly less than CONV, but difference was not significant. Altogether, 27.5 MeV proton beam at 40 Gy/s reduced SSB and not DSB, thus its effect may not be significant in reducing lethal DNA damage that become apparent in acute radiation effect.

The Impact of Sub-Millisecond Damage Fixation Kinetics on the In Vitro Sparing Effect at Ultra-High Dose Rate in UNIVERSE

The impact of the exact temporal pulse structure on the potential cell and tissue sparing of ultra-high dose-rate irradiation applied in FLASH studies has gained increasing attention. A previous version of our biophysical mechanistic model (UNIVERSE: UNIfied and VERSatile bio response Engine), based on the oxygen depletion hypothesis, has been extended in this work by considering oxygen-dependent damage fixation dynamics on the sub-milliseconds scale and introducing an explicit implementation of the temporal pulse structure. The model successfully reproduces in vitro experimental data on the fast kinetics of the oxygen effect in irradiated mammalian cells. The implemented changes result in a reduction in the assumed amount of oxygen depletion. Furthermore, its increase towards conventional dose-rates is parameterized based on experimental data from the literature. A recalculation of previous benchmarks shows that the model retains its predictive power, while the assumed amount of depleted oxygen approaches measured values. The updated UNIVERSE could be used to investigate the impact of different combinations of pulse structure parameters (e.g., dose per pulse, pulse frequency, number of pulses, etc.), thereby aiding the optimization of potential clinical application and the development of suitable accelerators.

Simultaneous dose and dose rate optimization (SDDRO) of the FLASH effect for pencil-beam-scanning proton therapy

PURPOSE

Compared to CONV-RT (with conventional dose rate), FLASH-RT (with ultra-high dose rate) can provide biological dose sparing for organs-at-risk (OARs) via the so-called FLASH effect, in addition to physical dose sparing. However, the FLASH effect only occurs, when both dose and dose rate meet certain minimum thresholds. This work will develop a simultaneous dose and dose rate optimization (SDDRO) method accounting for both FLASH dose and dose rate constraints during treatment planning for pencil-beam-scanning proton therapy.

METHODS

SDDRO optimizes the FLASH effect (specific to FLASH-RT) as well as the dose distribution (similar to CONV-RT). The nonlinear dose rate constraint is linearized, and the reformulated optimization problem is efficiently solved via iterative convex relaxation powered by alternating direction method of multipliers. To resolve and quantify the generic tradeoff of FLASH-RT between FLASH and dose optimization, we propose the use of FLASH effective dose based on dose modifying factor (DMF) owing to the FLASH effect.

RESULTS

FLASH-RT via transmission beams (TB) (IMPT-TB or SDDRO) and CONV-RT via Bragg peaks (BP) (IMPT-BP) were evaluated for clinical prostate, lung, head-and-neck (HN), and brain cases. Despite the use of TB, which is generally suboptimal to BP for normal tissue sparing, FLASH-RT via SDDRO considerably reduced FLASH effective dose for high-dose OAR adjacent to the target. For example, in the lung SBRT case, the max esophageal dose constraint 27 Gy was only met by SDDRO (24.8 Gy), compared to IMPT-BP (35.3 Gy) or IMPT-TB (36.6 Gy); in the brain SRS case, the brain constraint V12Gy≤15cc was also only met by SDDRO (13.7cc), compared to IMPT-BP (43.9cc) or IMPT-TB (18.4cc). In addition, SDDRO substantially improved the FLASH coverage from IMPT-TB, e.g., an increase from 37.2% to 67.1% for lung, from 39.1% to 58.3% for prostate, from 65.4% to 82.1% for HN, from 50.8% to 73.3% for the brain.

CONCLUSIONS

Both FLASH dose and dose rate constraints are incorporated into SDDRO for FLASH-RT that jointly optimizes the FLASH effect and physical dose distribution. FLASH effective dose via FLASH DMF is introduced to reconcile the tradeoff between physical dose sparing and FLASH sparing, and quantify the net effective gain from CONV-RT to FLASH-RT.

Radiobiology of the FLASH effect

Radiation exposures at ultrahigh dose rates (UHDRs) at several orders of magnitude greater than in current clinical radiotherapy (RT) have been shown to manifest differential radiobiological responses compared to conventional (CONV) dose rates. This has led to studies investigating the application of UHDR for therapeutic advantage (FLASH-RT) that have gained significant interest since the initial discovery in 2014 that demonstrated reduced lung toxicity with equivalent levels of tumor control compared with conventional dose-rate RT. Many subsequent studies have demonstrated the potential protective role of FLASH-RT in normal tissues, yet the underlying molecular and cellular mechanisms of the FLASH effect remain to be fully elucidated. Here, we summarize the current evidence of the FLASH effect and review FLASH-RT studies performed in preclinical models of normal tissue response. To critically examine the underlying biological mechanisms of responses to UHDR radiation exposures, we evaluate in vitro studies performed with normal and tumor cells. Differential responses to UHDR versus CONV irradiation recurrently involve reduced inflammatory processes and differential expression of pro- and anti-inflammatory genes. In addition, frequently reduced levels of DNA damage or misrepair products are seen after UHDR irradiation. So far, it is not clear what signal elicits these differential responses, but there are indications for involvement of reactive species. Different susceptibility to FLASH effects observed between normal and tumor cells may result from altered metabolic and detoxification pathways and/or repair pathways used by tumor cells. We summarize the current theories that may explain the FLASH effect and highlight important research questions that are key to a better mechanistic understanding and, thus, the future implementation of FLASH-RT in the clinic.

Model studies of the role of oxygen in the FLASH effect

Current radiotherapy facilities are standardized to deliver dose rates around 0.1-0.4 Gy/s in 2 Gy daily fractions, designed to deliver total accumulated doses to reach the tolerance limit of normal tissues undergoing irradiation. FLASH radiotherapy (FLASH-RT), on the other hand, relies on facilities capable of delivering ultrahigh dose rates in large doses in a single microsecond pulse, or in a few pulses given over a very short time sequence. For example, most studies to date have implemented 4-6 MeV electrons with intra-pulse dose rates in the range 10⁶ -10⁷  Gy/s. The proposed dependence of the FLASH effect on oxygen tension has stimulated several theoretical models based on three different hypotheses: (i) Radiation-induced transient oxygen depletion; (ii) cell-specific differences in the ability to detoxify and/or recover from injury caused by reactive oxygen species; (iii) self-annihilation of radicals by bimolecular recombination. This article focuses on the observations supporting or refuting these models in the frame of the chemical-biological bases of the impact of oxygen on the radiation response of cell free, in vitro and in vivo model systems.

Development of Ultra-High Dose-Rate (FLASH) Particle Therapy

Research efforts in FLASH radiotherapy have increased at an accelerated pace recently. FLASH radiotherapy involves ultra-high dose rates and has shown to reduce toxicity to normal tissue while maintaining tumor response in pre-clinical studies when compared to conventional dose rate radiotherapy. The goal of this review is to summarize the studies performed to-date with proton, electron, and heavy ion FLASH radiotherapy, with particular emphasis on the physical aspects of each study and the advantages and disadvantages of each modality. Beam delivery parameters, experimental set-up, and the dosimetry tools used are described for each FLASH modality. In addition, modeling efforts and treatment planning for FLASH radiotherapy is discussed along with potential drawbacks when translated into the clinical setting. The final section concludes with further questions that have yet to be answered before safe clinical implementation of FLASH radiotherapy.

In vitro assays for investigating the FLASH effect

FLASH radiotherapy is a novel technique that has been shown in numerous preclinical in vivo studies to have the potential to be the next important improvement in cancer treatment. However, the biological mechanisms responsible for the selective FLASH sparing effect of normal tissues are not yet known. An optimal translation of FLASH radiotherapy into the clinic would require a good understanding of the specific beam parameters that induces a FLASH effect, environmental conditions affecting the response, and the radiobiological mechanisms involved. Even though the FLASH effect has generally been considered as an in vivo effect, studies finding these answers would be difficult and ethically challenging to carry out solely in animals. Hence, suitable in vitro studies aimed towards finding these answers are needed. In this review, we describe and summarise several in vitro assays that have been used or could be used to finally elucidate the mechanisms behind the FLASH effect.

Non-Cancer Effects following Ionizing Irradiation Involving the Eye and Orbit

Simple Summary

The irradiation of tumors involving the eye or orbit represents a complex therapeutic challenge due to the proximity between the tumor and organs that are susceptible to radiation. The challenges include tumor control, as it is often a surrogate for survival; organ (usually the eyeball) preservation; and the minimization of damage of sensitive tissues surrounding the tumor in order to preserve vision. Anticipation of the spectrum and severity of radiation-induced complications is crucial to the decision of which technique to use for a given tumor. The aim of the present review is to report the non-cancer effects that may occur following ionizing irradiation involving the eye and orbit and their specific patterns of toxicity for a given radiotherapy modality. The pros and cons of conventional and advanced forms of radiation techniques and their clinical implementation are provided with a clinical perspective.

Abstract

The eye is an exemplarily challenging organ to treat when considering ocular tumors. It is at the crossroads of several major aims in oncology: tumor control, organ preservation, and functional outcomes including vision and quality of life. The proximity between the tumor and organs that are susceptible to radiation damage explain these challenges. Given a high enough dose of radiation, virtually any cancer will be destroyed with radiotherapy. Yet, the doses inevitably absorbed by normal tissues may lead to complications, the likelihood of which increases with the radiation dose and volume of normal tissues irradiated. Precision radiotherapy allows personalized decision-making algorithms based on patient and tumor characteristics by exploiting the full knowledge of the physics, radiobiology, and the modifications made to the radiotherapy equipment to adapt to the various ocular tumors. Anticipation of the spectrum and severity of radiation-induced complications is crucial to the decision of which technique to use for a given tumor. Radiation can damage the lacrimal gland, eyelashes/eyelids, cornea, lens, macula/retina, optic nerves and chiasma, each having specific dose–response characteristics. The present review is a report of non-cancer effects that may occur following ionizing irradiation involving the eye and orbit and their specific patterns of toxicity for a given radiotherapy modality.

The Therapeutic Potential of FLASH-RT for Pancreatic Cancer

Simple Summary

Ultra-high dose rate radiation, widely nicknamed FLASH-RT, kills tumors without significantly damaging nearby normal tissues. This selective sparing of normal tissue by FLASH-RT tissue is called the FLASH effect. This review explores some of the proposed mechanisms of the FLASH effect and the current data that might support its use in pancreatic cancer. Since radiation for pancreatic cancer treatment is limited by GI toxicity issues and is a disease with one of the lowest five-year survival rates, FLASH-RT could have a large impact in the treatment of this disease with further study.

Abstract

Recent preclinical evidence has shown that ionizing radiation given at an ultra-high dose rate (UHDR), also known as FLASH radiation therapy (FLASH-RT), can selectively reduce radiation injury to normal tissue while remaining isoeffective to conventional radiation therapy (CONV-RT) with respect to tumor killing. Unresectable pancreatic cancer is challenging to control without ablative doses of radiation, but this is difficult to achieve without significant gastrointestinal toxicity. In this review article, we explore the propsed mechanisms of FLASH-RT and its tissue-sparing effect, as well as its relevance and suitability for the treatment of pancreatic cancer. We also briefly discuss the challenges with regard to dosimetry, dose rate, and fractionation for using FLASH-RT to treat this disease.

Quantifying the DNA-damaging effects of FLASH irradiation with plasmid DNA

PURPOSE

To investigate a plasmid DNA nicking assay approach for isolating and quantifying the DNA damaging effects of ultra-high dose rate (i.e., FLASH) irradiation relative to conventional dose rate irradiation.

METHODS

We constructed and irradiated phantoms containing plasmid DNA to nominal doses of 20 Gy and 30 Gy using 16 MeV electrons at conventional (0.167 Gy/s) and FLASH (46.6 Gy/s and 93.2 Gy/s) dose rates. We delivered conventional dose rates using a standard clinical Varian iX linac and FLASH dose rates (FDR) using a modified Varian 21EX C-series linac. We ran the irradiated DNA and controls (0 Gy) through an agarose gel electrophoresis procedure that sorted and localized the DNA into bands associated with single strand breaks (SSBs), double strand breaks (DSBs), and undamaged DNA. We quantitatively analyzed the gel images to compute the relative yields of SSBs and DSBs, and applied a mathematical model of plasmid DNA damage as a function of dose to compute relative biological effectiveness (RBE) of SSB and DSB (RBE_SSBandRBE_DSB) damage for a given endpoint and FDR.

RESULTS

Both RBE_SSBandRBE_DSB were less than unity with the FDR irradiations, indicating FLASH sparing. With regard to the more deleterious DNA DSB damage, RBE_DSBs of FLASH beams at dose rates of 46.6 Gy/s and 93.2 Gy/s relative to the conventional 16 MeV beam dose rate were 0.54 ± 0.15 and 0.55 ± 0.17, respectively.

CONCLUSION

We have demonstrated the feasibility of using a DNA-based phantom to isolate and assess the FLASH sparing effect on DNA. We also found that FLASH irradiation causes less damage to DNA compared to a conventional dose rate. This result supports the notion that the protective effect of FLASH irradiation occurs at least partially via fundamental biochemical processes.

A quantitative FLASH effectiveness model to reveal potentials and pitfalls of high dose rate proton therapy

Purpose

In ultrahigh dose rate radiotherapy, the FLASH effect can lead to substantially reduced healthy tissue damage without affecting tumor control. Although many studies show promising results, the underlying biological mechanisms and the relevant delivery parameters are still largely unknown. It is unclear, particularly for scanned proton therapy, how treatment plans could be optimized to maximally exploit this protective FLASH effect.

Materials and Methods

To investigate the potential of pencil beam scanned proton therapy for FLASH treatments, we present a phenomenological model, which is purely based on experimentally observed phenomena such as potential dose rate and dose thresholds, and which estimates the biologically effective dose during FLASH radiotherapy based on several parameters. We applied this model to a wide variety of patient geometries and proton treatment planning scenarios, including transmission and Bragg peak plans as well as single‐ and multifield plans. Moreover, we performed a sensitivity analysis to estimate the importance of each model parameter.

Results

Our results showed an increased plan‐specific FLASH effect for transmission compared with Bragg peak plans (19.7% vs. 4.0%) and for single‐field compared with multifield plans (14.7% vs. 3.7%), typically at the cost of increased integral dose compared to the clinical reference plan. Similar FLASH magnitudes were found across the different treatment sites, whereas the clinical benefits with respect to the clinical reference plan varied strongly. The sensitivity analysis revealed that the threshold dose as well as the dose per fraction strongly impacted the FLASH effect, whereas the persistence time only marginally affected FLASH. An intermediate dependence of the FLASH effect on the dose rate threshold was found.

Conclusions

Our model provided a quantitative measure of the FLASH effect for various delivery and patient scenarios, supporting previous assumptions about potentially promising planning approaches for FLASH proton therapy. Positive clinical benefits compared to clinical plans were achieved using hypofractionated, single‐field transmission plans. The dose threshold was found to be an important factor, which may require more investigation.

FLASH Radiotherapy Using Single-Energy Proton PBS Transmission Beams for Hypofractionation Liver Cancer: Dose and Dose Rate Quantification

PURPOSE

This work aims to study the dose and ultra-high-dose rate characteristics of transmission proton pencil beam scanning (PBS) FLASH radiotherapy (RT) for hypofractionation liver cancer based on the parameters of a commercially available proton system operating under FLASH mode.

METHODS AND MATERIALS

An in-house treatment planning software (TPS) was developed to perform intensity-modulated proton therapy (IMPT) FLASH-RT planning. Single-energy transmission proton PBS plans of 4.5 Gy × 15 fractions were optimized for seven consecutive hepatocellular carcinoma patients, using 2 and 5 fields combined with 1) the minimum MU/spot chosen between 100 and 400, and minimum spot time (MST) of 2 ms, and 2) the minimum MU/spot of 100, and MST of 0.5 ms, based upon considerations in target uniformities, OAR dose constraints, and OAR FLASH dose rate coverage. Then, the 3D average dose rate distribution was calculated. The dose metrics for the mean dose of Liver-GTV and other major OARs were characterized to evaluate the dose quality for the different combinations of field numbers and minimum spot times compared to that of conventional IMPT plans. Dose rate quality was evaluated using 40 Gy/s volume coverage (V40Gy/s).

RESULTS

All plans achieved favorable and comparable target uniformities, and target uniformity improved as the number of fields increased. For OARs, no significant dose differences were observed between plans of different field numbers and the same MST. For plans using shorter MST and the same field numbers, better sparing was generally observed in most OARs and was statistically significant for the chest wall. However, the FLASH dose rate coverage V40Gy/s was increased by 20% for 2-field plans compared to 5-field plans in most OARs with 2-ms MST, which was less evident in the 0.5-ms cases. For 2-field plans, dose metrics and V40Gy/s of select OARs have large variations due to the beam angle selection and variable distances to the targets. The transmission plans generally yielded inferior dosimetric quality to the conventional IMPT plans.

CONCLUSION

This is the first attempt to assess liver FLASH treatment planning and demonstrates that it is challenging for hypofractionation with smaller fractional doses (4.5 Gy/fraction). Using fewer fields can allow higher minimum MU/spot, resulting in higher OAR FLASH dose rate coverages while achieving similar plan quality compared to plans with more fields. Shorter MST can result in better plan quality and comparable or even better FLASH dose rate coverage.

The importance of hypoxia in radiotherapy for the immune response, metastatic potential and FLASH-RT

PURPOSE

Hypoxia (low oxygen) is a common feature of solid tumors that has been intensely studied for more than six decades. Here we review the importance of hypoxia to radiotherapy with a particular focus on the contribution of hypoxia to immune responses, metastatic potential and FLASH radiotherapy, active areas of research by leading women in the field.

CONCLUSION

Although hypoxia-driven metastasis and immunosuppression can negatively impact clinical outcome, understanding these processes can also provide tumor-specific vulnerabilities that may be therapeutically exploited. The different oxygen tensions present in tumors and normal tissues may underpin the beneficial FLASH sparing effect seen in normal tissue and represents a perfect example of advances in the field that can leverage tumor hypoxia to improve future radiotherapy treatments.

Understanding the FLASH effect to unravel the potential of ultra-high dose rate irradiation

A reemergence of research implementing radiation delivery at ultra-high dose rates (UHDRs) has triggered intense interest in the radiation sciences and has opened a new field of investigation in radiobiology. Much of the promise of UHDR irradiation involves the FLASH effect, an in vivo biological response observed to maintain anti-tumor efficacy without the normal tissue complications associated with standard dose rates. The FLASH effect has been validated primarily, using intermediate energy electron beams able to deliver high doses (>7 Gy) in a very short period of time (<200 ms), but has also been found with photon and proton beams. The clinical implications of this new area of research are highly significant, as FLASH radiotherapy (FLASH-RT) has the potential to enhance the therapeutic index, opening new possibilities for eradicating radio-resistant tumors without toxicity. As pioneers in this field, our group has developed a multidisciplinary research team focused on investigating the mechanisms and clinical translation of the FLASH effect. Here, we review the field of UHDR, from the physico-chemical to the biological mechanisms.

Physics and biomedical challenges of cancer therapy with accelerated heavy ions

Radiotherapy should have low toxicity in the entrance channel (normal tissue) and be very effective in cell killing in the target region (tumour). In this regard, ions heavier than protons have both physical and radiobiological advantages over conventional X-rays. Carbon ions represent an excellent combination of physical and biological advantages. There are a dozen carbon-ion clinical centres in Europe and Asia, and more under construction or at the planning stage, including the first in the USA. Clinical results from Japan and Germany are promising, but a heated debate on the cost-effectiveness is ongoing in the clinical community, owing to the larger footprint and greater expense of heavy ion facilities compared with proton therapy centres. We review here the physical basis and the clinical data with carbon ions and the use of different ions, such as helium and oxygen. Research towards smaller and cheaper machines with more effective beam delivery is necessary to make particle therapy affordable. The potential of heavy ions has not been fully exploited in clinics and, rather than there being a single 'silver bullet', different particles and their combination can provide a breakthrough in radiotherapy treatments in specific cases.

Oxygen Depletion in Proton Spot Scanning: A Tool for Exploring the Conditions Needed for FLASH

FLASH radiotherapy is a rapidly developing field which promises improved normal tissue protection compared to conventional irradiation and no compromise on tumour control. The transient hypoxic state induced by the depletion of oxygen at high dose rates provides one possible explanation. However, studies have mostly focused on uniform fields of dose and there is a lack of investigation into the spatial and temporal variation of dose from proton pencil-beam scanning (PBS). A model of oxygen reaction and diffusion in tissue has been extended to simulate proton PBS delivery and its impact on oxygen levels. This provides a tool to predict oxygen effects from various PBS treatments, and explore potential delivery strategies. Here we present a number of case applications to demonstrate the use of this tool for FLASH-related investigations. We show that levels of oxygen depletion could vary significantly across a large parameter space for PBS treatments, and highlight the need for in silico models such as this to aid in the development and optimisation of FLASH radiotherapy.

Ultra-High Dose Rate (FLASH) Carbon Ion Irradiation: Dosimetry and First Cell Experiments

PURPOSE

To establish a beam monitoring and dosimetry system to enable the FLASH dose rate carbon ion irradiation and investigate, at different oxygen concentrations, the in vitro biological response in comparison to the conventional dose rate.

METHODS AND MATERIALS

CHO-K1 cell response to irradiation at different dose rates and at different levels of oxygenation was studied using clonogenic assay. The Heidelberg Ion-Beam Therapy Center (HIT) synchrotron, after technical improvements, was adjusted to extract ≥5 × 10^{8 12}C ions within approximately 150 milliseconds. The beam monitors were filled with helium.

RESULTS

The FLASH irradiation with beam scanning yields a dose of 7.5 Gy (homogeneity of ±5%) for a 280 MeV/u beam in a volume of at least 8 mm in diameter and a corresponding dose rate of 70 Gy/s (±20%). The dose repetition accuracy is better than 2%, the systematic uncertainty is better than 2%. Clonogenic assay demonstrates a significant FLASH sparing effect which is strongly oxygenation-dependent and mostly pronounced at 0.5% O₂ but absent at 0% and 21% O₂.

CONCLUSION

The FLASH dose rates >40 Gy/s were achieved with carbon beams. Cell survival analysis revealed FLASH dose rate sparing in hypoxia (0.5%-4% O₂).

A Computational Model for Oxygen Depletion Hypothesis in FLASH Effect

Experiments have reported low normal tissue toxicities during FLASH irradiation, but the mechanism has not been elaborated. Several hypotheses have been proposed to explain the mechanism. One hypothesis is oxygen depletion. We analyze the time-dependent change of oxygen concentration in the tissue to study the oxygen depletion hypothesis using a computational model. The effects of physical, chemical and physiological parameters on oxygen depletion were explored. The kinetic equation of the model is solved numerically using the finite difference method with rational boundary conditions. Results of oxygen distribution is supported by the experiments of oxygen-sensitivity electrodes and experiments on the expression and distribution of the hypoxia-inducible factors. The analysis of parameters shows that the steady-state oxygen distribution before irradiation is determined by the oxygen consumption rate of the tissue and the microvessel density. The change of oxygen concentration after irradiation has been found to follow a negative exponential function, and the time constant is mainly determined by the microvessel density. The change of oxygen during exposure increases with dose rate and tends to be saturated because of oxygen diffusion. When the dose rate is high enough, the same dose results in the same reduction of oxygen concentration regardless of dose rate. The analysis of the FLASH effect in the brain tissue based on this model does not support the explanation of the oxygen depletion hypothesis. The oxygen depletion hypothesis remains controversial because the oxygen in most normal tissues cannot be depleted to radiation resistance level by FLASH irradiation.

The current status of preclinical proton FLASH radiation and future directions

We review the current status of proton FLASH experimental systems, including preclinical physical and biological results. Technological limitations on preclinical investigation of FLASH biological mechanisms and determination of clinically relevant parameters are discussed. A review of the biological data reveals no reproduced proton FLASH effect in vitro and a significant in vivo FLASH sparing effect of normal tissue toxicity observed with multiple proton FLASH irradiation systems. Importantly, multiple studies suggest little or no difference in tumor growth delay for proton FLASH when compared to conventional dose rate proton radiation. A discussion follows on future areas of development with a focus on the determination of the optimal parameters for maximizing the therapeutic ratio between tumor and normal tissue response and ultimately clinical translation of proton FLASH radiation.

Transient hypoxia in water irradiated by swift carbon ions at ultra-high dose rates: implication for FLASH carbon-ion therapy

Large doses of ionizing radiation delivered to tumors at ultra-high dose rates (i.e., in a few milliseconds) paradoxically spare the surrounding healthy tissue while preserving anti-tumor activity (compared with conventional radiotherapy delivered at much lower dose rates). This new modality is known as “FLASH radiotherapy” (FLASH-RT). Although the molecular mechanisms underlying FLASH-RT are not yet fully understood, it has been suggested that radiation delivered at high dose rates spares normal tissue via oxygen depletion followed by subsequent radioresistance of the irradiated tissue. To date, FLASH-RT has been studied using electrons, photons, and protons in various basic biological experiments, pre-clinical studies, and recently in a human patient. However, the efficacy of heavy ions, such as energetic carbon ions, under FLASH-RT conditions remains unclear. Given that living cells and tissues consist mainly of water, we set out to study, from a pure radiation chemistry perspective, the effects of ultra-high dose rates on the transient yields and concentrations of radiolytic species formed in water irradiated by 300-MeV per nucleon carbon ions (LET ∼ 11.6 keV/µm). This mimics irradiation in the “plateau” region of the depth–dose distribution of ions, i.e., in the “normal” tissue region in which the LET is rather low. We used Monte Carlo simulations of multiple, simultaneously interacting radiation tracks together with an “instantaneous pulse” irradiation model. Our calculations show a pronounced oxygen depletion around 0.2 μs, strongly suggesting, as with electrons, photons, and protons, that irradiation with energetic carbon ions at ultra-high dose rates is suitable for FLASH-RT.

Modeling of cellular response after FLASH irradiation: a quantitative analysis based on the radiolytic oxygen depletion hypothesis

Purpose.Recent studies suggest ultra-high dose rate (FLASH) irradiation can spare normal tissues from radiotoxicity, while efficiently controlling the tumor, and this is known as the 'FLASH effect'. This study performed theoretical analyses about the impact of radiolytic oxygen depletion (ROD) on the cellular responses after FLASH irradiation.Methods.Monte Carlo simulation was used to model the ROD process, determine the DNA damage, and calculate the amount of oxygen depleted (L_ROD) during FLASH exposure. A mathematical model was applied to analyze oxygen tension (pO₂) distribution in human tissues and the recovery of pO₂after FLASH irradiation. DNA damage and cell survival fractions (SFs) after FLASH irradiation were calculated. The impact of initial cellular pO₂, FLASH pulse number, pulse interval, and radiation quality of the source particles on ROD and subsequent cellular responses were systematically evaluated.Results.The simulated electronL_RODrange was 0.38-0.43μM Gy^-1when pO₂ranged from 7.5 to 160 mmHg. The calculated DNA damage and SFs show that the radioprotective effect is only evident in cells with a low pO₂. Different irradiation setups alter the cellular responses by modifying the pO₂. Single pulse delivery or multi-pulse delivery with pulse intervals shorter than 10-50 ms resulted in fewer DNA damages and higher SFs. Source particles with a low linear energy transfer (LET) have a higher capacity to deplete oxygen, and thus, lead to a more conspicuous radioprotective effect.Conclusions. A systematic analysis of the cellular response following FLASH irradiation was performed to provided suggestions for future FLASH applications. The FLASH radioprotective effect due to ROD may only be observed in cells with a low pO₂. Single pulse delivery or multi-pulse delivery with short pulse intervals are suggested for FLASH irradiation to avoid oxygen tension recovery during pulse intervals. Source particles with low LET are preferred for their conspicuous radioprotective effects.

Demonstration of the FLASH Effect Within the Spread-out Bragg Peak After Abdominal Irradiation of Mice

Purpose

The effects of FLASH-level dose rates delivered at the spread-out Bragg peak (SOBP) on normal tissue damage in mice were investigated.

Materials and Methods

Fifty nontumor-bearing mice received abdominal irradiation, 30 at FLASH dose rates (100 Gy/s) and 20 at conventional dose rates (0.1 Gy/s). Total dose values ranged from 10 to 19 Gy, delivered in a single spot by a synchrocyclotron proton therapy system. Centered on the abdomen, the collimated field delivered was an 11-mm diameter circle with a water-equivalent depth of 2.4 cm from entrance to distal 80% dose. A ridge filter was used to provide dose uniformity over the full 2.4-cm range. The spatial distribution was identical for both the FLASH and conventional deliveries.

Results

Overall survival and individual mouse weights were tracked for 21 days after the exposure date, and LD50 values were compared for the FLASH and conventional dose rate groups. Mice exposed to FLASH dose rates had a higher LD50 value as compared with mice exposed to conventional dose rates, with a dose-dependent improvement in survivability of 10% to 20%. The FLASH cohort also showed greater or equal percent population survival for each day of the study.

Conclusion

These results are preliminary confirmation of the potential for the combination of the advantages of the Bragg peak with the normal tissue sparing benefits of FLASH treatments. This experiment also confirms that pulsed synchrocyclotrons can be used for the purpose of FLASH research and treatment.

Cancer Cells Can Exhibit a Sparing FLASH Effect at Low Doses Under Normoxic In Vitro-Conditions

Background

Irradiation with ultra-high dose rate (FLASH) has been shown to spare normal tissue without hampering tumor control in several in vivo studies. Few cell lines have been investigated in vitro, and previous results are inconsistent. Assuming that oxygen depletion accounts for the FLASH sparing effect, no sparing should appear for cells irradiated with low doses in normoxia.

Methods

Seven cancer cell lines (MDA-MB-231, MCF7, WiDr, LU-HNSCC4, HeLa [early passage and subclone]) and normal lung fibroblasts (MRC-5) were irradiated with doses ranging from 0 to 12 Gy using FLASH (≥800 Gy/s) or conventional dose rates (CONV, 14 Gy/min), with a 10 MeV electron beam from a clinical linear accelerator. Surviving fraction (SF) was determined with clonogenic assays. Three cell lines were further studied for radiation-induced DNA-damage foci using a 53BP1-marker and for cell cycle synchronization after irradiation.

Results

A tendency of increased survival following FLASH compared with CONV was suggested for all cell lines, with significant differences for 4/7 cell lines. The magnitude of the FLASH-sparing expressed as a dose-modifying factor at SF=0.1 was around 1.1 for 6/7 cell lines and around 1.3 for the HeLa_subclone. Similar cell cycle distributions and 53BP1-foci numbers were found comparing FLASH to CONV.

Conclusion

We have found a FLASH effect appearing at low doses under normoxic conditions for several cell lines in vitro. The magnitude of the FLASH effect differed between the cell lines, suggesting inherited biological susceptibilities for FLASH irradiation.

FLASH radiotherapy with carbon ion beams

FLASH radiotherapy is considered a new potential breakthrough in cancer treatment. Ultra-high dose rates (>40 Gy/s) have been shown to reduce toxicity in the normal tissue without compromising tumor control, resulting in a widened therapeutic window. These high dose rates are more easily achievable in the clinic with charged particles, and clinical trials are, indeed, ongoing using electrons or protons. FLASH could be an attractive solution also for heavier ions such as carbon and could even enhance the therapeutic window. However, it is not yet known whether the FLASH effect will be the same as for sparsely ionizing radiation when densely ionizing carbons ions are used. Here we discuss the technical challenges in beam delivery and present a promising solution using 3D range-modulators in order to apply ultra-high dose rates (UHDR) compatible with FLASH with carbon ions. Furthermore, we will discuss the possible outcome of C-ion therapy at UHDR on the level of the radiobiological and radiation chemical effects.

Quantitative Assessment of 3D Dose Rate for Proton Pencil Beam Scanning FLASH Radiotherapy and Its Application for Lung Hypofractionation Treatment Planning

To quantitatively assess target and organs-at-risk (OAR) dose rate based on three proposed proton PBS dose rate metrics and study FLASH intensity-modulated proton therapy (IMPT) treatment planning using transmission beams. An in-house FLASH planning platform was developed to optimize transmission (shoot-through) plans for nine consecutive lung cancer patients previously planned with proton SBRT. Dose and dose rate calculation codes were developed to quantify three types of dose rate calculation methods (dose-averaged dose rate (DADR), average dose rate (ADR), and dose-threshold dose rate (DTDR)) based on both phantom and patient treatment plans. Two different minimum MU/spot settings were used to optimize two different dose regimes, 34-Gy in one fraction and 45-Gy in three fractions. The OAR sparing and target coverage can be optimized with good uniformity (hotspot < 110% of prescription dose). ADR, accounting for the spot dwelling and scanning time, gives the lowest dose rate; DTDR, not considering this time but a dose-threshold, gives an intermediate dose rate, whereas DADR gives the highest dose rate without considering any time or dose-threshold. All three dose rates attenuate along the beam direction, and the highest dose rate regions often occur on the field edge for ADR and DTDR, whereas DADR has a better dose rate uniformity. The differences in dose rate metrics have led a large variation for OARs dose rate assessment, posing challenges to FLASH clinical implementation. This is the first attempt to study the impact of the dose rate models, and more investigations and evidence for the details of proton PBS FLASH parameters are needed to explore the correlation between FLASH efficacy and the dose rate metrics.

May oxygen depletion explain the FLASH effect? A chemical track structure analysis

BACKGROUND AND PURPOSE

Recent observations in animal models show that ultra-high dose rate ("FLASH") radiation treatment significantly reduces normal tissue toxicity maintaining an equivalent tumor control. The dependence of this "FLASH" effect on target oxygenation has led to the assumption that oxygen "depletion" could be its major driving force.

MATERIALS AND METHODS

In a bottom-up approach starting from the chemical track evolution of 1 MeV electrons in oxygenated water simulated with the TRAX-CHEM Monte Carlo code, we determine the oxygen consumption and radiolytic reactive oxygen species production following a short radiation pulse. Based on these values, the effective dose weighted by oxygen enhancement ratio (OER) or the in vitro cell survival under dynamic oxygen pressure is calculated and compared to that of conventional exposures, at constant OER.

RESULTS

We find an excellent agreement of our Monte Carlo predictions with the experimental value for radiolytic oxygen removal from oxygenated water. However, the application of the present model to published radiobiological experiment conditions shows that oxygen depletion can only have a negligible impact on radiosensitivity through oxygen enhancement, especially at typical experimental oxygenations where a FLASH effect has been observed.

CONCLUSION

We show that the magnitude and dependence of the "oxygen depletion" hypothesis are not consistent with the observed biological effects of FLASH irradiation. While oxygenation plays an undoubted role in mediating the FLASH effect, we conclude that state-of-the-art radiation chemistry models do not support oxygen depletion and radiation-induced transient hypoxia as the main mechanism.

SDDRO-joint: simultaneous dose and dose rate optimization with the joint use of transmission beams and Bragg peaks for FLASH proton therapy

Cancer radiotherapy (RT) with the irradiation at ultra-high dose rates, namely FLASH-RT, can substantially reduce radiation-induced normal tissue toxicities while maintaining tumor response. Currently, clinical FLASH-RT on deep-seated tumors can only be performed with proton beams. One way to achieve ultra-high dose rates at depth is through the use of high-energy transmission beams (TB), where the Bragg peaks (BP) fall outside the body. However, planning with TB alone does not fully leverage the degrees of freedom for dose shaping as traditional intensity modulated proton therapy (IMPT) which uses the BP of multi-energy proton beams at the tumor target. This work will develop a simultaneous dose and dose rate optimization (SDDRO) method with the joint use of TB and BP, namely SDDRO-Joint. Specifically, BP are placed inside tumor targets to improve the target dose conformality and sparse the normal-tissue dose, while TB primarily cover the tumor boundary to achieve ultra-high dose rate coverage of organs-at-risk (OAR) close to tumor targets. The sparing of OAR and other normal tissues via SDDRO-Joint is jointly by TB and BP, i.e. the FLASH sparing by TB and the dose sparing by BP. The results suggest that the addition of BP substantially increased the target dose conformality for SDDRO. Noticeably SDDRO-Joint also provided slightly higher conformal index values than the conventional IMPT method with BP alone.

Does FLASH deplete oxygen? Experimental evaluation for photons, protons, and carbon ions

PURPOSE

To investigate experimentally, if FLASH irradiation depletes oxygen within water for different radiation types such as photons, protons, and carbon ions.

METHODS

This study presents measurements of the oxygen consumption in sealed, 3D-printed water phantoms during irradiation with x-rays, protons, and carbon ions at varying dose rates up to 340 Gy/s. The oxygen measurement was performed using an optical sensor allowing for noninvasive measurements.

RESULTS

Oxygen consumption in water only depends on dose, dose rate, and linear energy transfer (LET) of the irradiation. The total amount of oxygen depleted per 10 Gy was found to be 0.04% atm - 0.18% atm for 225 kV photons, 0.04% atm - 0.25% atm for 224 MeV protons, and 0.09% atm - 0.17% atm for carbon ions. Consumption depends on dose rate by an inverse power law and saturates for higher dose rates because of self-interactions of radicals. Higher dose rates yield lower oxygen consumption. No total depletion of oxygen was found for clinical doses.

CONCLUSIONS

FLASH irradiation does consume oxygen, but not enough to deplete all the oxygen present. For higher dose rates, less oxygen was consumed than at standard radiotherapy dose rates. No total depletion was found for any of the analyzed radiation types for 10 Gy dose delivery using FLASH.

Ultra-high-dose-rate FLASH and Conventional-Dose-Rate Irradiation Differentially Affect Human Acute Lymphoblastic Leukemia and Normal Hematopoiesis

PURPOSE

Ultra-high-dose-rate FLASH radiation therapy has been shown to minimize side effects of irradiation in various organs while keeping antitumor efficacy. This property, called the FLASH effect, has caused enthusiasm in the radiation oncology community because it opens opportunities for safe dose escalation and improved radiation therapy outcome. Here, we investigated the impact of ultra-high-dose-rate FLASH versus conventional-dose-rate (CONV) total body irradiation (TBI) on humanized models of T-cell acute lymphoblastic leukemia (T-ALL) and normal human hematopoiesis.

METHODS AND MATERIALS

We optimized the geometry of irradiation to ensure reproducible and homogeneous procedures using eRT6/Oriatron. Three T-ALL patient-derived xenografts and hematopoietic stem/progenitor cells (HSPCs) and CD34⁺ cells isolated from umbilical cord blood were transplanted into immunocompromised mice, together or separately. After reconstitution, mice received 4 Gy FLASH and CONV-TBI, and tumor growth and normal hematopoiesis were studied. A retrospective study of clinical and gene-profiling data previously obtained on the 3 T-ALL patient-derived xenografts was performed.

RESULTS

FLASH-TBI was more efficient than CONV-TBI in controlling the propagation of 2 cases of T-ALL, whereas the third case of T-ALL was more responsive to CONV-TBI. The 2 FLASH-sensitive cases of T-ALL had similar genetic abnormalities, and a putative susceptibility imprint to FLASH-RT was found. In addition, FLASH-TBI was able to preserve some HSPC/CD34⁺ cell potential. Interestingly, when HSPC and T-ALL were present in the same animals, FLASH-TBI could control tumor development in most (3 of 4) of the secondary grafted animals, whereas among the mice receiving CONV-TBI, treated cells died with high leukemia infiltration.

CONCLUSIONS

Compared with CONV-TBI, FLASH-TBI reduced functional damage to human blood stem cells and had a therapeutic effect on human T-ALL with a common genetic and genomic profile. The validity of the defined susceptibility imprint needs to be investigated further; however, to our knowledge, the present findings are the first to show benefits of FLASH-TBI on human hematopoiesis and leukemia treatment.

Translational Research in FLASH Radiotherapy-From Radiobiological Mechanisms to In Vivo Results

FLASH radiotherapy, or the administration of ultra-high dose rate radiotherapy, is a new radiation delivery method that aims to widen the therapeutic window in radiotherapy. Thus far, most in vitro and in vivo results show a real potential of FLASH to offer superior normal tissue sparing compared to conventionally delivered radiation. While there are several postulations behind the differential behaviour among normal and cancer cells under FLASH, the full spectra of radiobiological mechanisms are yet to be clarified. Currently the number of devices delivering FLASH dose rate is few and is mainly limited to experimental and modified linear accelerators. Nevertheless, FLASH research is increasing with new developments in all the main areas: radiobiology, technology and clinical research. This paper presents the current status of FLASH radiotherapy with the aforementioned aspects in mind, but also to highlight the existing challenges and future prospects to overcome them.

Multicellular Spheroids as In Vitro Models of Oxygen Depletion During FLASH Irradiation

PURPOSE

The differential response of normal and tumor tissues to ultrahigh-dose-rate radiation (FLASH) has raised new hope for treating solid tumors but, to date, the mechanism remains elusive. One leading hypothesis is that FLASH radiochemically depletes oxygen from irradiated tissues faster than it is replenished through diffusion. The purpose of this study was to investigate these effects within hypoxic multicellular tumor spheroids through simulations and experiments.

METHODS AND MATERIALS

Physicobiological equations were derived to model (1) the diffusion and metabolism of oxygen within spheroids; (2) its depletion through reactions involving radiation-induced radicals; and (3) the increase in radioresistance of spheroids, modeled according to the classical oxygen enhancement ratio and linear-quadratic response. These predictions were then tested experimentally in A549 spheroids exposed to electron irradiation at conventional (0.075 Gy/s) or FLASH (90 Gy/s) dose rates. Clonogenic survival, cell viability, and spheroid growth were scored postradiation. Clonogenic survival of 2 other cell lines was also investigated.

RESULTS

The existence of a hypoxic core in unirradiated tumor spheroids is predicted by simulations and visualized by fluorescence microscopy. Upon FLASH irradiation, this hypoxic core transiently expands, engulfing a large number of well-oxygenated cells. In contrast, oxygen is steadily replenished during slower conventional irradiation. Experimentally, clonogenic survival was around 3-fold higher in FLASH-irradiated spheroids compared with conventional irradiation, but no significant difference was observed for well-oxygenated 2-dimensional cultured cells. This differential survival is consistent with the predictions of the computational model. FLASH irradiation of spheroids resulted in a dose-modifying factor of around 1.3 for doses above 10 Gy.

CONCLUSIONS

Tumor spheroids can be used as a model to study FLASH irradiation in vitro. The improved survival of tumor spheroids receiving FLASH radiation confirms that ultrafast radiochemical oxygen depletion and its slow replenishment are critical components of the FLASH effect.

Modeling the effect of oxygen on the chemical stage of water radiolysis using GPU-based microscopic Monte Carlo simulations, with an application in FLASH radiotherapy

Oxygen plays a critical role in determining the initial DNA damages induced by ionizing radiation. It is important to mechanistically model the oxygen effect in the water radiolysis process. However, due to the computational costs from the many body interaction problem, oxygen is often ignored or treated as a constant continuum radiolysis-scavenger background in the simulations using common microscopic Monte Carlo tools. In this work, we reported our recent progress on the modeling of the chemical stage of the water radiolysis with an explicit consideration of the oxygen effect, based upon our initial development of an open-source graphical processing unit (GPU)-based MC simulation tool, gMicroMC. The inclusion of oxygen mainly reduces the yields of [Formula: see text] and [Formula: see text] chemical radicals, turning them into highly toxic [Formula: see text] and [Formula: see text] species. To demonstrate the practical value of gMicroMC in large scale simulation problems, we applied the oxygen-simulation-enabled gMicroMC to compute the yields of chemical radicals under a high instantaneous dose rate [Formula: see text] to study the oxygen depletion hypothesis in FLASH radiotherapy. A decreased oxygen consumption rate (OCR) was found associated with a reduced initial oxygen concentration level due to reduced probabilities of reactions. With respect to dose rate, for the oxygen concentration of 21% and electron energy of 4.5 [Formula: see text], OCR remained approximately constant (∼0.22 [Formula: see text]) for [Formula: see text]'s of [Formula: see text], [Formula: see text] and reduced to 0.19 [Formula: see text] at [Formula: see text], because the increased dose rate improved the mutual reaction frequencies among radicals, hence reducing their reactions with oxygen. We computed the time evolution of oxygen concentration under the FLASH irradiation setups. At the dose rate of [Formula: see text] and initial oxygen concentrations from 0.01% to 21%, the oxygen is unlikely to be fully depleted with an accumulative dose of 30 Gy, which is a typical dose used in FLASH experiments. The computational efficiency of gMicroMC when considering oxygen molecules in the chemical stage was evaluated through benchmark work to GEANT4-DNA with simulating an equivalent number of radicals. With an initial oxygen concentration of 3% (∼10⁵ molecules), a speedup factor of 1228 was achieved for gMicroMC on a single GPU card when comparing with GEANT4-DNA on a single CPU.

Biological and Mechanical Synergies to Deal With Proton Therapy Pitfalls: Minibeams, FLASH, Arcs, and Gantryless Rooms

Proton therapy has advantages and pitfalls comparing with photon therapy in radiation therapy. Among the limitations of protons in clinical practice we can selectively mention: uncertainties in range, lateral penumbra, deposition of higher LET outside the target, entrance dose, dose in the beam path, dose constraints in critical organs close to the target volume, organ movements and cost. In this review, we combine proposals under study to mitigate those pitfalls by using individually or in combination: (a) biological approaches of beam management in time (very high dose rate “FLASH” irradiations in the order of 100 Gy/s) and (b) modulation in space (a combination of mini-beams of millimetric extent), together with mechanical approaches such as (c) rotational techniques (optimized in partial arcs) and, in an effort to reduce cost, (d) gantry-less delivery systems. In some cases, these proposals are synergic (e.g., FLASH and minibeams), in others they are hardly compatible (mini-beam and rotation). Fixed lines have been used in pioneer centers, or for specific indications (ophthalmic, radiosurgery,…), they logically evolved to isocentric gantries. The present proposals to produce fixed lines are somewhat controversial. Rotational techniques, minibeams and FLASH in proton therapy are making their way, with an increasing degree of complexity in these three approaches, but with a high interest in the basic science and clinical communities. All of them must be proven in clinical applications.

Significant changes in yields of 7-hydroxy-coumarin-3-carboxylic acid produced under FLASH radiotherapy conditions

FLASH radiotherapy appears to kill off tumor cells while sparing healthy tissues, by irradiation at ultra high dose rate (>40 Gy s⁻¹). The present study aims to clarify the mechanism of the sparing effect by proton irradiation under the FLASH conditions from a viewpoint of radiation chemistry. To do so, we evaluate radiation chemical yields (G values) of 7-hydroxy-coumarin-3-carboxylic acid (7OH–C3CA), which is produced by water radiolysis using coumarin-3-carboxylic acid (C3CA) solution as a radical scavenger of hydroxyl radicals. We shoot 27.5 MeV protons in the dose rate ranging from 0.05 to 160 Gy s⁻¹. The recombination process of hydroxyl radicals produced is followed by varying the concentration of C3CA from 0.2 to 20 mM, which corresponds to the scavenging time scale from 7.1 to 714 ns. The G value of 7OH–C3CA produced decreases with increasing dose rate on the same scavenging time scale. Additionally, the trend of the relative G value normalized at a scavenging time scale of 100 ns, where radical–radical reaction subsides, is consistent in the examined dose rate range. This finding implies that G values of 7OH–C3CA produced reduce with increasing dose rate due to the oxygen depletion. We experimentally present that the sparing effect for healthy tissues would be seen even with a proton beam under the FLASH conditions due to the depletion of oxygen.

Yield of 7-hydroxy-coumarin-3-carboxylic acid (7OH–C3CA) significantly decreases at FLASH condition with the dose rate of >40 Gy s⁻¹, compared to that at conventional condition of 0.05 Gy s⁻¹, due to the oxygen depletion in the solution.