Irradiation at Ultra-High (FLASH) Dose Rates Reduces Acute Normal Tissue Toxicity in the Mouse Gastrointestinal System

Proton FLASH: Impact of Dose Rate and Split Dose on Acute Skin Toxicity in a Murine Model

PURPOSE

Preclinical studies have shown a preferential normal tissue sparing effect of FLASH radiation therapy with ultra-high dose rates. The aim of the present study was to use a murine model of acute skin toxicity to investigate the biologic effect of varying dose rates, time structure, and introducing pauses in the dose delivery.

METHODS AND MATERIALS

The right hind limbs of nonanaesthetized mice were irradiated in the entrance plateau of a pencil beam scanning proton beam with 39.3 Gy. Experiment 1 was with varying field dose rates (0.7-80 Gy/s) without repainting, experiment 2 was with varying field dose rates (0.37-80 Gy/s) with repainting, and in experiment 3, the dose was split into 2, 3, 4, or 6 identical deliveries with 2-minute pauses. In total, 320 mice were included, with 6 to 25 mice per group. The endpoints were skin toxicity of different levels up to 25 days after irradiation.

RESULTS

The dose rate50, which is the dose rate to induce a response in 50% of the animals, depended on the level of skin toxicity, with the higher toxicity levels displaying a FLASH effect at 0.7-2 Gy/s. Repainting resulted in higher toxicity for the same field dose rate. Splitting the dose into 2 deliveries reduced the FLASH effect, and for 3 or more deliveries, the FLASH effect was almost abolished for lower grades of toxicity.

CONCLUSIONS

The dose rate that induced a FLASH effect varied for different skin toxicity levels, which are characterized by a differing degree of sensitivity to radiation dosage. Conclusions on a threshold for the dose rate needed to obtain a FLASH effect can therefore be influenced by the dose sensitivity of the used endpoint. Splitting the total dose into more deliveries compromised the FLASH effect. This can have an impact for fractionation as well as for regions where 2 or more FLASH fields overlap within the same treatment session.

Oxygen Enhancement Ratio-Weighted Dose Quantitatively Describes Acute Skin Toxicity Variations in Mice After Pencil Beam Scanning Proton FLASH Irradiation With Changing Doses and Time Structures

PURPOSE

The aim of this work was to investigate the ability of a biological oxygen enhancement ratio-weighted dose, DOER, to describe acute skin toxicity variations observed in mice after proton pencil beam scanning irradiations with changing doses and beam time structures.

METHODS AND MATERIALS

In five independent experiments, the right hind leg of a total of 621 CDF1 mice was irradiated previously in the entrance plateau of a pencil beam scanning proton beam. The incidence of acute skin toxicity (of level 1.5-2.0-2.5-3.0-3.5) was scored for 47 different mouse groups that mapped toxicity as function of dose for conventional and FLASH dose rate, toxicity as function of field dose rate with and without repainting, and toxicity when splitting the treatment into 1 to 6 identical deliveries separated by 2 minutes. DOER was calculated for all mouse groups using a simple oxygen kinetics model to describe oxygen depletion. The three independent model parameters (oxygen-depletion rate, oxygen-recovery rate, oxygen level without irradiation) were fitted to the experimental data. The ability of DOER to describe the toxicity variations across all experiments was investigated by comparing DOER-response curves across the five independent experiments.

RESULTS

After conversion from the independent variable tested in each experiment to DOER, all five experiments had similar MDDOER50 (DOER giving 50% toxicity incidence) with standard deviations of 0.45 - 1.6 Gy for the five toxicity levels. DOER could thus describe the observed toxicity variations across all experiments.

CONCLUSIONS

DOER described the varying FLASH-sparing effect observed for a wide range of conditions. Calculation of DOER for other irradiation conditions can quantitatively estimate the FLASH-sparing effect for arbitrary irradiations for the investigated murine model. With appropriate fitting parameters DOER also may be able to describe FLASH effect variations with dose and dose rate for other assays and endpoints.

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.

The sparing effect of ultra-high dose rate irradiation on the esophagus

Background and purpose

Current studies have substantiated the sparing effect of ultra-high dose rate irradiation (FLASH) in various organs including the brain, lungs, and intestines. Whether this sparing effect extends to esophageal tissue remains unexplored. This study aims to compare the different responses of esophageal tissue in histological and protein expression levels following conventional dose rate irradiation (CONV) and FLASH irradiation to ascertain the presence of a sparing effect.

Methods and materials

C57 female mice were randomly divided into three groups: control, CONV, and FLASH groups. The chest region of the mice in the radiation groups was exposed to a prescribed dose of 20 Gy using a modified electron linear accelerator. The CONV group received an average dose rate of 0.1 Gy/s, while the FLASH group received an average dose rate of 125 Gy/s. On the 10th day after irradiation, the mice were euthanized and their esophagi were collected for histopathological analysis. Subsequently, label-free proteomic quantification analysis was performed on esophageal tissue. The validation process involved analyzing transmission electron microscopy images and utilizing the parallel reaction monitoring method.

Results

Histopathology results indicated a significantly lower extent of esophageal tissue damage in the FLASH group compared to the CONV group (p < 0.05). Label-free quantitative proteomic analysis revealed that the sparing effect observed in the FLASH group may be attributed to a reduction in radiation-induced protein damage associated with mitochondrial functions, including proteins involved in the tricarboxylic acid cycle and oxidative phosphorylation, as well as a decrease in acute inflammatory responses.

Conclusions

Compared with CONV irradiation, a sparing effect on esophageal tissue can be observed after FLASH irradiation. This sparing effect is associated with alleviated mitochondria damage and acute inflammation.

Navigating the straits: realizing the potential of proton FLASH through physics advances and further pre-clinical characterization

Ultra-high dose-rate 'FLASH' radiotherapy may be a pivotal step forward for cancer treatment, widening the therapeutic window between radiation tumour killing and damage to neighbouring normal tissues. The extent of normal tissue sparing reported in pre-clinical FLASH studies typically corresponds to an increase in isotoxic dose-levels of 5-20%, though gains are larger at higher doses. Conditions currently thought necessary for FLASH normal tissue sparing are a dose-rate ≥40 Gy s-1, dose-per-fraction ≥5-10 Gy and irradiation duration ≤0.2-0.5 s. Cyclotron proton accelerators are the first clinical systems to be adapted to irradiate deep-seated tumours at FLASH dose-rates, but even using these machines it is challenging to meet the FLASH conditions. In this review we describe the challenges for delivering FLASH proton beam therapy, the compromises that ensue if these challenges are not addressed, and resulting dosimetric losses. Some of these losses are on the same scale as the gains from FLASH found pre-clinically. We therefore conclude that for FLASH to succeed clinically the challenges must be systematically overcome rather than accommodated, and we survey physical and pre-clinical routes for achieving this.

Exploring Deep Learning for Estimating the Isoeffective Dose of FLASH Irradiation From Mouse Intestinal Histological Images

PURPOSE

Ultrahigh-dose-rate (FLASH) irradiation has been reported to reduce normal tissue damage compared with conventional dose rate (CONV) irradiation without compromising tumor control. This proof-of-concept study aims to develop a deep learning (DL) approach to quantify the FLASH isoeffective dose (dose of CONV that would be required to produce the same effect as the given physical FLASH dose) with postirradiation mouse intestinal histology images.

METHODS AND MATERIALS

Eighty-four healthy C57BL/6J female mice underwent 16 MeV electron CONV (0.12 Gy/s; n = 41) or FLASH (200 Gy/s; n = 43) single fraction whole abdominal irradiation. Physical dose ranged from 12 to 16 Gy for FLASH and 11 to 15 Gy for CONV in 1 Gy increments. Four days after irradiation, 9 jejunum cross-sections from each mouse were hematoxylin and eosin stained and digitized for histological analysis. CONV data set was randomly split into training (n = 33) and testing (n = 8) data sets. ResNet101-based DL models were retrained using the CONV training data set to estimate the dose based on histological features. The classical manual crypt counting (CC) approach was implemented for model comparison. Cross-section-wise mean squared error was computed to evaluate the dose estimation accuracy of both approaches. The validated DL model was applied to the FLASH data set to map the physical FLASH dose into the isoeffective dose.

RESULTS

The DL model achieved a cross-section-wise mean squared error of 0.20 Gy2 on the CONV testing data set compared with 0.40 Gy2 of the CC approach. Isoeffective doses estimated by the DL model for FLASH doses of 12, 13, 14, 15, and 16 Gy were 12.19 ± 0.46, 12.54 ± 0.37, 12.69 ± 0.26, 12.84 ± 0.26, and 13.03 ± 0.28 Gy, respectively.

CONCLUSIONS

Our proposed DL model achieved accurate CONV dose estimation. The DL model results indicate that in the physical dose range of 13 to 16 Gy, the biologic dose response of small intestinal tissue to FLASH irradiation is represented by a lower isoeffective dose compared with the physical dose. Our DL approach can be a tool for studying isoeffective doses of other radiation dose modifying interventions.

Navigating the Critical Translational Questions for Implementing FLASH in the Clinic

The "FLASH effect" is an increased therapeutic index, that is, reduced normal tissue toxicity for a given degree of anti-cancer efficacy, produced by ultra-rapid irradiation delivered on time scales orders of magnitude shorter than currently conventional in the clinic for the same doses. This phenomenon has been observed in numerous preclinical in vivo tumor and normal tissue models. While the underlying biological mechanism(s) remain to be elucidated, a path to clinical implementation of FLASH can be paved by addressing several critical translational questions. Technological questions pertinent to each beam type (eg, electron, proton, photon) also dictate the logical progression of experimentation required to move forward in safe and decisive clinical trials. Here we review the available preclinical data pertaining to these questions and how they may inform strategies for FLASH cancer therapy clinical trials.

Development of an Ex Vivo Functional Assay for Prediction of Irradiation Related Toxicity in Healthy Oral Mucosa Tissue

Radiotherapy in the head-and-neck area is one of the main curative treatment options. However, this comes at the cost of varying levels of normal tissue toxicity, affecting up to 80% of patients. Mucositis can cause pain, weight loss and treatment delays, leading to worse outcomes and a decreased quality of life. Therefore, there is an urgent need for an approach to predicting normal mucosal responses in patients prior to treatment. We here describe an assay to detect irradiation responses in healthy oral mucosa tissue. Mucosa specimens from the oral cavity were obtained after surgical resection, cut into thin slices, irradiated and cultured for three days. Seven samples were irradiated with X-ray, and three additional samples were irradiated with both X-ray and protons. Healthy oral mucosa tissue slices maintained normal morphology and viability for three days. We measured a dose-dependent response to X-ray irradiation and compared X-ray and proton irradiation in the same mucosa sample using standardized automated image analysis. Furthermore, increased levels of inflammation-inducing factors-major drivers of mucositis development-could be detected after irradiation. This model can be utilized for investigating mechanistic aspects of mucositis development and can be developed into an assay to predict radiation-induced toxicity in normal mucosa.

How quickly does FLASH need to be delivered? A theoretical study of radiolytic oxygen depletion kinetics in tissues

Purpose. Radiation delivered over ultra-short timescales ('FLASH' radiotherapy) leads to a reduction in normal tissue toxicities for a range of tissues in the preclinical setting. Experiments have shown this reduction occurs for total delivery times less than a 'critical' time that varies by two orders of magnitude between brain (∼0.3 s) and skin (⪆10 s), and three orders of magnitude across different bowel experiments, from ∼0.01 to ⪆(1-10) s. Understanding the factors responsible for this broad variation may be important for translation of FLASH into the clinic and understanding the mechanisms behind FLASH.Methods.Assuming radiolytic oxygen depletion (ROD) to be the primary driver of FLASH effects, oxygen diffusion, consumption, and ROD were evaluated numerically for simulated tissues with pseudorandom vasculatures for a range of radiation delivery times, capillary densities, and oxygen consumption rates (OCR's). The resulting time-dependent oxygen partial pressure distribution histograms were used to estimate cell survival in these tissues using the linear quadratic model, modified to incorporate oxygen-enhancement ratio effects.Results. Independent of the capillary density, there was a substantial increase in predicted cell survival when the total delivery time was less than the capillary oxygen tension (mmHg) divided by the OCR (expressed in units of mmHg/s), setting the critical delivery time for FLASH in simulated tissues. Using literature OCR values for different normal tissues, the predicted range of critical delivery times agreed well with experimental values for skin and brain and, modifying our model to allow for fluctuating perfusion, bowel.Conclusions. The broad three-orders-of-magnitude variation in critical irradiation delivery times observed inin vivopreclinical experiments can be accounted for by the ROD hypothesis and differences in the OCR amongst simulated normal tissues. Characterization of these may help guide future experiments and open the door to optimized tissue-specific clinical protocols.

Dose and dose rate dependence of the tissue sparing effect at ultra-high dose rate studied for proton and electron beams using the zebrafish embryo model

PURPOSE

A better characterization of the dependence of the tissue sparing effect at ultra-high dose rate (UHDR) on physical beam parameters (dose, dose rate, radiation quality) would be helpful towards a mechanistic understanding of the FLASH effect and for its broader clinical translation. To address this, a comprehensive study on the normal tissue sparing at UHDR using the zebrafish embryo (ZFE) model was conducted.

METHODS

One-day-old ZFE were irradiated over a wide dose range (15-95 Gy) in three different beams (proton entrance channel, proton spread out Bragg peak and 30 MeV electrons) at UHDR and reference dose rate. After irradiation the ZFE were incubated for 4 days and then analyzed for four different biological endpoints (pericardial edema, curved spine, embryo length and eye diameter).

RESULTS

Dose-effect curves were obtained and a sparing effect at UHDR was observed for all three beams. It was demonstrated that proton relative biological effectiveness and UHDR sparing are both relevant to predict the resulting dose response. Dose dependent FLASH modifying factors (FMF) for ZFE were found to be compatible with rodent data from the literature. It was found that the UHDR sparing effect saturates at doses above ∼ 50 Gy with an FMF of ∼ 0.7-0.8. A strong dose rate dependence of the tissue sparing effect in ZFE was observed. The magnitude of the maximum sparing effect was comparable for all studied biological endpoints.

CONCLUSION

The ZFE model was shown to be a suitable pre-clinical high-throughput model for radiobiological studies on FLASH radiotherapy, providing results comparable to rodent models. This underlines the relevance of ZFE studies for FLASH radiotherapy research.

Very high-energy electron therapy as light-particle alternative to transmission proton FLASH therapy - An evaluation of dosimetric performances

PURPOSE

Clinical translation of FLASH-radiotherapy (RT) to deep-seated tumours is still a technological challenge. One proposed solution consists of using ultra-high dose rate transmission proton (TP) beams of about 200-250 MeV to irradiate the tumour with the flat entrance of the proton depth-dose profile. This work evaluates the dosimetric performance of very high-energy electron (VHEE)-based RT (50-250 MeV) as a potential alternative to TP-based RT for the clinical transfer of the FLASH effect.

METHODS

Basic physics characteristics of VHEE and TP beams were compared utilizing Monte Carlo simulations in water. A VHEE-enabled research treatment planning system was used to evaluate the plan quality achievable with VHEE beams of different energies, compared to 250 MeV TP beams for a glioblastoma, an oesophagus, and a prostate cancer case.

RESULTS

Like TP, VHEE above 100 MeV can treat targets with roughly flat (within ± 20 %) depth-dose distributions. The achievable dosimetric target conformity and adjacent organs-at-risk (OAR) sparing is consequently driven for both modalities by their lateral beam penumbrae. Electron beams of 400[500] MeV match the penumbra of 200[250] MeV TP beams and penumbra is increased for lower electron energies. For the investigated patient cases, VHEE plans with energies of 150 MeV and above achieved a dosimetric plan quality comparable to that of 250 MeV TP plans. For the glioblastoma and the oesophagus case, although having a decreased conformity, even 100 MeV VHEE plans provided a similar target coverage and OAR sparing compared to TP.

CONCLUSIONS

VHEE-based FLASH-RT using sufficiently high beam energies may provide a lighter-particle alternative to TP-based FLASH-RT with comparable dosimetric plan quality.

Feasibility and constraints of Bragg peak FLASH proton therapy treatment planning

Introduction

FLASH proton therapy (FLASH-PT) requires ultra-high dose rate (≥ 40 Gy/s) protons to be delivered in a short timescale whilst conforming to a patient-specific target. This study investigates the feasibility and constraints of Bragg peak FLASH-PT treatment planning, and compares the in silico results produced to plans for intensity modulated proton therapy (IMPT).

Materials and method

Bragg peak FLASH-PT and IMPT treatment plans were generated for bone (n=3), brain (n=3), and lung (n=4) targets using the MIROpt research treatment planning system and the Conformal FLASH library developed by Applications SA from the open-source version of UCLouvain. FLASH-PT beams were simulated using monoenergetic spot-scanned protons traversing through a conformal energy modulator, a range shifter, and an aperture. A dose rate constraint of ≥ 40 Gy/s was included in each FLASH-PT plan optimisation.

Results

Space limitations in the FLASH-PT adapted beam nozzle imposed a maximum target width constraint, excluding 4 cases from the study. FLASH-PT plans did not satisfy the imposed target dose constraints (D95% ≥ 95% and D2%≤ 105%) but achieved clinically acceptable doses to organs at risk (OARs). IMPT plans adhered to all target and OAR dose constraints. FLASH-PT plans showed a reduction in both target homogeneity (p < 0.001) and dose conformity (non-significant) compared to IMPT.

Conclusion

Without accounting for a sparing effect, IMPT plans were superior in target coverage, dose conformity, target homogeneity, and OAR sparing compared to FLASH-PT. Further research is warranted in treatment planning optimisation and beam delivery for clinical implementation of Bragg peak FLASH-PT.

Redefining FLASH RT: the impact of mean dose rate and dose per pulse in the gastrointestinal tract

Background

The understanding of how varying radiation beam parameter settings affect the induction and magnitude of the FLASH effect remains limited.

Purpose

We sought to evaluate how the magnitude of radiation-induced gastrointestinal (GI) toxicity (RIGIT) depends on the interplay between mean dose rate (MDR) and dose per pulse (DPP).

Methods

C57BL/6J mice were subjected to total abdominal irradiation (11-14 Gy single fraction) under conventional irradiation (low DPP and low MDR, CONV) and various combinations of DPP and MDR up to ultra-high-dose-rate (UHDR) beam conditions. The effects of DPP were evaluated for DPPs of 1-6 Gy while the total dose and MDR were kept constant; the effects of MDR were evaluated for the range 0.3- 1440 Gy/s while the total dose and DPP were kept constant. RIGIT was quantified in non-tumor-bearing mice through the regenerating crypt assay and survival assessment. Tumor response was evaluated through tumor growth delay.

Results

Within each tested total dose using a constant MDR (>100 Gy/s), increasing DPP led to better sparing of regenerating crypts, with a more prominent effect seen at 12 and 14 Gy TAI. However, at fixed DPPs >4 Gy, similar sparing of crypts was demonstrated irrespective of MDR (from 0.3 to 1440 Gy/s). At a fixed high DPP of 4.7 Gy, survival was equivalently improved relative to CONV for all MDRs from 0.3 Gy/s to 104 Gy/s, but at a lower DPP of 0.93 Gy, increasing MDR produced a greater survival effect. We also confirmed that high DPP, regardless of MDR, produced the same magnitude of tumor growth delay relative to CONV using a clinically relevant melanoma mouse model.

Conclusions

This study demonstrates the strong influence that the beam parameter settings have on the magnitude of the FLASH effect. Both high DPP and UHDR appeared independently sufficient to produce FLASH sparing of GI toxicity, while isoeffective tumor response was maintained across all conditions.

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

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

Electron FLASH radiotherapy in vivo studies. A systematic review

FLASH-radiotherapy delivers a radiation beam a thousand times faster compared to conventional radiotherapy, reducing radiation damage in healthy tissues with an equivalent tumor response. Although not completely understood, this radiobiological phenomenon has been proved in several animal models with a spectrum of all kinds of particles currently used in contemporary radiotherapy, especially electrons. However, all the research teams have performed FLASH preclinical studies using industrial linear accelerator or LINAC commonly employed in conventional radiotherapy and modified for the delivery of ultra-high-dose-rate (UHDRs). Unfortunately, the delivering and measuring of UHDR beams have been proved not to be completely reliable with such devices. Concerns arise regarding the accuracy of beam monitoring and dosimetry systems. Additionally, this LINAC totally lacks an integrated and dedicated Treatment Planning System (TPS) able to evaluate the internal dose distribution in the case of in vivo experiments. Finally, these devices cannot modify dose-time parameters of the beam relevant to the flash effect, such as average dose rate; dose per pulse; and instantaneous dose rate. This aspect also precludes the exploration of the quantitative relationship with biological phenomena. The dependence on these parameters need to be further investigated. A promising advancement is represented by a new generation of electron LINAC that has successfully overcome some of these technological challenges. In this review, we aim to provide a comprehensive summary of the existing literature on in vivo experiments using electron FLASH radiotherapy and explore the promising clinical perspectives associated with this technology.

The AsiDNA™ decoy mimicking DSBs protects the normal tissue from radiation toxicity through a DNA-PK/p53/p21-dependent G1/S arrest

AsiDNA™, a cholesterol-coupled oligonucleotide mimicking double-stranded DNA breaks, was developed to sensitize tumour cells to radio- and chemotherapy. This drug acts as a decoy hijacking the DNA damage response. Previous studies have demonstrated that standalone AsiDNA™ administration is well tolerated with no additional adverse effects when combined with chemo- and/or radiotherapy. The lack of normal tissue complication encouraged further examination into the role of AsiDNA™ in normal cells. This research demonstrates the radioprotective properties of AsiDNA™. In vitro, AsiDNA™ induces a DNA-PK/p53/p21-dependent G1/S arrest in normal epithelial cells and fibroblasts that is absent in p53 deficient and proficient tumour cells. This cell cycle arrest improved survival after irradiation only in p53 proficient normal cells. Combined administration of AsiDNA™ with conventional radiotherapy in mouse models of late and early radiation toxicity resulted in decreased onset of lung fibrosis and increased intestinal crypt survival. Similar results were observed following FLASH radiotherapy in standalone or combined with AsiDNA™. Mechanisms comparable to those identified in vitro were detected both in vivo, in the intestine and ex vivo, in precision cut lung slices. Collectively, the results suggest that AsiDNA™ can partially protect healthy tissues from radiation toxicity by triggering a G1/S arrest in normal cells.

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.

Mini-GRID radiotherapy on the CLEAR very-high-energy electron beamline: collimator optimization, film dosimetry, and Monte Carlo simulations

Objective.Spatially-fractionated radiotherapy (SFRT) delivered with a very-high-energy electron (VHEE) beam and a mini-GRID collimator was investigated to achieve synergistic normal tissue-sparing through spatial fractionation and the FLASH effect.Approach.A tungsten mini-GRID collimator for delivering VHEE SFRT was optimized using Monte Carlo (MC) simulations. Peak-to-valley dose ratios (PVDRs), depths of convergence (DoCs, PVDR ≤ 1.1), and peak and valley doses in a water phantom from a simulated 150 MeV VHEE source were evaluated. Collimator thickness, hole width, and septal width were varied to determine an optimal value for each parameter that maximized PVDR and DoC. The optimized collimator (20 mm thick rectangular prism with a 15 mm × 15 mm face with a 7 × 7 array of 0.5 mm holes separated by 1.1 mm septa) was 3D-printed and used for VHEE irradiations with the CERN linear electron accelerator for research beam. Open beam and mini-GRID irradiations were performed at 140, 175, and 200 MeV and dose was recorded with radiochromic films in a water tank. PVDR, central-axis (CAX) and valley dose rates and DoCs were evaluated.Main results.Films demonstrated peak and valley dose rates on the order of 100 s of MGy/s, which could promote FLASH-sparing effects. Across the three energies, PVDRs of 2-4 at 13 mm depth and DoCs between 39 and 47 mm were achieved. Open beam and mini-GRID MC simulations were run to replicate the film results at 200 MeV. For the mini-GRID irradiations, the film CAX dose was on average 15% higher, the film valley dose was 28% higher, and the film PVDR was 15% lower than calculated by MC.Significance.Ultimately, the PVDRs and DoCs were determined to be too low for a significant potential for SFRT tissue-sparing effects to be present, particularly at depth. Further beam delivery optimization and investigations of new means of spatial fractionation are warranted.

Dosimetric and biologic intercomparison between electron and proton FLASH beams

BACKGROUND AND PURPOSE

The FLASH effect has been validated in different preclinical experiments with electrons (eFLASH) and protons (pFLASH) operating at an average dose rate above 40 Gy/s. However, no systematic intercomparison of the FLASH effect produced by eFLASHvs. pFLASH has yet been performed and constitutes the aim of the present study.

MATERIALS AND METHODS

The electron eRT6/Oriatron/CHUV/5.5 MeV and proton Gantry1/PSI/170 MeV were used to deliver conventional (0.1 Gy/s eCONV and pCONV) and FLASH (≥110 Gy/s eFLASH and pFLASH) dose rates. Protons were delivered in transmission. Dosimetric and biologic intercomparisons were performed using previously validated dosimetric approaches and experimental murine models.

RESULTS

The difference between the average absorbed dose measured at Gantry 1 with PSI reference dosimeters and with CHUV/IRA dosimeters was -1.9 % (0.1 Gy/s) and + 2.5 % (110 Gy/s). The neurocognitive capacity of eFLASH and pFLASH irradiated mice was indistinguishable from the control, while both eCONV and pCONV irradiated cohorts showed cognitive decrements. Complete tumor response was obtained after an ablative dose of 20 Gy delivered with the two beams at CONV and FLASH dose rates. Tumor rejection upon rechallenge indicates that anti-tumor immunity was activated independently of the beam-type and the dose-rate.

CONCLUSION

Despite major differences in the temporal microstructure of proton and electron beams, this study shows that dosimetric standards can be established. Normal brain protection and tumor control were produced by the two beams. More specifically, normal brain protection was achieved when a single dose of 10 Gy was delivered in 90 ms or less, suggesting that the most important physical parameter driving the FLASH sparing effect might be the mean dose rate. In addition, a systemic anti-tumor immunological memory response was observed in mice exposed to high ablative dose of electron and proton delivered at CONV and FLASH dose rate.

The DRESDEN PLATFORM is a research hub for ultra-high dose rate radiobiology

The recently observed FLASH effect describes the observation of normal tissue protection by ultra-high dose rates (UHDR), or dose delivery in a fraction of a second, at similar tumor-killing efficacy of conventional dose delivery and promises great benefits for radiotherapy patients. Dedicated studies are now necessary to define a robust set of dose application parameters for FLASH radiotherapy and to identify underlying mechanisms. These studies require particle accelerators with variable temporal dose application characteristics for numerous radiation qualities, equipped for preclinical radiobiological research. Here we present the DRESDEN PLATFORM, a research hub for ultra-high dose rate radiobiology. By uniting clinical and research accelerators with radiobiology infrastructure and know-how, the DRESDEN PLATFORM offers a unique environment for studying the FLASH effect. We introduce its experimental capabilities and demonstrate the platform's suitability for systematic investigation of FLASH by presenting results from a concerted in vivo radiobiology study with zebrafish embryos. The comparative pre-clinical study was conducted across one electron and two proton accelerator facilities, including an advanced laser-driven proton source applied for FLASH-relevant in vivo irradiations for the first time. The data show a protective effect of UHDR irradiation up to [Formula: see text] and suggests consistency of the protective effect even at escalated dose rates of [Formula: see text]. With the first clinical FLASH studies underway, research facilities like the DRESDEN PLATFORM, addressing the open questions surrounding FLASH, are essential to accelerate FLASH's translation into clinical practice.

Effect of Conventional and Ultrahigh Dose Rate FLASH Irradiations on Preclinical Tumor Models: A Systematic Analysis

PURPOSE

Compared with conventional dose rate irradiation (CONV), ultrahigh dose rate irradiation (UHDR) has shown superior normal tissue sparing. However, a clinically relevant widening of the therapeutic window by UHDR, termed "FLASH effect," also depends on the tumor toxicity obtained by UHDR. Based on a combined analysis of published literature, the current study examined the hypothesis of tumor isoefficacy for UHDR versus CONV and aimed to identify potential knowledge gaps to inspire future in vivo studies.

METHODS AND MATERIALS

A systematic literature search identified publications assessing in vivo tumor responses comparing UHDR and CONV. Qualitative and quantitative analyses were performed, including combined analyses of tumor growth and survival data.

RESULTS

We identified 66 data sets from 15 publications that compared UHDR and CONV for tumor efficacy. The median number of animals per group was 9 (range 3-15) and the median follow-up period was 30.5 days (range 11-230) after the first irradiation. Tumor growth assays were the predominant model used. Combined statistical analyses of tumor growth and survival data are consistent with UHDR isoefficacy compared with CONV. Only 1 study determined tumor-controlling dose (TCD50) and reported statistically nonsignificant differences.

CONCLUSIONS

The combined quantitative analyses of tumor responses support the assumption of UHDR isoefficacy compared with CONV. However, the comparisons are primarily based on heterogeneous tumor growth assays with limited numbers of animals and short follow-up, and most studies do not assess long-term tumor control probability. Therefore, the assays may be insensitive in resolving smaller response differences, such as responses of radioresistant tumor subclones. Hence, tumor cure experiments, including additional TCD50 experiments, are needed to confirm the assumption of isoeffectiveness in curative settings.

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

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

Lung Organotypic Slices Enable Rapid Quantification of Acute Radiotherapy Induced Toxicity

To rapidly assess healthy tissue toxicities induced by new anti-cancer therapies (i.e., radiation alone or in combination with drugs), there is a critical need for relevant and easy-to-use models. Consistent with the ethical desire to reduce the use of animals in medical research, we propose to monitor lung toxicity using an ex vivo model. Briefly, freshly prepared organotypic lung slices from mice were irradiated, with or without being previously exposed to chemotherapy, and treatment toxicity was evaluated by analysis of cell division and viability of the slices. When exposed to different doses of radiation, this ex vivo model showed a dose-dependent decrease in cell division and viability. Interestingly, monitoring cell division was sensitive enough to detect a sparing effect induced by FLASH radiotherapy as well as the effect of combined treatment. Altogether, the organotypic lung slices can be used as a screening platform to rapidly determine in a quantitative manner the level of lung toxicity induced by different treatments alone or in combination with chemotherapy while drastically reducing the number of animals. Translated to human lung samples, this ex vivo assay could serve as an innovative method to investigate patients' sensitivity to radiation and drugs.

Flash Therapy for Cancer: A Potentially New Radiotherapy Methodology

In traditional treatment modalities and standard clinical practices, FLASH radiotherapy (FL-RT) administers radiation therapy at an exceptionally high dosage rate. When compared to standard dose rate radiation therapy, numerous preclinical investigations have demonstrated that FL-RT provides similar benefits in conserving normal tissue while maintaining equal antitumor efficacy, a phenomenon possible due to the 'FLASH effect' (FE) of FL-RT. The methodologies involve proton radiotherapy, intensity-modulated radiation treatment, and managing high-throughput damage by radiation to solid tissues. Recent results from animal studies indicate that FL-RT can reduce radiation-induced tissue damage, significantly enhancing anticancer potency. Focusing on the potential benefits of FL proton beam treatment in the years to come, this review details the FL-RT research that has been done so far and the existing theories illuminating the FL effects. This subject remains of interest, with many issues still needing to be answered. We offer a brief review to emphasize a few of the key efforts and difficulties in moving FL radiation research forward. The existing research state of FL-RT, its affecting variables, and its different specific impacts are presented in this current review. Key topics discussed include the biochemical mechanism during FL therapy, beam sources for FL therapy, the FL effect on immunity, clinical and preclinical studies on the protective effect of FL therapy, and parameters for effective FL therapy.

Evaluation of single-fraction high dose FLASH radiotherapy in a cohort of canine oral cancer patients

Background

FLASH radiotherapy (RT) is a novel method for delivering ionizing radiation, which has been shown in preclinical studies to have a normal tissue sparing effect and to maintain anticancer efficacy as compared to conventional RT. Treatment of head and neck tumors with conventional RT is commonly associated with severe toxicity, hence the normal tissue sparing effect of FLASH RT potentially makes it especially advantageous for treating oral tumors. In this work, the objective was to study the adverse effects of dogs with spontaneous oral tumors treated with FLASH RT.

Methods

Privately-owned dogs with macroscopic malignant tumors of the oral cavity were treated with a single fraction of ≥30Gy electron FLASH RT and subsequently followed for 12 months. A modified conventional linear accelerator was used to deliver the FLASH RT.

Results

Eleven dogs were enrolled in this prospective study. High grade adverse effects were common, especially if bone was included in the treatment field. Four out of six dogs, who had bone in their treatment field and lived at least 5 months after RT, developed osteoradionecrosis at 3-12 months post treatment. The treatment was overall effective with 8/11 complete clinical responses and 3/11 partial responses.

Conclusion

This study shows that single-fraction high dose FLASH RT was generally effective in this mixed group of malignant oral tumors, but the risk of osteoradionecrosis is a serious clinical concern. It is possible that the risk of osteonecrosis can be mitigated through fractionation and improved dose conformity, which needs to be addressed before moving forward with clinical trials in human cancer patients.

Effects of UHDR and Conventional Irradiation on Behavioral and Cognitive Performance and the Percentage of Ly6G+ CD45+ Cells in the Hippocampus

We assessed the effects of conventional and ultra-high dose rate (UHDR) electron irradiation on behavioral and cognitive performance one month following exposure and assessed whether these effects were associated with alterations in the number of immune cells in the hippocampus using flow cytometry. Two-month-old female and male C57BL/6J mice received whole-brain conventional or UHDR irradiation. UHDR mice were irradiated with 9 MeV electrons, delivered by the Linac-based/modified beam control. The mice were irradiated or sham-irradiated at Dartmouth, the following week shipped to OHSU, and behaviorally and cognitively tested between 27 and 41 days after exposure. Conventional- and UHDR-irradiated mice showed impaired novel object recognition. During fear learning, conventional- and UHDR-irradiated mice moved less during the inter-stimulus interval (ISI) and UHDR-irradiated mice also moved less during the baseline period (prior to the first tone). In irradiated mice, reduced activity levels were also seen in the home cage: conventional- and UHDR-irradiated mice moved less during the light period and UHDR-irradiated mice moved less during the dark period. Following behavioral and cognitive testing, infiltrating immune cells in the hippocampus were analyzed by flow cytometry. The percentage of Ly6G+ CD45+ cells in the hippocampus was lower in conventional- and UHDR-irradiated than sham-irradiated mice, suggesting that neutrophils might be particularly sensitive to radiation. The percentage of Ly6G+ CD45+ cells in the hippocampus was positively correlated with the time spent exploring the novel object in the object recognition test. Under the experimental conditions used, cognitive injury was comparable in conventional and UHDR mice. However, the percentage of CD45+ CD11b+ Ly6+ and CD45+ CD11b+ Ly6G- cells in the hippocampus cells in the hippocampus was altered in conventional- but not UHDR-irradiated mice and the reduced percentage of Ly6G+ CD45+ cells in the hippocampus might mediate some of the detrimental radiation-induced cognitive effects.

A feasibility study of ultra-high dose rate mini-GRID therapy using very-high-energy electron beams for a simulated pediatric brain case

Ultra-high dose rate (UHDR, >40 Gy/s), spatially-fractionated minibeam GRID (mini-GRID) therapy using very-high-energy electrons (VHEE) was investigated using Monte Carlo simulations. Multi-directional VHEE treatments with and without mini-GRID-fractionation were compared to a clinical 6 MV volumetric modulated arc therapy (VMAT) plan for a pediatric glioblastoma patient using dose-volume histograms, volume-averaged dose rates in critical patient structures, and planning target volume D98s. Peak-to-valley dose ratios (PVDRs) and dose rates in organs at risk (OARs) were evaluated due to their relevance for normal-tissue sparing in FLASH and spatially-fractionated techniques. Depths of convergence, defined where the PVDR is first ≤1.1, and depths at which dose rates fall below the UHDR threshold were also evaluated. In a water phantom, the VHEE mini-GRID treatments presented a surface (5 mm depth) PVDR of (51±2) and a depth of convergence of 42 mm at 150 MeV and a surface PVDR of (33±1) with a depth of convergence of 57 mm at 250 MeV. For a pediatric GBM case, VHEE treatments without mini-GRID-fractionation produced 25% and 22% lower volume-averaged doses to OARs compared to the 6 MV VMAT plan and 8/9 and 9/9 of the patient structures were exposed to volume-averaged dose rates >40 Gy/s for the 150 MeV and 250 MeV plans, respectively. The 150 MeV and 250 MeV mini-GRID treatments produced 17% and 38% higher volume-averaged doses to OARs and 3/9 patient structures had volume-averaged dose rates above 40 Gy/s. VHEE mini-GRID plans produced many comparable dose metrics to the clinical VMAT plan, encouraging further optimization.

N-acetyl-L-tryptophan (NAT) provides protection to intestinal epithelial cells (IEC-6) against radiation-induced apoptosis via modulation of oxidative stress and mitochondrial membrane integrity

BACKGROUND

Ionizing radiation generates oxidative stress in biological systems via inducing free radicals. Gastro-intestinal system has been known for its high radiosensitivity. Therefore, to develop an effective radiation countermeasure for gastrointestinal system, N-acetyl L-tryptophan was evaluated for its radioprotective efficacy using intestinal epithelial cells-6 (IEC-6) cells as the experimental model.

METHODS AND RESULTS

Cellular metabolic and lysosomal activity of L-NAT and L-NAT treated irradiated IEC-6 cells were assessed by MTT and NRU staining, respectively. ROS and mitochondrial superoxide levels along with mitochondrial disruption were detected using specific fluorescent probes.  Endogenous antioxidants (CAT, SOD, GST, GPx) activities were determined using calorimetric assay.  Apoptosis and DNA damage were assessed using flow cytometery and Comet assay, respectively. Results of the study were demonstrated that L-NAT pre-treatment (- 1 h) to irradiated IEC-6 cells significantly contribute to ensuring 84.36% to 87.68% (p < 0.0001) survival at 0.1 μg/mL concentration against LD50 radiation dose (LD50; 20 Gy). Similar level of radioprotection was observed with a clonogenic assay against γ radiation (LD50; 5 Gy). L-NAT was found to provide radioprotection by neutralizing radiation-induced oxidative stress, enhancing antioxidant enzymes (CAT, SOD, GST, and GPx), and protecting DNA from radiation-induced damage. Further, significant restoration of mitochondrial membrane integrity along with apoptosis inhibition was observed with irradiated IEC-6 cells upon L-NAT pretreatment.

Framework for Quality Assurance of Ultrahigh Dose Rate Clinical Trials Investigating FLASH Effects and Current Technology Gaps

FLASH radiation therapy (FLASH-RT), delivered with ultrahigh dose rate (UHDR), may allow patients to be treated with less normal tissue toxicity for a given tumor dose compared with currently used conventional dose rate. Clinical trials are being carried out and are needed to test whether this improved therapeutic ratio can be achieved clinically. During the clinical trials, quality assurance and credentialing of equipment and participating sites, particularly pertaining to UHDR-specific aspects, will be crucial for the validity of the outcomes of such trials. This report represents an initial framework proposed by the NRG Oncology Center for Innovation in Radiation Oncology FLASH working group on quality assurance of potential UHDR clinical trials and reviews current technology gaps to overcome. An important but separate consideration is the appropriate design of trials to most effectively answer clinical and scientific questions about FLASH. This paper begins with an overview of UHDR RT delivery methods. UHDR beam delivery parameters are then covered, with a focus on electron and proton modalities. The definition and control of safe UHDR beam delivery and current and needed dosimetry technologies are reviewed and discussed. System and site credentialing for large, multi-institution trials are reviewed. Quality assurance is then discussed, and new requirements are presented for treatment system standard analysis, patient positioning, and treatment planning. The tables and figures in this paper are meant to serve as reference points as we move toward FLASH-RT clinical trial performance. Some major questions regarding FLASH-RT are discussed, and next steps in this field are proposed. FLASH-RT has potential but is associated with significant risks and complexities. We need to redefine optimization to focus not only on the dose but also on the dose rate in a manner that is robust and understandable and that can be prescribed, validated, and confirmed in real time. Robust patient safety systems and access to treatment data will be critical as FLASH-RT moves into the clinical trials.

Comparison of Gonadal Toxicity of Single-Fraction Ultra-High Dose Rate and Conventional Radiation in Mice

Purpose

Increasing evidence suggests that ultra-high-dose-rate (UHDR) radiation could result in similar tumor control as conventional (CONV) radiation therapy (RT) while reducing toxicity to surrounding healthy tissues. Considering that radiation toxicity to gonadal tissues can cause hormone disturbances and infertility in young patients with cancer, the purpose of this study was to assess the possible role of UHDR-RT in reducing toxicity to healthy gonads in mice compared with CONV-RT.

Methods and Materials

Radiation was delivered to the abdomen or pelvis of female (8 or 16 Gy) and male (5 Gy) C57BL/6J mice, respectively, at conventional (∼0.4 Gy/s) or ultrahigh (>100 Gy/s) dose rates using an IntraOp Mobetron linear accelerator. Organ weights along with histopathology and immunostaining of irradiated gonads were used to compare toxicity between radiation modalities.

Results

CONV-RT and UHDR-RT induced a similar decrease in uterine weights at both studied doses (∼50% of controls), which indicated similarly reduced ovarian follicular activity. Histologically, ovaries of CONV- and UHDR-irradiated mice exhibited a comparable lack of follicles. Weights of CONV- and UHDR-irradiated testes were reduced to ∼30% of controls, and the percentage of degenerate seminiferous tubules was also similar between radiation modalities (∼80% above controls). Pairwise comparisons of all quantitative data indicated statistical significance between irradiated (CONV or UHDR) and control groups (from P ≤ .01 to P ≤ .0001) but not between radiation modalities.

Conclusions

The data presented here suggest that the short-term effects of UHDR-RT on the mouse gonads are comparable to those of CONV-RT.

Dosimetric and biologic intercomparison between electron and proton FLASH beams

Background and purpose

The FLASH effect has been validated in different preclinical experiments with electrons (eFLASH) and protons (pFLASH) operating at a mean dose rate above 40 Gy/s. However, no systematic intercomparison of the FLASH effect produced by e vs. pFLASH has yet been performed and constitutes the aim of the present study.

Materials and methods

The electron eRT6/Oriatron/CHUV/5.5 MeV and proton Gantry1/PSI/170 MeV were used to deliver conventional (0.1 Gy/s eCONV and pCONV) and FLASH (≥100 Gy/s eFLASH and pFLASH) irradiation. Protons were delivered in transmission. Dosimetric and biologic intercomparisons were performed with previously validated models.

Results

Doses measured at Gantry1 were in agreement (± 2.5%) with reference dosimeters calibrated at CHUV/IRA. The neurocognitive capacity of e and pFLASH irradiated mice was indistinguishable from the control while both e and pCONV irradiated cohorts showed cognitive decrements. Complete tumor response was obtained with the two beams and was similar between e and pFLASH vs. e and pCONV. Tumor rejection was similar indicating that T-cell memory response is beam-type and dose-rate independent.

Conclusion

Despite major differences in the temporal microstructure, this study shows that dosimetric standards can be established. The sparing of brain function and tumor control produced by the two beams were similar, suggesting that the most important physical parameter driving the FLASH effect is the overall time of exposure which should be in the range of hundreds of milliseconds for WBI in mice. In addition, we observed that immunological memory response is similar between electron and proton beams and is independent off the dose rate.

Absence of Tissue-Sparing Effects in Partial Proton FLASH Irradiation in Murine Intestine

Ultra-high dose rate irradiation has been reported to protect normal tissues more than conventional dose rate irradiation. This tissue sparing has been termed the FLASH effect. We investigated the FLASH effect of proton irradiation on the intestine as well as the hypothesis that lymphocyte depletion is a cause of the FLASH effect. A 16 × 12 mm² elliptical field with a dose rate of ~120 Gy/s was provided by a 228 MeV proton pencil beam. Partial abdominal irradiation was delivered to C57BL/6j and immunodeficient Rag1^-/-/C57 mice. Proliferating crypt cells were counted at 2 days post exposure, and the thickness of the muscularis externa was measured at 280 days following irradiation. FLASH irradiation did not reduce the morbidity or mortality of conventional irradiation in either strain of mice; in fact, a tendency for worse survival in FLASH-irradiated mice was observed. There were no significant differences in lymphocyte numbers between FLASH and conventional-dose-rate mice. A similar number of proliferating crypt cells and a similar thickness of the muscularis externa following FLASH and conventional dose rate irradiation were observed. Partial abdominal FLASH proton irradiation at 120 Gy/s did not spare normal intestinal tissue, and no difference in lymphocyte depletion was observed. This study suggests that the effect of FLASH irradiation may depend on multiple factors, and in some cases dose rates of over 100 Gy/s do not induce a FLASH effect and can even result in worse outcomes.

Human enteroids as a tool to study conventional and ultra-high dose rate radiation

Radiation therapy, one of the most effective therapies to treat cancer, is highly toxic to healthy tissue. The delivery of radiation at ultra-high dose rates, FLASH radiation therapy (FLASH), has been shown to maintain therapeutic anti-tumor efficacy while sparing normal tissues compared to conventional dose rate irradiation (CONV). Though promising, these studies have been limited mainly to murine models. Here, we leveraged enteroids, three-dimensional cell clusters that mimic the intestine, to study human-specific tissue response to radiation. We observed enteroids have a greater colony growth potential following FLASH compared with CONV. In addition, the enteroids that reformed following FLASH more frequently exhibited proper intestinal polarity. While we did not observe differences in enteroid damage across groups, we did see distinct transcriptomic changes. Specifically, the FLASH enteroids upregulated the expression of genes associated with the WNT-family, cell-cell adhesion, and hypoxia response. These studies validate human enteroids as a model to investigate FLASH and provide further evidence supporting clinical study of this therapy. Insight Box Promising work has been done to demonstrate the potential of ultra-high dose rate radiation (FLASH) to ablate cancerous tissue, while preserving healthy tissue. While encouraging, these findings have been primarily observed using pre-clinical murine and traditional two-dimensional cell culture. This study validates the use of human enteroids as a tool to investigate human-specific tissue response to FLASH. Specifically, the work described demonstrates the ability of enteroids to recapitulate previous in vivo findings, while also providing a lens through which to probe cellular and molecular-level responses to FLASH. The human enteroids described herein offer a powerful model that can be used to probe the underlying mechanisms of FLASH in future studies.

Effects of Flash Radiotherapy on Blood Lymphocytes in Humans and Small Laboratory Animals

A mathematical model, which describes the level of surviving lymphocytes in the blood after ultra-high (FLASH) and lower dose rates of partial-body irradiation, is developed. The model is represented by simple analytic formulae that involve a few parameters, namely, physiologic parameters (characteristics of the blood flow through the blood circulatory system and its irradiated part), a biophysical parameter (a characteristic of the blood lymphocytes radiosensitivity), and the physical parameters (characteristics of irradiation). The model predicts that the level of surviving blood lymphocytes increases as the dose rate increases and approaches the limiting level of (1 - vR), where vR is the fraction of the blood volume in the irradiated part of the blood circulatory system. The model also predicts that the level of surviving blood lymphocytes after the same exposure is higher for lower vR. It is found that FLASH irradiation in humans with doses of 10 to 40 Gy and with exposure times significantly less (<1 s) than the blood circulation time (∼60 s) leads to the maximal blood lymphocyte sparing. Simple formula, which determines effective dose rates for optimal blood lymphocyte sparing, is derived in the framework of the developed model. For the dose range specified above, the obtained modeling prediction of the range of effective dose rates for optimal blood lymphocyte sparing in humans (namely, N ≥40 Gy/s) coincides with the dose rate range in FLASH radiation therapy. It is revealed that the respective effective dose rates for mice are higher than those for humans (for the same dose range) due to the shorter blood circulation time in mice than in humans. Proceeding from the findings obtained in this paper, a hypothesis elucidating the mechanisms of the abscopal effect of FLASH radiation therapy (namely, an antitumor response on metastases located outside of irradiated part of a body) is proposed.

Gut microbiota: A novel and potential target for radioimmunotherapy in colorectal cancer

Colorectal cancer (CRC) is one of the most common cancers, with a high mortality rate, and is a major burden on human health worldwide. Gut microbiota regulate human immunity and metabolism through producing numerous metabolites, which act as signaling molecules and substrates for metabolic reactions in various biological processes. The importance of host-gut microbiota interactions in immunometabolic mechanisms in CRC is increasingly recognized, and interest in modulating the microbiota to improve patient's response to therapy has been raising. However, the specific mechanisms by which gut microbiota interact with immunotherapy and radiotherapy remain incongruent. Here we review recent advances and discuss the feasibility of gut microbiota as a regulatory target to enhance the immunogenicity of CRC, improve the radiosensitivity of colorectal tumor cells and ameliorate complications such as radiotoxicity. Currently, great breakthroughs in the treatment of non-small cell lung cancer and others have been achieved by radioimmunotherapy, but radioimmunotherapy alone has not been effective in CRC patients. By summarizing the recent preclinical and clinical evidence and considering regulatory roles played by microflora in the gut, such as anti-tumor immunity, we discuss the potential of targeting gut microbiota to enhance the efficacy of radioimmunotherapy in CRC and expect this review can provide references and fresh ideas for the clinical application of this novel strategy.

FLASHlab@PITZ: New R&D platform with unique capabilities for electron FLASH and VHEE radiation therapy and radiation biology under preparation at PITZ

At the Photo Injector Test facility at DESY in Zeuthen (PITZ), an R&D platform for electron FLASH and very high energy electron radiation therapy and radiation biology is being prepared (FLASHlab@PITZ). The beam parameters available at PITZ are worldwide unique. They are based on experiences from 20 + years of developing high brightness beam sources and an ultra-intensive THz light source demonstrator for ps scale electron bunches with up to 5 nC bunch charge at MHz repetition rate in bunch trains of up to 1 ms length, currently 22 MeV (upgrade to 250 MeV planned). Individual bunches can provide peak dose rates up to 10¹⁴ Gy/s, and 10 Gy can be delivered within picoseconds. Upon demand, each bunch of the bunch train can be guided to a different transverse location, so that either a "painting" with micro beams (comparable to pencil beam scanning in proton therapy) or a cumulative increase of absorbed dose, using a wide beam distribution, can be realized at the tumor. Full tumor treatment can hence be completed within 1 ms, mitigating organ movement issues. With extremely flexible beam manipulation capabilities, FLASHlab@PITZ will cover the current parameter range of successfully demonstrated FLASH effects and extend the parameter range towards yet unexploited short treatment times and high dose rates. A summary of the plans for FLASHlab@PITZ and the status of its realization will be presented.

FLASH X-ray spares intestinal crypts from pyroptosis initiated by cGAS-STING activation upon radioimmunotherapy

DNA-damaging treatments such as radiotherapy (RT) have become promising to improve the efficacy of immune checkpoint inhibitors by enhancing tumor immunogenicity. However, accompanying treatment-related detrimental events in normal tissues have posed a major obstacle to radioimmunotherapy and present new challenges to the dose delivery mode of clinical RT. In the present study, ultrahigh dose rate FLASH X-ray irradiation was applied to counteract the intestinal toxicity in the radioimmunotherapy. In the context of programmed cell death ligand-1 (PD-L1) blockade, FLASH X-ray minimized mouse enteritis by alleviating CD8⁺ T cell-mediated deleterious immune response compared with conventional dose rate (CONV) irradiation. Mechanistically, FLASH irradiation was less efficient than CONV X-ray in eliciting cytoplasmic double-stranded DNA (dsDNA) and in activating cyclic GMP-AMP synthase (cGAS) in the intestinal crypts, resulting in the suppression of the cascade feedback consisting of CD8⁺ T cell chemotaxis and gasdermin E-mediated intestinal pyroptosis in the case of PD-L1 blocking. Meanwhile, FLASH X-ray was as competent as CONV RT in boosting the antitumor immune response initiated by cGAS activation and achieved equal tumor control in metastasis burdens when combined with anti-PD-L1 administration. Together, the present study revealed an encouraging protective effect of FLASH X-ray upon the normal tissue without compromising the systemic antitumor response when combined with immunological checkpoint inhibitors, providing the rationale for testing this combination as a clinical application in radioimmunotherapy.

Potential Molecular Mechanisms behind the Ultra-High Dose Rate "FLASH" Effect

FLASH radiotherapy, or the delivery of a dose at an ultra-high dose rate (>40 Gy/s), has recently emerged as a promising tool to enhance the therapeutic index in cancer treatment. The remarkable sparing of normal tissues and equivalent tumor control by FLASH irradiation compared to conventional dose rate irradiation—the FLASH effect—has already been demonstrated in several preclinical models and even in a first patient with T-cell cutaneous lymphoma. However, the biological mechanisms responsible for the differential effect produced by FLASH irradiation in normal and cancer cells remain to be elucidated. This is of great importance because a good understanding of the underlying radiobiological mechanisms and characterization of the specific beam parameters is required for a successful clinical translation of FLASH radiotherapy. In this review, we summarize the FLASH investigations performed so far and critically evaluate the current hypotheses explaining the FLASH effect, including oxygen depletion, the production of reactive oxygen species, and an altered immune response. We also propose a new theory that assumes an important role of mitochondria in mediating the normal tissue and tumor response to FLASH dose rates.

A potential revolution in cancer treatment: A topical review of FLASH radiotherapy

FLASH radiotherapy (RT) is a novel technique in which the ultrahigh dose rate (UHDR) (≥40 Gy/s) is delivered to the entire treatment volume. Recent outcomes of in vivo studies show that the UHDR RT has the potential to spare normal tissue without sacrificing tumor control. There is a growing interest in the application of FLASH RT, and the ultrahigh dose irradiation delivery has been achieved by a few experimental and modified linear accelerators. The underlying mechanism of FLASH effect is yet to be fully understood, but the oxygen depletion in normal tissue providing extra protection during FLASH irradiation is a hypothesis that attracts most attention currently. Monte Carlo simulation is playing an important role in FLASH, enabling the understanding of its dosimetry calculations and hardware design. More advanced Monte Carlo simulation tools are under development to fulfill the challenge of reproducing the radiolysis and radiobiology processes in FLASH irradiation. FLASH RT may become one of standard treatment modalities for tumor treatment in the future. This paper presents the history and status of FLASH RT studies with a focus on FLASH irradiation delivery modalities, underlying mechanism of FLASH effect, in vivo and vitro experiments, and simulation studies. Existing challenges and prospects of this novel technique are discussed in this manuscript.

Radiobiological Aspects of FLASH Radiotherapy

Radiotherapy (RT) is one of the primary treatment modalities for cancer patients. The clinical use of RT requires a balance to be struck between tumor effect and the risk of toxicity. Sparing normal tissue is the cornerstone of reducing toxicity. Advances in physical targeting and dose-shaping technology have helped to achieve this. FLASH RT is a promising, novel treatment technique that seeks to exploit a potential normal tissue-sparing effect of ultra-high dose rate irradiation. A significant body of in vitro and in vivo data has highlighted a decrease in acute and late radiation toxicities, while preserving the radiation effect in tumor cells. The underlying biological mechanisms of FLASH RT, however, remain unclear. Three main mechanisms have been hypothesized to account for this differential FLASH RT effect between the tumor and healthy tissue: the oxygen depletion, the DNA damage, and the immune-mediated hypothesis. These hypotheses and molecular mechanisms have been evaluated both in vitro and in vivo. Furthermore, the effect of ultra-high dose rate radiation with extremely short delivery times on the dynamic tumor microenvironment involving circulating blood cells and immune cells in humans is essentially unknown. Therefore, while there is great interest in FLASH RT as a means of targeting tumors with the promise of an increased therapeutic ratio, evidence of a generalized FLASH effect in humans and data to show that FLASH in humans is safe and at least effective against tumors as standard photon RT is currently lacking. FLASH RT needs further preclinical investigation and well-designed in-human studies before it can be introduced into clinical practice.

Mechanisms of FLASH effect

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

Dose- and Volume-Limiting Late Toxicity of FLASH Radiotherapy in Cats with Squamous Cell Carcinoma of the Nasal Planum and in Mini Pigs

PURPOSE

The FLASH effect is characterized by normal tissue sparing without compromising tumor control. Although demonstrated in various preclinical models, safe translation of FLASH-radiotherapy stands to benefit from larger vertebrate animal models. Based on prior results, we designed a randomized phase III trial to investigate the FLASH effect in cat patients with spontaneous tumors. In parallel, the sparing capacity of FLASH-radiotherapy was studied on mini pigs by using large field irradiation.

EXPERIMENTAL DESIGN

Cats with T1-T2, N0 carcinomas of the nasal planum were randomly assigned to two arms of electron irradiation: arm 1 was the standard of care (SoC) and used 10 × 4.8 Gy (90% isodose); arm 2 used 1 × 30 Gy (90% isodose) FLASH. Mini pigs were irradiated using applicators of increasing size and a single surface dose of 31 Gy FLASH.

RESULTS

In cats, acute side effects were mild and similar in both arms. The trial was prematurely interrupted due to maxillary bone necrosis, which occurred 9 to 15 months after radiotherapy in 3 of 7 cats treated with FLASH-radiotherapy (43%), as compared with 0 of 9 cats treated with SoC. All cats were tumor-free at 1 year in both arms, with one cat progressing later in each arm. In pigs, no acute toxicity was recorded, but severe late skin necrosis occurred in a volume-dependent manner (7-9 months), which later resolved.

CONCLUSIONS

The reported outcomes point to the caveats of translating single-high-dose FLASH-radiotherapy and emphasizes the need for caution and further investigations. See related commentary by Maity and Koumenis, p. 3636.

Shining a FLASHlight on Ultrahigh Dose-Rate Radiation and Possible Late Toxicity

A recent study reported results from a clinical trial in cats and from experiments in mini-pigs in which a single dose of radiotherapy was delivered at ultrahigh dose rates (FLASH). There was acceptable acute toxicity; however, some animals suffered severe late toxicity, raising caution in the design of future trials. See related article by Rohrer Bley et al., p. 3814.

FLASH radiotherapy treatment planning and models for electron beams

The FLASH effect designates normal tissue sparing at ultra-high dose rate (UHDR, >40 Gy/s) compared to conventional dose rate (∼0.1 Gy/s) irradiation while maintaining tumour control and has the potential to improve the therapeutic ratio of radiotherapy (RT). UHDR high-energy electron (HEE, 4-20 MeV) beams are currently a mainstay for investigating the clinical potential of FLASH RT for superficial tumours. In the future very-high energy electron (VHEE, 50-250 MeV) UHDR beams may be used to treat deep-seated tumours. UHDR HEE treatment planning focused at its initial stage on accurate dosimetric modelling of converted and dedicated UHDR electron RT devices for the clinical transfer of FLASH RT. VHEE treatment planning demonstrated promising dosimetric performance compared to clinical photon RT techniques in silico and was used to evaluate and optimise the design of novel VHEE RT devices. Multiple metrics and models have been proposed for a quantitative description of the FLASH effect in treatment planning, but an improved experimental characterization and understanding of the FLASH effect is needed to allow for an accurate and validated modelling of the effect in treatment planning. The importance of treatment planning for electron FLASH RT will augment as the field moves forward to treat more complex clinical indications and target sites. In this review, TPS developments in HEE and VHEE are presented considering beam models, characteristics, and future FLASH applications.

Comparing radiolytic production of H2O2 and development of Zebrafish embryos after ultra high dose rate exposure with electron and transmission proton beams

The physico-chemical and biological response to conventional and UHDR electron and proton beams was investigated, along with conventional photons. The temporal structure and nature of the beam affected both, with electron beam at ≥1400 Gy/s and proton beam at 0.1 and 1260 Gy/s found to be isoefficient at sparing zebrafish embryos.

Normal tissue sparing by FLASH as a function of single fraction dose: A quantitative analysis

PURPOSE

The FLASH effect designates normal tissue sparing by ultra-high dose rate (UHDR) compared to conventional dose rate (CONV) irradiation without compromising tumor control. Understanding the magnitude of this effect and its dependency on dose are essential requirements for an optimized clinical translation of FLASH radiation therapy. In this context, we evaluated available experimental data on the magnitudes of normal tissue sparing provided by the FLASH effect as a function of dose, and followed a phenomenological data-driven approach for its parameterization.

METHODS

We gathered available in vivo data of the normal tissue sparing of CONV compared to UHDR single fraction doses and converted it to a common scale using isoeffect dose ratios, hereafter referred to as FLASH modifying factors (FMF). We then evaluated the suitability of a piecewise linear function with two pieces to parametrize FMF × D as a function of dose D.

RESULTS

We found that the magnitude of FMF generally decreases (i.e., sparing increases) as function of single fraction dose and that individual data series can be described by the piecewise linear function. The sparing magnitude appears organ specific. Pooled skin reaction data followed a consistent trend as a function of dose. Average FMF values and their standard deviations were 0.95±0.11 for all data below 10 Gy, 0.92±0.06 for mouse gut data between 10-25 Gy, and 0.96±0.07 and 0.71±0.06 for mammalian skin reaction data between 10-25 Gy and >25 Gy, respectively.

CONCLUSIONS

The magnitude of normal tissue sparing by FLASH is increasing with dose and is dependent on the irradiated tissue. A piecewise linear function can parameterize currently available individual data series.

Beam pulse structure and dose rate as determinants for the flash effect observed in zebrafish embryo

BACKGROUND AND PURPOSE

Continuing recent experiments at the research electron accelerator ELBE at the Helmholtz-Zentrum Dresden-Rossendorf the influence of beam pulse structure on the Flash effect was investigated.

MATERIALS AND METHODS

The proton beam pulse structure of an isochronous cyclotron (UHDR_iso) and a synchrocyclotron (UHDR_synchro) was mimicked at ELBE by quasi-continuous electron bunches at 13 MHz delivering mean dose rates of 287 Gy/s and 177 Gy/s and bunch dose rates of 10⁶Gy/s and 10⁹ Gy/s, respectively. For UHDR_synchro, 40 ms macro pulses at a frequency of 25 Hz superimposed the bunch delivery. For comparison, a maximum beam intensity (2.5 x 10⁵ Gy/s mean and ∼10⁹ Gy/s bunch dose rate) and a reference irradiation (of ∼8 Gy/min mean dose rate) were applied. Radiation induced changes were assessed in zebrafish embryos over four days post irradiation.

RESULTS

Relative to the reference a significant protecting Flash effect was observed for all electron beam pulse regimes with less severe damage the higher the mean dose rate of the electron beam. Accordingly, the macro pulsing induced prolongation of treatment time at UHDR_synchro regime reduces the protecting effect compared to the maximum regime delivered at same bunch but higher mean dose rate. The Flash effect of the UHDR_iso regime was confirmed at a clinical isochronous cyclotron comparing the damage induced by proton beams delivered at 300 Gy/s and ∼9 Gy/min.

CONCLUSION

The recent findings indicate that the mean dose rate or treatment time are decisive for the normal tissue protecting Flash effect in zebrafish embryo.