Neuroprotective Effects of Ultra-High Dose Rate FLASH Bragg Peak Proton Irradiation

Central Nervous System Response Against Ionizing Radiation Exposure: Cellular, Biochemical, and Molecular Perspectives

Gamma radiation is known to induce several detrimental effects on the nervous system. The hippocampus region, specifically the dentate gyrus (DG) and subventricular zone (SVZ), have been identified as a radiation-sensitive neurogenic niche. Radiation alters the endogenous redox status of neural stem cells (NSCs) and other proliferative cells, especially in the hippocampus region, leading to oxidative stress, neuroinflammation, and cell death. Planned (i.e., radiotherapy of brain tumor patients) or unplanned radiation exposure (i.e., accidental radiation exposure) can induce nonspecific damage to neuronal tissues, resulting in chronic or acute radiation syndrome. Although anatomical alterations in the neuronal tissues have been reported at higher doses of gamma radiation, biochemical and molecular perturbations may be evident even at much lower radiation doses. They may manifest in the form of neuronal deficits and cognitive impairment. In the present review, several molecular events and signaling pathways, such as oxidative stress, neuroinflammation, apoptosis, cognition, neuroplasticity, and neurotoxicity induced in neuronal cells upon ionizing radiation exposure, are reviewed. Furthermore, brain-specific radioprotectors and mitigators that protect normal neuronal cells and tissues against ionizing radiation during radiotherapy of cancer patients or nuclear emergencies are also discussed.

Ultrahigh Dose Rate Helium Ion Beams: Minimizing Brain Tissue Damage while Preserving Tumor Control

Ultrahigh dose rate radiotherapy (RT) with electrons and protons has shown potential for cancer treatment by effectively targeting tumors while sparing healthy tissues (FLASH effect). This study aimed to investigate the potential FLASH sparing effect of ultrahigh dose rate helium ion irradiation, focusing on acute brain injury and subcutaneous tumor response in a preclinical in vivo setting. Raster-scanned helium ion beams were used to compare the effects of standard dose rate (SDR; at 0.2 Gy/second) and FLASH (at 141 Gy/second) RT on healthy brain tissue. Irradiation-induced brain injury was studied in C57BL/6 mice via DNA damage response, using nuclear γH2AX as a marker for double-strand breaks. The integrity of neurovascular and immune compartments was assessed through CD31+ microvascular density and activation of microglia/macrophages. IBA1+ ramified and CD68+ phagocytic microglia/macrophages were quantified, along with the expression of inducible nitric oxide synthetase. Tumor response to SDR (0.2 Gy/second) and FLASH (250 Gy/second) RT was evaluated in an A549 carcinoma model, using tumor volume and Kaplan-Meier survival as endpoints. The results showed that helium FLASH RT significantly reduced acute brain tissue injury compared with SDR, evidenced by lower levels of double-strand breaks and preserved the neurovascular endothelium. Additionally, FLASH RT reduced neuroinflammatory signals compared with SDR, as indicated by fewer CD68+ inducible nitric oxide synthetase-positive microglia/macrophages. FLASH RT achieved tumor control comparable with that of SDR RT. To the best of our knowledge, this is the first study to report the FLASH sparing effect of raster scanning helium ion RT in vivo, highlighting its potential for neuroprotection and effective tumor control.

Targeting Active Microglia Alleviates Distal Edge of Proton Radiation-induced Neural Damage

Purpose

Proton therapy (PT) has distinct advantages in its ability to precisely target tumors while avoiding adjacent normal tissues. However, the distal edge effects of PT constrain its application. This study investigated the brain tissue response in the distal edge regions of protons and compared it with the effect of photons.

Methods and Materials

The occurrence of damage from photons and at the distal edge of protons was investigated in a murine model. Bragg peak treatment plans for murine models were optimized. Hematoxylin and eosin and immunofluorescence staining were performed along the distal margin. In addition, the approximate distance from the Bragg peak to the neuronal damage sites was calculated. Furthermore, a small-molecule inhibitor was studied for its ability to inhibit microglia activation.

Results

The distal edge brain injury murine model was successfully established. Reactive gliosis and granulovacuolar neuronal degeneration were observed in the right hemisphere of the brain in the proton irradiation group. Neuronal injuries were observed at multiple locations (the frontal lobe, thalamus, and cerebral cortex) along the distal border, but no injured neurons were detected along vertical photon irradiation exposed areas. Meanwhile, severe neural damage was seen with horizontal photon irradiation. At the distal edge of the Bragg peak (0.4633 ± 0.01856 cm), microglia with abnormal morphology accumulated. IBA1 and CD68 staining revealed activated microglia at the corresponding neuronal damage sites, indicating their involvement in irradiation-induced damage. Activated microglia were not observed with vertical photon irradiation, whereas many activated microglia were observed with horizontal photon irradiation. Moreover, asparagine endopeptidase inhibitors administered via intraperitoneal injection significantly reduced active microglia in the thalamus and cerebral cortex and alleviated brain damage.

Conclusions

This study demonstrated that proton radiation induces neuronal damage and accumulation of activated microglia at the distal edge. Targeting activated microglia may play a protective role in distal edge injury from radiation.

Physics-based energy spectrum optimization (PESO): a new method to model the energy spectrum of a compact ultra-high dose rate electron linac for Monte Carlo dose calculation

Objective.FLASH radiotherapy (FLASH-RT) is an emerging treatment modality that delivers ultra-high dose rates (UHDR) to achieve effective tumor control while minimizing damage to healthy tissues-a phenomenon known as the FLASH effect. Accurate modeling of the electron energy spectrum is essential for UHDR linacs used in FLASH-RT to ensure reliable dose calculations and effective treatment planning. This study introduces a novel, physics-based method to reconstruct electron energy spectra specifically tailored for compact UHDR linacs lacking bending magnets, which present unique challenges for beam modeling.Approach.A physics-based energy spectrum optimization (PESO) algorithm was developed to model electron beam dynamics within a compact linac with minimal free parameters. The PESO approach was evaluated against two conventional methods-simulated annealing (SA) and Gaussian regression (GR)-using radiochromic film measurements in solid water phantoms for three applicator sizes (25 mm, 40 mm, and 60 mm) in both conventional and FLASH modes. Accuracy of the reconstructed isodoses and robustness against measurement errors was evaluated for each method.Main results.We successfully implemented the PESO algorithm to resolve the electron beam dynamics as a function of the electric field within the waveguide. The method constrained the solution to physically plausible spectra and achieved superior dosimetric accuracy compared to both GR and SA for the 6 MeV UHDR beam, while producing results comparable to SA (and better than GR) for the 9 MeV UHDR beam. PESO also demonstrated reduced sensitivity to measurement errors and maintained consistency, even for the low-energy tail components of UHDR electron beams.Significance.By incorporating physically based constraints into the beam modeling process, PESO offers improvements in the reliability and precision of electron energy spectrum reconstruction for UHDR linacs. This development addresses challenges in electron FLASH-RT dose calculation and may aid in the clinical implementation of FLASH radiotherapy.

FLASH Radiotherapy: From In Vivo Data to Clinical Translation

Delivery of radiotherapy (RT) at ultra-high dose rates or FLASH radiotherapy (FLASH-RT) is an emerging treatment option for patients with cancer that could increase survival outcomes and quality of life. In vivo data across a multitude of normal tissues and associated tumors have been published demonstrating the FLASH effect while bringing attention to the need for additional research. Combination of FLASH-RT with other treatment options including spatially fractionated RT, immunotherapy, and usage in the setting of reirradiation could also provide additional benefit. Phase I clinical trials have shown promising results, yet research is warranted before routine clinical use of FLASH-RT.

Plasmid DNA Strand Breaks Are Dose Rate Independent at Clinically Relevant Proton Doses and Under Biological Conditions

We investigated the effect of proton FLASH radiation on plasmid DNA. Purified supercoiled pBR322 plasmids were irradiated with clinical doses (≤10 Gy) of protons at ultra-high and conventional dose rates using the Paul Scherrer Institute (PSI) isochronous cyclotron. The proton beam in this clinical facility has been validated to produce the FLASH effect in preclinical models. Plasmid samples were irradiated under various oxygen tensions, scavenger levels, pH conditions and Fe (II) concentrations as these biochemical parameters vary across tissues and tumors. Over the range of doses used, plasmid DNA strand breaks were found to be dose rate independent at all conditions investigated. Irradiation within the Bragg peak and spread-out Bragg peak increased clustered strand breaks, except in the presence of scavengers. With this model system, we demonstrate conclusively that plasmid DNA strand breakage is dose rate independent at doses below 10 Gy and does not constitute a high throughput assay endpoint predictive of the biological effect of FLASH.

FLASH radiotherapy: technical advances, evidence of the FLASH effect and mechanistic insights

Cancer is one of the leading causes of death worldwide, responsible for nearly 10 million deaths in 2020, with approximately 50% of patients receiving radiation therapy as part of their treatment (Baskaret al2012). Preclinical investigations studies have shown that FLASH radiotherapy (FLASH-RT), delivering radiation in ultra-high dose rates (UHDR), preserves healthy tissue integrity and reduces toxicity, all while maintaining an effective tumor response compared to conventional radiotherapy (CONV-RT), the combined biological benefit was termed as FLASH effect. This article comprehensively surveys pertinent research conducted within FLASH-RT, explores the facilities used in this realm, delves into hypothesized mechanism perspectives, and addresses the challenges to trigger the FLASH effect. In addition, we discuss the potential prospects of FLASH-RT and examine the obstacles that require resolution before its clinical implementation can become a reality.

Instantaneous dose rate as a crucial factor in reducing mortality and normal tissue toxicities in murine total-body irradiation: a comparative study of dose rate combinations

PURPOSE

The ultra-high dose rate (UHDR) radiation shows promise in eradicating tumors while reducing normal tissue toxicities. However, the biological outcomes of UHDR are influenced by various factors, particularly the mean dose rate and instantaneous dose rate. Additionally, the UHDR response at large field sizes is lacking. This study aimed to explore the impact of different dose rate combinations on gastrointestinal biological outcomes following total-body irradiations (TBI) and to examine the involved molecular signaling pathways.

METHOD

Female C57BL6/J mice received 10 Gy TBI using three modes: ultra-high mean and ultra-high instantaneous dose rate irradiation (HH mode), low mean and ultra-high instantaneous dose rate irradiation (LH mode), and low mean and low instantaneous dose rate irradiation (LL mode). Mice were euthanized at 3 h and 48 h post irradiation to assess acute normal tissue damage and perform transcriptome sequencing. Furthermore, a subset of mice was monitored for 30 days to evaluate survival.

RESULTS

We found that when the instantaneous dose rate is sufficiently high (> 105 Gy/s), both ultra-high or low mean dose rate irradiation reduced mice mortality, myelosuppression, DNA damage, and cell apoptosis. The survival probabilities 30 days after 10 Gy TBI were 4/7, 4/6, and 0/6 in the HH, LH, and LL groups, respectively. Myelosuppression was lower at 3 h and 48 h post HH and LH irradiations than LL irradiation. The better regulated inflammatory response was evident at 48 h post HH and LH irradiation compared to LL irradiation. Additionally, DNA damages and cell apoptosis in the intestinal tissue were significantly reduced after HH and LH irradiations compared to LL irradiation. Transcriptome sequencing of intestinal tissues revealed that HH irradiation activated immune response pathways and suppressed mitochondrial related pathways compared to LL irradiation.

CONCLUSION

Our findings underscore the pivotal role of instantaneous dose rate in reducing radiation damages. When the instantaneous dose rate is sufficiently high (> 105 Gy/s), both ultra-high or low mean dose rate irradiation (HH and LH mode) reduced mice mortality, myelosuppression, DNA damage, and cell apoptosis. Understanding these dose rate effects and biological responses are crucial for optimizing radiotherapy strategies and exploring the potential benefits of UHDR irradiation.

FLASH radiotherapy combined with immunotherapy: From biological mechanisms to blockbuster therapeutics

FLASH ultra-high dose rate radiotherapy (RT) can effectively exert the protective effect on normal tissue and reduce the risk of treatment-related toxicity, without compromising the killing effect on tumor tissue, resulting in a significant differential biological effect between tumor control and normal tissue damage, namely the FLASH effect. To date, the precise biological details of the FLASH effect remain uncertain. The currently mainstream mechanisms proposed by the academic community include the transient oxygen depletion hypothesis, free radical hypothesis, immune protection hypothesis, and DNA integrity hypothesis, which have attracted increasing attention in recent years. Based on these theoretical principles and numerous investigations on the FLASH effect in vivo and in vitro, the combined application of FLASH and immune checkpoint inhibitors (ICIs) has been considered synergistic and potentially practical. The primary underlying basis is that FLASH might actively preserve the number and function of circulating immune cells, thereby enhancing the efficacy of immune cell-mediated immunotherapy. Meanwhile, FLASH RT could activate the tumor immune microenvironment and transform "cold'' tumors into ''hot'' ones, consequently boosting local and systemic anti-tumor immunity and expanding the therapeutic benefits of ICIs. Moreover, FLASH might attenuate immunoinflammatory responses and minimize the incidence of radiation-related adverse events, allowing for the potentially safer and promising clinical application of combing FLASH RT with ICI therapy. Nevertheless, data on this treatment modality is currently lacking, and several barriers remain to be addressed, including the logistical bottlenecks, technical hurdles, limited availability, and unclear biological mechanisms. Further research is warranted in the future.

Reply to Comments on 'Modeling for predicting survival fraction of cells after ultra-high dose rate irradiation'

Liew and Mairani (2024Phys. Med. Biol.69248001) commented on our previous reply to comments on our paper, 'Modeling for predicting survival fraction of cells after ultra-high dose rate irradiation'. We appreciate their comments on the choice of experimental data on DNA damage for cell survival and agree that the estimate of the dose-response curve on cell survival depends on the selection of DNA damage data. As an additional benchmark test, we compared the relative biological effectiveness (RBE) predicted using the recommended DNA damage data measured in normoxia with those reported in our original paper, and confirmed that the difference in RBE was less than 8%. Although our model allows for the estimation of cell survival and RBE under ultra-high dose rate (UHDR) irradiation, we highlight that a further accumulation of experimental data on DNA damage under UHDR irradiation is necessary for the further development of biophysical models concerning the mechanistical estimation of biological effects.

Second Comment on 'Modeling for predicting survival fraction of cells after ultra-high dose rate irradiation'

We comment on the reply by Shiraishiet alto our comments regarding their recently published study 'Modeling for Predicting Survival Fraction of Cells after Ultra-High Dose Rate Irradiation'. While we appreciate the effort of the authors to consider our comments, we see ourselves compelled to add another short comment as we believe that some of our suggestions have been misrepresented. This may have resulted in a misguiding re-evaluation of the model.

Ultra-High Dose Rate Helium Ion Beams: Minimizing Brain Tissue Damage while Preserving Tumor Control

Ultra-high dose rate radiotherapy with electrons and protons has shown potential for cancer treatment by effectively targeting tumors while sparing healthy tissues (FLASH effect). This study aimed to investigate the potential FLASH sparing effect of ultra-high-dose rate helium ion irradiation, focusing on acute brain injury and subcutaneous tumor response in a preclinical in vivo setting. Raster-scanned helium ion beams were used to compare the effects of standard dose rate (SDR at 0.2 Gy/s) and FLASH (at 141 Gy/s) radiotherapy on healthy brain tissue. Irradiation-induced brain injury was studied in C57BL/6 mice via DNA damage response, using nuclear γH2AX as a marker for double-strand breaks (DSB). The integrity of neurovascular and immune compartments was assessed through CD31 + microvascular density and activation of microglia/macrophages. Iba1+ ramified and CD68 + phagocytic microglia/macrophages were quantified, along with the expression of inducible nitric oxide synthetase (iNOS). Tumor response to SDR (0.2 Gy/s) and FLASH (250 Gy/s) radiotherapy was evaluated in A549 carcinoma model, using tumor volume and Kaplan-Meier survival as endpoints. The results showed that helium FLASH radiotherapy significantly reduced acute brain tissue injury compared to SDR, evidenced by lower levels of DSB and preserved neurovascular endothelium. Additionally, FLASH radiotherapy reduced neuroinflammatory signals compared to SDR, as indicated by fewer CD68+ iNOS+ microglia/macrophages. FLASH radiotherapy achieved tumor control comparable to that of SDR radiotherapy. This study is the first to report the FLASH sparing effect of raster scanning helium ion radiotherapy in vivo, highlighting its potential for neuroprotection and effective tumor control.

Current views on mechanisms of the FLASH effect in cancer radiotherapy

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

A multidisciplinary view of flash irradiation

The delivery of ultra-high dose rates of radiation, called flash irradiation or flash-RT, has emerged as a new modality of radiotherapy shaking up the paradigm of proportionality of effect and dose whatever the method of delivery of the radiation. The hallmark of flash-RT is healthy tissue sparing from the side effects of radiation without decrease of the antitumor efficiency in animal models. In this review we will define its specificities, the molecular mechanisms underlying the flash effect and the ongoing developments to bring this new modality to patient treatment.

Spread-out Bragg peak FLASH: quantifying normal tissue toxicity in a murine model

Objective

A favorable effect of ultra-high dose rate (FLASH) radiation on normal tissue-sparing has been indicated in several preclinical studies. In these studies, the adverse effects of radiation damage were reduced without compromising tumor control. Most studies of proton FLASH investigate these effects within the entrance of a proton beam. However, the real advantage of proton therapy lies in the Spread-out Bragg Peak (SOBP), which allows for giving a high dose to a target with a limited dose to healthy tissue at the entrance of the beam. Therefore, a clinically relevant investigation of the FLASH effect would be of healthy tissues within a SOBP. Our study quantified the tissue-sparing effect of FLASH radiation on acute and late toxicity within an SOBP in a murine model.

Material/Methods

Radiation-induced damage was assessed for acute and late toxicity in the same mice following irradiation with FLASH (Field dose rate of 60 Gy/s) or conventional (CONV, 0.34 Gy/s) dose rates. The right hindleg of unanesthetized female CDF1 mice was irradiated with single-fraction doses between 19.9-49.7 Gy for CONV and 30.4-65.9 Gy for FLASH with 5-8 mice per dose. The leg was placed in the middle of a 5 cm SOBP generated from a mono-energetic beam using a 2D range modulator. Acute skin toxicity quantified by hair loss, moist desquamation and toe separation was monitored daily within 29 days post-treatment. Late toxicity of fibrotic development measured by leg extendibility was monitored biweekly until 30 weeks post-treatment.

Results

Comparison of acute skin toxicity following radiation indicated a tissue-sparing effect of FLASH compared to conventional single-fraction radiation with a mean protection ratio of 1.40 (1.35-1.46). Fibrotic development similarly indicated normal tissue sparing with a 1.18 (1.17-1.18) protection ratio. The acute skin toxicity tissue sparing was similar to data from entrance-beam irradiations of Sørensen et al. (4).

Conclusion

Full dose-response curves for acute and late toxicity after CONV and FLASH radiation were obtained. Radiation within the SOBP retains the normal-tissue-sparing effect of FLASH with a dose-modifying factor of 40% for acute skin damage and 18% for fibrotic development.

Effects of Partial-Body, Continuous/Pulse Irradiation at Dose Rates from FLASH to Conventional Rates on the Level of Surviving Blood Lymphocytes: Modeling Approach. I. Continuous Irradiation

A mathematical model developed by Cucinotta and Smirnova is extended to describe effects of continuous, partial-body irradiation at high doses D and at dose rates N from FLASH to conventional rates on the level of surviving blood lymphocytes in humans and small laboratory animals (mice). Specifically, whereas the applicability of the model is limited to the exposure times shorter than a single cardiac cycle T0, the extended model is capable of describing such effects for the aforementioned and longer exposure times. The extended model is implemented as the algebraic equations. It predicts that the level of surviving blood lymphocytes in humans and mice increases with increasing the dose rate from N= D/T0 to FLASH rates and approaches the upper limiting level of 1-vR, where vR is the fraction of blood volume in the irradiated part of the blood circulatory system. Levels of surviving blood lymphocytes computed at doses from 10 Gy to 40 Gy and at dose rates N, which equal or exceed 40 Gy/s for humans and 400 Gy/s for mice, are nearly indistinguishable from the upper limiting level. In turn, the level of surviving blood lymphocytes in humans and mice decreases with decreasing the dose rate from N= D/T0 to conventional rates and approaches a lower limiting level. This level strongly depends on the dose D (it is smaller at larger values of D) with a slight dependence on the dose rate N. The model with the parameters specified for mice (together with the model of the dynamics of lymphopoietic system in mice after partial-body irradiation) reproduce, on a quantitative level, the experimental data, according to which the concentration of blood lymphocytes measured in mice in one day after continuous, partial-body irradiation at a high dose and conventional dose rate is smaller at the larger value of vR. Additionally, the model predicts at the same high dose (>10 Gy) a faster restoration of the blood lymphocyte population in humans exposed to continuous, partial-body irradiation at a FLASH dose rate compared to a conventional dose rate.

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

We comment on the recently published study 'Modeling for predicting survival fraction of cells after ultra-high dose rate irradiation' by Shiraishiet al. While the general approach of the study may be appropriate, we wish to comment on its limitations and point out issues concerning their choice of the benchmarking and fitting data. The approach by the authors could become viable in an extended form once more comprehensive data is available.

Reply to comment on 'Modeling for predicting survival fraction of cells after ultra-high dose rate irradiation'

Liew and Mairani commented on our paper 'Modeling for predicting survival fraction of cells after ultra-high dose rate irradiation' (Shiraishiet al2024aPhys. Med. Biol.69015017), which proposed a biophysical model to predict the dose-response curve of surviving cell fractions after ultra-high dose rate irradiation following conventional dose rate irradiation by considering DNA damage yields. They suggested the need to consider oxygen concentration in our prediction model and possible issues related to the data selection process used for the benchmarking test in our paper. In this reply, we discuss the limitations of both the present model and the available experimental data for determining the model's parameters. We also demonstrate that our proposed model can reproduce the experimental survival data even when using only the experimental DNA damage data measured reliably under normoxic conditions.

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.

Evaluation of specific RBE in different cells of hippocampus under high-dose proton irradiation in rats

The study aimed to determine the specific relative biological effectiveness (RBE) of various cells in the hippocampus following proton irradiation. Sixty Sprague-Dawley rats were randomly allocated to 5 groups receiving 20 or 30 Gy of proton or photon irradiation. Pathomorphological neuronal damage in the hippocampus was assessed using Hematoxylin-eosin (HE) staining. The expression level of NeuN, Nestin, Caspase-3, Olig2, CD68 and CD45 were determined by immunohistochemistry (IHC). The RBE range established by comparing the effects of proton and photon irradiation at equivalent biological outcomes. Proton20Gy induced more severe damage to neurons than photon20Gy, but showed no difference compared to photon30Gy. The RBE of neuron was determined to be 1.65. Similarly, both proton20Gy and proton30Gy resulted in more inhibition of oligodendrocytes and activation of microglia in the hippocampal regions than photon20Gy and photon30Gy. However, the expression of Olig2 was higher and CD68 was lower in the proton20Gy group than in the photon30Gy group. The RBE of oligodendrocyte and microglia was estimated to be between 1.1 to 1.65. For neural stem cells (NSCs) and immune cells, there were no significant difference in the expression of Nestin and CD45 between proton and photon irradiation (both 20 and 30 Gy). Therefore, the RBE for NSCs and immune cell was determined to be 1.1. These findings highlight the varying RBE values of different cells in the hippocampus in vivo. Moreover, the actual RBE of the hippocampus may be higher than 1.1, suggesting that using as RBE value of 1.1 in clinical practice may underestimate the toxicities induced by proton radiation.

Diamond detectors for dose and instantaneous dose-rate measurements for ultra-high dose-rate scanned helium ion beams

BACKGROUND

The possible emergence of the FLASH effect-the sparing of normal tissue while maintaining tumor control-after irradiations at dose-rates exceeding several tens of Gy per second, has recently spurred a surge of studies attempting to characterize and rationalize the phenomenon. Investigating and reporting the dose and instantaneous dose-rate of ultra-high dose-rate (UHDR) particle radiotherapy beams is crucial for understanding and assessing the FLASH effect, towards pre-clinical application and quality assurance programs.

PURPOSE

The purpose of the present work is to investigate a novel diamond-based detector system for dose and instantaneous dose-rate measurements in UHDR particle beams.

METHODS

Two types of diamond detectors, a microDiamond (PTW 60019) and a diamond detector prototype specifically designed for operation in UHDR beams (flashDiamond), and two different readout electronic chains, were investigated for absorbed dose and instantaneous dose-rate measurements. The detectors were irradiated with a helium beam of 145.7 MeV/u under conventional and UHDR delivery. Dose-rate delivery records by the monitoring ionization chamber and diamond detectors were studied for single spot irradiations. Dose linearity at 5 cm depth and in-depth dose response from 2 to 16 cm were investigated for both measurement chains and both detectors in a water tank. Measurements with cylindrical and plane-parallel ionization chambers as well as Monte-Carlo simulations were performed for comparisons.

RESULTS

Diamond detectors allowed for recording the temporal structure of the beam, in good agreement with the one obtained by the monitoring ionization chamber. A better time resolution of the order of few μs was observed as compared to the approximately 50 μs of the monitoring ionization chamber. Both diamonds detectors show an excellent linearity response in both delivery modalities. Dose values derived by integrating the measured instantaneous dose-rates are in very good agreement with the ones obtained by the standard electrometer readings. Bragg peak curves confirmed the consistency of the charge measurements by the two systems.

CONCLUSIONS

The proposed novel dosimetric system allows for a detailed investigation of the temporal evolution of UHDR beams. As a result, reliable and accurate determinations of dose and instantaneous dose-rate are possible, both required for a comprehensive characterization of UHDR beams and relevant for FLASH effect assessment in clinical treatments.

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

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

Impact of Multiple Beams on the FLASH Effect in Soft Tissue and Skin in Mice

PURPOSE

FLASH proton pencil beam scanning (p-PBS) showed a reduction in mouse skin toxicity and fibrosis when delivered as a single, uninterrupted, high-dose fraction. Clinical p-PBS treatment usually requires multiple beams to achieve good conformality, and these beams are separated by minutes to allow patient and equipment repositioning. We evaluate the impact of multibeam versus single-beam proton radiation on the FLASH sparing effect on skin toxicity.

METHODS AND MATERIALS

The right hind leg of 10-week-old female C57Bl/6j mice was irradiated using a Varian ProBeam proton beam scanning gantry system at conventional (1 Gy/s) or FLASH (100 Gy/s) average field dose rate. We scored the skin toxicity after different doses for 7 weeks. The treatment was delivered as 1, 2, or 3 equal beams with an interruption of 2 minutes. For each beam delivery, the equipment remained in the same position so that there was a full overlap of beams administered.

RESULTS

Single-beam delivery confirmed a benefit for p-PBS FLASH in this model at 30, 35, and 40 Gy. At 30 and 35 Gy, a single beam interruption of 2 minutes (2 × 15 Gy or 2 × 17.5 Gy) reduced the FLASH sparing effect, which remained significant (P < .001). However, 2 interruptions (3 × 10 Gy or 3 × 11.6 Gy) abrogated the normal tissue sparing effect.

CONCLUSIONS

Our results indicate that the FLASH sparing effect in areas of beam overlap can be compromised by interruptions in delivery time. Time gap between overlapping beams and spatial arrangement of the delivered beams are important parameters for FLASH studies. The effect of multibeam needs to be studied on different organs of interest.

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

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

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

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

Proton Bragg Peak FLASH Enables Organ Sparing and Ultra-High Dose-Rate Delivery: Proof of Principle in Recurrent Head and Neck Cancer

Proton pencil-beam scanning (PBS) Bragg peak FLASH combines ultra-high dose rate delivery and organ-at-risk (OAR) sparing. This proof-of-principle study compared dosimetry and dose rate coverage between PBS Bragg peak FLASH and PBS transmission FLASH in head and neck reirradiation. PBS Bragg peak FLASH plans were created via the highest beam single energy, range shifter, and range compensator, and were compared to PBS transmission FLASH plans for 6 GyE/fraction and 10 GyE/fraction in eight recurrent head and neck patients originally treated with quad shot reirradiation (14.8/3.7 CGE). The 6 GyE/fraction and 10 GyE/fraction plans were also created using conventional-rate intensity-modulated proton therapy techniques. PBS Bragg peak FLASH, PBS transmission FLASH, and conventional plans were compared for OAR sparing, FLASH dose rate coverage, and target coverage. All FLASH OAR V40 Gy/s dose rate coverage was 90-100% at 6 GyE and 10 GyE for both FLASH modalities. PBS Bragg peak FLASH generated dose volume histograms (DVHs) like those of conventional therapy and demonstrated improved OAR dose sparing over PBS transmission FLASH. All the modalities had similar CTV coverage. PBS Bragg peak FLASH can deliver conformal, ultra-high dose rate FLASH with a two-millisecond delivery of the minimum MU per spot. PBS Bragg peak FLASH demonstrated similar dose rate coverage to PBS transmission FLASH with improved OAR dose-sparing, which was more pronounced in the 10 GyE/fraction than in the 6 GyE/fraction. This feasibility study generates hypotheses for the benefits of FLASH in head and neck reirradiation and developing biological models.

The current status of FLASH particle therapy: a systematic review

Particle therapies are becoming increasingly available clinically due to their beneficial energy deposition profile, sparing healthy tissues. This may be further promoted with ultra-high dose rates, termed FLASH. This review comprehensively summarises current knowledge based on studies relevant to proton- and carbon-FLASH therapy. As electron-FLASH literature presents important radiobiological findings that form the basis of proton and carbon-based FLASH studies, a summary of key electron-FLASH papers is also included. Preclinical data suggest three key mechanisms by which proton and carbon-FLASH are able to reduce normal tissue toxicities compared to conventional dose rates, with equipotent, or enhanced, tumour kill efficacy. However, a degree of caution is needed in clinically translating these findings as: most studies use transmission and do not conform the Bragg peak to tumour volume; mechanistic understanding is still in its infancy; stringent verification of dosimetry is rarely provided; biological assays are prone to limitations which need greater acknowledgement.

Single Ultra-High Dose Rate Proton Transmission Beam for Whole Breast FLASH-Irradiation: Quantification of FLASH-Dose and Relation with Beam Parameters

Healthy tissue-sparing effects of FLASH (≥40 Gy/s, ≥4-8 Gy/fraction) radiotherapy (RT) make it potentially useful for whole breast irradiation (WBI), since there is often a lot of normal tissue within the planning target volume (PTV). We investigated WBI plan quality and determined FLASH-dose for various machine settings using ultra-high dose rate (UHDR) proton transmission beams (TBs). While five-fraction WBI is commonplace, a potential FLASH-effect might facilitate shorter treatments, so hypothetical 2- and 1-fraction schedules were also analyzed. Using one tangential 250 MeV TB delivering 5 × 5.7 Gy, 2 × 9.74 Gy or 1 × 14.32 Gy, we evaluated: (1) spots with equal monitor units (MUs) in a uniform square grid with variable spacing; (2) spot MUs optimized with a minimum MU-threshold; and (3) splitting the optimized TB into two sub-beams: one delivering spots above an MU-threshold, i.e., at UHDRs; the other delivering the remaining spots necessary to improve plan quality. Scenarios 1-3 were planned for a test case, and scenario 3 was also planned for three other patients. Dose rates were calculated using the pencil beam scanning dose rate and the sliding-window dose rate. Various machine parameters were considered: minimum spot irradiation time (minST): 2 ms/1 ms/0.5 ms; maximum nozzle current (maxN): 200 nA/400 nA/800 nA; two gantry-current (GC) techniques: energy-layer and spot-based. For the test case (PTV = 819 cc) we found: (1) a 7 mm grid achieved the best balance between plan quality and FLASH-dose for equal-MU spots; (2) near the target boundary, lower-MU spots are necessary for homogeneity but decrease FLASH-dose; (3) the non-split beam achieved >95% FLASH for favorable (not clinically available) machine parameters (SB GC, low minST, high maxN), but <5% for clinically available settings (EB GC, minST = 2 ms, maxN = 200 nA); and (4) splitting gave better plan quality and higher FLASH-dose (~50%) for available settings. The clinical cases achieved ~50% (PTV = 1047 cc) or >95% (PTV = 477/677 cc) FLASH after splitting. A single UHDR-TB for WBI can achieve acceptable plan quality. Current machine parameters limit FLASH-dose, which can be partially overcome using beam-splitting. WBI FLASH-RT is technically feasible.

Reinventing Radiobiology in the Light of FLASH Radiotherapy

Ultrahigh-dose rate FLASH radiotherapy (FLASH-RT) is a potentially paradigm-shifting treatment modality that holds the promise of expanding the therapeutic index for nearly any cancer. At the heart of this exciting technology comes the capability to ameliorate major normal tissue complications without compromising the efficacy of tumor killing. This combination of benefits has now been termed the FLASH effect and relies on an in vivo validation to rigorously demonstrate the absence of normal tissue toxicity. The FLASH effect occurs when the overall irradiation time is extremely short (<500 ms), and in this review we attempt to understand how FLASH-RT can kill tumors but spare normal tissues-likely the single most pressing question confronting the field today.

Proton therapy for pediatric diencephalic tumors

Diencephalic tumors tend to be low grade tumors located near several critical structures, including the optic nerves, optic chiasm, pituitary, hypothalamus, Circle of Willis, and hippocampi. In children, damage to these structures can impact physical and cognitive development over time. Thus, the goal of radiotherapy is to maximize long term survival while minimizing late effects, including endocrine disruption leading to precocious puberty, height loss, hypogonadotropic hypogonadism, and primary amenorrhea; visual disruption including blindness; and vascular damage resulting in cerebral vasculopathy. Compared to photon therapy, proton therapy offers the potential to decrease unnecessary dose to these critical structures while maintaining adequate dose to the tumor. In this article, we review the acute and chronic toxicities associated with radiation for pediatric diencephalic tumors, focusing on the use of proton therapy to minimize treatment-related morbidity. Emerging strategies to further reduce radiation dose to critical structures will also be considered.

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.

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.