Revisiting the ultra-high dose rate effect: implications for charged particle radiotherapy using protons and light ions

Possible mechanisms and simulation modeling of FLASH radiotherapy

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

Pencil Beam Scanning Proton Bragg Peak Conformal FLASH in Prostate Cancer Stereotactic Body Radiotherapy

Bragg peak FLASH radiotherapy (RT) uses a distal tracking method to eliminate exit doses and can achieve superior OAR sparing. This study explores the application of this novel method in stereotactic body radiotherapy prostate FLASH-RT. An in-house platform was developed to enable intensity-modulated proton therapy (IMPT) planning using a single-energy Bragg peak distal tracking method. The patients involved in the study were previously treated with proton stereotactic body radiotherapy (SBRT) using the pencil beam scanning (PBS) technique to 40 Gy in five fractions. FLASH plans were optimized using a four-beam arrangement to generate a dose distribution similar to the conventional opposing beams. All of the beams had a small angle of two degrees from the lateral direction to increase the dosimetry quality. Dose metrics were compared between the conventional PBS and the Bragg peak FLASH plans. The dose rate histogram (DRVH) and FLASH metrics of 40 Gy/s coverage (V40Gy/s) were investigated for the Bragg peak plans. There was no significant difference between the clinical and Bragg peak plans in rectum, bladder, femur heads, large bowel, and penile bulb dose metrics, except for Dmax. For the CTV, the FLASH plans resulted in a higher Dmax than the clinical plans (116.9% vs. 103.3%). For the rectum, the V40Gy/s reached 94% and 93% for 1 Gy dose thresholds in composite and single-field evaluations, respectively. Additionally, the FLASH ratio reached close to 100% after the application of the 5 Gy threshold in composite dose rate assessment. In conclusion, the Bragg peak distal tracking method can yield comparable plan quality in most OARs while preserving sufficient FLASH dose rate coverage, demonstrating that the ultra-high dose technique can be applied in prostate FLASH SBRT.

Pencil Beam Scanning Bragg Peak FLASH Technique for Ultra-High Dose Rate Intensity-Modulated Proton Therapy in Early-Stage Breast Cancer Treatment

Bragg peak FLASH-RT can deliver highly conformal treatment and potentially offer improved normal tissue protection for radiotherapy patients. This study focused on developing ultra-high dose rate (≥40 Gy × RBE/s) intensity-modulated proton therapy (IMPT) for hypofractionated treatment of early-stage breast cancer. A novel tracking technique was developed to enable pencil beaming scanning (PBS) of single-energy protons to adapt the Bragg peak (BP) to the target distally. Standard-of-care PBS treatment plans of consecutively treated early-stage breast cancer patients using multiple energy layers were reoptimized using this technique, and dose metrics were compared between single-energy layer BP FLASH and conventional IMPT plans. FLASH dose rate coverage by volume (V40Gy/s) was also evaluated for the FLASH sparing effect. Distal tracking can precisely stop BP at the target distal edge. All plans (n = 10) achieved conformal IMPT-like dose distributions under clinical machine parameters. No statistically significant differences were observed in any dose metrics for heart, ipsilateral lung, most ipsilateral breast, and CTV metrics (p > 0.05 for all). Conventional plans yielded slightly superior target and skin dose uniformities with 4.5% and 12.9% lower dose maxes, respectively. FLASH-RT plans reached 46.7% and 61.9% average-dose rate FLASH coverage for tissues receiving more than 1 and 5 Gy plan dose total under the 250 minimum MU condition. Bragg peak FLASH-RT techniques achieved comparable plan quality to conventional IMPT while reaching adequate dose rate ratios, demonstrating the feasibility of early-stage breast cancer clinical applications.

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.

Investigating the potential contribution of inter-track interactions within ultra-high dose-rate proton therapy

Objective. Laser-accelerated protons offer an alternative delivery mechanism for proton therapy. This technique delivers dose-rates of ≥10⁹Gy s^-1, many orders of magnitude greater than used clinically. Such ultra-high dose-rates reduce delivery time to nanoseconds, equivalent to the lifetime of reactive chemical species within a biological medium. This leads to the possibility of inter-track interactions between successive protons within a pulse, potentially altering the yields of damaging radicals if they are in sufficient spatial proximity. This work investigates the temporal evolution of chemical species for a range of proton energies and doses to quantify the circumstances required for inter-track interactions, and determine any relevance within ultra-high dose-rate proton therapy.Approach. The TOPAS-nBio Monte Carlo toolkit was used to investigate possible inter-track interactions. Firstly, protons between 0.5 and 100 MeV were simulated to record the radial track dimensions throughout the chemical stage from 1 ps to 1μs. Using the track areas, the geometric probability of track overlap was calculated for various exposures and timescales. A sample of irradiations were then simulated in detail to compare any change in chemical yields for independently and instantaneously delivered tracks, and validate the analytic model.Main results. Track overlap for a clinical 2 Gy dose was negligible for biologically relevant timepoints for all energies. Overlap probability increased with time after irradiation, proton energy and dose, with a minimum 23 Gy dose required before significant track overlap occurred. Simulating chemical interactions confirmed these results with no change in radical yields seen up to 8 Gy for independently and instantaneously delivered tracks.Significance. These observations suggest that the spatial separation between incident protons is too large for physico-chemical inter-track interactions, regardless of the delivery time, indicating such interactions would not play a role in any potential changes in biological response between laser-accelerated and conventional proton therapy.

FLASH radiotherapy: A promising new method for radiotherapy

Among the treatments for malignant tumors, radiotherapy is of great significance both as a main treatment and as an adjuvant treatment. Radiation therapy damages cancer cells with ionizing radiation, leading to their death. However, radiation-induced toxicity limits the dose delivered to the tumor, thereby constraining the control effect of radiotherapy on tumor growth. In addition, the delayed toxicity caused by radiotherapy significantly harms the physical and mental health of patients. FLASH-RT, an emerging class of radiotherapy, causes a phenomenon known as the 'FLASH effect', which delivers radiotherapy at an ultra-high dose rate with lower toxicity to normal tissue than conventional radiotherapy to achieve local tumor control. Although its mechanism remains to be fully elucidated, this modality constitutes a potential new approach to treating malignant tumors. In the present review, the current research progress of FLASH-RT and its various particular effects are described, including the status of research on FLASH-RT and its influencing factors. The hypothetic mechanism of action of FLASH-RT is also summarized, providing insight into future tumor treatments.

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.

Validation of Monte Carlo-based calculations for megavolt electron beams for IORT and FLASH-IORT

In Intra-Operative Radiation Therapy (IORT) the tumour site is surgically exposed and normal tissue located around the tumour may be avoided. Electron applicators would require large surgical incisions; therefore, the preferred mechanism for beam collimation is the IORT cone system. FLASH radiotherapy (FLASH-RT) involves the treatment of tumours at ultra-high dose rates and the IORT cone system can also be used. This study validates the Monte Carlo-based calculations for these small electron beams to accurately determine the dose characteristics of each possible cone-energy combination as well as custom-built alloy cutouts attached to the end of the IORT cone. This will contribute to accurate dose distribution and output factor calculations that are essential to all radiation therapy treatments. A Monte Carlo (MC) model was modelled for electron beams produced by a Siemens Primus LINAC and the IORT cones. The accelerator was built with the component modules available in the BEAMnrc code. The phase-space file generated by the BEAM simulation was used as the source input for the subsequent DOSXYZnrc simulations. Percentage Depth Dose (PDD) data and profiles were extracted from the dose distributions obtained with the DOSXYZnrc simulations. These beam characteristics were compared with measured data for 6, 12, and 18 MeV electron beams for the IORT open cones of diameters 19, 45, and 64 mm and irregularly shaped cutouts. The MC simulations could replicate electron beams within a criterion of 3%/3 mm. Applicator factors were within 0.7%, and cone factors showed good agreement, except for the 9 mm cone size.

Based on the successful comparisons between measurement and MC-calculated dose distributions, output factors for the open cones and for small irregularly shaped IORT beams, it may be concluded that the Monte Carlo based dose calculation could replicate electron beams used for IORT and FLASH-IORT.

Dose rate and dose robustness for proton transmission FLASH-RT treatment in lung cancer

Purposes

To evaluate the plan quality and robustness of both dose and dose rate of proton pencil beam scanning (PBS) transmission FLASH delivery in lung cancer treatment.

Methods and materials

An in-house FLASH planning platform was used to optimize 10 lung cancer patients previously consecutively treated with proton stereotactic body radiation therapy (SBRT) to receive 3 and 5 transmission beams (Trx-3fds and Trx-5fds, respectively) to 34 Gy in a single fraction. Perturbation scenarios (n=12) for setup and range uncertainties (5 mm and 3.5%) were introduced, and dose-volume histogram and dose-rate-volume histogram bands were generated. Conventional proton SBRT clinical plans were used as a reference. RTOG 0915 dose metrics and 40 Gy/s dose rate coverage (V_40Gy/s) were used to assess the dose and dose rate robustness.

Results

Trx-5fds yields a comparable iCTV D_2% of 105.3%, whereas Trx-3fds resulted in inferior D_2% of 111.9% to the clinical SBRT plans with D_2% of 105.6% (p<0.05). Both Trx-5fds and Trx-3fds plans had slightly worse dose metrics to organs at risk than SBRT plans. Trx-5fds achieved superior dosimetry robustness for iCTV, esophagus, and spinal cord doses than both Trx-3fds and conventional SBRT plans. There was no significant difference in dose rate robustness for V_40Gy/s coverage between Trx-3fds and Trx-5fds. Dose rate distribution has similar distributions to the dose when perturbation exists.

Conclusion

Transmission plans yield overall modestly inferior plan quality compared to the conventional proton SBRT plans but provide improved robustness and the potential for a toxicity-sparing FLASH effect. By using more beams (5- versus 3-field), both dose and dose rate robustness for transmission plans can be achieved.

Advanced pencil beam scanning Bragg peak FLASH-RT delivery technique can enhance lung cancer planning treatment outcomes compared to conventional multiple-energy proton PBS techniques

PURPOSE

To investigate the dosimetric characteristics between an advanced proton pencil beam scanning (PBS) Bragg peak FLASH technique and conventional PBS planning technique in lung tumors. To evaluate the "FLASHness" of single-field in a multiple-field delivery scheme for a hypofractionation regimen and move a step forward to clinical application.

METHODS

Single-energy PBS Bragg peak FLASH treatment plans were optimized based on a novel Bragg peak tracking technique to enable Bragg peaks to stop at the distal edge of the target. Inverse treatment planning using multiple-field optimization (MFO) can achieve sufficient FLASH dose rate and intensity-modulated proton therapy (IMPT)-equivalent dosimetric quality. The dose rate of organs-at-risk (OARs) and the target were calculated under FLASH machine parameters. A group of 10 consecutive lung SBRT patients was optimized to 34 Gy/fraction using a standard treatment of PBS technique with multiple energy layers as references to the Bragg peak plans. The dosimetric quality was compared between Bragg peak FLASH and conventional plans based on RTOG0915 dose metrics. FLASH dose rate ratios (V_40Gy/s) were calculated as a metric of the FLASH-sparing effect.

RESULTS

For higher dose thresholds, the Bragg peak plans achieved greater V_40Gy/s FLASH coverage for all major OARs. The V40Gy/s was close to 100% for all OARs when the dose thresholds were > 5 Gy for full plan and single beam evaluations. The less "FLASHness" region was associated with a low dose distribution, mainly occurring in the PBS field penumbra region. The conventional IMPT treatment plans yielded slightly superior target dose uniformity with a D_2%(%) of 108.02% versus that of Bragg peak 300 MU plans of 111.81% (p < 0.01) and that of Bragg peak 1200 MU plans of 115.95% (p < 0.01). No significant difference in dose metrics was found between Bragg peak and IMPT treatment plans for the spinal cord, esophagus, heart, or lung-GTV (all p > 0.05).

CONCLUSION

Hypofractionated lung Bragg peak plans can maintain comparable plan quality to conventional PBS while achieving sufficient FLASH dose rate coverage for major OARs for each field under the multiple-field delivery scheme. The novel Bragg peak FLASH technique has the potential to enhance lung cancer planning treatment outcomes compared to standard PBS treatment techniques.

Use of single-energy proton pencil beam scanning Bragg peak for intensity-modulated proton therapy FLASH treatment planning in liver hypofractionated radiation therapy

PURPOSE

The transmission proton FLASH technique delivers high doses to the normal tissue distal to the target, which is less conformal compared to the Bragg peak technique. To investigate FLASH RT planning using single-energy Bragg peak beams with a similar beam arrangement as clinical intensity-modulated proton therapy (IMPT) in liver stereotactic body radiation therapy (SBRT) and to characterize the plan quality, dose sparing of organs-at-risk (OARs), and FLASH dose rate percentage.

MATERIALS AND METHODS

An in-house platform was developed to enable inverse IMPT-FLASH planning using single-energy Bragg peaks. A universal range shifter and range compensators were utilized to effectively align the Bragg peak to the distal edge of the target. Two different minimum MU settings of 400 and 800 MU/spot (Bragg-400MU and Bragg-800MU) plans were investigated on 10 consecutive hepatocellular carcinoma patients previously treated by IMPT-SBRT to evaluate the FLASH dose and dose rate coverage for OARs. The IMPT-FLASH using single-energy Bragg peaks delivered 50 Gy in 5 fractions with similar or identical beam arrangement to the clinical IMPT-SBRT plans. NRG GI003 dose constraint metrics were used. Three dose rate calculation methods, including average dose rate (ADR), dose threshold dose rate (DTDR), and dose-averaged dose rate (DADR), were all studied.

RESULTS

The novel spot map optimization can fulfill the inverse planning using single-energy Bragg peaks. All the Bragg peak FLASH plans achieved similar results for the liver-GTV D_mean and heart D_0.5cc , compared to SBRT-IMPT. The Bragg-800MU plans resulted in 18.3% higher CTV D_2cc compared with SBRT (p < 0.05), and no significant difference was found between Bragg-400MU and SBRT plans. For the CTV D_max , SBRT plans resulted in 10.3% (p<0.01) less than Bragg-400MU plans and 16.6% (p<0.01) less than Bragg-800MU plans. The Bragg-800MU plans generally achieved higher ADR, DADR, and DTDR dose rates than Bragg-400MU plans, and DADR mostly led to the highest V_40Gy/s compared to other dose rate calculation methods, whereas ADR led to the lowest. The lower dose rate portions in certain OARs are related to the lower dose deposited due to the farther distances from targets, especially in the penumbra of the beams.

CONCLUSION

Single-energy Bragg peak IMPT-FLASH plans eliminate the exit dose in normal tissues, maintaining comparable dose metrics to the conventional IMPT-SBRT plans while achieving a sufficient FLASH dose rate for liver cancers. This study demonstrates the feasibility of and sufficiently high dose rate when applying Bragg peak FLASH treatment for liver cancer hypofractionated FLASH therapy. The advancement of this novel method has the potential to optimize treatment for liver cancer patients. This article is protected by copyright. All rights reserved.

The influence of hypoxia on LET and RBE relationships with implications for ultra-high dose rates and FLASH modelling

Objective. To investigate relationships between linear energy transfer (LET), fluence rates, changes in radiosensitivity and the oxygen enhancement ratio (OER) in different ion beams and extend these concepts to ultra-high dose rate (UHDR) or FLASH effects. Approach. LET values providing maximum relative biological effect (RBE), designated as LETU, are found for neon, carbon and helium beams. Proton experiments show reduced RBEs with depth in scattered (divergent) beams, but not with scanned beams, suggesting that instantaneous fluence rates (related to track separation distances) can modify RBE, all other RBE-determining factors being equal. Micro-volumetric energy transfer per μm3 (mVET) is defined by LET × fluence. High fluence rates will increase mVET rates, with proportional shifts of LETU to lower values due to more rapid energy transfer. From the relationship between LETU and OER at conventional dose rates, OER reductions in UHDR/FLASH exposures can be estimated and biological effective dose analysis of experimental lung and skin reactions becomes feasible. Main results. The Furusawa et al data show that hypoxic LETU values exceed their oxic counterparts. OER reduces from around 3–1.25 at LETU, although the relative radiosensitivities of the oxic and hypoxic α parameters (the OER(α)) exceed those of the standard OER values. Increased fluence rates are predicted to reduce LETU and OER. Large FLASH single doses will minimise RBE increments due to the β parameter reducing by a factor of 0.5–0.25 consistent with oxygen depletion, causing radioresistance. Similar results will occur for photons. Tissue α/β ratios increase by around 10 in FLASH conditions, agreeing with derived ion-beam dose rate equations. Significance. Increasing dose rates enhance local energy deposition rate per unit volume, probably causing oxygen depletion and radioresistance in pre-existing hypoxic sites during UHDR/FLASH exposures. The modelled equations provide testable hypotheses for further dose rate investigations in photon, proton and ion beams.

Model studies of the role of oxygen in the FLASH effect

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

A Universal Range Shifter and Range Compensator Can Enable Proton Pencil Beam Scanning Single-Energy Bragg Peak FLASH-RT Treatment Using Current Commercially Available Proton Systems

PURPOSE

Transmission beams have been proposed for ultra-high dose (or FLASH) proton planning, limiting the organ sparing potentials of proton therapy. By pulling back the ranges of the highest energy proton beams and compensating proton ranges to adapt to the target distally, the exit dose of proton beams can be eliminated to better protect organs at risk while still preserving FLASH dose rate delivery.

METHOD AND MATERIALS

An inverse planning tool was developed to optimize intensity modulated proton therapy using a single-energy layer for FLASH radiation therapy planning. The range pull-backs were calculated to stop single-energy proton beams at the distal edge of the target. The spot map and weights of each field were optimized to achieve a sufficient dose rate using proton beam Bragg peaks. A C-shape target in phantom, along with 6 consecutive lung cancer patients previously treated using proton stereotactic body radiation therapy were planned using this novel Bragg Peak method and also transmission technique. Dosimetry characteristics and 3-dimensional dose rate were investigated.

RESULTS

The minimum monitor units (MU) for transmission and Bragg peak plans were 400 MU/spot and 1200 MU/spot, respectively, corresponding to spot peak dose rates of 670 GyRBE (relative biological effectiveness) per second and 1950 GyRBE per second. Bragg peak plans yield a generally comparable target uniformity while significantly reducing dose spillage volume from the low to medium dose level. For all the 6 lung cases delivery of 34 GyRBE in 1 fraction, assessing Radiation Therapy Oncology Group 0915 constraints, the lung V_7GyRBE volume was reduced by up to 32% (P = .001) for Bragg peak plans. The transmission plans tended to generate 2.4% higher FLASH dose rate coverage (V_40GyRBE/s) versus Bragg peak plans over the major organs at risk. However, Bragg peak plans could also reach the FLASH radiation therapy threshold of V_40GyRBE/s using a higher MU/spot and sophisticated dose-rate optimization algorithm.

CONCLUSIONS

This first proof-of-concept study has demonstrated this novel method of combining range pull-back and powerful inverse optimization capable of achieving FLASH dose rate based on currently available machine parameters using a single-energy Bragg peak. Similar target coverage and uniformity can be maintained by Bragg peak FLASH plans while substantially improving the sparing of organs at risk compared with transmission plans.

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

PURPOSE

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

METHODS AND MATERIALS

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

RESULTS

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

CONCLUSION

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

FLASH ultra-high dose rates in radiotherapy: preclinical and radiobiological evidence

PURPOSE

Flash radiotherapy (FLASH-RT) is currently being regarded as the next breakthrough in radiation treatment of cancer, delivering ultrahigh radiation doses in a very short time, and sparing normal tissues from detrimental injury. Here we review the current evidence on the preclinical findings as well as the radiobiological mechanisms underlying the FLASH effect. We also briefly examine the scenario of available technologies for delivering FLASH dose-rates for research and their implications for future clinical use.

CONCLUSIONS

Preclinical studies report that the FLASH-RT reduces radiation-induced toxicity whilst maintaining an equivalent tumor response across different animal models. However, the molecular radiobiology underlying FLASH effect is not fully understood and further experiments are necessary to understand the biological response. Future studies also includes the design of a FLASH delivery system able to produce beams appropriate for treatment of tumors with ultra-high dose rates. All these research activities will greatly benefit from a multidisciplinary collaboration across biology, physics and clinical oncology, increasing the potential of a rapid clinical translation of FLASH-RT.

In-vitro 3D modelling for charged particle therapy - Uncertainties and opportunities

Radiation therapy is a critical component of oncologic management, with more than half of all cancer patients requiring radiotherapy at some point during their disease course. Over the last decade, there has been increasing interest in charged particle therapy due to its advantageous physical and radiobiologic properties, with the therapeutic use of proton beam therapy (PBT) expanding worldwide. However, there remain large gaps in our knowledge of the radiobiologic mechanisms that underlie key aspects of PBT, such as variations in relative biologic effectiveness (RBE), radioresistance, DNA damage response and repair pathways, as well as immunologic effects. In addition, while the emerging technique of ultra-high dose rate or FLASH radiotherapy, with its potential to further reduce normal tissue toxicities, is an exciting development, in-depth study is needed into the postulated biochemical mechanisms that underpin the FLASH effect such as the oxygen depletion hypothesis as well as the relative contributions of immune responses and the tumor microenvironment. Further investigation is also required to ensure that the FLASH effect is not diminished or lost in PBT. Current methods to evaluate the biologic effects of charged particle therapy rely heavily on 2D cell culture systems and/or animal models. However, both of these methods have well-recognized limitations which limit translatability of findings from bench to bedside. The advent of novel three-dimensional in-vitro tumor models offers a more physiologically relevant and high throughput in-vitro system for the study of tumor development as well as novel therapeutic approaches such as PBT. Advances in 3D cell culture methods, together with knowledge of disease mechanism, biomarkers, and genomic data, can be used to design personalized 3D models that most closely recapitulate tumor microenvironmental factors promoting a particular disease phenotype, moving 3D models and PBT into the age of precision medicine.

A Novel Proton Pencil Beam Scanning FLASH RT Delivery Method Enables Optimal OAR Sparing and Ultra-High Dose Rate Delivery: A Comprehensive Dosimetry Study for Lung Tumors

PURPOSE

While transmission proton beams have been demonstrated to achieve ultra-high dose rate FLASH therapy delivery, they are unable to spare normal tissues distal to the target. This study aims to compare FLASH treatment planning using single energy Bragg peak proton beams versus transmission proton beams in lung tumors and to evaluate Bragg peak plan optimization, characterize plan quality, and quantify organ-at-risk (OAR) sparing.

MATERIALS AND METHODS

Both Bragg peak and transmission plans were optimized using an in-house platform for 10 consecutive lung patients previously treated with proton stereotactic body radiation therapy (SBRT). To bring the dose rate up to the FLASH-RT threshold, Bragg peak plans with a minimum MU/spot of 1200 and transmission plans with a minimum MU/spot of 400 were developed. Two common prescriptions, 34 Gy in 1 fraction and 54 Gy in 3 fractions, were studied with the same beam arrangement for both Bragg peak and transmission plans (n = 40 plans). RTOG 0915 dosimetry metrics and dose rate metrics based on different dose rate calculations, including average dose rate (ADR), dose-averaged dose rate (DADR), and dose threshold dose rate (DTDR), were investigated. We then evaluated the effect of beam angular optimization on the Bragg peak plans to explore the potential for superior OAR sparing.

RESULTS

Bragg peak plans significantly reduced doses to several OAR dose parameters, including lung V7.4Gy and V7Gy by 32.0% (p < 0.01) and 30.4% (p < 0.01) for 34Gy/fx plans, respectively; and by 40.8% (p < 0.01) and 41.2% (p < 0.01) for 18Gy/fx plans, respectively, compared with transmission plans. Bragg peak plans have ~3% less in DADR and ~10% differences in mean OARs in DTDR and DADR relative to transmission plans due to the larger portion of lower dose regions of Bragg peak plans. With angular optimization, optimized Bragg peak plans can further reduce the lung V7Gy by 20.7% (p < 0.01) and V7.4Gy by 19.7% (p < 0.01) compared with Bragg peak plans without angular optimization while achieving a similar 3D dose rate distribution.

CONCLUSION

The single-energy Bragg peak plans achieve superior dosimetry performances in OARs to transmission plans with comparable dose rate performances for lung cancer FLASH therapy. Beam angle optimization can further improve the OAR dosimetry parameters with similar 3D FLASH dose rate coverage.

A Computational Model for Oxygen Depletion Hypothesis in FLASH Effect

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

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

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

FLASH radiotherapy with carbon ion beams

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

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

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

Ultra-High Dose Rate FLASH Irradiation Induced Radio-Resistance of Normal Fibroblast Cells Can Be Enhanced by Hypoxia and Mitochondrial Dysfunction Resulting From Loss of Cytochrome C

Ultra-high dose rate FLASH irradiation (FLASH-IR) has got extensive attention since it may provide better protection on normal tissues while maintain tumor killing effect compared with conventional dose rate irradiation. The FLASH-IR induced protection effect on normal tissues is exhibited as radio-resistance of the irradiated normal cells, and is suggested to be related to oxygen depletion. However, the detailed cell death profile and pathways are still unclear. Presently normal mouse embryonic fibroblast cells were FLASH irradiated (∼10⁹ Gy/s) at the dose of ∼10–40 Gy in hypoxic and normoxic condition, with ultra-fast laser-generated particles. The early apoptosis, late apoptosis and necrosis of cells were detected and analyzed at 6, 12, and 24 h post FLASH-IR. The results showed that FLASH-IR induced significant early apoptosis, late apoptosis and necrosis in normal fibroblast cells, and the apoptosis level increased with time, in either hypoxic or normoxic conditions. In addition, the proportion of early apoptosis, late apoptosis and necrosis were significantly lower in hypoxia than that of normoxia, indicating that radio-resistance of normal fibroblast cells under FLASH-IR can be enhanced by hypoxia. To further investigate the apoptosis related profile and potential pathways, mitochondria dysfunction cells resulting from loss of cytochrome c (cyt c^–/–) were also irradiated. The results showed that compared with irradiated normal cells (cyt c^+/+), the late apoptosis and necrosis but not early apoptosis proportions of irradiated cyt c^–/– cells were significant decreased in both hypoxia and normoxia, indicating mitochondrial dysfunction increased radio-resistance of FLASH irradiated cells. Taken together, to our limited knowledge, this is the first report shedding light on the death profile and pathway of normal and cyt c^–/– cells under FLASH-IR in hypoxic and normoxic circumstances, which might help us improve the understanding of the FLASH-IR induced protection effect in normal cells, and thus might potentially help to optimize the future clinical FLASH treatment.

Ultra-High Dose-Rate, Pulsed (FLASH) Radiotherapy with Carbon Ions: Generation of Early, Transient, Highly Oxygenated Conditions in the Tumor Environment

It is well known that molecular oxygen is a product of the radiolysis of water with high-linear energy transfer (LET) radiation, which is distinct from low-LET radiation wherein O2 radiolytic yield is negligible. Since O2 is a powerful radiosensitizer, this fact is of practical relevance in cancer therapy with energetic heavy ions, such as carbon ions. It has recently been discovered that large doses of ionizing radiation delivered to tumors at very high dose rates (i.e., in a few milliseconds) have remarkable benefits in sparing healthy tissue while preserving anti-tumor activity compared to radiotherapy delivered at conventional, lower dose rates. This new method is called "FLASH radiotherapy" and has been tested using low-LET radiation (i.e., electrons and photons) in various pre-clinical studies and recently in a human patient. Although the exact mechanism(s) underlying FLASH are still unclear, it has been suggested that radiation delivered at high dose rates spares normal tissue via oxygen depletion. In addition, heavy-ion radiation achieves tumor control with reduced normal tissue toxicity due to its favorable physical depth-dose profile and increased radiobiological effectiveness in the Bragg peak region. To date, however, biological research with energetic heavy ions delivered at ultra-high dose rates has not been performed and it is not known whether heavy ions are suitable for FLASH radiotherapy. Here we present the additive or even synergistic advantages of integrating the FLASH dose rates into carbon-ion therapy. These benefits result from the ability of heavy ions at high LET to generate an oxygenated microenvironment around their track due to the occurrence of multiple (mainly double) ionization of water. This oxygen is abundant immediately in the tumor region where the LET of the carbon ions is very high, near the end of the carbon-ion path (i.e., in the Bragg peak region). In contrast, in the "plateau" region of the depth-dose distribution of ions (i.e., in the normal tissue region), in which the LET is significantly lower, this generation of molecular oxygen is insignificant. Under FLASH irradiation, it is shown that this early generation of O2 extends evenly over the entire irradiated tumor volume, with concentrations estimated to be several orders of magnitude higher than the oxygen levels present in hypoxic tumor cells. Theoretically, these results indicate that FLASH radiotherapy using carbon ions would have a markedly improved therapeutic ratio with greater toxicity in the tumor due to the generation of oxygen at the spread-out Bragg peak.

FLASH Irradiation with Proton Beams: Beam Characteristics and Their Implications for Beam Diagnostics

FLASH irradiations use dose-rates orders of magnitude higher than commonly used in patient treatments. Such irradiations have shown interesting normal tissue sparing in cell and animal experiments, and, as such, their potential application to clinical practice is being investigated. Clinical accelerators used in proton therapy facilities can potentially provide FLASH beams; therefore, the topic is of high interest in this field. However, a clear FLASH effect has so far been observed in presence of high dose rates (>40 Gy/s), high delivered dose (tens of Gy), and very short irradiation times (<300 ms). Fulfilling these requirements poses a serious challenge to the beam diagnostics system of clinical facilities. We will review the status and proposed solutions, from the point of view of the beam definitions for FLASH and their implications for beam diagnostics. We will devote particular attention to the topics of beam monitoring and control, as well as absolute dose measurements, since finding viable solutions in these two aspects will be of utmost importance to guarantee that the technique can be adopted quickly and safely in clinical practice.

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

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

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

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

Immunomodulatory Effects of Radiotherapy

Radiation therapy (RT), an integral component of curative treatment for many malignancies, can be administered via an increasing array of techniques. In this review, we summarize the properties and application of different types of RT, specifically, conventional therapy with x-rays, stereotactic body RT, and proton and carbon particle therapies. We highlight how low-linear energy transfer (LET) radiation induces simple DNA lesions that are efficiently repaired by cells, whereas high-LET radiation causes complex DNA lesions that are difficult to repair and that ultimately enhance cancer cell killing. Additionally, we discuss the immunogenicity of radiation-induced tumor death, elucidate the molecular mechanisms by which radiation mounts innate and adaptive immune responses and explore strategies by which we can increase the efficacy of these mechanisms. Understanding the mechanisms by which RT modulates immune signaling and the key players involved in modulating the RT-mediated immune response will help to improve therapeutic efficacy and to identify novel immunomodulatory drugs that will benefit cancer patients undergoing targeted RT.

Technical challenges for FLASH proton therapy

There is growing interest in the radiotherapy community in the application of FLASH radiotherapy, wherein the dose is delivered to the entire treatment volume in less than a second. Early pre-clinical evidence suggests that these extremely high dose rates provide significant sparing of healthy tissue compared to conventional radiotherapy without reducing the damage to cancerous cells. This interest has been reflected in the proton therapy community, with early tests indicating that the FLASH effect is also present with high dose rate proton irradiation. In order to deliver clinically relevant doses at FLASH dose rates significant technical hurdles must be overcome in the accelerator technology before FLASH proton therapy can be realised. Of these challenges, increasing the average current from the present clinical range of 1-10 nA to in excess of 100 nA is at least feasible with existing technology, while the necessity for rapid energy adjustment on the order of a few milliseconds is much more challenging, particularly for synchrotron-based systems. However, the greatest challenge is to implement full pencil beam scanning, where scanning speeds 2 orders of magnitude faster than the existing state-of-the-art will be necessary, along with similar improvements in the speed and accuracy of associated dosimetry. Hybrid systems utilising 3D-printed patient specific range modulators present the most likely route to clinical delivery. However, to correctly adapt and develop existing technology to meet the challenges of FLASH, more pre-clinical studies are needed to properly establish the beam parameters that are necessary to produce the FLASH effect.

A Quantitative Analysis of the Role of Oxygen Tension in FLASH Radiation Therapy

PURPOSE

Recent demonstrations of normal tissue sparing by high-dose, high-dose-rate FLASH radiation therapy have driven considerable interest in its application to improve clinical outcomes. However, significant uncertainty remains about the underlying mechanisms of FLASH sparing and how deliveries can be optimized to maximize benefit from this effect. Rapid oxygen depletion has been suggested as a potential mechanism by which these effects occur, but this has yet to be quantitatively tested against experimental data.

METHODS AND MATERIALS

Models of oxygen kinetics during irradiation were used to develop a time-dependent model of the oxygen enhancement ratio in mammalian cells that incorporates oxygen depletion. The characteristics of this model were then explored in terms of the dose and dose-rate dependence of the oxygen enhancement ratio. This model was also fit to experimental data from both in vitro and in vivo data sets.

RESULTS

In cases of FLASH radiation therapy, this model suggests that oxygen levels can be depleted by amounts that are sufficient to affect radiosensitivity only in conditions of intermediate oxygen tension, with no effect seen at high or very low initial oxygen levels. The model also effectively reproduced the dose, dose rate, and oxygen tension dependence of responses to FLASH radiation therapy in a range of systems, with model parameters compatible with published data.

CONCLUSIONS

Oxygen depletion provides a credible quantitative model to understand the biological effects of FLASH radiation therapy and is compatible with a range of experimental observations of FLASH sparing. These results highlight the need for more detailed quantification of oxygen depletion under high-dose-rate radiation exposures in relevant systems and the importance of oxygen tension in target tissues for FLASH sparing to be observed.

FLASH and minibeams in radiation therapy: the effect of microstructures on time and space and their potential application to protontherapy

After years of lethargy, studies on two non-conventional microstructures in time and space of the beams used in radiation therapy are enjoying a huge revival. The first effect called “FLASH” is based on very high dose-rate irradiation (pulse amplitude ≥10⁶ Gy/s), short beam-on times (≤100 ms) and large single doses (≥10 Gy) as experimental parameters established so far to give biological and potential clinical effects. The second effect relies on the use of arrays of minibeams (e.g., 0.5–1 mm, spaced 1–3.5 mm). Both approaches have been shown to protect healthy tissues as an endpoint that must be clearly specified and could be combined with each other (e.g., minibeams under FLASH conditions). FLASH depends on the presence of oxygen and could proceed from the chemistry of peroxyradicals and a reduced incidence on DNA and membrane damage. Minibeams action could be based on abscopal effects, cell signalling and/or migration of cells between “valleys and hills” present in the non-uniform irradiation field as well as faster repair of vascular damage. Both effects are expected to maintain intact the tumour control probability and might even preserve antitumoural immunological reactions. FLASH in vivo experiments involving Zebrafish, mice, pig and cats have been done with electron beams, while minibeams are an intermediate approach between X-GRID and synchrotron X-ray microbeams radiation. Both have an excellent rationale to converge and be applied with proton beams, combining focusing properties and high dose rates in the beam path of pencil beams, and the inherent advantage of a controlled limited range. A first treatment with electron FLASH (cutaneous lymphoma) has recently been achieved, but clinical trials have neither been presented for FLASH with protons, nor under the minibeam conditions. Better understanding of physical, chemical and biological mechanisms of both effects is essential to optimize the technical developments and devise clinical trials.

The FLASH effect depends on oxygen concentration

OBJECTIVE

Recent in vivo results have shown prominent tissue sparing effect of radiotherapy with ultra-high dose rates (FLASH) compared to conventional dose rates (CONV). Oxygen depletion has been proposed as the underlying mechanism, but in vitro data to support this have been lacking. The aim of the current study was to compare FLASH to CONV irradiation under different oxygen concentrations in vitro.

METHODS

Prostate cancer cells were irradiated at different oxygen concentrations (relative partial pressure ranging between 1.6 and 20%) with a 10 MeV electron beam at a dose rate of either 600 Gy/s (FLASH) or 14 Gy/min (CONV), using a modified clinical linear accelerator. We evaluated the surviving fraction of cells using clonogenic assays after irradiation with doses ranging from 0 to 25 Gy.

RESULTS

Under normoxic conditions, no differences between FLASH and CONV irradiation were found. For hypoxic cells (1.6%), the radiation response was similar up to a dose of about 5-10 Gy, above which increased survival was shown for FLASH compared to CONV irradiation. The increased survival was shown to be significant at 18 Gy, and the effect was shown to depend on oxygen concentration.

CONCLUSION

The in vitro FLASH effect depends on oxygen concentration. Further studies to characterize and optimize the use of FLASH in order to widen the therapeutic window are indicated.

ADVANCES IN KNOWLEDGE

This paper shows in vitro evidence for the role of oxygen concentration underlying the difference between FLASH and CONV irradiation.

Ultra-High Dose Rate (FLASH) Radiotherapy: Silver Bullet or Fool's Gold?

Radiotherapy is a cornerstone of both curative and palliative cancer care. However, radiotherapy is severely limited by radiation-induced toxicities. If these toxicities could be reduced, a greater dose of radiation could be given therefore facilitating a better tumor response. Initial pre-clinical studies have shown that irradiation at dose rates far exceeding those currently used in clinical contexts reduce radiation-induced toxicities whilst maintaining an equivalent tumor response. This is known as the FLASH effect. To date, a single patient has been subjected to FLASH radiotherapy for the treatment of subcutaneous T-cell lymphoma resulting in complete response and minimal toxicities. The mechanism responsible for reduced tissue toxicity following FLASH radiotherapy is yet to be elucidated, but the most prominent hypothesis so far proposed is that acute oxygen depletion occurs within the irradiated tissue. This review examines the tissue response to FLASH radiotherapy, critically evaluates the evidence supporting hypotheses surrounding the biological basis of the FLASH effect, and considers the potential for FLASH radiotherapy to be translated into clinical contexts.

The Importance and Clinical Implications of FLASH Ultra-High Dose-Rate Studies for Proton and Heavy Ion Radiotherapy

The goal of radiation therapy is to provide the highest probability of tumor control while minimizing normal tissue toxicity. Recently, it has been discovered that ultra-high dose rates of ionizing radiation may preferentially spare normal tissue over tumor tissue. This effect, referred to as FLASH radiotherapy, has been observed in various animal models as well as, more recently, in a human patient. This effect may be related to the cell sparing found in vitro at ultra-high dose rates of photons and electrons dating back to the 1960s. Conditions representative of physiologic oxygen were found to be essential for this process to occur. However, there is no conclusive data on whether this effect occurs with protons, as all results to date have been in cells irradiated at ambient oxygen conditions. There have been no ultra-high dose-rate experiments with heavy ions, which would be relevant to the implementation of FLASH to carbon-ion therapy. These basic science results are critical in guiding this rapidly advancing field, since clinical particle therapy machines capable of FLASH dose rates have already been promoted for protons. To help ensure FLASH radiotherapy is reliable and maximally effective, the radiobiology must keep ahead of the clinical implementation to help guide it. In this context, in vitro and in vivo proton and heavy ion experiments involving FLASH dose rates need to be performed to evaluate not only short-term consequences, but also sequelae related to long-term health risks. Critical to these future studies is consideration of relevant oxygen tensions at the time of irradiation, as well as appropriate in silico modeling to assist in understanding the initial physicochemical events.

Laser-driven radiation: Biomarkers for molecular imaging of high dose-rate effects

Recently developed short‐pulsed laser sources garner high dose‐rate beams such as energetic ions and electrons, x rays, and gamma rays. The biological effects of laser‐generated ion beams observed in recent studies are different from those triggered by radiation generated using classical accelerators or sources, and this difference can be used to develop new strategies for cancer radiotherapy. High‐power lasers can now deliver particles in doses of up to several Gy within nanoseconds. The fast interaction of laser‐generated particles with cells alters cell viability via distinct molecular pathways compared to traditional, prolonged radiation exposure. The emerging consensus of recent literature is that the differences are due to the timescales on which reactive molecules are generated and persist, in various forms. Suitable molecular markers have to be adopted to monitor radiation effects, addressing relevant endogenous molecules that are accessible for investigation by noninvasive procedures and enable translation to clinical imaging. High sensitivity has to be attained for imaging molecular biomarkers in cells and in vivo to follow radiation‐induced functional changes. Signal‐enhanced MRI biomarkers enriched with stable magnetic nuclear isotopes can be used to monitor radiation effects, as demonstrated recently by the use of dynamic nuclear polarization (DNP) for biomolecular observations in vivo. In this context, nanoparticles can also be used as radiation enhancers or biomarker carriers. The radiobiology‐relevant features of high dose‐rate secondary radiation generated using high‐power lasers and the importance of noninvasive biomarkers for real‐time monitoring the biological effects of radiation early on during radiation pulse sequences are discussed.

Feasibility of proton FLASH effect tested by zebrafish embryo irradiation

BACKGROUND AND PURPOSE

Motivated by first animal trials showing the normal tissue protecting effect of electron and photon Flash irradiation, i.e. at mean dose rates of 100 Gy/s and higher, relative to conventional beam delivery over minutes the feasibility of proton Flash should be assessed.

MATERIALS AND METHODS

A setup and beam parameter settings for the treatment of zebrafish embryo with proton Flash and proton beams of conventional dose rate were established at the University Proton Therapy Dresden. Zebrafish embryos were treated with graded doses and the differential effect on embryonic survival and the induction of morphological malformations was followed for up to four days after irradiation.

RESULTS

Beam parameters for the realization of proton Flash were set and tested with respect to controlled dose delivery to biological samples. Analyzing the dose dependent embryonic survival and the rate of spinal curvature as one type of developmental abnormality, no significant influence of proton dose rate was revealed. For the rate of pericardial edema as acute radiation effect, a significant difference (p < 0.05) between proton Flash and protons delivered at conventional dose rate of 5 Gy/min was observed for one dose point only.

CONCLUSION

The feasibility of Flash proton irradiation was successfully shown, whereas more experiments are required to confirm the presence or absence of a protecting effect and to figure out the limits and requirements for the Flash effect.

DNA Base Modifications Mediated by Femtosecond Laser-Induced Cold Low-Density Plasma in Aqueous Solutions

Applications based on near-infrared femtosecond laser-induced plasma in biological materials involve numerous ionization events that inevitably mediate physicochemical effects. Here, the physical chemistry underlying the action of such plasma is characterized in a system of biological interest. We have implemented wavefront shaping techniques to control the generation of laser-induced low electron density plasma channels in DNA aqueous solutions, which minimize the unwanted thermo-mechanical effects associated with plasma of higher density. The number of DNA base modifications per unit of absolute energy deposited by such cold plasma is compared to those induced by either ultraviolet or standard ionizing radiation (γ-rays). Analyses of various photoinduced, oxidative, and reductive DNA base products show that the effects of laser-induced cold plasma are mainly mediated by reactive radical species produced upon the ionization of water, rather than by the direct interaction of the strong laser field with DNA. In the plasma environment, reactions among densely produced primary radicals result in a dramatic decrease in the yields of DNA damages relative to sparse ionizing radiation. This intense radical production also drives the local depletion of oxygen.

DNA DSB Repair Dynamics following Irradiation with Laser-Driven Protons at Ultra-High Dose Rates

Protontherapy has emerged as more effective in the treatment of certain tumors than photon based therapies. However, significant capital and operational costs make protontherapy less accessible. This has stimulated interest in alternative proton delivery approaches, and in this context the use of laser-based technologies for the generation of ultra-high dose rate ion beams has been proposed as a prospective route. A better understanding of the radiobiological effects at ultra-high dose-rates is important for any future clinical adoption of this technology. In this study, we irradiated human skin fibroblasts-AG01522B cells with laser-accelerated protons at a dose rate of 10⁹ Gy/s, generated using the Gemini laser system at the Rutherford Appleton Laboratory, UK. We studied DNA double strand break (DSB) repair kinetics using the p53 binding protein-1(53BP1) foci formation assay and observed a close similarity in the 53BP1 foci repair kinetics in the cells irradiated with 225 kVp X-rays and ultra- high dose rate protons for the initial time points. At the microdosimetric scale, foci per cell per track values showed a good correlation between the laser and cyclotron-accelerated protons indicating similarity in the DNA DSB induction and repair, independent of the time duration over which the dose was delivered.

Enhancing radiosensitivity of melanoma cells through very high dose rate pulses released by a plasma focus device

Radiation therapy is a useful and standard tumor treatment strategy. Despite recent advances in delivery of ionizing radiation, survival rates for some cancer patients are still low because of recurrence and radioresistance. This is why many novel approaches have been explored to improve radiotherapy outcome. Some strategies are focused on enhancement of accuracy in ionizing radiation delivery and on the generation of greater radiation beams, for example with a higher dose rate. In the present study we proposed an in vitro research of the biological effects of very high dose rate beam on SK-Mel28 and A375, two radioresistant human melanoma cell lines. The beam was delivered by a pulsed plasma device, a "Mather type" Plasma Focus for medical applications. We hypothesized that this pulsed X-rays generator is significantly more effective to impair melanoma cells survival compared to conventional X-ray tube. Very high dose rate treatments were able to reduce clonogenic efficiency of SK-Mel28 and A375 more than the X-ray tube and to induce a greater, less easy-to-repair DNA double-strand breaks. Very little is known about biological consequences of such dose rate. Our characterization is preliminary but is the first step toward future clinical considerations.

Dose-Rate Effects in Breaking DNA Strands by Short Pulses of Extreme Ultraviolet Radiation

In this study, we examined dose-rate effects on strand break formation in plasmid DNA induced by pulsed extreme ultraviolet (XUV) radiation. Dose delivered to the target molecule was controlled by attenuating the incident photon flux using aluminum filters as well as by changing the DNA/buffer-salt ratio in the irradiated sample. Irradiated samples were examined using agarose gel electrophoresis. Yields of single- and double-strand breaks (SSBs and DSBs) were determined as a function of the incident photon fluence. In addition, electrophoresis also revealed DNA cross-linking. Damaged DNA was inspected by means of atomic force microscopy (AFM). Both SSB and DSB yields decreased with dose rate increase. Quantum yields of SSBs at the highest photon fluence were comparable to yields of DSBs found after synchrotron irradiation. The average SSB/DSB ratio decreased only slightly at elevated dose rates. In conclusion, complex and/or clustered damages other than cross-links do not appear to be induced under the radiation conditions applied in this study.

Faster and safer? FLASH ultra-high dose rate in radiotherapy

Recent results from the Franco-Swiss team of Institute Curie and Centre Hospitalier Universitaire Vaudois demonstrate a remarkable sparing of normal tissue after irradiation at ultra-high dose rate (>40 Gy s^-1). The "FLASH" radiotherapy maintains tumour control level, suggesting that ultra-high dose rate can substantially enhance the therapeutic window in radiotherapy. The results have been obtained so far only with 4-6 MeV electrons in lung and brain mouse model. Nevertheless, they have attracted a great attention for the potential clinical applications. Oxygen depletion had been discussed many years ago as a possible mechanism for reduction of the damage after exposure to ultra-high dose rate. However, the mechanism underlying the effect observed in the FLASH radiotherapy remains to be elucidated.

Thermal limits on MV x-ray production by bremsstrahlung targets in the context of novel linear accelerators

PURPOSE

To study the impact of target geometrical and linac operational parameters, such as target material and thickness, electron beam size, repetition rate, and mean current on the ability of the radiotherapy treatment head to deliver high-dose-rate x-ray irradiation in the context of novel linear accelerators capable of higher repetition rates/duty cycle than conventional clinical linacs.

METHODS

The depth dose in a water phantom without a flattening filter and heat deposition in an x-ray target by 10 MeV pulsed electron beams were calculated using the Monte-Carlo code MCNPX, and the transient temperature behavior of the target was simulated by ANSYS. Several parameters that affect both the dose distribution and temperature behavior were investigated. The target was tungsten with a thickness ranging from 0 to 3 mm and a copper heat remover layer. An electron beam with full width at half maximum (FWHM) between 0 and3 mm and mean current of 0.05-2 mA was used as the primary beam at repetition rates of 100, 200, 400, and 800 Hz.

RESULTS

For a 10 MeV electron beam with FWHM of 1 mm, pulse length of 5 μs, by using a thin tungsten target with thickness of 0.2 mm instead of 1 mm, and by employing a high repetition rate of 800 Hz instead of 100 Hz, the maximum dose rate delivered can increase two times from 0.57 to 1.16 Gy/s. In this simple model, the limiting factor on dose rate is the copper heat remover's softening temperature, which was considered to be 500°C in our study.

CONCLUSIONS

A high dose rate can be obtained by employing thin targets together with high repetition rate electron beams enabled by novel linac designs, whereas the benefit of thin targets is marginal at conventional repetition rates. Next generation linacs used to increase dose rate need different target designs compared to conventional linacs.

Dose-rate effect of ultrashort electron beam radiation on DNA damage and repair in vitro

Laser-generated electron beams are distinguished from conventional accelerated particles by ultrashort beam pulses in the femtoseconds to picoseconds duration range, and their application may elucidate primary radiobiological effects. The aim of the present study was to determine the dose-rate effect of laser-generated ultrashort pulses of 4 MeV electron beam radiation on DNA damage and repair in human cells. The dose rate was increased via changing the pulse repetition frequency, without increasing the electron energy. The human chronic myeloid leukemia K-562 cell line was used to estimate the DNA damage and repair after irradiation, via the comet assay. A distribution analysis of the DNA damage was performed. The same mean level of initial DNA damages was observed at low (3.6 Gy/min) and high (36 Gy/min) dose-rate irradiation. In the case of low-dose-rate irradiation, the detected DNA damages were completely repairable, whereas the high-dose-rate irradiation demonstrated a lower level of reparability. The distribution analysis of initial DNA damages after high-dose-rate irradiation revealed a shift towards higher amounts of damage and a broadening in distribution. Thus, increasing the dose rate via changing the pulse frequency of ultrafast electrons leads to an increase in the complexity of DNA damages, with a consequent decrease in their reparability. Since the application of an ultrashort pulsed electron beam permits us to describe the primary radiobiological effects, it can be assumed that the observed dose-rate effect on DNA damage/repair is mainly caused by primary lesions appearing at the moment of irradiation.