Back to the Future: Very High-Energy Electrons (VHEEs) and Their Potential Application in Radiation Therapy

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.

Design, Construction, and Test of Compact, Distributed-Charge, X-Band Accelerator Systems that Enable Image-Guided, VHEE FLASH Radiotherapy

The design and optimization of laser-Compton x-ray systems based on compact distributed charge accelerator structures can enable micron-scale imaging of disease and the concomitant production of beams of Very High Energy Electrons (VHEEs) capable of producing FLASH-relevant dose rates. The physics of laser-Compton x-ray scattering ensures that the scattered x-rays follow exactly the trajectory of the incident electrons, thus providing a route to image-guided, VHEE FLASH radiotherapy. The keys to a compact architecture capable of producing both laser-Compton x-rays and VHEEs are the use of X-band RF accelerator structures which have been demonstrated to operate with over 100 MeV/m acceleration gradients. The operation of these structures in a distributed charge mode in which each radiofrequency (RF) cycle of the drive RF pulse is filled with a low-charge, high-brightness electron bunch is enabled by the illumination of a high-brightness photogun with a train of UV laser pulses synchronized to the frequency of the underlying accelerator system. The UV pulse trains are created by a patented pulse synthesis approach which utilizes the RF clock of the accelerator to phase and amplitude modulate a narrow band continuous wave (CW) seed laser. In this way it is possible to produce up to 10 μA of average beam current from the accelerator. Such high current from a compact accelerator enables production of sufficient x-rays via laser-Compton scattering for clinical imaging and does so from a machine of "clinical" footprint. At the same time, the production of 1000 or greater individual micro-bunches per RF pulse enables > 10 nC of charge to be produced in a macrobunch of < 100 ns. The design, construction, and test of the 100-MeV class prototype system in Irvine, CA is also presented.

Democratizing FLASH Radiotherapy

FLASH radiotherapy (RT) is emerging as a potentially revolutionary advancement in cancer treatment, offering the potential to deliver RT at ultra-high dose rates (>40 Gy/s) while significantly reducing damage to healthy tissues. Democratizing FLASH RT by making this cutting-edge approach more accessible and affordable for healthcare systems worldwide would have a substantial impact in global health. Here, we review recent developments in FLASH RT and present perspective on further developments that could facilitate the democratizing of FLASH RT. These include upgrading and validating current technologies that can deliver and measure the FLASH radiation dose with high accuracy and precision, establishing a deeper mechanistic understanding of the FLASH effect, and optimizing dose delivery conditions and parameters for different types of tumors and normal tissues, such as the dose rate, dose fractionation, and beam quality for high efficacy. Furthermore, we examine the potential for democratizing FLASH radioimmunotherapy leveraging evidence that FLASH RT can make the tumor microenvironment more immunogenic, and parallel developments in nanomedicine or use of smart radiotherapy biomaterials for combining RT and immunotherapy. We conclude that the democratization of FLASH radiotherapy represents a major opportunity for concerted cross-disciplinary research collaborations with potential for tremendous impact in reducing radiotherapy disparities and extending the cancer moonshot globally.

CERN-based experiments and Monte-Carlo studies on focused dose delivery with very high energy electron (VHEE) beams for radiotherapy applications

Very High Energy Electron (VHEE) beams are a promising alternative to conventional radiotherapy due to their highly penetrating nature and their applicability as a modality for FLASH (ultra-high dose-rate) radiotherapy. The dose distributions due to VHEE need to be optimised; one option is through the use of quadrupole magnets to focus the beam, reducing the dose to healthy tissue and allowing for targeted dose delivery at conventional or FLASH dose-rates. This paper presents an in depth exploration of the focusing achievable at the current CLEAR (CERN Linear Electron Accelerator for Research) facility, for beam energies >200 MeV. A shorter, more optimal quadrupole setup was also investigated using the TOPAS code in Monte Carlo simulations, with dimensions and beam parameters more appropriate to a clinical situation. This work provides insight into how a focused VHEE radiotherapy beam delivery system might be achieved.

First in vitro measurement of VHEE relative biological effectiveness (RBE) in lung and prostate cancer cells using the ARES linac at DESY

Very high energy electrons (VHEE) are a potential candidate for radiotherapy applications. This includes tumours in inhomogeneous regions such as lung and prostate cancers, due to the insensitivity of VHEE to inhomogeneities. This study explores how electrons in the VHEE range can be used to perform successful in vitro radiobiological studies. The ARES (accelerator research experiment at SINBAD) facility at DESY, Hamburg, Germany was used to deliver 154 MeV electrons to both prostate (PC3) and lung (A549) cancer cells in suspension. Dose was delivered to samples with repeatability and uniformity, quantified with Gafchromic film. Cell survival in response to VHEE was measured using the clonogenic assay to determine the biological effectiveness of VHEE in cancer cells for the first time using this method. Equivalent experiments were performed using 300 kVp X-rays, to enable VHEE irradiated cells to be compared with conventional photons. VHEE irradiated cancer cell survival was fitted to the linear quadratic (LQ) model (R2 = 0.96-0.97). The damage from VHEE and X-ray irradiated cells at doses between 1.41 and 6.33 Gy are comparable, suggesting similar relative biological effectiveness (RBE) between the two modalities. This suggests VHEE is as damaging as photon radiotherapy and therefore could be used to successfully damage cancer cells during radiotherapy. The RBE of VHEE was quantified as the relative doses required for 50% (D0.5) and 10% (D0.1) cell survival. Using these values, VHEE RBE was measured as 0.93 (D0.5) and 0.99 (D0.1) for A549 and 0.74 (D0.5) and 0.93 (D0.1) for PC3 cell lines respectively. For the first time, this study has shown that 154 MeV electrons can be used to effectively kill lung and prostate cancer cells, suggesting that VHEE would be a viable radiotherapy modality. Several studies have shown that VHEE has characteristics that would offer significant improvements over conventional photon radiotherapy for example, electrons are relatively easy to steer and can be used to deliver dose rapidly and with high efficiency. Studies have shown improved dose distribution with VHEE in treatment plans, in comparison to VMAT, indicating that VHEE can offer improved and safer treatment plans with reduced side effects. The biological response of cancer cells to VHEE has not been sufficiently studied as of yet, however this initial study provides some initial insights into cell damage. VHEE offers significant benefits over photon radiotherapy and therefore more studies are required to fully understand the biological effectiveness of VHEE.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Extending deterministic transport capabilities for very-high and ultra-high energy electron beams

Focused Very-High Energy Electron (VHEE, 50-300 MeV) and Ultra-High Energy Electron (UHEE, > 300 MeV) beams can accurately target both large and deeply seated human tumors with high sparing properties, while avoiding the spatial requirements and cost of proton and heavy ion facilities. Advanced testing phases are underway at the CLEAR facilities at CERN (Switzerland), NLCTA at Stanford (USA), and SPARC at INFN (Italy), aiming to accelerate the transition to clinical application. Currently, Monte Carlo (MC) transport is the sole paradigm supporting preclinical trials and imminent clinical deployment. In this paper, we propose an alternative: the first extension of the nuclear-reactor deterministic chain NJOY-DRAGON for VHEE and UHEE applications. We have extended the Boltzmann-Fokker-Planck (BFP) multigroup formalism and validated it using standard radio-oncology benchmarks, complex assemblies with a wide range of atomic numbers, and comprehensive irradiation of the entire periodic table. We report that [Formula: see text] of water voxels exhibit a BFP-MC deviation below [Formula: see text] for electron energies under [Formula: see text]. Additionally, we demonstrate that at least [Formula: see text] of voxels of bone, lung, adipose tissue, muscle, soft tissue, tumor, steel, and aluminum meet the same criterion between [Formula: see text] and [Formula: see text]. For water, the thorax, and the breast intra-operative benchmark, typical average BFP-MC deviations of [Formula: see text] and [Formula: see text] were observed at [Formula: see text] and [Formula: see text], respectively. By irradiating the entire periodic table, we observed similar performance between lithium ([Formula: see text]) and cerium ([Formula: see text]). Deficiencies observed between praseodymium ([Formula: see text]) and einsteinium ([Formula: see text]) have been reported, analyzed, and quantified, offering critical insights for the ongoing development of the Evaluated Nuclear Data File mode in NJOY.

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.

Very high-energy electron dose calculation using the Fermi-Eyges theory of multiple scattering and a simplified pencil beam model

BACKGROUND

Very high-energy electrons (VHEE) radiotherapy, in the energy range of 100-200 MeV is currently considered a promising technique for the future of radiation therapy and could benefit from the promises of ultra-high dose rate FLASH therapy. However, to our knowledge, no analytical calculation models have been tested for this type of application and the approximations proposed for multiple scattering with electron beams have not been extensively evaluated at these high energies.

PURPOSE

In this work, we discuss the derivation of a simple and fast algorithm based on the Fermi-Eyges theory of multiple Coulomb scattering for fast dose calculation for VHEE beams (up to 200 MeV). Similar to the Gaussian pencil beam models used for electron or proton beams, this pencil beam kernel is separated into a central and an off-axis term. Monte Carlo simulations are performed to compare the analytical calculations with simulations and to determine the parametrizations used in the model at the highest electron energies.

METHODS

The normalized electron planar fluence distribution is described in water according to the Fermi-Eyges theory of multiple Coulomb scattering and a double Gaussian distribution model. The main quantities used in the model and their calculation (mass angular scattering power, mean energy, range straggling) are discussed and tested for electron energies up to 200 MeV. The TOPAS/Geant4 Monte Carlo (MC) toolkit is used to compare analytical calculations with MC simulations for a theoretical pencil beam irradiation and to find the best parameters describing the range straggling. The model is then tested on a realistic simulation of a pencil beam scanning beamline with treatment field dimensions up to 15 × 15 cm2  and for deep-seated targets.

RESULTS

Radial dose distributions of a pencil beam in water were calculated with the model and compared with the results of a complete Monte Carlo simulation. A good agreement (within 2%/2 mm gamma passing rate superior to 90%, and a mean deviation between calculated and simulated pencil beam radial spread smaller than 0.6 mm) was observed between analytical dose distributions and simulations for energies up to 200 MeV and field sizes up to 15 × 15 cm2 .

CONCLUSIONS

A parameterization of an electron source and an analytical pencil beam model were proposed in this work, thereby allowing a suitable reproduction of the lateral fluence of a VHEE beam and good agreement between calculations and simulated data. Further improvement of the method would require the consideration of a model describing the large-angle scattering of the electrons. The results of this work could support future research into VHEE radiotherapy and might be of interest for use together with VHEE broad beams produced by scanned narrow pencil beams.

3D-conformal very-high energy electron therapy as candidate modality for FLASH-RT: A treatment planning study for glioblastoma and lung cancer

BACKGROUND

Pre-clinical ultra-high dose rate (UHDR) electron irradiations on time scales of 100 ms have demonstrated a remarkable sparing of brain and lung tissues while retaining tumor efficacy when compared to conventional dose rate irradiations. While clinically-used gantries and intensity modulation techniques are too slow to match such time scales, novel very-high energy electron (VHEE, 50-250 MeV) radiotherapy (RT) devices using 3D-conformed broad VHEE beams are designed to deliver UHDR treatments that fulfill these timing requirements.

PURPOSE

To assess the dosimetric plan quality obtained using VHEE-based 3D-conformal RT (3D-CRT) for treatments of glioblastoma and lung cancer patients and compare the resulting treatment plans to those delivered by standard-of-care intensity modulated photon RT (IMRT) techniques.

METHODS

Seven glioblastoma patients and seven lung cancer patients were planned with VHEE-based 3D-CRT using 3 to 16 coplanar beams with equidistant angular spacing and energies of 100 and 200 MeV using a forward planning approach. Dose distributions, dose-volume histograms, coverage (V95% ) and homogeneity (HI98% ) for the planning target volume (PTV), as well as near-maximum doses (D2% ) and mean doses (Dmean ) for organs-at-risk (OAR) were evaluated and compared to clinical IMRT plans.

RESULTS

Mean differences of V95% and HI98% of all VHEE plans were within 2% or better of the IMRT reference plans. Glioblastoma plan dose metrics obtained with VHEE configurations of 200 MeV and 3-16 beams were either not significantly different or were significantly improved compared to the clinical IMRT reference plans. All OAR plan dose metrics evaluated for VHEE plans created using 5 beams of 100 MeV were either not significantly different or within 3% on average, except for Dmean for the body, Dmean for the brain, D2% for the brain stem, and D2% for the chiasm, which were significantly increased by 1, 2, 6, and 8 Gy, respectively (however below clinical constraints). Similarly, the dose metrics for lung cancer patients were also either not significantly different or were significantly improved compared to the reference plans for VHEE configurations with 200 MeV and 5 to 16 beams with the exception of D2% and Dmean to the spinal canal (however below clinical constraints). For the lung cancer cases, the VHEE configurations using 100 MeV or only 3 beams resulted in significantly worse dose metrics for some OAR. Differences in dose metrics were, however, strongly patient-specific and similar for some patient cases.

CONCLUSIONS

VHEE-based 3D-CRT may deliver conformal treatments to simple, mostly convex target shapes in the brain and the thorax with a limited number of critical adjacent OAR using a limited number of beams (as low as 3 to 7). Using such treatment techniques, a dosimetric plan quality comparable to that of standard-of-care IMRT can be achieved. Hence, from a treatment planning perspective, 3D-conformal UHDR VHEE treatments delivered on time scales of 100 ms represent a promising candidate technique for the clinical transfer of the FLASH effect.

Nonlinear pulse compression of a 200 mJ and 1 kW ultrafast thin-disk amplifier

We present a high-energy laser source consisting of an ultrafast thin-disk amplifier followed by a nonlinear compression stage. At a repetition rate of 5 kHz, the drive laser provides a pulse energy of up to 200 mJ with a pulse duration below 500 fs. Nonlinear broadening is implemented inside a Herriott-type multipass cell purged with noble gas, allowing us to operate under different seeding conditions. Firstly, the nonlinear broadening of 64 mJ pulses is demonstrated in an argon-filled cell, showing a compressibility down to 32 fs. Finally, we employ helium as a nonlinear medium to increase the energy up to 200 mJ while maintaining compressibility below 50 fs. Such high-energy pulses with sub-50 fs duration hold great promise as drivers of secondary sources.

Treatment planning consideration for very high-energy electron FLASH radiotherapy

PURPOSE

Very high-energy electron (VHEE) can make up the insufficient treatment depth of the low-energy electron while offering an intermediate dosimetric advantage between photon and proton. Combining FLASH with VHEE, a quantitative comparison between different energies was made, with regard to plan quality, dose rate distribution (both in PTV and OAR), and total duration of treatment (beam-on time).

METHODS

In two patient cases (head and lung), we created the treatment plans utilizing the scanning pencil beam via the Monte Carlo simulation and a PTV-based optimization algorithm. Geant4 was used to simulate VHEE pencil beams and sizes of 0.3-5 mm defined by the full width at half maximum (FWHM). Monoenergetic beams with Gaussian distribution in x and y directions (ISOURC = 19) were used as the source of electrons. A large-scale non-linear solver (IPOPT) was used to calculate the optimal spot weights. After optimization, a quantitative comparison between different energies was made regarding treatment plan quality, dose rate distribution (both in PTV and OAR), and total beam duration.

RESULTS

For head (80 MeV, 100 MeV, and 120 MeV) and lung cases (100 MeV, 120 MeV, and 140 MeV), the minimum beam intensity needs to be ∼2.5 × 10¹¹ electrons/s and ∼9.375 × 10¹¹ electrons/s to allow > 90 % volume of PTV reaching the average dose rate (DADR) higher than 40 Gy/s. At this beam intensity (fraction dose: 10 Gy), the overall irradiation time for the head case is 5258.75 ms (80 MeV), 5149.75 ms (100 MeV), and 4976.75 ms (120 MeV), including scanning time 872.75 ms. For lung cases, this number is 1034.25 ms (100 MeV), 981.55 ms (120 MeV), and 928.15 ms (140 MeV), including scanning time 298.75 ms. The plan of higher energy always performs with a higher dose rate (both in PTV and OAR) and thereby costs less delivery time (beam-on time).

CONCLUSION

The study systematically investigated the currently known FLASH parameters for VHEE radiotherapy and successfully established a benchmark reference for its FLASH dose rate performance.

Development of a compact linear accelerator to generate ultrahigh dose rate high-energy X-rays for FLASH radiotherapy applications

PURPOSE

In recent years, the FLASH effect, in which ultrahigh dose rate (UHDR) radiotherapy (RT) can significantly reduce toxicity to normal tissue while maintaining antitumor efficacy, has been verified in many studies and even applied in human clinical cases. This work evaluates whether a room-temperature radio-frequency (RF) linear accelerator (linac) system can produce UHDR high-energy X-rays exceeding a dose rate of 40 Gy/s at a clinical source-surface distance (SSD), exploring the possibility of a compact and economical clinical FLASH RT machine suitable for most hospital treatmentrooms.

METHODS

A 1.65 m long S-band backward-traveling-wave (BTW) electron linac was developed to generate high-current electron beams, supplied by a commercial klystron-based power source. A tungsten-copper electron-to-photon conversion target for UHDR X-rays was designed and optimized with Monte Carlo (MC) simulations using Geant4 and thermal finite element analysis (FEA) simulations using ANSYS. EBT3 and EBT-XD radiochromic films, which were calibrated with a clinical machine Varian VitalBeam, were used for absolute dose measurements. A PTW ionization chamber detector was used to measure the relative total dose and a plane-parallel ionization chamber detector was used to measure the relative normalized dose of each pulse.

RESULTS

The BTW linac generated 300-mA-pulse-current 11 MeV electron beams with 29 kW mean beam power, and the conversion target could sustain this high beam power within a maximum irradiation duration of 0.75 s. The mean energy of the produced X-rays was 1.66 MeV in the MC simulation. The measured flat-filter-free (FFF) maximum mean dose rate of the room-temperature linac exceeded 80 Gy/s at an SSD of 50 cm and 45 Gy/s at an SSD of 67.9 cm, both at a 2.1 cm depth of the water phantom. The FFF radiation fields at 50 cm and 67.9 cm SSD at a 2.1 cm depth of the water phantom showed Gaussian-like distributions with 14.3 and 20 cm full-width at half-maximum (FWHM) values, respectively.

CONCLUSION

This work demonstrated the feasibility of UHDR X-rays produced by a room-temperature RF linac, and explored the further optimization of system stability. It shows that a simple and compact UHDR X-ray solution can be facilitated for both FLASH-RT scientific research and clinical applications.

GPU-accelerated Monte Carlo simulation of electron and photon interactions for radiotherapy applications

Objective. The Monte Carlo simulation software is a valuable tool in radiation therapy, in particular to achieve the needed accuracy in the dose evaluation for the treatment plans optimisation. The current challenge in this field is the time reduction to open the way to many clinical applications for which the computational time is an issue. In this manuscript we present an innovative GPU-accelerated Monte Carlo software for dose valuation in electron and photon based radiotherapy, developed as an update of the FRED (Fast paRticle thErapy Dose evaluator) software.Approach. The code transports particles through a 3D voxel grid, while scoring their energy deposition along their trajectory. The models of electromagnetic interactions in the energy region between 1 MeV-1 GeV available in literature have been implemented to efficiently run on GPUs, allowing to combine a fast tracking while keeping high accuracy in dose assessment. The FRED software has been bench-marked against state-of-art full MC (FLUKA, GEANT4) in the realm of two different radiotherapy applications: Intra-Operative Radio Therapy and Very High Electron Energy radiotherapy applications.Results. The single pencil beam dose-depth profiles in water as well as the dose map computed on non-homogeneous phantom agree with full-MCs at 2% level, observing a gain in processing time from 200 to 5000.Significance. Such performance allows for computing a plan with electron beams in few minutes with an accuracy of ∼%, demonstrating the FRED potential to be adopted for fast plan re-calculation in photon or electron radiotherapy applications.

Proton FLASH Radiation Therapy and Immune Infiltration: Evaluation in an Orthotopic Glioma Rat Model

PURPOSE

FLASH radiation therapy (FLASH-RT) is a promising radiation technique that uses ultrahigh doses of radiation to increase the therapeutic window of the treatment. FLASH-RT has been observed to provide normal tissue sparing at high dose rates and similar tumor control compared with conventional RT, yet the biological processes governing these radiobiological effects are still unknown. In this study, we sought to investigate the potential immune response generated by FLASH-RT in a high dose of proton therapy in an orthotopic glioma rat model.

METHODS AND MATERIALS

We cranially irradiated rats with a single high dose (25 Gy) using FLASH dose rate proton irradiation (257 ± 2 Gy/s) or conventional dose rate proton irradiation (4 ± 0.02 Gy/s). We first assessed the protective FLASH effect that resulted in our setup through behavioral studies in naïve rats. This was followed by a comprehensive analysis of immune cells in blood, healthy tissue of the brain, and tumor microenvironment by flow cytometry.

RESULTS

Proton FLASH-RT spared memory impairment produced by conventional high-dose proton therapy and induced a similar tumor infiltrating lymphocyte recruitment. Additionally, a general neuroinflammation that was similar in both dose rates was observed.

CONCLUSIONS

Overall, this study demonstrated that FLASH proton therapy offers a neuro-protective effect even at high doses while mounting an effective lymphoid immune response in the tumor.

FLASH radiotherapy treatment planning and models for electron beams

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

Treatment Planning in Intraoperative Radiation Therapy (IORT): Where Should We Go?

As opposed to external beam radiation therapy (EBRT), treatment planning systems (TPS) dedicated to intraoperative radiation therapy (IORT) were not subject to radical modifications in the last two decades. However, new treatment regimens such as ultrahigh dose rates and combination with multiple treatment modalities, as well as the prospected availability of dedicated in-room imaging, call for important new features in the next generation of treatment planning systems in IORT. Dosimetric accuracy should be guaranteed by means of advanced dose calculation algorithms, capable of modelling complex scattering phenomena and accounting for the non-tissue equivalent materials used to shape and compensate electron beams. Kilovoltage X-ray based IORT also presents special needs, including the correct description of extremely steep dose gradients and the accurate simulation of applicators. TPSs dedicated to IORT should also allow real-time imaging to be used for treatment adaptation at the time of irradiation. Other features implemented in TPSs should include deformable registration and capability of radiobiological planning, especially if unconventional irradiation schemes are used. Finally, patient safety requires that the multiple features be integrated in a comprehensive system in order to facilitate control of the whole process.

Prospect of radiotherapy technology development in the era of immunotherapy

Radiotherapy (RT) is one of the important modalities for cancer treatments. Mounting evidence suggests that the host immune system is involved in the tumor cell killing during RT, and future RT technology development should aim to minimize radiation dose to the immune system while maintaining a sufficient dose to the tumor. A brief history of RT technology development is first summarized. Three RT technologies, namely FLASH RT, proton therapy, and spatially fractionated RT (SFRT), are singled out for the era of immunotherapy. Besides the technical aspects, the mechanism of FLASH effect is discussed, which is likely the combined results of the recombination effect, oxygen depletion effect and immune sparing effect. The proton therapy should have the advantage of causing much less immune damage in comparison to X-ray based RT due to the Bragg peak. However, the relative biological effectiveness (RBE) uncertainty and range uncertainty may hinder the translation of this advantage into clinical benefit. Research approaches to overcome these two technical hurdles are discussed. Various SFRT approaches and their application are reviewed. These approaches are categorized as single-field 1D/2D SFRT, multi-field 3D SFRT and quasi-3D SFRT techniques. A 3D SFRT approach, which is achieved by placing the Bragg peak of a proton 2D SFRT field in discrete depths, may have special potential because all 3 technologies (FLASH RT, proton therapy and SFRT) may be used in this approach.

A high spatiotemporal resolution 2D strip ionization chamber array for proton pencil beam scanning FLASH radiotherapy

PURPOSE

Experimental measurements of 2D dose rate distributions in proton pencil beam scanning (PBS) FLASH radiation therapy (RT) are currently lacking. In this study, we characterize a newly designed 2D strip-segmented ionization chamber array (SICA) with high spatial and temporal resolution and demonstrate its applications in a modern proton PBS delivery system at both conventional and ultra-high dose rates.

METHODS

A dedicated research beamline of the Varian ProBeam system was employed to deliver a 250 MeV proton PBS beam with nozzle currents up to 215 nA. In the research and clinical beamlines, the spatial, temporal, and dosimetric performance of the SICA was characterized and compared with measurements using parallel-plate ion chambers (IBA PPC05 and PTW Advanced Markus chamber), a 2D scintillator camera (IBA Lynx), Gafchromic films (EBT-XD), and a Faraday Cup. A novel reconstruction approach was proposed to enable the measurement of 2D dose and dose rate distributions using such a strip-type detector.

RESULTS

The SICA demonstrated a position accuracy of 0.12 ± 0.02 mm at a 20 kHz sampling rate (50 μs per event) and a linearity of R² > 0.99 for both dose and dose rate with nozzle beam currents ranging from 1 nA to 215 nA. The 2D dose comparison to the film measurement resulted in a gamma passing rate of 99.8% (2 mm/2%). A measurement-based proton PBS 2D FLASH dose rate distribution was compared to simulation results and showed a gamma passing rate of 97.3% (2 mm/2%).

CONCLUSIONS

The newly designed SICA demonstrated excellent spatial, temporal, and dosimetric performance and is well suited for commissioning, quality assurance (QA), and a wide range of clinical applications in proton PBS clinical and FLASH radiotherapy. This article is protected by copyright. All rights reserved.

Determination of the ion collection efficiency of the Razor Nano Chamber for ultra-high dose-rate electron beams

Background

Ultra‐high dose‐rate (UHDR) irradiations (>40 Gy/s) have recently garnered interest in radiotherapy (RT) as they can trigger the so‐called “FLASH” effect, namely a higher tolerance of normal tissues in comparison with conventional dose rates when a sufficiently high dose is delivered to the tissue. To transfer this to clinical RT treatments, adapted methods and practical tools for online dosimetry need to be developed. Ionization chambers remain the gold standards in RT but the charge recombination effects may be very significant at such high dose rates, limiting the use of some of these dosimeters. The reduction of the sensitive volume size can be an interesting characteristic to reduce such effects.

Purpose

In that context, we have investigated the charge collection behavior of the recent IBA Razor™ Nano Chamber (RNC) in UHDR pulses to evaluate its potential interest for FLASH RT.

Methods

In order to quantify the RNC ion collection efficiency (ICE), simultaneous dose measurements were performed under UHDR electron beams with dose‐rate‐independent Gafchromic™ EBT3 films that were used as the dose reference. A dose‐per‐pulse range from 0.01 to 30 Gy was investigated, varying the source‐to‐surface distance, the pulse duration (1 and 3 μs investigated) and the LINAC gun grid tension as irradiation parameters. In addition, the RNC measurements were corrected from the inherent beam shot‐to‐shot variations using an independent current transformer. An empirical logistic model was used to fit the RNC collection efficiency measurements and the results were compared with the Advanced Markus plane parallel ion chamber.

Results

The RNC ICE was found to decrease as the dose‐per‐pulse increases, starting from doses above 0.2 Gy/pulse and down to 40% of efficiency at 30 Gy/pulse. The RNC resulted in a higher ICE for a given dose‐per‐pulse in comparison with the Markus chamber, with a measured efficiency found higher than 85 and 55% for 1 and 10 Gy/pulse, respectively, whereas the Markus ICE was of 60 and 25% for the same doses. However, the RNC shows a higher sensitivity to the pulse duration than the Advanced Markus chamber, with a lower efficiency found at 1 μs than at 3 μs, suggesting that this chamber could be more sensitive to the dose rate within the pulse.

Conclusions

The results confirmed that the small sensitive volume of the RNC ensures higher ICE compared with larger chambers. The RNC was thus found to be a promising online dosimetry tool for FLASH RT and we proposed an ion recombination model to correct its response up to extreme dose‐per‐pulses of 30 Gy.

Deep Seated Tumour Treatments With Electrons of High Energy Delivered at FLASH Rates: The Example of Prostate Cancer

Different therapies are adopted for the treatment of deep seated tumours in combination or as an alternative to surgical removal or chemotherapy: radiotherapy with photons (RT), particle therapy (PT) with protons or even heavier ions like ¹²C, are now available in clinical centres. In addition to these irradiation modalities, the use of Very High Energy Electron (VHEE) beams (100–200 MeV) has been suggested in the past, but the diffusion of that technique was delayed due to the needed space and budget, with respect to standard photon devices. These disadvantages were not paired by an increased therapeutic efficacy, at least when comparing to proton or carbon ion beams. In this contribution we investigate how recent developments in electron beam therapy could reshape the treatments of deep seated tumours. In this respect we carefully explored the application of VHEE beams to the prostate cancer, a well-known and studied example of deep seated tumour currently treated with high efficacy both using RT and PT. The VHEE Treatment Planning System was obtained by means of an accurate Monte Carlo (MC) simulation of the electrons interactions with the patient body. A simple model of the FLASH effect (healthy tissues sparing at ultra-high dose rates), has been introduced and the results have been compared with conventional RT. The study demonstrates that VHEE beams, even in absence of a significant FLASH effect and with a reduced energy range (70–130 MeV) with respect to implementations already explored in literature, could be a good alternative to standard RT, even in the framework of technological developments that are nowadays affordable.

Verification of electron beam parameters in an intraoperative linear accelerator using dosimetric and radiobiological response methods

BACKGROUND

The availability of linear accelerators (linac) for research purposes is often limited and therefore alternative radiation sources are needed to conduct radiobiological research. The National Centre for Radiation Research in Poland recently developed an intraoperative mobile linac that enables electron irradiation at energies ranging from 4 to 12 MeV and dose rates of 5 or 10 Gy/min. The present study was conducted to evaluate the electron beam parameters of this intraoperative linac and to verify the set-up to evaluate out-of-field doses in a water phantom, which were determined through dosimetric and biological response measurements.

MATERIALS AND METHODS

The distribution of radiation doses along and across the radiation beam were measured in a water phantom using a semiconductor detector and absolute doses using an ionisation chamber. Two luminal breast cancer cell lines (T-47D and HER2 positive SK-BR-3) were placed in the phantom to study radiation response at doses ranging from 2 to 10 Gy. Cell response was measured by clonogenic assays.

RESULTS AND CONCLUSION

The electron beam properties, including depth doses and profiles, were within expected range for the stated energies. These results confirm the viability of this device and set-up as a source of megavoltage electrons to evaluate the radiobiological response of tumour cells.