Ultra-high Dose Research Articles

The PTB water calorimeter for determining the absolute absorbed dose to water in ultra-high pulse dose rate electron beams

Objective. The purpose of this investigation is to establish the water calorimeter as the primary standard in PTB’s ultra-high pulse dose rate (UHPDR) 20 MeV reference electron beams. Approach. The calorimetric measurements were performed at the PTB research linac facility using the UHPDR reference electron beam setups that enable a dose per pulse between about 0.1 Gy and 6 Gy. The beam is monitored by an in-flange integrating current transformer. The correction factors required to determine the absorbed dose to water were evaluated using thermal and Monte Carlo simulations. Measurements were performed with different total doses delivered per pulse by modifying the instantaneous dose rate within a pulse and by changing the pulse length. The obtained temperature–time traces were compared to the simulated ones to validate the thermal simulations. In addition, absorbed-dose-to-water measurements obtained using the secondary standard alanine dosimeter system were compared to measurements performed with the primary standard. Main results. The simulated and measured temperature–time traces were shown to be consistent, within combined uncertainties, with one another. Measurements with alanine dosimeters proved to be consistent within k = 1 of the total combined uncertainty with the absorbed dose to water determined using the primary standard. Significance. The total relative standard uncertainty of absorbed dose to water determined using the PTB water calorimeter primary standard in UHPDR electron beams was estimated to be less than 0.5%, and the combined correction factors were found to deviate from 1 by less than 1% for both PTB UHPDR 20 MeV reference electron beams. The water calorimeter is therefore considered to be an established primary standard for the higher energy UHPDR reference electron beams.

Detailed Monte-Carlo characterization of a Faraday cup for proton therapy.

Experiments with ultra-high dose rates in proton therapy are of increasing interest for potential treatment benefits. The Faraday Cup (FC) is an important detector for the dosimetry of such ultra-high dose rate beams. So far, there is no consensus on the optimal design of a FC, or on the influence of beam properties and magnetic fields on shielding of the FC from secondary charged particles. To perform detailed Monte Carlo simulations of a Faraday cup to identify and quantify all the charge contributions from primary protons and secondary particles that modify the efficiency of the FC response as a function of a magnetic field employed to improve the detector's reading. In this paper, a Monte Carlo (MC) approach was used to investigate the Paul Scherrer Institute (PSI) FC and quantify contributions of charged particles to its signal for beam energies of 70, 150, and 228 MeV and magnetic fields between 0 and 25 mT. Finally, we compared our MC simulations to measurements of the response of the PSI FC. For maximum magnetic fields, the efficiency (signal of the FC normalized to charged delivered by protons) of the PSI FC varied between 99.97% and 100.22% for the lowest and highest beam energy. We have shown that this beam energy-dependence is mainly caused by contributions of secondary charged particles, which cannot be fully suppressed by the magnetic field. Additionally, it has been demonstrated that these contributions persist, making the FC efficiency beam energy dependent for fields up to 250 mT, posing inevitable limits on the accuracy of FC measurements if not corrected. In particular, we have identified a so far unreported loss of electrons via the outer surfaces of the absorber block and show the energy distributions of secondary electrons ejected from the vacuum window (VW) (up to several hundred keV), together with electrons ejected from the absorber block (up to several MeV). Even though, in general, simulations and measurements were well in agreement, the limitation of the current MC calculations to produce secondary electrons below 990eV posed a limit in the efficiency simulations in the absence of a magnetic field as compared to the experimental data. TOPAS-based MC simulations allowed to identify various and previously unreported contributions to the FC signal, which are likely to be present in other FC designs. Estimating the beam energy dependence of the PSI FC for additional beam energies could allow for the implementation of an energy-dependent correction factor to the signal. Dose estimates, based on accurate measurements of the number of delivered protons, provided a valid instrument to challenge the dose determined by reference ionization chambers, not only at ultra-high dose rates but also at conventional dose rates.

Secondary radiation dose modeling in passive scattering and pencil beam scanning very high energy electron (VHEE) radiation therapy.

Electrons with kinetic energy up to a few hundred MeV, also called very high energy electrons (VHEE), are currently considered a promising technique for the future of radiation therapy (RT) and in particular ultra-high dose rate (UHDR) therapy. However, the feasibility of a clinical application is still being debated and VHEE therapy remains an active area of research for which the optimal conformal technique is also yet to be determined. In this work, we will apply two existing formalisms based on analytical Gaussian multiple-Coulomb scattering theory and Monte Carlo (MC) simulations to study and compare the electron and bremsstrahlung photon dose distributions arising from two beam delivery systems (passive scattering with or without a collimator or active scanning). We therefore tested the application of analytical and MC models to VHEE beams and assessed their performance and parameterization in the energy range of 6-200MeV. The optimized electron beam fluence, the bremsstrahlung, an estimation of central-axis and off-axis x-ray dose at the practical range and neutron contributions to the total dose, along with an extended parameterization for the photon dose model were developed, together with a comparison between double scattering (DS) and pencil beam scanning (PBS) techniques. MC simulations were performed with the TOPAS/Geant4 toolkit to verify the dose distributions predicted by the analytical calculations. The results for the clinical energy range (between 6 and 20MeV) as well as for higher energies (VHEE range between 20 and 200MeV) and for two treatment field sizes (5×5 and 10×10 cm2 ) are reported, showing a reasonable agreement with MC simulations with mean differences below 2.1%. The relative contributions of photons generated in the medium or by the scattering system along the central-axis (up to 50% of the total dose) are also illustrated, along with their relative variations with electron energy. The fast analytical models parametrized in this study allow an estimation of the amount of photons produced behind the practical range by a DS system with an accuracy lower than 3%, providing important information for the eventual design of a VHEE system. The results of this work could support future research on VHEE radiotherapy.

Towards quantitative in vivo dosimetry using x-ray acoustic computed tomography.

Radiation dosimetry is essential for radiation therapy (RT) to ensure that radiation dose is accurately delivered to the tumor. Despite its wide use in clinical intervention, the delivered radiation dose can only be planned and verified via simulation. This makes precision radiotherapy challenging while in-line verification of the delivered dose is still absent in the clinic. X-ray-induced acoustic computed tomography (XACT) has recently been proposed as an imaging tool for in vivo dosimetry. Most of the XACT studies focus on localizing the radiation beam. However, it has not been studied for its potential for quantitative dosimetry. The aim of this study was to investigate the feasibility of using XACT for quantitative in vivo dose reconstruction during radiotherapy. Varian Eclipse system was used to generate simulated uniform and wedged 3D radiation field with a size of 4 cm 4 cm. In order to use XACT for quantitative dosimetry measurements, we have deconvoluted the effects of both the x-ray pulse shape and the finite frequency response of the ultrasound detector. We developed a model-based image reconstruction algorithm to quantify radiation dose in vivo using XACT imaging, and universal back-projection (UBP) reconstruction is used as comparison. The reconstructed dose was calibrated before comparing it to the percent depth dose (PDD) profile. Structural similarity index matrix (SSIM) and root mean squared error (RMSE) are used for numeric evaluation. Experimental signals were acquired from 4 cm 4 cm radiation field created by Linear Accelerator (LINAC) at depths of 6, 8, and 10 cm beneath the water surface. The acquired signals were processed before reconstruction to achieve accurate results. Applying model-based reconstruction algorithm with non-negative constraints successfully reconstructed accurate radiation dose in 3D simulation study. The reconstructed dose matches well with the PDD profile after calibration in experiments. The SSIMs between the model-based reconstructions and initial doses are over 85%, and the RMSEs of model-based reconstructions are eight times lower than the UBP reconstructions. We have also shown that XACT images can be displayed as pseudo-color maps of acoustic intensity, which correspond to different radiation doses in the clinic. Our results show that the XACT imaging by model-based reconstruction algorithm is considerably more accurate than the dose reconstructed by UBP algorithm. With proper calibration, XACT is potentially applicable to the clinic for quantitative in vivo dosimetry across a wide range of radiation modalities. In addition, XACT's capability of real-time, volumetric dose imaging seems well-suited for the emerging field of ultrahigh dose rate "FLASH" radiotherapy.

Implementation of novel measurement-based patient-specific QA for pencil beam scanning proton FLASH radiotherapy.

Several studies have shown pencil beam scanning (PBS) proton therapy is a feasible and safe modality to deliver conformal and ultra-high dose rate (UHDR) FLASH radiation therapy. However, it would be challenging and burdensome to conduct the quality assurance (QA) of the dose rate along with conventional patient-specific QA (psQA). To demonstrate a novel measurement-based psQA program for UHDR PBS proton transmission FLASH radiotherapy (FLASH-RT) using a high spatiotemporal resolution 2D strip ionization chamber array (SICA). The SICA is a newly designed open-air strip-segmented parallel plate ionization chamber, which is capable of measuring spot position and profile through 2mm-spacing-strip electrodes at a 20kHz sampling rate (50μs per event) and has been characterized to exhibit excellent dose and dose rate linearity under UHDR conditions. A SICA-based delivery log was collected for each irradiation containing the measured position, size, dwell time, and delivered MU for each planned spot. Such spot-level information was compared with the corresponding quantities in the treatment planning system (TPS). The dose and dose rate distributions were reconstructed on patient CT using the measured SICA log and compared to the planned values in volume histograms and 3D gamma analysis. Furthermore, the 2D dose and dose rate measurements were compared with the TPS calculations of the same depth. In addition, simulations using different machine-delivery uncertainties were performed, and QA tolerances were deduced. A transmission proton plan of 250MeV for a lung lesion was planned and measured in a dedicated ProBeam research beamline (Varian Medical System) with a nozzle beam current between 100 to 215nA. The worst gamma passing rates for dose and dose rate of the 2D SICA measurements (four fields) compared to TPS prediction (3%/3mm criterion) were 96.6% and 98.8%, respectively, whereas the SICA-log reconstructed 3D dose distribution achieved a gamma passing rate of 99.1% (2%/2mm criterion) compared to TPS. The deviations between SICA measured log, and TPS were within 0.3ms for spot dwell time with a mean difference of 0.069±0.11 s, within 0.2mm for spot position with a mean difference of -0.016±0.03mm in the x-direction, and -0.036±0.059mm in the y-direction, and within 3% for delivered spot MUs. Volume histogram metric of dose (D95) and dose rate (V40Gy/s ) showed minimal differences, within less than 1%. This work is the first to describe and validate an all-in-one measurement-based psQA framework that can fulfill the goals of validating the dose rate accuracy in addition to dosimetric accuracy for proton PBS transmission FLASH-RT. The successful implementation of this novel QA program can provide future clinical practice with more confidence in the FLASH application.

Dose rate assessment of spot-scanning very high energy electrons radiotherapy driven by laser plasma acceleration

Laser plasma accelerators (LPA) can produce very high-energy electrons (VHEE) with ultra-short bunch duration, which may facilitate the application of ultra-high dose rate radiotherapy (FLASH-RT) to treat deep-seated tumors. The study aims to evaluate the dose rate delivery by spot-scanning VHEE beams produced by LPA and to discuss the feasibility and beam specifications for FLASH-RT implementation. Various dose rate metrics, including averaged dose rate (ADR), dose-averaged dose rate (DADR), and dose-threshold dose rate (DTDR), are examined in the context of spot-scanning. Theoretical analysis and Monte Carlo simulations are employed to quantify the dose rate distribution for a water phantom and explore the impact of beam parameters. All the beam parameters are based on experimental results. With a lower pulse repetition rate of 5 Hz, ADR can only reach a dose rate in the order of 10−1Gy/s, while attaining the FLASH-RT dose rate of 40Gy/s necessitates the utilization of high-power lasers with a kilohertz working repetition rate. In contrast to ADR, DADR and DTDR remain independent of the scanning path and can reach the ultra-high dose rate surpassing 1014Gy/s at the phantom surface. Meanwhile, the ultrashort electron bunch can be stretched during scattering within the water, resulting in a dependence of DADR and DTDR on the penetration depth. Both the charge per shot and angular spread are important parameters in dose rate calculations. This investigation offers insights into practical beam parameters for preclinical applications and supplies guidance for designing the LPAs suitable for future spot-scanning VHEE FLASH-RT.

In situ correction of recombination effects in ultra-high dose rate irradiations with protons

Background. At the Center for Proton Therapy at the Paul Scherrer Institute (PSI) the delivery of proton radiation is controlled via gas-based ionization chambers: the beam is turned off when a certain amount of preset charge has been collected. At low dose rates the charge collection efficiency in these detectors is unity, at ultra-high dose rates it is less due to induced charge recombination effects. If not corrected, the latter would lead to an overdosage. Purpose. In the scope of this work, we developed a novel approach to an in situ charge recombination correction for our dose defining detectors, when irradiated with a proton beam at ultra-high dose rates. This approach is based on the Two-Voltage-Method. Methods. We have translated this method to two separate devices operated simultaneously at different conditions. By doing so, the charge collection losses can be corrected directly and without the need for empirical correction values. This approach has been tested at ultra-high dose rates; proton beam was delivered by the COMET cyclotron to Gantry 1 at PSI. Results. We were able to correct the charge losses caused by recombination effects at local beam currents of approximately 700 nA (i.e. instantaneous dose rate of 3600 Gy s–1 at isocenter). The corrected collected charges in our gaseous detectors were compared against recombination-free measurements with a Faraday cup. The ratio of both quantities shows no significant dose rate dependence within their respective combined uncertainties. Conclusions. Correcting recombination effects in our gas-based detectors with the novel method greatly eases the handling of Gantry 1 as ‘FLASH test bench’. Not only is the application of a preset dose more accurate compared to using an empirical correction curve, also the re-determination of empirical correction curves in the case of a beam phase space change can be omitted.

Development of a Real-Time Pixel Array-Type Detector for Ultrahigh Dose-Rate Beams

Although research into ultrahigh dose-rate (UHDR) radiation therapy is ongoing, there is a significant lack of experimental measurements for two-dimensional (2D) dose-rate distributions. Additionally, conventional pixel-type detectors result in significant beam loss. In this study, we developed a pixel array-type detector with adjustable gaps and a data acquisition system to evaluate its effectiveness in measuring UHDR proton beams in real time. We measured a UHDR beam at the Korea Institute of Radiological and Medical Sciences using an MC-50 cyclotron, which produced a 45-MeV energy beam with a current range of 10–70 nA, to confirm the UHDR beam conditions. To minimize beam loss during measurement, we adjusted the gap and high voltage on the detector and determined the collection efficiency of the developed detector through Monte Carlo simulation and experimental measurements of the 2D dose-rate distribution. We also verified the accuracy of the real-time position measurement using the developed detector with a 226.29-MeV PBS beam at the National Cancer Center of the Republic of Korea. Our results indicate that, for a current of 70 nA with an energy beam of 45 MeV generated using the MC-50 cyclotron, the dose rate exceeded 300 Gy/s at the center of the beam, indicating UHDR conditions. Simulation and experimental measurements show that fixing the gap at 2 mm and the high voltage at 1000 V resulted in a less than 1% loss of collection efficiency when measuring UHDR beams. Furthermore, we achieved real-time measurements of the beam position with an accuracy of within 2% at five reference points. In conclusion, our study developed a beam monitoring system that can measure UHDR proton beams and confirmed the accuracy of the beam position and profile through real-time data transmission.

High-LET targeted microbeam irradiation induces local chromatin reorganization in living cells showing active basal mechanisms at highly complex DNA damage sites

DNA repair eukaryotic cells have additional protective mechanisms that avoid uncontrolled interaction of different parts of the chromatin and damaged regions. Key factors here are the regulation of chromatin density and mobility. The 4D (temporal and spatial) organization of chromatin is controlling this security barrier by regulating the accessibility of genes, flexibility of DNA, and its ability to move inside the nucleus. How this regulation mechanisms are involved in DNA repair upon radiation damage is until now rarely known but an important part to understand the enhanced effectiveness of high linear energy transfer (LET) particles. The damage recognition via PARP1 and the subsequent chromatin decondensation via PARylation is a crucial step in the DNA damage response (DDR). Upon We used the SNAKE microbeam with a beam spot size of <1 µm to induce highly localized DNA damage in living cells using 55 MeV Carbon ions to investigate the chromatin rearrangements in the early stage of DDR. The nuclei were irradiated with a cross pattern consisting of 1000 ions per spot and 25 spots per cell either with one (11 000 Gy), two (22 000 Gy), or three crosses (33 000 Gy). The chromatin rearrangement was imaged live for several minutes after irradiation at the beam using SiR chromatin stain. Upon 91% of the cells show a localized decondensation starting from a few seconds up to minutes after irradiation. The chromatin is decondensed by 6%-8% in the beam path with a local condensation at the edges of up to 8%. Our results suggest that chromatin decondensation is a fast process in the first few seconds after damage induction. Furthermore, decondensation status does not change over minutes, which gives evidence that this process and therefore DDR is paused or even stopped. In combination with the existing knowledge about early reactions to damage induction our data support the model of PARP induced chromatin decondensation. Furthermore, it is evident that also ultra-high doses of radiation are, in first place not able to inactivate initial basal mechanisms as response to damage induction.

A stochastic reaction–diffusion modeling investigation of FLASH ultra-high dose rate response in different tissues

Purpose: The aim of the study was to propose a theory based on topology and geometry of diffusion channels in tissue to contribute to the mechanistic understanding of normal tissue sparing at ultra-high dose rates (UHDRs) and explore an interplay between intra- and inter-track radical recombination through a reaction–diffusion mechanism.Methods: We calculate the time evolution of particle track structures using a system of coupled reaction–diffusion equations on a random network designed for molecular transport in porous and disordered media. The network is representative of the intra- and inter-cellular diffusion channels in tissues. Spatial cellular heterogeneities over the scale of track spacing are constructed by incorporating random fluctuations in the connectivity between network sites, resembling molecular mass and charge heterogeneities at the cellular level.Results: We demonstrate the occurrence of phase separation among the tracks as the complexity in intra- and inter-cellular structure increases. At the strong limit of structural disorder, tracks evolve individually like isolated islands with negligible inter-track as they propagate like localized waves in space, analogous to the Anderson localization in quantum mechanics. In contrast, at the limit of weak disorder in a homogeneous medium, such as water, the neighboring tracks melt into each other and form a percolated network of non-reactive species. Thus, the spatiotemporal correlation among chemically active domains vanishes as the inter-cellular complexity of the tissue increases from normal tissue structure to fractal-type malignancy.Conclusion: Differential FLASH normal tissue sparing may result from the interplay of the proximity of tracks over the intra- and inter-cellular landscape, a transition in the spatial distribution of chemical reactivity, and molecular crowding. In this context, insensitivities in the radiobiological responses of the tumors to FLASH-UHDR are interpreted via a lack of geometrical correlation among isolated tracks. The structural and geometrical complexities of cancerous cells prevent the clustering of tracks over a timescale, in which inter-track chemical reactivities presumably prevail in normal tissues. A series of systematic experiments on radiolysis-induced diffusivity and reactivity in actual normal and cancerous tissues must be performed to classify the tissues potentially spared by FLASH-UHDRs and verify our theory.

Development and characterization of a versatile mini-beam collimator for pre-clinical photon beam irradiation.

Interest in spatial fractionation radiotherapy has exponentially increased over the last decade as a significant reduction of healthy tissue toxicity was observed by mini-beam irradiation. Published studies, however, mostly use rigid mini-beam collimators dedicated to their exact experimental arrangement such that changing the setup or testing new mini-beam collimator configurations becomes challenging andexpensive. In this work, a versatile, low-cost mini-beam collimator was designed and manufactured for pre-clinical applications with X-ray beams. The mini-beam collimator enables variability of the full width at half maximum (FWHM), the center-to-center distance (ctc), the peak-to-valley dose ratio (PVDR), and the source-to-collimator distance (SCD). The mini-beam collimator is an in-house development, which was constructed of 10× 40mm2 tungsten or brass plates. These metal plates were combined with 3D-printed plastic plates that can be stacked together in the desired order. A standard X-ray source was used for the dosimetric characterization of four different configurations of the collimator, including a combination of plastic plates of 0.5, 1, or 2mm width, assembled with 1 or 2mm thick metal plates. Irradiations were done at three different SCDs for characterizing the performance of the collimator. For the SCDs closer to the radiation source, the plastic plates were 3D-printed with a dedicated angle to compensate for the X-ray beam divergence, making it possible to study ultra-high dose rates of around 40Gy/s. All dosimetric quantifications were performed using EBT-XD films. Additionally, in vitro studies with H460 cells were carriedout. Characteristic mini-beam dose distributions were obtained with the developed collimator using a conventional X-ray source. With the exchangeable 3D-printed plates, FWHM and ctc from 0.52 to 2.11mm, and from 1.77 to 4.61mm were achieved, with uncertainties ranging from 0.01% to 8.98%, respectively. The FWHM and ctc obtained with the EBT-XD films are in agreement with the design of each mini-beam collimator configuration. For dose rates in the order of several Gy/min, the highest PVDR of 10.09 ± 1.08 was achieved with a collimator configuration of 0.5mm thick plastic plates and 2mm thick metal plates. Exchanging the tungsten plates with the lower-density metal brass reduced the PVDR by approximately 50%. Also, increasing the dose rate to ultra-high dose rates was feasible with the mini-beam collimator, where a PVDR of 24.26 ± 2.10 was achieved. Finally, it was possible to deliver and quantify mini-beam dose distribution patterns in vitro. With the developed collimator, we achieved various mini-beam dose distributions that can be adjusted according to the needs of the user in regards to FWHM, ctc, PVDR and SCD, while accounting for beam divergence. Therefore, the designed mini-beam collimator may enable low-cost and versatile pre-clinical research on mini-beamirradiation.

Fast Electrochemical Micropump for Portable Drug Delivery Module

Microfluidic devices are capable of precise drug delivery to the human body. For this purpose, they must be equipped with a compact pump that provides a high flow rate and precise dosing. In this paper, we present a micropump based on a fast electrochemical actuator that meets these requirements. It contains three actuators operating in the peristaltic mode. The device is fabricated from glass and silicon wafers using the standard microfabrication processes. The working part of the pump has a size of about 3 mm3, which is an order of magnitude smaller than other types of diaphragm pumps. The small size of the actuators ensures ultrahigh liquid dosing accuracy of 0.14 nL. At the same time, the high frequency of operation of the actuators makes it possible to develop a specific pumping velocity comparable to other types of pumps.

PO-1773 Characterization of new detectors for FLASH radiotherapy with ultra-high dose rates proton beams

PO-1773 Characterization of new detectors for FLASH radiotherapy with ultra-high dose rates proton beams

OC-0929 Scintillation imaging for time-resolved 2D monitoring of ultra-high dose rate proton beam scanning

OC-0929 Scintillation imaging for time-resolved 2D monitoring of ultra-high dose rate proton beam scanning

OC-0623 Ultra-High dose rate irradiation spares a large quantity of circulating lymphocytes in brain tumours

OC-0623 Ultra-High dose rate irradiation spares a large quantity of circulating lymphocytes in brain tumours

PO-1838 Characterization of a ultra-high dose rate proton beam for preclinical experiments.

PO-1838 Characterization of a ultra-high dose rate proton beam for preclinical experiments.

PD-0905 Electron Ultra-High Dose-Rate (FLASH) beam monitoring and control through beam current transformers

PD-0905 Electron Ultra-High Dose-Rate (FLASH) beam monitoring and control through beam current transformers

PO-2221 The effect of Ultra-High Dose-Rate Carbon ion irradiation to cell invasion on breast tumor cells.

PO-2221 The effect of Ultra-High Dose-Rate Carbon ion irradiation to cell invasion on breast tumor cells.

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

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

Radiation Protection at Petawatt Laser-Driven Accelerator Facilities: The ELI Beamlines Case

ELI Beamlines is a newly constructed petawatt (PW) laser-based accelerator facility. Its flagship laser has a nominal peak power of 10 PW with a pulse duration of 150 fs. The radiation fields emerging from the laser-target interactions will be pulsed and made of mixed particles of high intensity and energy, thus posing new and daunting challenges compared to conventional radiation protection. The civil engineering constraints of a laser facility bring localized weaknesses to the shielding structures. The ultrashort pulses in conjunction with ultrahigh dose rates strain the capabilities of the commercially available radiation detectors. The complexity of modeling laser-target interactions and the resulting ionizing radiation complicates the radiation protection calculations. This paper is a review of the radiation protection challenges at ELI Beamlines, the adopted solutions, and the practices to minimize the harmful effects of ionizing radiation linked to its activities. Some of the presented solutions may have universal validity, while others are specific to this site and can only serve as an inspiration.