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Background: The extremely fast delivery of doses with ultra high dose rate (UHDR) beams necessitates the investigation of novel approaches for real‐time dosimetry and beam monitoring. This aspect is fundamental in the perspective of the clinical application of FLASH radiotherapy (FLASH‐RT), as conventional dosimeters tend to saturate at such extreme dose rates. Purpose: This study aims to experimentally characterize newly developed silicon carbide (SiC) detectors of various active volumes at UHDRs and systematically assesses their response to establish their suitability for dosimetry in FLASH‐RT. Methods: SiC PiN junction detectors, recently realized and provided by STLab company, with different active areas (ranging from 4.5 to 10 mm2) and thicknesses (10–20 µm), were irradiated using 9 MeV UHDR pulsed electron beams accelerated by the ElectronFLASH linac at the Centro Pisano for FLASH Radiotherapy (CPFR). The linearity of the SiC response as a function of the delivered dose per pulse (DPP), which in turn corresponds to a specific instantaneous dose rate, was studied under various experimental conditions by measuring the produced charge within the SiC active layer with an electrometer. Due to the extremely high peak currents, an external customized electronic RC circuit was built and used in conjunction with the electrometer to avoid saturation. Results: The study revealed a linear response for the different SiC detectors employed up to 21 Gy/pulse for SiC detectors with 4.5 mm2/10 µm active area and thickness. These values correspond to a maximum instantaneous dose rate of 5.5 MGy/s and are indicative of the maximum achievable monitored DPP and instantaneous dose rate of the linac used during the measurements. Conclusions: The results clearly demonstrate that the developed devices exhibit a dose‐rate independent response even under extreme instantaneous dose rates and dose per pulse values. A systematic study of the SiC response was also performed as a function of the applied voltage bias, demonstrating the reliability of these dosimeters with UHDR also without any applied voltage. This demonstrates the great potential of SiC detectors for accurate dosimetry in the context of FLASH‐RT. [ABSTRACT FROM AUTHOR]

This study investigates how scattering foil materials and sampling holder placement affect electron energy distribution in electron beams from a modified medical linear accelerator for FLASH radiotherapy. We analyze electron energy spectra at various positions—ionization chamber, mirror, and jaw—to evaluate the impact of Cu, Pb-Cu, Pb, and Ta foils. Our findings show that close proximity to the source intensifies the dependence of electron energy distribution on foil material, enabling precise beam control through material selection. Monte Carlo simulations are effective for designing foils to achieve desired energy distributions. Moving the sampling holder farther from the source reduces foil material influence, promoting more uniform energy spreads, particularly in the 0.5–10 MeV range for 12 MeV electron beams. These insights emphasize the critical role of tailored material selection and sampling holder positioning in optimizing electron energy distribution and fluence intensity for FLASH radiotherapy research, benefiting both experimental design and clinical applications. [ABSTRACT FROM AUTHOR]

Monitoring Ultra-High Dose Rate (UHDR) beams is one of the multiple challenges posed by the emergent FLASH radiotherapy. Technologies (i.e., gas-filled ionization chambers) nowadays used in conventional radiotherapy are no longer effective when applied to UHDR regimes, due to the recombination effect they are affected by, and the time required to collect charges. Exploiting the expertise in the field of silicon sensors' applications into clinics, the medical physics group of the University and INFN Torino is investigating thin silicon sensors as possible candidates for UHDR beam monitoring, exploiting their excellent spatial resolution and well-developed technology. Silicon sensors of 30 and 45 μιτι active thicknesses and 0.25, 1 and 2 mm[sup 2] active areas were tested at the SIT ElectronFlash machine (CPFR, Pisa) on 9 MeV electron beams, featuring a pulse duration of 4 μ^ a frequency of 1 Hz, and a dose-per-pulse ranging from 1.62 to 10.22 Gy/pulse. The silicon sensors were positioned at the exit of the ElectronFlash applicator, after a solid water build-up slab, and were readout both with an oscilloscope and with a multi-channel front-end readout chip (TERA08). A response linearity extending beyond 10 Gy/pulse was demonstrated by comparison with a reference dosimeter (FlashDiamond), thus fulfilling the first requirement of a potential application in UHDR beam monitoring. [ABSTRACT FROM AUTHOR]

Background: The potential reduction of normal tissue toxicities during FLASH radiotherapy (FLASH‐RT) has inspired many efforts to investigate its underlying mechanism and to translate it into the clinic. Such investigations require experimental platforms of FLASH‐RT capabilities. Purpose: To commission and characterize a 250 MeV proton research beamline with a saturated nozzle monitor ionization chamber for proton FLASH‐RT small animal experiments. Methods: A 2D strip ionization chamber array (SICA) with high spatiotemporal resolution was used to measure spot dwell times under various beam currents and to quantify dose rates for various field sizes. An Advanced Markus chamber and a Faraday cup were irradiated with spot‐scanned uniform fields and nozzle currents from 50 to 215 nA to investigate dose scaling relations. The SICA detector was set up upstream to establish a correlation between SICA signal and delivered dose at isocenter to serve as an in vivo dosimeter and monitor the delivered dose rate. Two off‐the‐shelf brass blocks were used as apertures to shape the dose laterally. Dose profiles in 2D were measured with an amorphous silicon detector array at a low current of 2 nA and validated with Gafchromic films EBT‐XD at high currents of up to 215 nA. Results: Spot dwell times become asymptotically constant as a function of the requested beam current at the nozzle of greater than 30 nA due to the saturation of monitor ionization chamber (MIC). With a saturated nozzle MIC, the delivered dose is always greater than the planned dose, but the desired dose can be achieved by scaling the MU of the field. The delivered doses exhibit excellent linearity with R2>0.99${R^2} > 0.99$ with respect to MU, beam current, and the product of MU and beam current. If the total number of spots is less than 100 at a nozzle current of 215 nA, a field‐averaged dose rate greater than 40 Gy/s can be achieved. The SICA‐based in vivo dosimetry system achieved excellent estimates of the delivered dose with an average (maximum) deviation of 0.02 Gy (0.05 Gy) over a range of delivered doses from 3 to 44 Gy. Using brass aperture blocks reduced the 80%‐20% penumbra by 64% from 7.55 to 2.75 mm. The 2D dose profiles measured by the Phoenix detector at 2 nA and the EBT‐XD film at 215 nA showed great agreement, with a gamma passing rate of 95.99% using 1 mm/2% criterion. Conclusion: A 250 MeV proton research beamline was successfully commissioned and characterized. Challenges due to the saturated monitor ionization chamber were mitigated by scaling MU and using an in vivo dosimetry system. A simple aperture system was designed and validated to provide sharp dose fall‐off for small animal experiments. This experience can serve as a foundation for other centers interested in implementing FLASH radiotherapy preclinical research, especially those equipped with a similar saturated MIC. [ABSTRACT FROM AUTHOR]

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 treatment rooms. 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 cmSSD 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. [ABSTRACT FROM AUTHOR]

Ultra-high dose rate (UHDR) beams for FLASH radiotherapy present significant dosimetric challenges. Although novel approaches for decreasing or correcting ion recombination in ionization chambers are being proposed, applicability of ionimetric dosimetry to UHDR beams is still under investigation. Solid-state sensors have been recently investigated as a valuable alternative for real-time measurements, especially for relative dosimetry and beam monitoring. Among them, Silicon Carbide (SiC) represents a very promising candidate, compromising between the maturity of Silicon and the robustness of diamond. Its features allow for large area sensors and high electric fields, required to avoid ion recombination in UHDR beams. In this study, we present simulations and experimental measurements with the low energy UHDR electron beams accelerated with the ElectronFLASH machine developed by the SIT Sordina company (IT). The response of a newly developed 1 × 1 cm2 SiC sensor in charge as a function of the dose-per-pulse and its radiation hardness up to a total delivered dose of 90 kGy, was investigated during a dedicated experimental campaign, which is, to our knowledge, the first characterization ever done of SiC with UHDR-pulsed beams accelerated by a dedicated ElectronFLASH LINAC. Results are encouraging and show a linear response of the SiC detector up to 2 Gy/pulse and a variation in the charge per pulse measured for a cumulative delivered dose of 90 kGy, within ±0.75%. [ABSTRACT FROM AUTHOR]

The FRIDA INFN project characterized thin silicon sensors for monitoring electron FLASH beams, showing a response linearity up to ∼ 10 Gy/pulse on 9 MeV electrons. The use of segmented silicon configurations could provide the spatial resolution useful in Spatially Fractionated Radiotherapy (SFRT). Within the MIRO INFN project, a strip sensor integrated with a multichannel readout chip is being developed and tested for monitoring FLASH and SFRT electron and proton beams. One hundred and twenty-eight strips are independently read by two front-end TERA chips. Preliminary tests were performed on 6–10 MeV electron beams at the LINAC at Physics Department of the University of Torino, with and without using a tungsten minibeam collimator, and on 62–226 MeV proton beams at CNAO (Pavia). The capability to spatially resolve electron and proton beams at conventional dose-rates has been proved. [ABSTRACT FROM AUTHOR]

Purpose: A diamond detector prototype was recently proposed by Marinelli et al. (Medical Physics 2022, https://doi.org/10.1002/mp.15473) for applications in ultrahigh‐dose‐per‐pulse (UH‐DPP) and ultrahigh‐dose‐rate (UH‐DR) beams, as used in FLASH radiotherapy (FLASH‐RT). In the present study, such so‐called flashDiamond (fD) was investigated from the dosimetric point of view, under pulsed electron beam irradiation. It was then used for the commissioning of an ElectronFlash linac (SIT S.p.A., Italy) both in conventional and UH‐DPP modalities. Methods: Detector calibration was performed in reference conditions, under 60Co and electron beam irradiation. Its response linearity was investigated in UH‐DPP conditions. For this purpose, the DPP was varied in the 1.2–11.9 Gy range, by changing either the beam applicator or the pulse duration from 1 to 4 μs. Dosimetric validation of the fD detector prototype was then performed in conventional modality, by measuring percentage depth dose (PDD) curves, beam profiles, and output factors (OFs). All such measurements were carried out in a motorized water phantom. The obtained results were compared with the ones from commercially available dosimeters, namely, a microDiamond, an Advanced Markus ionization chamber, a silicon diode detector, and EBT‐XD GAFchromic films. Finally, the fD detector was used to fully characterize the 7 and 9 MeV UH‐DPP electron beams delivered by the ElectronFlash linac. In particular, PDDs, beam profiles, and OFs were measured, for both energies and all the applicators, and compared with the ones from EBT‐XD films irradiated in the same experimental conditions. Results: The fD calibration coefficient resulted to be independent from the investigated beam qualities. The detector response was found to be linear in the whole investigated DPP range. A very good agreement was observed among PDDs, beam profiles, and OFs measured by the fD prototype and reference detectors, both in conventional and UH‐DPP irradiation modalities. Conclusions: The fD detector prototype was validated from the dosimetric point of view against several commercial dosimeters in conventional beams. It was proved to be suitable in UH‐DPP and UH‐DR conditions, for which no other commercial real‐time active detector is available to date. It was shown to be a very useful tool to perform fast and reproducible beam characterizations in standard clinical motorized water phantom setups. All of the previously mentioned demonstrate the suitability of the proposed detector for the commissioning of UH‐DR linac beams for preclinical FLASH‐RT applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Background: Conventional air ionization chambers (ICs) exhibit ion recombination correction factors that deviate substantially from unity when irradiated with dose per pulse magnitudes higher than those used in conventional radiotherapy. This fact makes these devices unsuitable for the dosimetric characterization of beams in ultra‐high dose per pulse as used for FLASH radiotherapy. Purpose: We present the design, development, and characterization of an ultra‐thin parallel plate IC that can be used in ultra‐high dose rate (UHDR) deliveries with minimal recombination. Methods: The charge collection efficiency (CCE) of parallel plate ICs was modeled through a numerical solution of the coupled differential equations governing the transport of charged carriers produced by ionizing radiation. It was used to find out the optimal parameters for the purpose of designing an IC capable of exhibiting a linear response with dose (deviation less than 1%) up to 10 Gy per pulse at 4 μ$\umu$s pulse duration. As a proof of concept, two vented parallel plate IC prototypes have been built and tested in different ultra‐high pulse dose rate electron beams. Results: It has been found that by reducing the distance between electrodes to a value of 0.25 mm it is possible to extend the dose rate operating range of parallel plate ICs to ultra‐high dose per pulse range, at standard voltage of clinical grade electrometers, well into several Gy per pulse. The two IC prototypes exhibit behavior as predicted by the numerical simulation. One of the so‐called ultra‐thin parallel plate ionization chamber (UTIC) prototypes was able to measure up to 10 Gy per pulse, 4 μ$\umu$s pulse duration, operated at 300 V with no significant deviation from linearity within the uncertainties (ElectronFlash Linac, SIT). The other prototype was tested up to 5.4 Gy per pulse, 2.5 μ$\umu$s pulse duration, operated at 250 V with CCE higher than 98.6% (Metrological Electron Accelerator Facility, MELAF at Physikalisch‐Technische Bundesanstalt, PTB). Conclusions: This work demonstrates the ability to extend the dose rate operating range of ICs to ultra‐high dose per pulse range by reducing the spacing between electrodes. The results show that UTICs are suitable for measurement in UHDR electron beams. [ABSTRACT FROM AUTHOR]

Purpose: FLASH radiotherapy (RT) is an emerging technique in which beams with ultra‐high dose rates (UH‐DR) and dose per pulse (UH‐DPP) are used. Commercially available active real‐time dosimeters have been shown to be unsuitable in such conditions, due to severe response nonlinearities. In the present study, a novel diamond‐based Schottky diode detector was specifically designed and realized to match the stringent requirements of FLASH‐RT. Methods: A systematic investigation of the main features affecting the diamond response in UH‐DPP conditions was carried out. Several diamond Schottky diode detector prototypes with different layouts were produced at Rome Tor Vergata University in cooperation with PTW‐Freiburg. Such devices were tested under electron UH‐DPP beams. The linearity of the prototypes was investigated up to DPPs of about 26 Gy/pulse and dose rates of approximately 1 kGy/s. In addition, percentage depth dose (PDD) measurements were performed in different irradiation conditions. Radiochromic films were used for reference dosimetry. Results: The response linearity of the diamond prototypes was shown to be strongly affected by the size of their active volume as well as by their series resistance. By properly tuning the design layout, the detector response was found to be linear up to at least 20 Gy/pulse, well into the UH‐DPP range conditions. PDD measurements were performed by three different linac applicators, characterized by DPP values at the point of maximum dose of 3.5, 17.2, and 20.6 Gy/pulse, respectively. The very good superimposition of three curves confirmed the diamond response linearity. It is worth mentioning that UH‐DPP irradiation conditions may lead to instantaneous detector currents as high as several mA, thus possibly exceeding the electrometer specifications. This issue was properly addressed in the case of the PTW UNIDOS electrometers. Conclusions: The results of the present study clearly demonstrate the feasibility of a diamond detector for FLASH‐RT applications. [ABSTRACT FROM AUTHOR]

Preclinical studies have shown that radiotherapy treatments benefit from ultra-high dose rate (UHDR) irradiations. These trigger the FLASH-effect, resulting in a strong decrease in normal tissue toxicity with conserved tumor control probability, compared with conventional irradiations. However, the beam parameters to trigger the FLASH-effect and the radiobiological mechanisms behind it remain to be elucidated. A limiting factor in many studies is the lack of accurate real time dosimetry. In this work an innovative solution for 2D real time dosimetry in UHDR electron beams is presented and characterized. The in-house developed ImageDosis system consists of a scientific camera, with high temporal resolution, and a coating, containing 12% of Y 3 Al 5 O 12 :Ce 3 + as scintillating material. Reference dosimetry was performed by means of radiochromic film, and a (C 38 H 34 P 2)MnBr 4 point scintillator was used to validate the pulse discrimination properties of the ImageDosis system. Irradiations were performed in two centers (Antwerp and Orsay), with an ElectronFlash accelerator. The ImageDosis system was tested with varying number of pulses, pulse length, pulse repetition frequency (PRF) and energy. In addition, its temporal resolution and 2D properties were investigated. The ImageDosis system showed negligible dose, dose rate and energy dependence for doses up to 13 Gy and average dose rates up to 140 Gy/s, with a dose per pulse up to 2 Gy and a PRF up to 300 Hz. The system is capable of discriminating and measuring the dose of individual pulses and shows promising 2D characteristics that need further optimization. [Display omitted] • The YAG-based ImageDosis system is a promising tool for real time UHDR dosimetry. • The ImageDosis system has shown not to be dose rate dependent. • The ImageDosis system provides 2D characteristics that are relevant for FLASH-RT. • The ImageDosis system has shown not to be energy dependent. • The ImageDosis system is capable of measuring individual pulses. [ABSTRACT FROM AUTHOR]

FLASH Radiotherapy (RT) delivers an average dose-rate >40 Gy/s in less than 200 ms with extremely high instantaneous dose-rates, and preclinical studies demonstrated a tumoricidal effect comparable to conventional RT with an increased sparing effect on healthy tissues (FLASH effect). Within the INFN-FRIDA project, we are exploring thin silicon sensors and polycrystalline CVD diamond (pCVD) sensors as real-time beam monitors for electron and proton FLASH beams. Planar silicon sensors were first tested on conventional electron beams and subsequently on high-dose electron beams (ElectronFLASH machine, funded by the Pisa Foundation). Diamond, due to some of its properties including radiation resistance, could also be a viable alternative for monitoring FLASH beams. The first tests were carried out in Turin, and the sensor response was studied in terms of polarization voltage, integrated charge, charge collection efficiency and sensitivity. [ABSTRACT FROM AUTHOR]

FLASH radiotherapy is an emerging radiotherapy technique used to spare normal tissues. It employs ultra-high dose rate radiation beams over 40 Gy/s, which is significantly higher than those of conventional radiotherapy. In this study, a fiber-optic radiation sensor (FORS) was fabricated using a plastic scintillator, an optical filter, and a plastic optical fiber to measure the ultra-high dose rate electron beams over 40 Gy/s used in FLASH radiotherapy. The radiation-induced emissions, such as Cherenkov radiation and fluorescence generated in a transmitting optical fiber, were spectrally discriminated from the light outputs of the FORS. To evaluate the linearity and dose rate dependence of the FORS, the outputs of the fiber-optic radiation sensor were measured according to distances from an electron scattering device, and the results were compared with those of an ionization chamber and radiochromic films. Finally, the percentage depth doses were obtained using the FORS as a function of depth in a water phantom. This study found that ultra-high dose rate electron beams over 40 Gy/s could be measured in real time using a FORS. [ABSTRACT FROM AUTHOR]

RadiaBeam has developed a novel, flexible linac for electrons and X-rays (FLEX) capable of energy variation from 2 to 9 MeV within a single RF pulse. This accelerator employs a robust traveling wave accelerating structure with a large energy and power acceptance that allows the unique features of deep energy and current variation required for novel adaptive cargo inspection techniques and Computed Tomography. Robust RF and water-cooling systems allow average beam powers up to 15 kW, upgradable to 57 kW, at 6 MeV, potentially making this accelerator useful for electron beam irradiation applications. The high peak currents of the electron beam produced by FLEX enable novel FLASH radiotherapy methods with fast dose delivery. In this paper, we present the linac design and its components, as well the results of the experimental demonstration of beam acceleration with energy variation. • Novel security, radiotherapy and industrial applications require new accelerators. • Adaptive cargo inspection needs beam parameters variation within a single pulse. • FLASH radiotherapy requires delivery of high X-ray doses within a few seconds. • RadiaBeam developed accelerator with rapid beam energy and current variation. [ABSTRACT FROM AUTHOR]