Background and purpose:Robust optimization mitigates the effects of range and setup uncertainties in intensity-modulated proton therapy (IMPT) through robustness settings. However, it increases the dose to organs-at-risk (OARs). This study investigated how reducing range robustness (RR) and setup robustness (SR) settings impacts plan adaptation frequency in IMPT of head-and-neck cancer (HNC) patients. Material and methods:A cohort of twenty-six HNC patients treated with IMPT were retrospectively analyzed. Original plans used RR/SR settings of 3%/3 mm. For each patient, four plans were generated with reduced robustness settings. All plans were robustly evaluated on weekly repeat computed tomography scans (rCTs) across 28 error scenarios, applying 2.7-3.2% range and 1 mm setup uncertainty. The failure rate of the voxel-wise minimum D98%≥94% criterion was used as a surrogate for plan adaptation frequency. Differences and equivalence in failure rates between settings were assessed using both two-proportion z-test and Two-One Side Test. Results:Fixing SR at 3 mm, each 1% reduction in RR setting reduced the mean OAR dose by 0.45  Gy (RBE) and normal tissue complication probability (NTCP) by 1.1%-points at a 3.5%-points increase in adaptation triggering rate. Implementing a RR of 3%, each 1 mm reduction in SR settings reduced the OAR dose by 1.0 Gy (RBE) and NTCP by 2.2%-points at a 7.0%-points increase in adaptation triggering rate. Conclusions:Reducing robustness margins decreased OAR dose and NTCP with a manageable increase in plan adaptation; however, larger reductions would require adaptive workflows and implementation of range uncertainty mitigation strategies before safe clinical implementation.

Spent nuclear fuel imaging before disposal is of utmost importance before long term disposal in dedicated storage facilities. Passive Gamma Emission Tomography (PGET) is an approved method by the International Atomic Energy Agency. The present detection system is based on small CZT detectors behind a tungsten-based collimator consisting of a linear array of slits. Small scale CZT crystals limit the detection efficiency of high energetic gamma rays from the fuel rods, mainly the 662 keV emissions from Cs-137. In our study based on full Monte-Carlo simulations as well as on experiments, we explore the capabilities of large pixelated CZT detectors to be used for PGET. We will discuss the theoretical advantages and practical challenges of the larger crystals. We demonstrate that the larger crystals, depending on their orientation, will increase the detection efficiency by a factor of 7 to 13. Due to the pixelated sensor signal readout we also explore the possibility to employ Compton imaging to improve the information on the location of origin of gamma rays. In addition we explore the usefulness of commercial gamma-ray imagers for waste characterisation and decommissioning. In particular we report on the performance of the GeGI imager from PHDS Co and the H420 imager from H3D Inc in measuring nuclear waste drums at Svafo, Sweden.

The Passive Gamma Emission Tomography (PGET) instrument, authorized by the International Atomic Energy Agency (IAEA) for verification of spent nuclear fuel, aims to reconstruct 2D cross-sectional images of spent fuel assemblies (SFAs), identify missing or present fuel pins, and quantify fuel pin activities. Although the first two objectives are reliably achieved, accurate determination of fuel pin activities remains a challenge due to intense self-shielding and scattering effects. We have developed a linear inverse approach that addresses these effects and demonstrated superior image quality and identification accuracy in simulation studies. This approach frames the image reconstruction process as an inverse problem, relying on a physics-based forward model of the PGET system. We improved our forward model by incorporating collimator septal penetration and detector scattering effects. The enhanced forward model enables near-real-time sinogram simulation and system matrix calculation, which is> 100,000 times faster than 3D Monte Carlo simulations. The model was validated through simulations of VVER-1000 and VVER-440 SFA, and a relative difference of 3.7% in counts was achieved between MCNP and our forward model. Based on this enhanced model, we successfully reconstructed images from the simulated data, identified 100% of the fuel pins, and achieved an average uncertainty of 2.3% in activity quantification. We applied the reconstruction method to measured data of VVER-440 SFAs, successfully imaging all the pins, including the innermost ones, and identifying the water channel within the SFA. The high accuracy and low computational cost of our forward model demonstrate its potential for real-world inspection scenarios and enable future algorithm development.

BACKGROUND:Intensity-modulated proton therapy (IMPT) holds promise for improving outcomes in head-and-neck cancer (HNC) patients by enhancing organ-at-risk (OAR) sparing. A key challenge in IMPT is ensuring an accurate dose delivery at the distal edge of the tumor, where the steep dose gradients make treatment precision highly sensitive to uncertainties in both proton range and patient setup. Thus, IMPT conformality is increased by incorporating robust margins in the treatment optimization. However, an increment in the plan robustness could lead to an OAR overdosing. Therefore, an accurate distal edge verification during dose delivery is crucial to increase IMPT conformality by reducing optimization settings in treatment planning. PURPOSE:This work aims to evaluate, in a quasi-clinical setting, a novel approach for accurate instantaneous proton beam distal edge verification in IMPT by means of spot-by-spot positron emission tomography (PET) imaging. METHODS:An anthropomorphic head and neck phantom CIRS-731 HN was irradiated at the head and neck region. The targets were defined as 4 cm diameter spheres. A 60-ms delay was introduced between the proton beam spots in order to enable the spot-by-spot coincidence detection of the 511-keV photons resulting from positron annihilation following the positron emission from very short-lived positron-emitting, mainly 12N (T1/2  = 11.0 ms). Additionally, modified irradiations were carried out using solid water slabs of 2 and 5 mm thickness in the beam path to assess the precision of the approach for detecting range deviations. The positron activity range (PAR) was determined from the 50% distal fall-off position of the 1D longitudinal positron activity profile derived from the 2D image reconstructions. Furthermore, Monte Carlo (MC) simulations were performed using an in-house RayStation/GATE MC framework to predict the positron activity images and verify the PAR measurements. RESULTS:PAR measurements achieved a precision between 1.5 and 3.6 mm (at 1.5σ clinical level) at the beam spot level within sub-second time scales. Measured PAR shifts of 1.6-2.1  and 4.2--.7 mm were observed with the 2- and 5-mm thickness range shifters, respectively, aligning with the corresponding proton dose range (PDR) shifts of 1.3-1.8 and 3.9-4.3 mm. The simulated PAR agrees with the measured PARs, showing an average range difference of ∼0.4 mm. CONCLUSION:This study demonstrated the feasibility of instantaneous distal edge verification using PET imaging by introducing beam spot delays during dose delivery. The findings represent a first step toward the clinical implementation of instantaneous in vivo distal edge verification. The approach contributes to the development of real-time range verification aimed at improving IMPT treatments by mitigating range and setup uncertainties, thereby reducing dose to organs-at-risk and ultimately enhancing patient outcomes.

We are evaluating the performance of a Passive Gamma Emission Tomography (PGET) device [1] equipped with 3D position-sensitive cadmium zinc telluride (CZT) gamma-ray detectors when used for inspecting spent nuclear fuel assemblies (SFAs). Before their disposal in a geological repository, SFAs undergo verification using the PGET device, which was developed under the guidance of the IAEA and approved by the IAEA for safeguards inspections. Recent advancements in imaging detector technology may offer a method to extend the capabilities of such devices beyond standard safeguards applications, allowing an efficient non-invasive way to accurately characterise the properties of nuclear fuel assemblies. The efficiency of the currently used small CZT detectors is restricted by the limited likelihood of full gamma-ray absorption, which is needed for optimal imaging information. Employing larger CZT detectors would increase the probability of capturing the full energy of gamma rays, thereby enhancing the sensitivity of the PGET device and the quality of the reconstructed images. Large CZT detectors need to be position-sensitive to determine through which collimator slit a gamma ray travelled. Position sensitivity results from the pixelated readout of the CZT crystals. Pixelation potentially increases the spatial resolution of the system, which is currently determined by the collimator used. Pixelation allows resolving the position of arrival up to (readout pitch)/√(12). We are additionally exploring the potential of utilising Compton imaging to provide information on the origin of gamma rays along the SFA. Monte Carlo simulations are used to estimate the increase in full photon absorption efficiency when comparing large and current, small, crystals. A dedicated simulation is created using Geant4, where gamma rays of energy 661.7 keV and 1274 keV are targeted to the model describing the approved apparatus now equipped with 22 mm × 22 mm × 10 mm crystals of CZT. It is observed that the efficiency for photon absorption in this case is greatly increased when compared to the existing detectors.

Precision experiments with thermalized exotic nuclei will be possible at the super-conducting fragment separator Super-FRS at the Facility for Antiproton and Ion Research (FAIR). In the Early Science/First Science programs of FAIR, they will be performed at the focal plane FHF1 of the Super-FRS (in front of the High-Energy Branch) and, at a later stage during First Science++, also at the Low-Energy Branch. Exotic nuclei will be produced in flight and separated in the Super-FRS and their momentum spread will be reduced by energy-bunching. The ions will be further slowed down in a homogeneous degrader, thermalized in a gas-filled stopping cell, and extracted and transferred to the experimental setups. The stopping cell is, thus, a key device for experiments with thermalized exotic nuclei, and its performance characteristics will have a strong impact on the range of nuclides available and their yields, since its stopping and extraction efficiencies and extraction times strongly influence the rate of the extracted nuclei and put a limit on their lifetimes. In combination with a multiple-reflection time-of-flight mass spectrometer, the stopping cell will enable the measurement of masses, branching ratios, e.g., β -delayed (multi-)neutron emission probabilities, and lifetimes, as well as the in-cell production of exotic nuclei by multi-nucleon transfer with primary and secondary beams. Moreover, it will be a tool for the absolute calibration of the particle identification in the Super-FRS. Furthermore, using the combination of accurate mass determination with the PID of the Super-FRS on an event-by-event basis, completely new experimental possibilities will become available, such as the identification of millisecond isomers at the Super-FRS and the measurement of the dependence of the isomer-to-ground-state ratios on the production mechanism. These studies will be pursued in the context of the Super-FRS Experiment Collaboration. At the Low-Energy Branch, high-accuracy mass measurements, in-trap conversion electron and alpha spectroscopy, and trap-assisted spectroscopy will be performed with MATS (Precision Measurements of very short-lived nuclei using an Advanced Trapping System for highly charged ions). Measurements of nuclear spins, magnetic dipole and electric quadrupole moments, and root-mean-square charge radii will be carried out using collinear laser spectroscopy on ions and atoms and beta-NMR experiments with LaSpec (Laser Spectroscopy of short-lived nuclei). These experiments will address a wide scientific field ranging from nuclear structure and nuclear astrophysics to tests of the weak interaction and of the Standard Model. For the Super-FRS, a stopping cell with an areal density of 20 mg/cm^2 is required, with a cross-section of the stopping volume of 200 cm^2 , a high extraction efficiency that is element-independent, an extraction time on the order of 10 ms, and a rate capability of 10^7 ions/s up to a nuclear charge of Z=92. Furthermore, the stopping cell needs to deliver bunches of ions with high purity. No existing stopping cell is capable of reaching these performance characteristics simultaneously. To fulfill these requirements, a novel concept for gas-filled stopping cells has been developed. It is based on the cryogenic stopping cell of the FRS Ion Catcher, and, in addition, it implements the two-stage extraction of the thermalized ions in a direction orthogonal to the incoming ion beam (high-areal-density orthogonal-extraction cryogenic stopping cell, HADO-CSC). This will boost all performance characteristics of the stopping cell and thus remove the performance bottleneck of present stopping cells for the thermalization of exotic nuclei produced at relativistic energies.

Postirradiation examination of nuclear fuel is routinely performed to characterize the important properties of current and future fuel. Gamma emission tomography is a proven noninvasive technique for this purpose. Among various measurement elements of the technique, a gamma-ray detector is an important element whose spectroscopic abilities and detection efficiency affect the overall results. Finding a combination of high detection efficiency and excellent energy resolution in a single detector is often a challenge. We have designed a novel planar segmented high-purity germanium detector that offers simultaneous measurement in six lines of sight with excellent energy resolution. The simultaneous detection ability enables faster data acquisition in a tomographic measurement, which may facilitate achieving higher spatial resolution. In this work, we have demonstrated the first use of the detector by performing a full tomographic measurement of mockup fuel rods. Two methods of detector data analysis were used to make spectra, and the images (tomograms) were reconstructed using the filtered back projection algorithm. The reconstructed images validate the successful use of the detector for tomographic measurement. The use of the detector for real fuel measurement is being planned and will be performed in the near future.

Objective. Proton therapy currently faces challenges from clinical complications on organs-at-risk due to range uncertainties. To address this issue, positron emission tomography (PET) of the proton-induced11C and15O activity has been used to provide feedback on the proton range. However, this approach is not instantaneous due to the relatively long half-lives of these nuclides. An alternative nuclide,12N (half-life 11 ms), shows promise for real-timein vivoproton range verification. Development of12N imaging requires better knowledge of its production reaction cross section.Approach. The12C(p,n)12N reaction cross section was measured by detecting positron activity of graphite targets irradiated with 66.5, 120, and 150 MeV protons. A pulsed beam delivery with 0.7-2 × 108protons per pulse was used. The positron activity was measured during the beam-off periods using a dual-head Siemens Biograph mCT PET scanner. The12N production was determined from activity time histograms.Main results. The cross section was calculated for 11 energies, ranging from 23.5 to 147 MeV, using information on the experimental setup and beam delivery. Through a comprehensive uncertainty propagation analysis, a statistical uncertainty of 2.6%-5.8% and a systematic uncertainty of 3.3%-4.6% were achieved. Additionally, a comparison between measured and simulated scanner sensitivity showed a scaling factor of 1.25 (±3%). Despite this, there was an improvement in the precision of the cross section measurement compared to values reported by the only previous study.Significance. Short-lived12N imaging is promising for real-timein vivoverification of the proton range to reduce clinical complications in proton therapy. The verification procedure requires experimental knowledge of the12N production cross section for proton energies of clinical importance, to be incorporated in a Monte Carlo framework for12N imaging prediction. This study is the first to achieve a precise measurement of the12C(p,n)12N nuclear cross section for such proton energies.

Objective.12N, having a half-life of 11 ms, is a highly effective positron emitter that can potentially provide near real-time feedback in proton therapy. There is currently no framework for comparing and validating positron emission imaging of12N. This work describes the development and validation of a Monte Carlo (MC) framework to calculate the images of12N, as well as long-lived isotopes, originating from activation by protons.Approach. The available dual-panel Biograph mCT PET scanner was modeled in GATE and validated by comparing the simulated sensitivity map with the measured one. The distributions of12N and long-lived isotopes were calculated by RayStation and used as the input of GATE simulations. The RayStation/GATE combination was verified using proton beam irradiations of homogeneous phantoms. A 120 MeV pulsed pencil beam with 108protons per pulse was used. Two-dimensional images were created from the GATE output and compared with the images based on the measurements and the 1D longitudinal projection of the full 2D image was used to calculate the12N activity range.Main results. The simulated sensitivity in the center of the FoV (5.44%) agrees well with the measured one (5.41%). The simulated and measured 2D sensitivity maps agree in good detail. The relative difference between the measured and simulated positron activity range for both12N and long-lived isotopes is less than 1%. The broadening of the12N images relative to those of the longer-lived isotopes can be understood in terms of the large positron range of12N.Significance. We developed and validated a MC framework based on RayStation/GATE to support the in-beam PET method for quality assurance of proton therapy. The inclusion of the very short-lived isotope12N makes the framework useful for developing near real-time verification. This represents a significant step towards translating12N real-time in vivo verification to the clinic.

Hadron therapy is a radiotherapy modality which offers a precise energy deposition to the tumors and a dose reduction to healthy tissue as compared to conventional methods. However, methods for real-time monitoring are required to ensure that the radiation dose is deposited on the target. The IRIS group of IFIC-Valencia developed a Compton camera prototype for this purpose, intending to image the Prompt Gammas emitted by the tissue during irradiation. The system detectors are composed of Lanthanum (III) bromide scintillator crystals coupled to silicon photomultipliers. After an initial characterization in the laboratory, in order to assess the system capabilities for future experiments in proton therapy centers, different tests were carried out in two facilities: PARTREC (Groningen, The Netherlands) and the CNA cyclotron (Sevilla, Spain). Characterization studies performed at PARTREC indicated that the detectors linearity was improved with respect to the previous version and an energy resolution of 5.2 % FWHM at 511 keV was achieved. Moreover, the imaging capabilities of the system were evaluated with a line source of 68Ge and a point-like source of 241Am-9Be. Images at 4.439 MeV were obtained from irradiation of a graphite target with an 18 MeV proton beam at CNA, to perform a study of the system potential to detect shifts at different intensities. In this sense, the system was able to distinguish 1 mm variations in the target position at different beam current intensities for measurement times of 1800 and 600 s.

Abstract A fast and reliable range monitoring method is required to take full advantage of the high linear energy transfer provided by therapeutic ion beams like carbon and oxygen while minimizing damage to healthy tissue due to range uncertainties. Quasi-real-time range monitoring using in-beam positron emission tomography (PET) with therapeutic beams of positron-emitters of carbon and oxygen is a promising approach. The number of implanted ions and the time required for an unambiguous range verification are decisive factors for choosing a candidate isotope. An experimental study was performed at the FRS fragment-separator of GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany, to investigate the evolution of positron annihilation activity profiles during the implantation of $$^{14}$$ 14 O and $$^{15}$$ 15 O ion beams in a PMMA phantom. The positron activity profile was imaged by a dual-panel version of a Siemens Biograph mCT PET scanner. Results from a similar experiment using ion beams of carbon positron-emitters $$^{11}$$ 11 C and $$^{10}$$ 10 C performed at the same experimental setup were used for comparison. Owing to their shorter half-lives, the number of implanted ions required for a precise positron annihilation activity peak determination is lower for $$^{10}$$ 10 C compared to $$^{11}$$ 11 C and likewise for $$^{14}$$ 14 O compared to $$^{15}$$ 15 O, but their lower production cross-sections make it difficult to produce them at therapeutically relevant intensities. With a similar production cross-section and a 10 times shorter half-life than $$^{11}$$ 11 C, $$^{15}$$ 15 O provides a faster conclusive positron annihilation activity peak position determination for a lower number of implanted ions compared to $$^{11}$$ 11 C. A figure of merit formulation was developed for the quantitative comparison of therapy-relevant positron-emitting beams in the context of quasi-real-time beam monitoring. In conclusion, this study demonstrates that among the positron emitters of carbon and oxygen, $$^{15}$$ 15 O is the most feasible candidate for quasi-real-time range monitoring by in-beam PET that can be produced at therapeutically relevant intensities. Additionally, this study demonstrated that the in-flight production and separation method can produce beams of therapeutic quality, in terms of purity, energy, and energy spread.

Early warning networks are used for detecting abnormal radioactivity levels in the environment. State-of-the-art networks are equipped with both dose rate detectors and spectrometric stations. Current networks don’t automatically discriminate between radioactivity on the ground and in the air. A novel directional sensing gamma radiation detector utilizing a collimated phoswich scintillator was developed. The signals from the two scintillator materials are separated using a pulse shape discrimination. The separated signals are employed to determine the radioactivity concentrations on the ground and in the air assuming specific concentration distributions. Limitations related to imperfect directional sensing and dead time are discussed.

A first-of-a-kind geological repository for spent nuclear fuel is being built in Finland and will soon start operations. To make sure all nuclear material stays in peaceful use, the fuel is measured with two complementary non-destructive methods to verify the integrity and the fissile content of the fuel prior to disposal. For pin-wise identification of active fuel material, a Passive Gamma Emission Tomography (PGET) device is used. Gamma radiation emitted by the fuel is assayed from 360 angles around the assembly with highly collimated CdZnTe detectors, and a 2D cross-sectional image is reconstructed from the data. At the encapsulation plant in Finland, there will be the possibility to measure in air. Since the performance of the method has only been studied in water, measurements with mock-up fuel were conducted at the Atominstitut in Vienna, Austria. Four different arrangements of activated Co-60 rods, steel rods and empty positions were investigated both in air and in water to confirm the functionality of the method. The measurement medium was not observed to affect the ability of the method to distinguish modified rod positions from filled rod positions. More extended conclusions about the method performance with real spent nuclear fuel cannot be drawn from the mock-up studies, since the gamma energies, activities, material attenuations and assembly dimensions are different, but full-scale measurements with spent nuclear fuel are planned for 2023.