Relative Stopping Power Research Articles

A Monte-Carlo-based study of a single-2D-detector proton-radiography system

PurposeTo assess the feasibility of a proton radiography (pRG) system based on a single thin pixelated detector for water-equivalent path length (WEPL) and relative stopping power (RSP) measurements. MethodsA model of a pRG system consisting of a single pixelated detector measuring energy deposition and proton fluence was investigated in a Geant4-based Monte Carlo study. At the position directly after an object traversed by a broad proton beam, spatial 2D distributions are calculated of the energy deposition in, and the number of protons entering the detector. Their ratio relates to the 2D distribution of the average stopping power of protons in the detector. The system response is calibrated against the residual range in water of the protons to provide the 2D distribution of the WEPL of the object. The WEPL distribution is converted into the distribution of the RSP of the object. Simulations have been done, where the system has been tested on 13 samples of homogeneous materials of which the RSPs have been calculated and compared with RSPs determined from simulations of residual-range-in-water, which we refer to as reference RSPs. ResultsFor both human-tissue- and non-human-tissue-equivalent materials, the RSPs derived with the detector agree with the reference values within 1%. ConclusionThe study shows that a pRG system based on one thin pixelated detection screen has the potential to provide RSP predictions with an accuracy of 1%.

The INFN proton computed tomography system for relative stopping power measurements: calibration and verification

Objective. This paper describes the procedure to calibrate the three-dimensional (3D) proton stopping power relative to water (SPR) maps measured by the proton computed tomography (pCT) apparatus of the Istituto Nazionale di Fisica Nucleare (INFN, Italy). Measurements performed on water phantoms are used to validate the method. The calibration allowed for achieving measurement accuracy and reproducibility to levels below 1%. Approach. The INFN pCT system is made of a silicon tracker for proton trajectory determination followed by a YAG:Ce calorimeter for energy measurement. To perform the calibration, the apparatus has been exposed to protons of energies ranging from 83 to 210 MeV. Using the tracker, a position-dependent calibration has been implemented to keep the energy response uniform across the calorimeter. Moreover, correction algorithms have been developed to reconstruct the proton energy when this is shared in more than one crystal and to consider the energy loss in the non-uniform apparatus material. To verify the calibration and its reproducibility, water phantoms have been imaged with the pCT system during two data-taking sessions. Main results. The energy resolution of the pCT calorimeter resulted to be at 196.5 MeV. The average values of the water SPR in fiducial volumes of the control phantoms have been calculated to be 0.995±0.002. The image non-uniformities were below 1%. No appreciable variation of the SPR and uniformity values between the two data-taking sessions could be identified. Significance. This work demonstrates the accuracy and reproducibility of the calibration of the INFN pCT system at a level below 1%. Moreover, the uniformity of the energy response keeps the image artifacts at a low level even in the presence of calorimeter segmentation and tracker material non-uniformities. The implemented calibration technique allows the INFN-pCT system to face applications where the precision of the SPR 3D maps is of paramount importance.

Detection of range shifts in proton beam therapy using the J-PET scanner: a patient simulation study

Objective. The Jagiellonian positron emission tomography (J-PET) technology, based on plastic scintillators, has been proposed as a cost effective tool for detecting range deviations during proton therapy. This study investigates the feasibility of using J-PET for range monitoring by means of a detailed Monte Carlo simulation study of 95 patients who underwent proton therapy at the Cyclotron Centre Bronowice (CCB) in Krakow, Poland. Approach. Discrepancies between prescribed and delivered treatments were artificially introduced in the simulations by means of shifts in patient positioning and in the Hounsfield unit to the relative proton stopping power calibration curve. A dual-layer, cylindrical J-PET geometry was simulated in an in-room monitoring scenario and a triple-layer, dual-head geometry in an in-beam protocol. The distribution of range shifts in reconstructed PET activity was visualized in the beam’s eye view. Linear prediction models were constructed from all patients in the cohort, using the mean shift in reconstructed PET activity as a predictor of the mean proton range deviation. Main results. Maps of deviations in the range of reconstructed PET distributions showed agreement with those of deviations in dose range in most patients. The linear prediction model showed a good fit, with coefficient of determination r 2 = 0.84 (in-room) and 0.75 (in-beam). Residual standard error was below 1 mm: 0.33 mm (in-room) and 0.23 mm (in-beam). Significance. The precision of the proposed prediction models shows the sensitivity of the proposed J-PET scanners to shifts in proton range for a wide range of clinical treatment plans. Furthermore, it motivates the use of such models as a tool for predicting proton range deviations and opens up new prospects for investigations into the use of intra-treatment PET images for predicting clinical metrics that aid in the assessment of the quality of delivered treatment.

Multi-stage image registration based on list-mode proton radiographies for small animal proton irradiation: A simulation study

We present a multi-stage and multi-resolution deformable image registration framework for image-guidance at a small animal proton irradiation platform. The framework is based on list-mode proton radiographies acquired at different angles, which are used to deform a 3D treatment planning CT relying on normalized mutual information (NMI) or root mean square error (RMSE) in the projection domain.We utilized a mouse X-ray micro-CT expressed in relative stopping power (RSP), and obtained Monte Carlo simulations of proton images in list-mode for three different treatment sites (brain, head and neck, lung). Rigid transformations and controlled artificial deformation were applied to mimic position misalignments, weight loss and breathing changes. Results were evaluated based on the residual RMSE of RSP in the image domain including the comparison of extracted local features, i.e. between the reference micro-CT and the one transformed taking into account the calculated deformation.The residual RMSE of the RSP showed that the accuracy of the registration framework is promising for compensating rigid (>97% accuracy) and non-rigid (∼95% accuracy) transformations with respect to a conventional 3D-3D registration. Results showed that the registration accuracy is degraded when considering the realistic detector performance and NMI as a metric, whereas the RMSE in projection domain is rather insensitive.This work demonstrates the pre-clinical feasibility of the registration framework on different treatment sites and its use for small animal imaging with a realistic detector. Further computational optimization of the framework is required to enable the use of this tool for online estimation of the deformation.

Clinical Characterization of a Table Mounted Range Shifter Board for Synchrotron-Based Intensity Modulated Proton Therapy for Pediatric Craniospinal Irradiation.

Purpose: To report our design, manufacturing, commissioning and initial clinical experience with a table-mounted range shifter board (RSB) intended to replace the machine-mounted range shifter (MRS) in a synchrotron-based pencil beam scanning (PBS) system to reduce penumbra and normal tissue dose for image-guided pediatric craniospinal irradiation (CSI). Methods: A custom RSB was designed and manufactured from a 3.5 cm thick slab of polymethyl methacrylate (PMMA) to be placed directly under patients, on top of our existing couch top. The relative linear stopping power (RLSP) of the RSB was measured using a multi-layer ionization chamber, and output constancy was measured using an ion chamber. End-to-end tests were performed using the MRS and RSB approaches using an anthropomorphic phantom and radiochromic film measurements. Cone beam CT (CBCT) and 2D planar kV X-ray image quality were compared with and without the RSB present using image quality phantoms. CSI plans were produced using MRS and RSB approaches for two retrospective pediatric patients, and the resultant normal tissue doses were compared. Results: The RLSP of the RSB was found to be 1.163 and provided computed penumbra of 6.9 mm in the phantom compared to 11.8 mm using the MRS. Phantom measurements using the RSB demonstrated errors in output constancy, range, and penumbra of 0.3%, -0.8%, and 0.6 mm, respectively. The RSB reduced mean kidney and lung dose compared to the MRS by 57.7% and 46.3%, respectively. The RSB decreased mean CBCT image intensities by 86.8 HU but did not significantly impact CBCT or kV spatial resolution providing acceptable image quality for patient setup. Conclusions: A custom RSB for pediatric proton CSI was designed, manufactured, modeled in our TPS, and found to significantly reduce lateral proton beam penumbra compared to a standard MRS while maintaining CBCT and kV image-quality and is in routine use at our center.

Experimental comparison of relative stopping power evaluation between proton CT and x-ray CT for pre-clinical proton irradiation studies of small animals

Objective. Proton therapy experiments in small animals are useful not only for pre-clinical and translational studies, but also for the development of advanced technologies for high-precision proton therapy. While treatment planning for proton therapy is currently based on the stopping power of protons relative to water (i.e. the relative stopping power (RSP)), estimated by converting the CT number into RSP (Hounsfield unit (HU)-RSP conversion) in reconstructed x-ray computed tomography (XCT) images, the HU-RSP conversion causes uncertainties in RSP, which affect the accuracy of dose simulation in patients. Proton computed tomography (pCT) has attracted a great deal of attention due to its potential to reduce RSP uncertainties in clinical treatment planning. However, as the proton energies for irradiating small animals are much lower than those used clinically, the energy dependence of RSP may negatively affect pCT-based RSP evaluation. Here, we explored whether the low-energy pCT approach provided more accurate RSPs when planning proton therapy treatment for small animals. Approach. We evaluated the RSPs of 10 water- and tissue-equivalent materials with known constituent elements based on pCT measurements conducted at 73.6 MeV, then compared them with XCT-based and calculated RSPs to investigate energy dependence and achieve more accurate RSPs for treatment planning in small animals. Main results. Despite the low proton energy, the pCT approach for RSP evaluation yields a smaller root mean square deviation (1.9%) of RSP from the theoretical prediction, compared to conventional HU-RSP conversion with XCT (6.1%). Significance. Low-energy pCT is expected to improve the accuracy of proton therapy treatment planning in pre-clinical studies of small animals if the RSP variation that can be attributed to energy dependence is identical to the variation in the clinical proton energy region.

OPTIma: a tracking solution for proton computed tomography in high proton flux environments

Currently there is a large discrepancy between the currents that are used for treatments in proton beam therapy facilities and the ultra low beam currents required for many proton CT imaging systems. Here we provide details of the OPTIma silicon strip based tracking system, which has been designed for performing proton CT imaging in conditions closer to the high proton flux environments of modern spot scanning treatment facilities. Details on the physical design, sensor testing, modelling, and track reconstruction are provided along with Monte-Carlo simulation studies of the expected performance for proton beam currents of up to 50 pA at the nozzle when using a σ= ∼10 mm spot scanning cyclotron system. Using a detailed simulation of the proposed OPTIma system, a discrepancy of less than 1% on the Relative Stopping Power is found for various tissues when embedded within a 150 mm diameter Perspex sphere. It is found that by accepting up to 7 protons per bunch it is possible to operate at cyclotron beam currents up to 5 times higher than would be possible with a single proton based readout, significantly reducing the total beam time required to produce an image, while also reducing the discrepancy between the beam currents required for treatment and those used for proton CT.

The effect of common dental fixtures on treatment planning and delivery for head and neck intensity modulated proton therapy.

Proton treatment plan perturbation by common dental fixtures such as amalgams (Am) and porcelain-fused-to-metal (PFM) crowns has, to date, been uncharacterized. Previous studies have been conducted to determine the physical effect of these materials within the beam path for single spots, but their effects on complex treatment plans and clinical anatomy have not yet been quantified. The present manuscript aims to study the effect of Am and PFM fixtures on proton treatment planning in a clinical setting. An anthropomorphic phantom with removable tongue, maxilla, and mandible modules was simulated on a clinical computed tomography (CT) scanner. Spare maxilla modules were modified to include either a 1.5mm depth central groove occlusal amalgam (Am) or a porcelain-fused-to-metal (PFM) crown, implanted on the first right molar. Modified tongue modules were 3D printed to accommodate several axial or sagittal oriented pieces of EBT-3 film. Clinically representative spot-scanning proton plans were generated in Eclipse v.15.6 using the proton convolution superposition (PCS) algorithm v.15.6.06 using a multi-field optimization (MFO) technique with the goal of delivering a uniform 54Gy dose to a clinical target volume (CTV) typical of a base-of-tongue (BoT) treatment. A typical geometric beam arrangement of two anterior oblique (AO) beams and a posterior beam was employed. Plans optimized without any material overrides were delivered to the phantom A) without implants; B) with Am fixture; or C) with PFM crown. Plans were also reoptimized and delivered with inclusion of material overrides to equate relative stopping power of the fixture with that of a previously measured result. Plans exhibit slightly greater dose weight towards AO beams. The optimizer accounted for inclusion of fixture overrides by increasing beam weights to the beam closest to the implant. Film measurements exhibited cold spots directly within the beam path through the fixture in plans with and without overridden materials. Cold spots were somewhat mitigated in plans including overridden materials in the structure set but were not entirely eliminated. Cold spots associated with Am and PFM fixtures were quantified at 17% and 14% for plans without overrides, respectively, and 11% and 9% with using Monte Carlo simulation. Compared with film measurements and Monte Carlo simulation, the treatment planning system underestimates the dose shadowing effect in plans including material overrides. Dental fixtures create a dose shadowing effect directly in line with the beam path through the material. This cold spot is partially mitigated by overriding the material to measured relative stopping powers. Due to uncertainties in modeling perturbation through the fixture, the magnitude of the cold spot is underestimated using the institutional TPS when compared to measurement and MC simulation.

Development of optimised tissue-equivalent materials for proton therapy

Objective. In proton therapy there is a need for proton optimised tissue-equivalent materials as existing phantom materials can produce large uncertainties in the determination of absorbed dose and range measurements. The aim of this work is to develop and characterise optimised tissue-equivalent materials for proton therapy. Approach. A mathematical model was developed to enable the formulation of epoxy-resin based tissue-equivalent materials that are optimised for all relevant interactions of protons with matter, as well as photon interactions, which play a role in the acquisition of CT numbers. This model developed formulations for vertebra bone- and skeletal muscle-equivalent plastic materials. The tissue equivalence of these new materials and commercial bone- and muscle-equivalent plastic materials were theoretical compared against biological tissue compositions. The new materials were manufactured and characterised by their mass density, relative stopping power (RSP) measurements, and CT scans to evaluate their tissue-equivalence. Main results. Results showed that existing tissue-equivalent materials can produce large uncertainties in proton therapy dosimetry. In particular commercial bone materials showed to have a relative difference up to 8% for range. On the contrary, the best optimised formulations were shown to mimic their target human tissues within 1%–2% for the mass density and RSP. Furthermore, their CT-predicted RSP agreed within 1%–2% of the experimental RSP, confirming their suitability as clinical phantom materials. Significance. We have developed a tool for the formulation of tissue-equivalent materials optimised for proton dosimetry. Our model has enabled the development of proton optimised tissue-equivalent materials which perform better than existing tissue-equivalent materials. These new materials will enable the advancement of clinical proton phantoms for accurate proton dosimetry.

A likelihood-based particle imaging filter using prior information.

Particle imaging can increase precision in proton and ion therapy. Interactions with nuclei in the imaged object increase image noise and reduce image quality, especially for multinucleon ions that can fragment, such as helium. This work proposes a particle imaging filter, referred to as the Prior Filter, based on using prior information in the form of an estimated relative stopping power (RSP) map and the principles of electromagnetic interaction, to identify particles that have undergone nuclear interaction. The particles identified as having undergone nuclear interactions are then excluded from the image reconstruction, reducing the imagenoise. The Prior Filter uses Fermi-Eyges scattering and Tschalär straggling theories to determine the likelihood that a particle only interacts electromagnetically. A threshold is then set to reject those particles with a low likelihood. The filter was evaluated and compared with a filter that estimates this likelihood based on the measured distribution of energy and scattering angle within pixels, commonly implemented as the 3σ filter. Reconstructed radiographs from simulated data of a 20-cm water cylinder and an anthropomorphic chest phantom were generated with both protons and helium ions to assess the effect of the filters on noise reduction. The simulation also allowed assessment of secondary particle removal through the particle histories. Experimental data were acquired of the Catphan CTP 404 Sensitometry phantom using the U.S. proton CT (pCT) collaboration prototype scanner. The proton and helium images were filtered with both the prior filtering method and a state-of-the-art method including an implementation of the 3σ filter. For both cases, a dE-E telescope filter, designed for this type of detector, was alsoapplied. The proton radiographs showed a small reduction in noise (1mm of water-equivalent thickness [WET]) but a larger reduction in helium radiographs (up to 5-6mm of WET) due to better secondary filtering. The proton and helium CT images reflected this, with similar noise at the center of the phantom (0.02 RSP) for the proton images and an RSP noise of 0.03 for the proposed filter and 0.06 for the 3σ filter in the helium images. Images reconstructed from data with a dose reduction, up to a factor of 9, maintained a lower noise level using the Prior Filter over the state-of-the-art filteringmethod. The proposed filter results in images with equal or reduced noise compared to those that have undergone a filtering method typical of current particle imaging studies. This work also demonstrates that the proposed filter maintains better performance against the state of the art with up to a nine-fold dose reduction.

Evaluation of Carbon Fiber and Titanium Surgical Implants for Proton and Photon Therapy

Impending and actual pathologic fractures secondary to metastatic bone disease, lymphoma, or multiple myeloma often require intramedullary fixation followed by radiation therapy. Because of carbon's low atomic number, there are reduced computed tomography (CT) imaging artifacts and dose perturbation when planning postoperative radiation for carbon fiber (CF) rods. Herein, we characterize the dosimetric properties of CF implants compared with titanium alloy (TA) for proton and photon. TA and CF samples were acquired from an implant manufacturer. Material characteristics were evaluated by CT scans with and without metal artifact reduction (MAR). Relative stopping power (RSP) was determined from the range pull-back of each sample in a 20-cm range proton beam. Photon transmission measurements were made in a solid water phantom and compared with the modeled dosimetry from the RayStation planning system. CF caused no visible CT artifacts, and MAR was not necessary for Hounsfield unit (HU) determination (median, 364 HU) or contouring, whereas TA (median, 3071 HU) caused substantial artifacts, which were improved, but not eliminated by MAR. The proton RSP was measured as 3.204 for TA and 1.414 for CF. For 6 MV photons, the measured transmission was 89.3% for TA and 98% for CF. CF RSP calculation and transmission from CT HU showed a physical density overestimate compared with measurements, which would cause a slight, but acceptable, dose uncertainty (<10% proton range or 1% photon transmission). With a density similar to bone, CF implants did not cause imaging artifacts and minimal dose perturbation compared with TA. Although the CF proton RSP is underestimated and the photon attenuation is overestimated by the HU, both effects are relatively small and may be most easily accounted for by planning with a 2-mm expansion around organs at risk beyond or in close proximity to the implant.

Novel ion imaging concept based on time-of-flight measurements with low gain avalanche detectors

Treatment planning in ion beam therapy requires accurate knowledge of the relative stopping power (RSP) distribution within the patient. Currently, RSP maps are obtained via conventional x-ray computed tomography (CT) by converting the measured attenuation coefficients of photons into RSP values for ions. Alternatively, to avoid conversion errors that are inherent to this method, ion computed tomography (iCT) can be used since it allows determining the RSP directly. In typical iCT systems, which usually consist of a tracking system and a separate residual energy detector, the RSP is obtained by measuring the particle trajectory and the corresponding water equivalent path length (WEPL) of single ions travelling through the patient. In this work, we explore a novel iCT approach which does not require a residual energy detector. Instead, the WEPL is estimated indirectly by determining the change in time of flight (TOF) due to the energy loss along the ion’s path. For this purpose, we have created a Geant4 model of a TOF-iCT system based on low gain avalanche detectors (LGADs), which are fast 4D-tracking detectors that can measure the time of arrival and position of individual particles with high spatial and time precision. To assess the performance of this TOF-iCT concept, we determined the RSP resolution and accuracy for different system settings using the Catphan® CTP404 sensitometry phantom. Within the set of investigated system parameters, the lower limit of the RSP accuracy was found at 0.91%, demonstrating the proof-of-principle of this novel TOF-iCT concept. The main advantage of using this approach is that it could potentially facilitate clinical integration due to its compact design, which, however, requires experimental verification and an improvement of the current WEPL calibration procedure.

Evaluation of water equivalent thicknesses using the IEM-CSIC scanner prototype

Proton imaging has been proposed as an alternative low-dose imaging technique for acquiring relative stopping power (RSP) maps and for patient positioning. In proton therapy and hadrontherapy, the water equivalent thickness (WET) of a material represents its radiological thickness, however, its calculation requires computationally intensive methods or approximate solutions. At IEM-CSIC, we have developed a scanner prototype for imaging with protons composed of two double-sided silicon strip detectors (DSSSD) and fast scintillators with high energy resolution, detectors that are commonly used in Experimental Nuclear Physics. Two custom-made samples of aluminum, air, and polymethyl methacrylate (PMMA) were imaged and their proton radiography images (pRads) are presented in terms of WET. Pattern reproduction, spatial resolution, and sensitivity to di↵erent materials were studied. We present pRads of our samples obtained using a rather traditional approach to compute WET values. All three materials of the studied samples, aluminum, PMMA, and air, were well distinguished; the resulting WET values were in good agreement between samples.

Improvement of proton beam range uncertainty in breast treatment using tissue samples

Objective. Proton therapy after breast-conserving surgery (BCS) can substantially reduce the dose to lung and cardiac structures. However, these dosimetric benefits are subject to beam range uncertainty in patient. The conversion of the CT-Hounsfield unit (HU) into relative stopping power (RSP) is the primary contribution to range uncertainty. Hence, an accurate HU-RSP conversion is essential. Approach. Real tissue samples, including muscle and adipose, were prepared. The water equivalent path length (WEPL) of these samples was measured under homogeneous conditions using a 12-diode detector array of our time-resolved in vivo range verification system (IRVS). The HU-RSP conversion was improved using the measured WEPL and HU for adipose tissue. The measured WEPL values were compared with the treatment planning calculation results based on the stoichiometric CT-HU calibration technique. The effect was investigated for both with and without adipose tissue in HU-RSP conversion. Main results. The IRVS was calibrated based on the solid water phantom. The relative differences in WEPL (RSP) between measurements and calculations for muscle, adipose, and water was −1.19% (−0.75%), −4.25%(−4%), and −0.23%(−0.07%), respectively. Based on the improved HU-RSP conversion, the relative differences in WEPL was reduced to −0.97%(−0.62%), −1.50%(−1.46%), and −0.22% (0.00%), respectively. Significance. The WEPL deviation of adipose tissue is larger than the testing limit of 3.5% for beam range robustness in current clinical practice. However, the improved HU-RSP conversion reduced this deviation. The main component of breast tissue is adipose. Hence, the proton treatment of BCS can be undershooting if no proper measures are taken against this specific uncertainty.

A Proton Computed Tomography Demonstrator for Stopping Power Measurements

Particle therapy is an established method to treat deep-seated tumours using accelerator-produced ion beams. For treatment planning, the precise knowledge of the relative stopping power (RSP) within the patient is vital. Conversion errors from x-ray computed tomography (CT) measurements to RSP introduce uncertainties in the applied dose distribution. Using a proton computed tomography (pCT) system to measure the SP directly could potentially increase the accuracy of treatment planning. A pCT demonstrator, consisting of double-sided silicon strip detectors (DSSD) as tracker and plastic scintillator slabs coupled to silicon photomultipliers (SiPM) as a range telescope, was developed. After a significant hardware upgrade of the range telescope, a 3D tomogram of an aluminium stair phantom was recorded at the MedAustron facility in Wiener Neustadt, Austria. In total, 80 projections with 6.5 × 105 primary events were acquired and used for the reconstruction of the RSP distribution in the phantom. After applying a straight-line approximation for the particle path inside the phantom, the most probable value (MPV) of the RSP distribution could be measured with an accuracy of 0.59%. The RSP resolution inside the phantom was only 9.3% due to a limited amount of projections and measured events per projection.

Quantitative and qualitative evaluation of density resolution of pCT images using Cirs062m phantom simulation in Geant4

Accuracy in the treatment planning (TP) of proton therapy (PT) depends on the precision of the information employed to calculate the relative stopping power (RSP) of the tissues in the patient's body. This information is obtained from the x-ray computed tomography (xCT) images and the calibration curve needed to convert the Hounsfield unit to RSP values. Using xCT bears errors in proton range estimation and dose calculation of TP. Proton computed tomography (pCT) can decrease the inherent uncertainty in PT by directly measuring RSP. Though by using pCT in the integral mode of energy loss, this error is removed, and the RSP map of tissues is calculated directly. This study simulated a modern pCT imaging system with particle-by-particle tracking capability using Geant4 Monte Carlo code. This simulation sought to improve the density resolution of tissue images without increasing the dose. The CIRS062M standard phantom was irradiated with 300 MeV protons, and the energy, position, and direction of the particle were recorded before and after the phantom. The matrix of the RSP image was modified using the normalization algorithm of the amount of integral energy loss and the correction of the angles of the projections.

Assessment of quantitative information for radiation therapy at a first-generation clinical photon-counting computed tomography scanner.

As one of the latest developments in X-ray computed tomography (CT), photon-counting technology allows spectral detection, demonstrating considerable advantages as compared to conventional CT. In this study, we investigated the use of a first-generation clinical photon-counting computed tomography (PCCT) scanner and estimated proton relative (to water) stopping power (RSP) of tissue-equivalent materials from virtual monoenergetic reconstructions provided by the scanner. A set of calibration and evaluation tissue-equivalent inserts were scanned at 120 kVp. Maps of relative electron density (RED) and effective atomic number (EAN) were estimated from the reconstructed virtual monoenergetic images (VMI) using an approach previously applied to a spectral CT scanner with dual-layer detector technology, which allows direct calculation of RSP using the Bethe-Bloch formula. The accuracy of RED, EAN, and RSP was evaluated by root-mean-square errors (RMSE) averaged over the phantom inserts. The reference RSP values were obtained experimentally using a water column in an ion beam. For RED and EAN, the reference values were calculated based on the mass density and the chemical composition of the inserts. Different combinations of low- and high-energy VMIs were investigated in this study, ranging from 40 to 190 keV. The overall lowest error was achieved using VMIs at 60 and 180 keV, with an RSP accuracy of 1.27% and 0.71% for the calibration and the evaluation phantom, respectively.

Relative stopping power resolution in time-of-flight proton CT

Objective. Proton computed tomography (CT) is similar to x-ray CT but relies on protons rather than photons to form an image. In its most common operation mode, the measured quantity is the amount of energy that a proton has lost while traversing the imaged object from which a relative stopping power map can be obtained via tomographic reconstruction. To this end, a calorimeter which measures the energy deposited by protons downstream of the scanned object has been studied or implemented as energy detector in several proton CT prototypes. An alternative method is to measure the proton’s residual velocity and thus its kinetic energy via the time of flight (TOF) between at least two sensor planes. In this work, we study the RSP resolution, seen as image noise, which can be expected from TOF proton CT systems. Approach. We rely on physics models on the one hand and statistical models of the relevant uncertainties on the other to derive closed form expressions for the noise in projection images. The TOF measurement error scales with the distance between the TOF sensor planes and is reported as velocity error in ps/m. We use variance reconstruction to obtain noise maps of a water cylinder phantom given the scanner characteristics and additionally reconstruct noise maps for a calorimeter-based proton CT system as reference. We use Monte Carlo simulations to verify our model and to estimate the noise due to multiple Coulomb scattering inside the object. We also provide a comparison of TOF helium and proton CT. Main results. We find that TOF proton CT with 30 ps m−1 velocity error reaches similar image noise as a calorimeter-based proton CT system with 1% energy error (1 sigma error). A TOF proton CT system with a 50 ps m−1 velocity error produces slightly less noise than a 2% calorimeter system. Noise in a reconstructed TOF proton CT image is spatially inhomogeneous with a marked increase towards the object periphery. Our modelled noise was consistent with Monte Carlo simulated images. TOF helium CT offers lower RSP noise at equal fluence, but is less advantageous at equal imaging dose. Significance. This systematic study of image noise in TOF proton CT can serve as a guide for future developments of this alternative solution for estimating the residual energy of protons and helium ions after the scanned object.

Impact of beam-hardening corrections on proton relative stopping power estimates from single- and dual-energy CT.

A major contributing factor to proton range uncertainty is the conversion of computed tomography (CT) Hounsfield units (HU) to proton relative stopping power (RSP). This uncertainty is heightened in the presence of X‐ray beam‐hardening artifact (BHA), which has two manifestations: cupping and streaking, especially in and near bone tissue. This uncertainty can affect the accuracy of proton RSP calculation for treatment planning in proton radiotherapy. Dual‐energy CT (DECT) and iterative beam‐hardening correction (iBHC) both show promise in mitigating CT BHA. This present work attempts to analyze the relative robustness of iBHC and DECT techniques on both manifestations of BHA.The stoichiometric method for HU to RSP conversion was used for single‐energy CT (SECT) and DECT‐based monochromatic techniques using a tissue substitute phantom. Cupping BHA was simulated by measuring the HU of a bone substitute plug in wax/3D‐printed phantoms of increasing size. Streaking BHA was simulated by placing a solid water plug between two bone plugs in a wax phantom. Finally, the effect of varying calibration phantom size on RSP was calculated in an anthropomorphic head phantom.The RSP decreased −0.002 cm–1 as phantom size increased for SECT but remained largely constant when iBHC applied or with DECT techniques. The RSP varied a maximum of 2.60% in the presence of streaking BHA in SECT but was reduced to 1.40% with iBHC. For DECT techniques, the maximum difference was 2.40%, reduced to 0.6% with iBHC. Comparing calibration phantoms of 20‐ and 33‐cm diameter, maximum voxel differences of 5 mm in the water‐equivalent thickness were observed in the skull but reduced to 1.3 mm with iBHC. The DECT techniques excelled in mitigating cupping BHA, but streaking BHA still could be observed. The use of iBHC reduced RSP variation with BHA in both SECT and DECT techniques.

Optimal Configuration of Proton-Therapy Accelerators for Relative-Stopping-Power Resolution in Proton Computed Tomography

The determination of relative stopping power (RSP) via proton computed tomography (pCT) of a patient is dependent in part on the knowledge of the incoming proton kinetic energies; the uncertainty in these energies is in turn determined by the proton source---typically a cyclotron. Here, we show that reducing the incident proton beam energy spread may significantly improve RSP determination in pCT. We demonstrate that the reduction of beam energy spread from the typical 1.0% (at 70 MeV) down to 0.2% can be achieved at the proton currents needed for imaging at the Paul Scherrer Institut 250-MeV cyclotron. Through a simulated pCT imaging system, we find that this effect results in RSP resolutions as low as 0.2% for materials such as cortical bone, up to 1% for lung tissue. Several materials offer further improvement when the beam (residual) energy is also chosen such that the detection mechanisms used provide the optimal RSP resolution.