Technical Note: SpekPy v2.0-a software toolkit for modeling x-ray tube spectra

Feasibility of proton dosimetry overriding planning CT with daily CBCT elaborated through generative artificial intelligence tools

Radiotherapy commonly utilizes cone beam computed tomography (CBCT) for patient positioning and treatment monitoring. CBCT is deemed to be secure for patients, making it suitable for the delivery of fractional doses. However, limitations such as a narrow field of view, beam hardening, scattered radiation artifacts, and variability in pixel intensity hinder the direct use of raw CBCT for dose recalculation during treatment. To address this issue, reliable correction techniques are necessary to remove artifacts and remap pixel intensity into Hounsfield Units (HU) values. This study proposes a deep-learning framework for calibrating CBCT images acquired with narrow field of view (FOV) systems and demonstrates its potential use in proton treatment planning updates. Cycle-consistent generative adversarial networks (cGAN) processes raw CBCT to reduce scatter and remap HU. Monte Carlo simulation is used to generate CBCT scans, enabling the possibility to focus solely on the algorithm's ability to reduce artifacts and cupping effects without considering intra-patient longitudinal variability and producing a fair comparison between planning CT (pCT) and calibrated CBCT dosimetry. To showcase the viability of the approach using real-world data, experiments were also conducted using real CBCT. Tests were performed on a publicly available dataset of 40 patients who received ablative radiation therapy for pancreatic cancer. The simulated CBCT calibration led to a difference in proton dosimetry of less than 2%, compared to the planning CT. The potential toxicity effect on the organs at risk decreased from about 50% (uncalibrated) up the 2% (calibrated). The gamma pass rate at 3%/2 mm produced an improvement of about 37% in replicating the prescribed dose before and after calibration (53.78% vs 90.26%). Real data also confirmed this with slightly inferior performances for the same criteria (65.36% vs 87.20%). These results may confirm that generative artificial intelligence brings the use of narrow FOV CBCT scans incrementally closer to clinical translation in proton therapy planning updates.

Determination of kilovoltage x-ray therapy depth doses with open-ended applicators

The purpose of this work was to determine percentage depth dose (PDD) curves for kilovoltage x-rays from the WOmed-T105 unit, with open-ended steel applicators and beam qualities ranging from 0.5 to 4.2 mm Al. Measurements were made with parallel plate chambers in a water phantom, with extrapolation based on a fifth order polynomial used to estimate the surface dose. Measurements were also made with parallel plate chambers in a plastic water phantom, with thin plastic sheets used to obtain detailed measurements at shallow depths (less than 1 mm). Monte Carlo simulations were performed using the EGSnrc package, with two different sources as input: a SpekPy simulation of the x-ray beam and a full simulation of the x-ray tube, treatment head and applicators. Results showed that all four methods (two measurements and two simulations) agreed within the measurement uncertainty at depths greater than 2 mm. At shallow depths, significant differences were noted. At depths less than 0.1 mm, the full Monte Carlo simulation and the solid water measurements showed a sharp spike in surface dose which is attributed to electron contamination, which was not seen in the SpekPy Monte Carlo simulation or the extrapolated water measurements. At depths between 0.1 mm and 2 mm, beyond the range of contaminant electrons, the extrapolated water measurements underestimate the dose by up to 13% compared to the full Monte Carlo simulation and the solid water measurements, attributed to fluorescent photons generated in the applicators. This work demonstrates that for open-ended applicators, measurement of depth doses in water with extrapolation of surface dose has the potential to significantly underestimate the dose at shallow depths between the surface and 2 mm, even after eliminating electron contamination from the beam.

Extension of the cone-beam CT field-of-view using two complementary short scans

BACKGROUND

Robotic C-arm cone-beam computed tomography (CBCT) scanners provide fast in-room imaging in radiotherapy. Their mobility extends beyond performing a gantry rotation, but they might encounter obstructions to their motion which limit the gantry angle range. The axial field-of-view (FOV) of a reconstructed CBCT image depends on the acquisition geometry. When imaging a large anatomical location, such as the thorax, abdomen, or pelvis, a centered cone beam might be insufficient to acquire untruncated projection images. Some CBCT scanners can laterally displace their detector and collimate the beam to increase the FOV, but the gantry must then perform a360 ∘ $360^{\circ}$ rotation to provide complete data for reconstruction.

PURPOSE

To extend the FOV of a CBCT image with a single short scan (gantry angle range of180 ∘ + $180^{\circ}+$ fan angle) using two complementary short scans.

METHODS

We defined an acquisition protocol using two short scans during which the source follows the same trajectory and where the detector has equal and opposite tilt and/or offset between the two scans, which we refer to as complementary scans. We created virtual acquisitions using a Monte Carlo simulator on a digital anthropomorphic phantom and on a computed tomography (CT) scan of a patient abdomen. For our proposed method, each simulation produced two complementary sets of projections, which were weighted for redundancies and used to reconstruct one CBCT image. We compared the resulting images to the ground truth phantoms and simulations of conventional scans.

RESULTS

Reconstruction artifacts were slightly more prominent in the complementary scans w.r.t. a complete scan with untruncated projections but matched those in a single short scan without truncation. When analyzing reconstructed scans from simulated projections with scatter and corrected with prior CT information, we found a global agreement between complementary and conventional scan approaches.

CONCLUSIONS

When dealing with a limited range of motion of the gantry of a CBCT scanner, two complementary short scans are a technically valid alternative to a full 360∘ $^{\circ}$ scan with equal FOV. This approach enables FOV extension without collisions or hardware upgrades.

Implementation of a new EGSnrc particle source class for computed tomography: validation and uncertainty quantification

Objective. Personalized dose monitoring and risk management are of increasing significance with the growing number of computer tomography (CT) examinations. These require high-quality Monte Carlo (MC) simulations that are of the utmost importance for the new developments in personalized CT dosimetry. This work aims to extend the MC framework EGSnrc source code with a new particle source. This, in turn, allows CT-scanner-specific dose and image calculations for any CT scanner. The novel method can be used with all modern EGSnrc user codes, particularly for the simulation of the effective dose based on DICOM images and the calculation of CT images.Approach. The new particle source can be used with input data derived by the user. The input data can be generated by the user based on a previously developed method for the experimental characterization of any CT scanner (doi.org/10.1016/j.ejmp.2015.09.006). Furthermore, the new particle source was benchmarked by air kerma measurements in an ionization chamber at a clinical CT scanner. For this, the simulated angular distribution and attenuation characteristics were compared to measurements to verify the source output free in air. In a second validation step, simulations of air kerma in a homogenous cylindrical and an anthropomorphic thorax phantom were performed and validated against experimentally determined results. A detailed uncertainty evaluation of the simulated air kerma values was developed.Main results. We successfully implemented a new particle source class for the simulation of realistic CT scans. This method can be adapted to any CT scanner. For the attenuation characteristics, there was a maximal deviation of 6.86% between the measurement and the simulation. The mean deviation for all tube voltages was 2.36% (σ= 1.6%). For the phantom measurements and simulations, all the values agreed within 5.0%. The uncertainty evaluation resulted in an uncertainty of 5.5% (k=1).

Dosimetric validation of SmART-RAD Monte Carlo modelling for x-ray cabinet radiobiology irradiators

Objective. Accuracy and reproducibility in the measurement of radiation dose and associated reporting are critically important for the validity of basic and preclinical radiobiological studies performed with kilovolt x-ray radiation cabinets. This is essential to enable results of radiobiological studies to be repeated, as well as enable valid comparisons between laboratories. In addition, the commonly used single point dose value hides the 3D dose heterogeneity across the irradiated sample. This is particularly true for preclinical rodent models, and is generally difficult to measure directly. Radiation transport simulations integrated in an easy to use application could help researchers improve quality of dosimetry and reporting.Approach. This paper describes the use and dosimetric validation of a newly-developed Monte Carlo (MC) tool, SmART-RAD, to simulate the x-ray field in a range of standard commercial x-ray cabinet irradiators used for preclinical irradiations. Comparisons are made between simulated and experimentally determined dose distributions for a range of configurations to assess the potential use of this tool in determining dose distributions through samples, based on more readily available air-kerma calibration point measurements.Main results. Simulations gave very good dosimetric agreement with measured depth dose distributions in phantoms containing both water and bone equivalent materials. Good spatial and dosimetric agreement between simulated and measured dose distributions was obtained when using beam-shaping shielding.Significance. The MC simulations provided by SmART-RAD provide a useful tool to go from a limited number of dosimetry measurements to detailed 3D dose distributions through a non-homogeneous irradiated sample. This is particularly important when trying to determine the dose distribution in more complex geometries. The use of such a tool can improve reproducibility and dosimetry reporting in preclinical radiobiological research.

On the robustness of detective quantum efficiency within the limits of IEC 61267 RQA standard radiation qualities

IEC 61267 allows a certain leeway regarding the establishment of radiation qualities in order to enable the use of X-ray tubes having different anode angles and inherent filtrations. This allowance has a direct impact on the calculation of the detective quantum efficiency and may potentially complicate any comparison of different imaging detectors based on this quantity. This work investigates this effect by applying computational methods. To this end, an algorithm was implemented to calculate the variation of the squared signal-to-noise ratio per air kerma for RQA standard radiation qualities and to deduce corresponding uncertainties based on GUM Supplement 2. For RQA standard radiation qualities, the results show standard uncertainties for the squared signal-to-noise ratio per air kerma of between 0.05 and 2.1%. Comparing imaging detectors based on detective quantum efficiency is associated with substantial uncertainty for some radiation qualities. This is due to the different photon fluences with respect to energy that are allowed by IEC 61267 for identical standard radiation qualities.

Optimizing dual-energy CT technique for iodine-based contrast-to-noise ratio, a theoretical study

BACKGROUND

Dual-energy CT (DECT) systems provide valuable material-specific information by simultaneously acquiring two spectral measurements, resulting in superior image quality and contrast-to-noise ratio (CNR) while reducing radiation exposure and contrast agent usage. The selection of DECT scan parameters, including x-ray tube settings and fluence, is critical for the stability of the reconstruction process and hence the overall image quality.

PURPOSE

The goal of this study is to propose a systematic theoretical method for determining the optimal DECT parameters for minimal noise and maximum CNR in virtual monochromatic images (VMIs) for fixed subject size and total radiation dose.

METHODS

The noise propagation in the process of projection based material estimation from DECT measurements is analyzed. The main components of the study are the mean pixel variances for the sinogram and monochromatic image and the CNR, which were shown to depend on the Jacobian matrix of the sinograms-to-DECT measurements map. Analytic estimates for the mean sinogram and monochromatic image pixel variances and the CNR as functions of tube potentials, fluence, and VMI energy are derived, and then used in a virtual phantom experiment as an objective function for optimizing the tube settings and VMI energy to minimize the image noise and maximize the CNR.

RESULTS

It was shown that DECT measurements corresponding to kV settings that maximize the square of Jacobian determinant values over a domain of interest lead to improved stability of basis material reconstructions. Instances of non-uniqueness in DECT were addressed, focusing on scenarios where the Jacobian determinant becomes zero within the domain of interest despite significant spectral separation. The presence of non-uniqueness can lead to singular solutions during the inversion of sinograms-to-DECT measurements, underscoring the importance of considering uniqueness properties in parameter selection. Additionally, the optimal VMI energy and tube potentials for maximal CNR was determined. When the x-ray beam filter material was fixed at 2 mm of aluminum and the photon fluence for low and high kV scans were considered equal, the tube potential pair of 60/120 kV led to the maximal iodine CNR in the VMI at 53 keV.

CONCLUSIONS

Optimizing DECT scan parameters to maximize the CNR can be done in a systematic way. Also, choosing the parameters that maximize the Jacobian determinant over the set of expected line integrals leads to more stable reconstructions due to the reduced amplification of the measurement noise. Since the values of the Jacobian determinant depend strongly on the imaging task, careful consideration of all of the relevant factors is needed when implementing the proposed framework.

Dual energy CT reconstruction using the constrained one step spectral image reconstruction algorithm

BACKGROUND

The constrained one-step spectral CT Image Reconstruction method (cOSSCIR) has been developed to estimate basis material maps directly from spectral CT data using a model of the polyenergetic x-ray transmissions and incorporating convex constraints into the inversion problem. This 'one-step' approach has been shown to stabilize the inversion in the case of photon-counting CT, and may provide similar benefits to dual-kV systems that utilize integrating detectors. Since the approach does not require the same rays be acquired for every spectral measurement, cOSSCIR can apply to dual energy protocols and systems used clinically, such as fast and slow kV switching systems and dual source scanning.

PURPOSE

The purpose of this study is to investigate the use of cOSSCIR applied to dual-kV data, using both registered and unregistered spectral acquisitions, specifically slow and fast kV switching imaging protocols. For this application, cOSSCIR is investigated using inverse crime simulations and dual-kV experiments. This study is the first demonstration of cOSSCIR on the dual-kV reconstruction problem.

METHODS

An integrating detector model was developed for the purpose of reconstructing dual-kV data, and an inverse crime study was used to validate the detector model within the cOSSCIR framework using a simulated pelvic phantom. Experiments were also used to evaluate cOSSCIR on the dual energy problem. Dual-kV data was obtained from a physical phantom containing analogs of adipose, bone, and liver tissues, with the aim of recovering the material coefficients in the bone and adipose basis material maps. cOSSCIR was applied to acquisitions where all rays performed both spectral measurements (registered) and fast and slow kV switching acquisitions (unregistered). cOSSCIR was also compared to two image-domain decomposition approaches, where image-domain methods are the conventional approach for decomposing unregistered spectral data.

RESULTS

Simulations demonstrate the application of cOSSCIR to the dual-kV inversion problem by successfully recovering the material basis maps on ideal data, while further showing that unregistered data presents a more challenging inversion problem. In our experimental reconstructions, the recovered basis material coefficient errors were found to be less than 6.5% in the bone, adipose, and liver regions for both registered and unregistered protocols. Similarly, the errors were less than 4% in the 50 keV virtual mono-energetic images, and the recovered material decomposition vectors nearly overlap their corresponding ground-truth vectors. Additionally, a preliminary two material decomposition study of iodine quantification recovered an average concentration of 9.2 mg/mL from a 10 mg/mL experimental iodine analog.

CONCLUSIONS

Using our integrating detector and spectral models, cOSCCIR is capable of accurately recovering material basis maps from dual-kV data for both registered and unregistered data. The material decomposition quantification compare favorably to the image domain approaches, and our results were not affected by the imaging protocol. Our results also suggest the extension of cOSSCIR to iodine quantification using two material decomposition.

X-Ray Dark-Field Signal Reduction Due to Hardening of the Visibility Spectrum

X-ray dark-field imaging enables a spatially-resolved visualization of ultra-small-angle X-ray scattering. Using phantom measurements, we demonstrate that a material's effective dark-field signal may be reduced by modification of the visibility spectrum by other dark-field-active objects in the beam. This is the dark-field equivalent of conventional beam-hardening, and is distinct from related, known effects, where the dark-field signal is modified by attenuation or phase shifts. We present a theoretical model for this group of effects and verify it by comparison to the measurements. These findings have significant implications for the interpretation of dark-field signal strength in polychromatic measurements.

Technical note: SpekPy Web-online x-ray spectrum calculations using an interface to the SpekPy toolkit

Knowledge of the photon spectrum emitted from an x-ray tube is frequently needed in imaging and dosimetry contexts. As the spectrum characteristics are influenced by several parameters and routine measurement of a spectrum is often impractical, a variety of software programs have been developed over the decades for convenient calculations. SpekPy is a state-of-the-art software package containing several spectrum models, and was created to estimate photon spectra originating from x-ray tubes using a small set of input parameters (e.g., anode material, anode angle, tube potential, filtration, etc.). SpekPy is distributed as a Python toolkit and is available free of charge. The toolkit does, however, lack a graphical user interface and a user is required to write a Python script to make use of it. In this work this limitation is addressed by introducing a web application called SpekPy Web: a graphical user interface together with an application programmable interface (API). These developments both make the SpekPy spectrum models accessible to a broader set of users and increases the ease of use for existing users. SpekPy Web is hosted at: https://spekpy.smile.ki.se. The functionality of the software is demonstrated, using its API, by estimating first half-value layers (HVLs) for 15 standard beam qualities from the International Bureau of Weights and Measures (BIPM). The estimated HVLs were found to all be within 3.5% agreement when compared to experimental values, with an average calculation time of 2.5 s per spectrum. half-value-layer, software, x-ray spectrum.

Material decomposition with a prototype photon-counting detector CT system: expanding a stoichiometric dual-energy CT method via energy bin optimization and K-edge imaging

Objective.Computed tomography (CT) has advanced since its inception, with breakthroughs such as dual-energy CT (DECT), which extracts additional information by acquiring two sets of data at different energies. As high-flux photon-counting detectors (PCDs) become available, PCD-CT is also becoming a reality. PCD-CT can acquire multi-energy data sets in a single scan by spectrally binning the incident x-ray beam. With this, K-edge imaging becomes possible, allowing high atomic number (high-Z) contrast materials to be distinguished and quantified. In this study, we demonstrated that DECT methods can be converted to PCD-CT systems by extending the method of Bourqueet al(2014). We optimized the energy bins of the PCD for this purpose and expanded the capabilities by employing K-edge subtraction imaging to separate a high-atomic number contrast material.Approach.The method decomposes materials into their effective atomic number (Zeff) and electron density relative to water (ρe). The model was calibrated and evaluated using tissue-equivalent materials from the RMI Gammex electron density phantom with knownρevalues and elemental compositions. TheoreticalZeffvalues were found for the appropriate energy ranges using the elemental composition of the materials.Zeffvaried slightly with energy but was considered a systematic error. Anex vivobovine tissue sample was decomposed to evaluate the model further and was injected with gold chloride to demonstrate the separation of a K-edge contrast agent.Main results.The mean root mean squared percent errors on the extractedZeffandρefor PCD-CT were 0.76% and 0.72%, respectively and 1.77% and 1.98% for DECT. The tissue types in theex vivobovine tissue sample were also correctly identified after decomposition. Additionally, gold chloride was separated from theex vivotissue sample with K-edge imaging.Significance.PCD-CT offers the ability to employ DECT material decomposition methods, along with providing additional capabilities such as K-edge imaging.

A simulation study on the radiosensitization properties of gold nanorods

Objective. Gold nanorods (GNRs) have emerged as versatile nanoparticles with unique properties, holding promise in various modalities of cancer treatment through drug delivery and photothermal therapy. In the rapidly evolving field of nanoparticle radiosensitization (NPRS) for cancer therapy, this study assessed the potential of gold nanorods as radiosensitizing agents by quantifying the key features of NPRS, such as secondary electron emission and dose enhancement, using Monte Carlo simulations.Approach. Employing the TOPAS track structure code, we conducted a comprehensive evaluation of the radiosensitization behavior of spherical gold nanoparticles and gold nanorods. We systematically explored the impact of nanorod geometry (in particular size and aspect ratio) and orientation on secondary electron emission and deposited energy ratio, providing validated results against previously published simulations.Main results. Our findings demonstrate that gold nanorods exhibit comparable secondary electron emission to their spherical counterparts. Notably, nanorods with smaller surface-area-to-volume ratios (SA:V) and alignment with the incident photon beam proved to be more efficient radiosensitizing agents, showing superiority in emitted electron fluence. However, in the microscale, the deposited energy ratio (DER) was not markedly influenced by the SA:V of the nanorod. Additionally, our findings revealed that the geometry of gold nanoparticles has a more significant impact on the emission of M-shell Auger electrons (with energies below 3.5 keV) than on higher-energy electrons.Significance. This research investigated the radiosensitization properties of gold nanorods, positioning them as promising alternatives to the more conventionally studied spherical gold nanoparticles in the context of cancer research. With increasing interest in multimodal cancer therapy, our findings have the potential to contribute valuable insights into the perspective of gold nanorods as effective multipurpose agents for synergistic photothermal therapy and radiotherapy. Future directions may involve exploring alternative metallic nanorods as well as further optimizing the geometry and coating materials, opening new possibilities for more effective cancer treatments.

Peripheral blood lymphocytes differ in DNA damage response after exposure to X-rays with different physical properties

Introduction: In radiology, low X-ray energies (<140 keV) are used to obtain an optimal image while in radiotherapy, higher X-ray energies (MeV) are used to eradicate tumor tissue. In radiation research, both these X-ray energies being used to extrapolate in vitro research to clinical practice. However, the energy deposition of X-rays depends on their energy spectrum, which might lead to changes in biological response. Therefore, this study compared the DNA damage response (DDR) in peripheral blood lymphocytes (PBLs) exposed to X-rays with varying beam quality, mean photon energy (MPE) and dose rate.Methods: The DDR was evaluated in peripheral blood lymphocytes (PBLs) by the ɣ-H2AX foci assay, the cytokinesis-block micronucleus assay and an SYTOX-based cell death assay, combined with specific cell death inhibitors. Cell cultures were irradiated with a 220 kV X-ray research cabinet (SARRP, X-Strahl) or a 6 MV X-ray linear accelerator (Elekta Synergy). Three main physical parameters were investigated: beam quality (V), MPE (eV) and dose rate (Gy/min). Additional copper (Cu) filtration caused variation in the MPE (78 keV, 94 keV, 118 keV) at SARRP; dose rates were varied by adjusting tube current for 220 kV X-rays (0.33-3 Gy/min) or water-phantom depth in the 6 MV set-up (3-6 Gy/min).Results: The induction of chromosomal damage and initial (30 min) DNA double-stranded breaks (DSBs) were significantly higher for 220 kV X-rays compared to 6 MV X-rays, while cell death induction was similar. Specific cell death inhibitors for apoptosis, necroptosis and ferroptosis were not capable of blocking cell death after irradiation using low or high-energy X-rays. Additional Cu filtration increased the MPE, which significantly decreased the amount of chromosomal damage and DSBs. Within the tested ranges no specific effects of dose rate variation were observed.Conclusion: The DDR in PBLs is influenced by the beam quality and MPE. This study reinforces the need for consideration and inclusion of all physical parameters in radiation-related studies.

Portable Single-Exposure Dual-Energy X-ray Detector for Improved Point-of-Care Diagnostic Imaging

INTRODUCTION

Dual-energy subtraction (DES) imaging is well known to reduce anatomical noise and enable material classification. The current approaches to DES imaging have trade-offs, such as motion artifacts, low sensitivity because of losses in a mid-filter, and lack of portability. Recently, a portable triple-layer flat-panel detector (FPD) was proposed for use in single-shot DES imaging that can provide improved sensitivity and removal of motion artifacts in a point-of-care setting. The purpose of this study is to evaluate the feasibility of such a detector. Various image quality metrics and clinical images are provided.

MATERIALS AND METHODS

An FDA-cleared single-exposure DES FPD consisting of three stacked sensors was used for all measurements. This detector generates three images per exposure: A digital radiography (DR) image, i.e., as would be produced with a conventional detector, and two DES images, bone and soft tissue. To evaluate DR image quality, detective quantum efficiency (DQE) and modulation transfer function were measured for multiple radiation quality beams. Digital radiography and DES images obtained from this FPD were evaluated in previously reported fixed and portable clinical studies. Digital radiography and DES images from case studies are presented for qualitative assessment.

RESULTS

Modulation transfer function and DQE were measured across multiple radiation quality beams for the DR image. The DES images showed good tissue separation and uniformity with no visible motion or alignment artifacts. The DES images, when read in conjunction with the DR image, resulted in increased reader confidence and revealed abnormalities or details that were sometimes overlooked in the DR image.

CONCLUSIONS

The proposed panel produces high-quality DR images as indicated by the DQE and modulation transfer function. The DES images have been shown to improve sensitivity in clinical applications and increase reader confidence. This detector can enable DES in portable or otherwise difficult applications, opening new doors for improved patient care.

Simulation study on the conversion between CT and CBCT dose quantities via the effective dose

In recent years, cone-beam computed tomography (CBCT) has been used in many imaging tasks traditionally performed by computed tomography (CT). This has created challenges for dosimetry, as the dose quantities in CBCT and CT, i.e. the dose-area product (DAP) and dose-length product (DLP), are not mutually convertible. Convertibility would be desirable to compare doses in similar clinical studies performed using CT or CBCT and ultimately for the application of diagnostic reference levels (DRLs). In this work, the conversion of the DAP into the DLP and vice versa via the effective doseEis investigated with the aim of finding common diagnostic reference levels. The dose calculation was performed using Monte Carlo simulations for scan regions with imaging tasks, which can be carried out either with CT or CBCT scanners. Four regions in the head and four in the trunk were chosen. The calculations resulted in conversion coefficientsk=DAPDLPof 30(4) cm for the cranium, 22(4) cm for the facial bones, 24(2) cm for the paranasal sinuses, 18(2) cm for the cervical spine, 78(12) cm for the thorax, 85(13) cm for the upper abdomen, 57(6) cm for the lumbar spine and 70(12) cm for the pelvis.

Monte Carlo modelling of experimental setup used for biodosimetry intercomparison

When using biodosimetry techniques to assess absorbed dose from an ionising radiation exposure, a calibration curve is required. At Health Canada (HC), these curves are generated for a variety of radiation qualities and assays to translate biological damage into absorbed dose. They are produced by irradiating biological samples in custom-designed water-equivalent phantoms inside a cabinet X-ray machine. In the HC lab, two different phantoms can be used for irradiation that differs in material composition and internal geometry. To ensure consistency, the impact of using the phantoms interchangeably was investigated. This was done through lab measurements and the development of a Monte Carlo (MC) model. Differences up to 6.7% were found between the two experimental setups, indicating the need for careful consideration if using these setups interchangeably in the laboratory. Once validated, the MC model can be used to investigate different aspects of the experimental setup without the need for laboratory measurements.

A formalism for traceable dosimetry in superficial electronic brachytherapy (eBT)

Objective. Despite the number of treatments performed with electronic brachytherapy (eBT) there is no uniform methodology for reference dosimetry for international traceability to primary dosimetry standards in different eBT systems. The objective of this study is to propose a formalism for traceability reference dosimetry in superficial eBT, that is easy to apply in the clinic. This method was investigated for an Elekta Esteya with one applicator.Approach. The calibration x-ray spectrum at the primary standards dosimetry laboratory was matched to the measured eBT photon spectrum. Subsequently, two ionization chambers of different types were calibrated at the primary standard dosimetry laboratory (PSDL) in terms of air kerma against a primary standard. The chambers were used to measure ionization chamber reading ratios in-air at different distances from the applicator. Monte Carlo based air kerma ratios were calculated at different positions from the eBT applicator as well as backscatter factors in water and average mass energy absorption ratios in water and in air. Relative measurements with radiochromic films were performed in a water phantom to determine the ratio of absorbed dose to water,Dw, at the surface toDwat 1 cm depth in water. These were compared with Monte Carlo calculations.Main results. Calculations and measurements were combined to estimate theDwat the surface and at 1 cm depth in water. Ionization chamber agreement of the surface dose was 1.7%, within an uncertainty of 6.8% (k= 2). They agreed with the manufacturer dosimetry within 1.8%, with an uncertainty of 5.0% (k= 2). The feasibility of the formalism and methodology for the Esteya system was demonstrated.Significance. This study proposes a method for harmonization of traceable reference dosimetry for eBT contact treatments which does not involve a detailed simulation of the ionization chamber. The method demonstrated feasibility for one eBT system using one surface applicator. In the future the method could be applied for different eBT systems.

A new formula for converting dose-area product to effective dose in dental cone-beam computed tomography

PURPOSE

The purpose of this study was to determine a dedicated conversion formula between dose-area product (DAP) and effective dose (E) for dental CBCT, which incorporates X-ray beam energy as well as geometric factors.

METHODS

CBCT exposures of an adult phantom were simulated using PCXMC 2.0 (STUK, Finland). Fifty-seven fields of view (FOV) were included, ranging from Ø4x4cm to Ø21x19cm. Six tube voltages (70 kV to 120 kV), eight combinations of Al (2.5 to 10 mm) and Cu (0 to 0.5 mm) filtration, and four source-isocentre distances (35 to 65 cm) were used, resulting in 10 896 simulated scan protocols. In addition, 10 944 random combinations of scan parameters within the aforementioned ranges were simulated, resulting in 21 840 scan protocols that were used for fitting a formula using multiple linear regression with 8 independent variables. Finally, 2304 random scan protocols were simulated as validation data to evaluate the formula's generalizability.

RESULTS

E/DAP ranged between 0.031 µSv/mGy.cm2 and 0.294 µSv/mGy.cm2, with a mean of 0.150 µSv/mGy.cm2. Due to extensive clustering of the E/DAP, three formulas were determined according to FOV categories (Small: <100 cm2, Medium: 100-400 cm2, Large: >400 cm2). The resulting formulas showed mean absolute errors of 7.9%, 4.0% and 3.6%, respectively, for the validation data.

CONCLUSION

The new formulas allow for a straightforward, yet accurate, estimation of the effective dose for dental CBCT based on DAP. Further fine-tuning of the model could be achieved by incorporating, for example, bowtie filtration and off-axis beam geometries.

Simulation and Optimization of Optical Fiber Irradiation with X-rays at Different Energies

We investigated the influence of modifying the voltage of an X-ray tube with a tungsten anode between 30 kV and 225 kV, and therefore its photon energy spectrum (up to 225 keV), on the Total Ionizing Dose deposited in a single-mode, phosphorus-doped optical fiber, already identified as a promising dosimeter. Simulation data, obtained using a toolchain combining SpekPy and Geant4 software, are compared to experimental results obtained on this radiosensitive optical fiber and demonstrate an increase of the deposited dose with operating voltage, at a factor of 4.5 between 30 kV and 225 kV, while keeping the same operating current of 20 mA. Analysis of simulation results shows that dose deposition in such optical fibers is mainly caused by the low-energy part of the spectrum, with 90% of the deposited energy originating from photons with an energy below 30 keV. Comparison between simulation and various experimental measurements indicates that phosphosilicate fibers are adapted for performing X-ray dosimetry at different voltages.

Dose estimation for cone-beam computed tomography in image-guided radiation therapy for pelvic cancer using adult mesh-type reference computational phantoms

The use of cone-beam computed tomography (CBCT) is expanding owing to its installation in linear accelerators for radiation therapy, and the imaging dose induced by this system has become the center of attention. Here, the dose to patients caused by the CBCT imager was investigated. Organ doses and effective doses for male and female mesh-type reference computational phantoms (MRCPs) and pelvis CBCT mode, routinely used for pelvic irradiation, were estimated using the Particle and Heavy Ion Transport Code System. The simulation results were confirmed based on the point-dose measurements. The estimated organ doses for male MRCPs with/without raised arms and for female MRCPs with/without raised arms were 0.00286-35.6 mGy, 0.00286-35.1 mGy, 0.00933-39.5 mGy, and 0.00931-39.0 mGy, respectively. The anticipated effective doses for male MRCPs with/without raised arms and female MRCPs with/without raised arms irradiated by pelvis CBCT mode were 4.25 mSv, 4.16 mSv, 7.66 mSv, and 7.48 mSv, respectively. The results of this study will be useful for patients who undergo image-guided radiotherapy with CBCT. However, because this study only covered one type of cancer with one type of imager, and image quality was not considered, more studies should be conducted to estimate the radiation dose from imaging devices in radiation therapy.

Machine characterization and central axis depth dose data of a superficial x-ray radiotherapy unit

Objectives. The purpose of this study is to present data from the clinical commissioning of an Xstrahl 150 x-ray unit used for superficial radiotherapy,Methods. Commissioning tasks included vendor acceptance tests, timer reproducibility, linearity and end-effect measurements, half-value layer (HVL) measurements, inverse square law verification, head-leakage measurements, and beam output calibration. In addition, percent depth dose (PDD) curves were determined for different combinations of filter/kV settings and applicators. Automated PDD water phantom scans were performed utilizing four contemporary detectors: a microDiamond detector, a microSilicon detector, an EDGE detector, and a PinPoint ionization chamber. The measured PDD data were compared to the published values in BJR Supplement 25,Results. The x-ray unit's mechanical, safety, and radiation characteristics were within vendor-stated specifications. Across sixty commissioned x-ray beams, the PDDs determined in water using solid state detectors were in excellent agreement with the BJR 25 data. For the lower (<100 kVp) and medium-energy (≥100 kVp) superficial beams the average agreement was within [-3.6,+0.4]% and [-3.7,+1.4]% range, respectively. For the high-energy superficial (low-energy orthovoltage) x-rays at 150 kVp, the average difference for the largest 20 × 20 cm²collimator was (-0.7 ± 1.0)%,Conclusions. This study presents machine characterization data collected for clinical use of a superficial x-ray unit. Special focus was placed on utilizing contemporary detectors and techniques for the relative PDD measurements using a motorized water phantom. The results in this study confirm that the aggregate values published in the BJR 25 report still serve as a valid benchmark when comparing data from site-specific measurements, or the reference data for clinical utilization without such measurements,Advances in knowledge. This paper presents comprehensive data from the acceptance and commissioning of a modern kilovoltage superficial x-ray radiotherapy machine. Comparisons between the PDD data measured in this study using different detectors and BJR 25 data are highlighted.

Radiation quality correction factors for improved dosimetry in preclinical minibeam radiotherapy

BACKGROUND

In reference dosimetry, radiation quality correction factors are used in order to account for changes in the detector's response among different radiation qualities, improving dosimetric accuracy.

PURPOSE

Reference dosimetry radiation quality corrections factors for the PTW microDiamond were calculated for preclinical X-ray and proton minibeams, and their impact in dosimetric accuracy was evaluated.

METHODS

A formalism for the calculation of radiation quality correction factors for absolute dosimetry in minibeam fields was developed. Following our formalism, radiation quality correction factors were calculated for the PTW microDiamond detector, using the Monte Carlo method. Models of the detector, and X-ray and proton irradiation platform, were imported into the TOPAS Monte Carlo simulation toolkit. The radiation quality correction factors were calculated in the following scenarios: (i) reference dosimetry open field to minibeam center of the central peak, (ii) different positions at the minibeam profile (along the peaks and valleys direction) to the center of the central minibeam, and (iii) some representative depth positions. In addition, the radiation quality correction factors needed for the calculation of the peak-to-valley dose ratio at different depths were calculated.

RESULTS

An important overestimation of the dose (about 10%) was found in the case of the open to minibeam field for both X-rays and proton beams, when the correction factors were used. Smaller differences were observed in the other cases.

CONCLUSIONS

The usage of the PTW microDiamond detector requires radiation quality correction factors in order to be used in minibeam reference dosimetry.

Hard x-ray spectrometer calibrations using a portable 120 kV x-ray source

A Cauchois transmission-crystal hard x-ray spectrometer was calibrated by using a portable, compact, battery-powered tungsten x-ray source having 120 peak kilovoltage. The source emission region was characterized by recording high-resolution 2D x-ray images and was found to be composed of three emission regions having a 400 µm overall extent. The absolutely calibrated source fluence was measured by using a calibrated silicon drift detector and was in good agreement with the spectrum calculated by the SpekPy code. High-resolution spectra of the W Kα and Kβ lines in the 57-70 keV energy range were recorded on image plate detectors by the Cauchois spectrometer and provided excellent calibrations of the spectrometer's dispersion and spectral resolution. The minimal effect of the source size in the spectral lines recorded on the spectrometer's Rowland circle and the source-size broadening of the spatial lines recorded well beyond the Rowland circle were analyzed. The integrated reflectivity of the spectrometer's quartz (101) crystal was measured by using the absolutely calibrated 59.318 keV W Kα₁ spectral line emission and was in agreement with previous integrated reflectivity measurements performed at the National Institute of Standards and Technology. The well-characterized portable 120 kV x-ray source provides a convenient and cost-effective way to accurately calibrate the sensitivity, dispersion, spectral resolution, and source-size broadening in the spectra recorded by high-resolution x-ray spectrometers operating in the hard x-ray range. The absolutely calibrated source fluence can also be used to calibrate x-ray detectors at energies in the 40-100 keV energy range.

Statistical iterative spectral CT imaging method based on blind separation of polychromatic projections

Spectral computed tomography (CT) can provide narrow-energy-width reconstructed images, thereby suppressing beam hardening artifacts and providing rich attenuation information for component characterization. We propose a statistical iterative spectral CT imaging method based on blind separation of polychromatic projections to improve the accuracy of narrow-energy-width image decomposition. For direct inversion in blind scenarios, we introduce the system matrix into the X-ray multispectral forward model to reduce indirect errors. A constrained optimization problem with edge-preserving regularization is established and decomposed into two sub-problems to be alternately solved. Experiments indicate that the novel algorithm obtains more accurate narrow-energy-width images than the state-of-the-art method.

Use of calculations to validate beam quality and relative dose measurements for a kilovoltage X-ray therapy unit

Relative dosimetry measurements are required to fully commission kilovoltage X-ray units for superficial and orthovoltage X-ray therapy. Validation of these relative dosimetry measurements with Monte Carlo methods is advantageous being independent of the measurement process. In this study use is made of the X-ray spectrum generating program SpekPy along with the EGSnrc Monte Carlo code to calculate depth doses and explore the dosimetry effect of changes in backscatter. These calculations are compared with previously reported measurements for the Pantak SXT 150 X-ray therapy unit. SpekPy can also be used to generate half value layer (HVL) values and these are also compared to previously reported HVL measurements for the same X-ray therapy unit. It was found that agreements of the order of 5% in HVL, 3% in depth dose and 1% in backscatter doses were found between Monte Carlo calculations and the previously published measured data. Exit doses in conditions of lack of full backscatter were explored with Monte Carlo calculations demonstrating reduced exit dose up to 20% in these conditions. It is concluded that SpekPy with Monte Carlo codes such as EGSnrc provides a straightforward approach to validating various relative dosimetry measurements in kilovoltage X-ray dosimetry.

Femtosecond Direct Laser Writing of Silver Clusters in Phosphate Glasses for X-ray Spatially-Resolved Dosimetry

Radio-photoluminescence in silver-doped phosphate glasses has been extensively used for X-ray dosimetry. In this paper, we present the potential of silver clusters for X-ray spatially resolved dosimetry. Those clusters are generated in phosphate glasses containing a high concentration of silver oxide by femtosecond direct laser writing technique. Two phosphate glasses of different compositions were investigated. First, the spectroscopic properties of the pristine glasses were studied after X-ray irradiation at different doses to assess their dosimetry potential. Second, the impact of X-rays on the three-dimensional inscribed silver clusters has been analyzed using several spectroscopies methods. Our analysis highlights the resilience of embedded silver clusters acting as local probes of the deposited doses. We demonstrate that these inscribed glasses can define the range and sensitivity of X-ray doses and consider the realization of spatially-resolved dosimeters.

Numerical evaluation of high-energy, laser-Compton x-ray sources for contrast enhancement and dose reduction in clinical imaging via gadolinium-based K-edge subtraction

Conventional x-ray sources for medical imaging utilize bremsstrahlung radiation. These sources generate large bandwidth (BW) x-ray spectra with large fractions of photons that impart a dose, but do not contribute to image production. X-ray sources based on laser-Compton scattering can have inherently small energy BWs and can be tuned to low dose-imparting energies, allowing them to take advantage of atomic K-edge contrast enhancement. This paper investigates the use of gadolinium-based K-edge subtraction imaging in the context of mammography using a laser-Compton source through simulations quantifying contrast and dose in such imaging systems as a function of laser-Compton source parameters. Our simulations indicate that a K-edge subtraction image generated with a 0.5% BW (FWHM) laser-Compton x-ray source can obtain an equal contrast to a bremsstrahlung image with only 3% of the dose.