Validation of a deterministic linear Boltzmann transport equation solver for rapid CT dose computation using physical dose measurements in pediatric phantoms.

PURPOSE: The risk of inducing cancer to patients undergoing CT examinations has motivated efforts for CT dose estimation, monitoring, and reduction, especially among pediatric population. The method investigated in this study is Acuros CTD (Varian Medical Systems, Palo Alto, CA), a deterministic linear Boltzmann transport equation (LBTE) solver aimed at generating rapid and reliable dose maps of CT exams. By applying organ contours, organ doses can also be obtained, thus patient-specific organ dose estimates can be provided. This study experimentally validated Acuros against measurements performed on a clinical CT system using a range of physical pediatric anthropomorphic phantoms and acquisition protocols. METHODS: The study consisted of (1) the acquisition of dose measurements on a clinical CT scanner through thermoluminescent dosimeters (TLDs), and (2) the modeling in the Acuros platform of the measurement set up, which includes the modeling of the CT scanner and of the anthropomorphic phantoms. For the measurements, 1-year-old, 5-year-old, and 10-year-old anthropomorphic phantoms of the CIRS ATOM family were used. TLDs were placed in selected organ locations such as stomach, liver, lungs, and heart. The pediatric phantoms were scanned helically with the GE Discovery 750 HD clinical scanner for several examination protocols. For the simulations in Acuros, scanner-specific input, such as bowtie filters, overrange collimation, and tube current modulation schemes, were modeled. These scanner complexities were implemented by defining discretized X-ray beams whose spectral distribution, defined in Acuros by only six energy bins, varied across fan angle, cone angle, and slice position. The images generated during the CT acquisitions were used to create the geometrical models, by applying thresholding algorithms and assigning materials to the HU values. The TLDs were contoured in the phantom models as sensitive cylindrical volumes at the locations selected for dosimeters placement, to provide dose estimates, in terms of dose per unit photon. To compare measured doses with dose estimates, a calibration factor was derived from the CTDIvol displayed by the scanner, to account for the number of photons emitted by the X-ray tube during the procedure. RESULTS: The differences of the measured and estimated doses, in terms of absolute % errors, were within 13% for 153 TLD locations, with an error of 17% at the stomach for one study with the 10-year-old phantom. Root-mean-squared-errors (RMSE) across all TLD locations for all configurations were in the range of 3%-8%, with Acuros providing dose estimates in a time range of a few seconds up to 2 min. CONCLUSIONS: An overall good agreement between measurements and simulations was achieved, with average RMSE of 6% across all cases. The results demonstrate that Acuros can model a specific clinical scanner despite the required discretization in spatial and energy domains. The proposed deterministic tool has the potential to be part of a near real-time individualized dosimetry monitoring system for CT applications, providing patient-specific organ dose estimates.

Validation of the MC-GPU Monte Carlo code against the PENELOPE/penEasy code system and benchmarking against experimental conditions for typical radiation qualities and setups in interventional radiology and cardiology.

INTRODUCTION: Interventional procedures are associated with potentially high radiation doses to the skin. The 2013/59/EURATOM Directive establishes that the equipment used for interventional radiology must have a device or a feature informing the practitioner of relevant parameters for assessing patient dose at the end of the procedure. Monte Carlo codes of radiation transport are considered to be one of the most reliable tools available to assess doses. However, they are usually too time consuming for use in clinical practice. This work presents the validation of the fast Monte Carlo code MC-GPU for application in interventional radiology. METHODOLOGIES: MC-GPU calculations were compared against the well-validated Monte Carlo simulation code PENELOPE/penEasy by simulating the organ dose distribution in a voxelized anthropomorphic phantom. In a second phase, the code was compared against thermoluminescent measurements performed on slab phantoms, both in a calibration laboratory and at a hospital. RESULTS: The results obtained from the two simulation codes show very good agreement, differences in the output were within 1%, whereas the calculation time on the MC-GPU was 2500 times shorter. Comparison with measurements is of the order of 10%, within the associated uncertainty. CONCLUSIONS: It has been verified that MC-GPU provides good estimates of the dose when compared to PENELOPE program. It is also shown that it presents very good performance when assessing organ doses in very short times, less than one minute, in real clinical set-ups. Future steps would be to simulate complex procedures with several projections.

Deterministic linear Boltzmann transport equation solver for patient-specific CT dose estimation: Comparison against a Monte Carlo benchmark for realistic scanner configurations and patient models.

PURPOSE: Epidemiological evidence suggests an increased risk of cancer related to computed tomography (CT) scans, with children exposed to greater risk. The purpose of this work is to test the reliability of a linear Boltzmann transport equation (LBTE) solver for rapid and patient-specific CT dose estimation. This includes building a flexible LBTE framework for modeling modern clinical CT scanners and to validate the resulting dose maps across a range of realistic scanner configurations and patient models. METHODS: In this study, computational tools were developed for modeling CT scanners, including a bowtie filter, overrange collimation, and tube current modulation. The LBTE solver requires discretization in the spatial, angular, and spectral dimensions, which may affect the accuracy of scanner modeling. To investigate these effects, this study evaluated the LBTE dose accuracy for different discretization parameters, scanner configurations, and patient models (male, female, adults, pediatric). The method used to validate the LBTE dose maps was the Monte Carlo code Geant4, which provided ground truth dose maps. LBTE simulations were implemented on a GeForce GTX 1080 graphic unit, while Geant4 was implemented on a distributed cluster of CPUs. RESULTS: The agreement between Geant4 and the LBTE solver quantifies the accuracy of the LBTE, which was similar across the different protocols and phantoms. The results suggest that 18 views per rotation provides sufficient accuracy, as no significant improvement in the accuracy was observed by increasing the number of projection views. Considering this discretization, the LBTE solver average simulation time was approximately 30 s. However, in the LBTE solver the phantom model was implemented with a lower voxel resolution with respect to Geant4, as it is limited by the memory of the GPU. Despite this discretization, the results showed a good agreement between the LBTE and Geant4, with root mean square error of the dose in organs of approximately 3.5% for most of the studied configurations. CONCLUSIONS: The LBTE solver is proposed as an alternative to Monte Carlo for patient-specific organ dose estimation. This study demonstrated accurate organ dose estimates for the rapid LBTE solver when considering realistic aspects of CT scanners and a range of phantom models. Future plans will combine the LBTE framework with deep learning autosegmentation algorithms to provide near real-time patient-specific organ dose estimation.

Absorbed dose in the operator's brain in interventional radiology practices: evaluation through KAP value conversion factors.

In order to address the recent concerns over a possible increasing in brain tumour mortality among interventional radiologists and cardiologist, this work evaluated the exposure conditions of the operator's brain during interventional procedures using Monte Carlo simulations with anthropomorphic phantoms. The absorbed doses in several predefined segments of the operator's brain were estimated in a typical interventional radiology irradiation scenario. The doses were normalized to the KAP values simulated for ten X-ray beam qualities and four projections (PA, RAO 25°, LAO 25° and CRA 25°). For the interventional radiology scenario, because of the position of the operator, no difference was found in the exposure between the left and right regions of the brain for the first operator. However, for the second operator standing at a farer distance from the tube, the exposure of the left part of the brain is up to two times higher than that of the right part. The results are in agreement with dose measurements reported in the literature. The conversion factors, obtained as the absorbed dose per KAP, can be used to obtain a first estimate of the exposure of the brain of the operators during interventional procedures.

Monte Carlo study of the scattered radiation field near the eyes of the operator in interventional procedures.

Interventional radiology and cardiology guarantee high benefits for patients, but are known to be associated with a high level of radiation exposure of medical staff. The recently suggested decrease of the annual dose limit for the eye lens, from 150 to 20 mSv, caused a need for a reconsideration of practices ensuring sufficient protection for the lens of the eyes of medical staff. In such context the study of the scattered radiation around the operator's head could help in finding the best solutions to be adopted for the ceiling-suspended shield and lead glasses in the most common situations in interventional practices. MCNPX Monte Carlo code was employed with anthropomorphic mathematical phantoms to simulate interventional practice projections. For each projection the effect of changing selected parameters on the evaluated scattered radiation towards the operator's head has been calculated. The variety of modelled situations provides plentiful material regarding the spatial distribution of the scattered radiation, useful to improve eye lens radiation protection, such as the following:  (a) Glasses, which provide shielding from both lateral and bottom-up scattered radiation, can reduce by ten times the exposure to the most exposed eyes;  (b) The ceiling-suspended shield offers valuable protection, but such effectiveness can diminish by 90% if the shielding is not correctly positioned;  (c) The transition from femoral to radial access usually intensifies the scattered radiation toward the operator head (a factor of 1.5 for AP projection), but for RAO projections, a reduction of the order by two to three times, in the case of radial access, can be seen, due to the protection provided by the image receptor. The detailed fluence outcomes show that there is a preferential direction of the impinging scattered radiation that should be considered when radiation protection options are evaluated or when a dedicated eye lens dosemeter is used for monitoring.

Influence of dosemeter position for the assessment of eye lens dose during interventional cardiology.

The equivalent dose limit for the eye lens for occupational exposure recommended by the ICRP has been reduced to 20 mSv y(-1) averaged over defined periods of 5 y, with no single year exceeding 50 mSv. The compliance with this new requirement could not be easy in some workplace such as interventional radiology and cardiology. The aim of this study is to evaluate different possible approaches in order to have a good estimate of the eye lens dose during interventional procedures. Measurements were performed with an X-ray system Philips Allura FD-10, using a PMMA phantom to simulate the patient scattered radiation and a Rando phantom to simulate the cardiologist. Thermoluminescence (TL) whole-body and TL eye lens dosemeters together with Philips DoseAware active dosemeters were located on different positions of the Rando phantom to estimate the eye lens dose in typical cardiology procedures. The results show that, for the studied conditions, any of the analysed dosemeter positions are suitable for eye lens dose assessment. However, the centre of the thyroid collar and the left ear position provide a better estimate. Furthermore, in practice, improper use of the ceiling-suspended screen can produce partial protection of some parts of the body, and thus large differences between the measured doses and the actual exposure of the eye could arise if the dosemeter is not situated close to the eye.