Dosimetry applications in GATE Monte Carlo toolkit

DosePatch: physics-inspired cropping layout for patch-based Monte Carlo simulations to provide fast and accurate internal dosimetry

BACKGROUND

Dosimetry-based personalized therapy was shown to have clinical benefits e.g. in liver selective internal radiation therapy (SIRT). Yet, there is no consensus about its introduction into clinical practice, mainly as Monte Carlo simulations (gold standard for dosimetry) involve massive computation time. We addressed the problem of computation time and tested a patch-based approach for Monte Carlo simulations for internal dosimetry to improve parallelization. We introduce a physics-inspired cropping layout for patch-based MC dosimetry, and compare it to cropping layouts of the literature as well as dosimetry using organ-S-values, and dose kernels, taking whole-body Monte Carlo simulations as ground truth. This was evaluated in five patients receiving Yttrium-90 liver SIRT.

RESULTS

The patch-based Monte Carlo approach yielded the closest results to the ground truth, making it a valid alternative to the conventional approach. Our physics-inspired cropping layout and mosaicking scheme yielded a voxel-wise error of < 2% compared to whole-body Monte Carlo in soft tissue, while requiring only ≈  10% of the time.

CONCLUSIONS

This work demonstrates the feasibility and accuracy of physics-inspired cropping layouts for patch-based Monte Carlo simulations.

Impact of deep learning-based multiorgan segmentation methods on patient-specific internal dosimetry in PET/CT imaging: A comparative study

PURPOSE

Accurate and fast multiorgan segmentation is essential in image-based internal dosimetry in nuclear medicine. While conventional manual PET image segmentation is widely used, it suffers from both being time-consuming as well as subject to human error. This study exploited 2D and 3D deep learning (DL) models. Key organs in the trunk of the body were segmented and then used as a reference for networks.

METHODS

The pre-trained p2p-U-Net-GAN and HighRes3D architectures were fine-tuned with PET-only images as inputs. Additionally, the HighRes3D model was alternatively trained with PET/CT images. Evaluation metrics such as sensitivity (SEN), specificity (SPC), intersection over union (IoU), and Dice scores were considered to assess the performance of the networks. The impact of DL-assisted PET image segmentation methods was further assessed using the Monte Carlo (MC)-derived S-values to be used for internal dosimetry.

RESULTS

A fair comparison with manual low-dose CT-aided segmentation of the PET images was also conducted. Although both 2D and 3D models performed well, the HighRes3D offers superior performance with Dice scores higher than 0.90. Key evaluation metrics such as SEN, SPC, and IoU vary between 0.89-0.93, 0.98-0.99, and 0.87-0.89 intervals, respectively, indicating the encouraging performance of the models. The percentage differences between the manual and DL segmentation methods in the calculated S-values varied between 0.1% and 6% with a maximum attributed to the stomach.

CONCLUSION

The findings prove while the incorporation of anatomical information provided by the CT data offers superior performance in terms of Dice score, the performance of HighRes3D remains comparable without the extra CT channel. It is concluded that both proposed DL-based methods provide automated and fast segmentation of whole-body PET/CT images with promising evaluation metrics. Between them, the HighRes3D is more pronounced by providing better performance and can therefore be the method of choice for 18F-FDG-PET image segmentation.

Assessing radiation dosimetry for microorganisms in naturally radioactive mineral springs using GATE and Geant4-DNA Monte Carlo simulations

Mineral springs in Massif Central, France can be characterized by higher levels of natural radioactivity in comparison to the background. The biota in these waters is constantly under radiation exposure mainly from the α-emitters of the natural decay chains, with 226Ra in sediments ranging from 21 Bq/g to 43 Bq/g and 222Rn activity concentrations in water up to 4600 Bq/L. This study couples for the first time micro- and nanodosimetric approaches to radioecology by combining GATE and Geant4-DNA to assess the dose rates and DNA damages to microorganisms living in these naturally radioactive ecosystems. It focuses on unicellular eukaryotic microalgae (diatoms) which display an exceptional abundance of teratological forms in the most radioactive mineral springs in Auvergne. Using spherical geometries for the microorganisms and based on γ-spectrometric analyses, we evaluate the impact of the external exposure to 1000 Bq/L 222Rn dissolved in the water and 30 Bq/g 226Ra in the sediments. Our results show that the external dose rates for diatoms are significant (9.7 μGy/h) and comparable to the threshold (10 μGy/h) for the protection of the ecosystems suggested by the literature. In a first attempt of simulating the radiation induced DNA damage on this species, the rate of DNA Double Strand Breaks per day is estimated to 1.11E-04. Our study confirms the significant mutational pressure from natural radioactivity to which microbial biodiversity has been exposed since Earth origin in hydrothermal springs.

Updating 90Y Voxel S-Values including internal Bremsstrahlung: Monte Carlo study and development of an analytical model

PURPOSE

Internal Bremsstrahlung (IB) is a process accompanying β-decay but neglected in Voxel S-Values (VSVs) calculation. Aims of this work were to calculate, through Monte Carlo (MC) simulation, updated 90Y-VSVs including IB, and to develop an analytical model to evaluate 90Y-VSVs for any voxel size of practical interest.

METHODS

GATE (Geant4 Application for Tomographic Emission) was employed for simulating voxelized geometries of soft tissue, with voxels sides l ranging from 2 to 6 mm, in steps of 0.5 mm. The central voxel was set as a homogeneous source of 90Y when IB photons are not modelled. For each l, the VSVs were computed for 90Y decays alone and for 90Y + IB. The analytical model was then built through fitting procedures of the VSVs including IB contribution.

RESULTS

Comparing GATE-VSVs with and without IB, differences between + 25% and + 30% were found for distances from the central voxel larger than the maximum β-range. The analytical model showed an agreement with MC simulations within ± 5% in the central voxel and in the Bremsstrahlung tails, for any l value examined, and relative differences lower than ± 40%, for other distances from the source.

CONCLUSIONS

The presented 90Y-VSVs include for the first time the contribution due to IB, thus providing a more accurate set of dosimetric factors for three-dimensional internal dosimetry of 90Y-labelled radiopharmaceuticals and medical devices. Furthermore, the analytical model constitutes an easy and fast alternative approach for 90Y-VSVs estimation for non-standard voxel dimensions.

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.

Spatial Scattering Radiation to the Radiological Technologist during Medical Mobile Radiography

Mobile radiography allows for the diagnostic imaging of patients who cannot move to the X-ray examination room. Therefore, mobile X-ray equipment is useful for patients who have difficulty with movement. However, staff are exposed to scattered radiation from the patient, and they can receive potentially harmful radiation doses during radiography. We estimated occupational exposure during mobile radiography using phantom measurements. Scattered radiation distribution during mobile radiography was investigated using a radiation survey meter. The efficacy of radiation-reducing methods for mobile radiography was also evaluated. The dose decreased as the distance from the X-ray center increased. When the distance was more than 150 cm, the dose decreased to less than 1 μSv. It is extremely important for radiological technologists (RTs) to maintain a sufficient distance from the patient to reduce radiation exposure. The spatial dose at eye-lens height increases when the bed height is high, and when the RT is short in stature and abdominal imaging is performed. Maintaining sufficient distance from the patient is also particularly effective in limiting radiation exposure of the eye lens. Our results suggest that the doses of radiation received by staff during mobile radiography are not significant when appropriate radiation protection is used. To reduce exposure, it is important to maintain a sufficient distance from the patient. Therefore, RTs should bear this is mind during mobile radiography.

Development of a voxel S-value database for patient internal radiation dosimetry

PURPOSE

Personalized dosimetry with high accuracy drew great attention in clinical practices. Voxel S-value (VSV) convolution has been proposed to speed up absorbed dose calculations. However, the VSV method is efficient for personalized internal radiation dosimetry only when there are pre-calculated VSVs of the radioisotope. In this work, we propose a new method for VSV calculation based on the developed mono-energetic particle VSV database of γ, β, α, and X-ray for any radioisotopes.

METHODS

Mono-energetic VSV database for γ, β, α, and X-ray was calculated using Monte Carlo methods. Radiation dose was first calculated based on mono-energetic VSVs for [F-18]-FDG in 10 patients. The estimated doses were compared with the values obtained from direct Monte Carlo simulation for validation of the proposed method. The number of VSVs used in calculation was optimized based on the estimated dose accuracy and computation time.

RESULTS

The generated VSVs showed a great consistency with the results calculated using direct Monte Carlo simulation. For [F-18]-FDG, the proposed VSV method with number of VSV of 9 shows the best relative average organ absorbed dose uncertainty of 3.25% while the calculation time was reduced by 99% and 97% compared to the Monte Carlo simulation and traditional multiple VSV methods, respectively.

CONCLUSIONS

In this work, we provided a method to generate the VSV kernels for any radioisotope based on the pre-calculated mono-energetic VSV database and significantly reduced the time cost for the multiple VSVs dosimetry approach. A software was developed to generate VSV kernels for any radioisotope in 19 mediums.

Impact of the anode tilt on photon emission spectrum and temperature rise in an x-ray tube

In radiology, the photon fluence and the energy spectrum generated from an x-ray tube may depend on the anode tilt angle. In this contribution, a Monte Carlo investigation is performed to quantify this effect by modeling an x-ray tube based on published data Bujila R.et al(2020Physica. Med.7544-54). The GATE simulation code is used for this purpose. The calculations have moreover confirmed this dependence; the tilt of the anode could be used to increase the photon fluence. The thermal analysis has shown that the hot spot size is dependent as well on the anode tilt angle. The thermal focus temperature (ΔT) decreases when the anode tilt angle increases. Finally, by moving the acquisition angle from 293°-337° to 248°-292° and changing the anode tilt angle from 8° to 28°, the photon fluence can be increased by 55%.

Utilizing 3D Slicer to incorporate tomographic images into GATE Monte Carlo simulation for personalized dosimetry in yttrium-90 radioembolization

PURPOSE

Monte Carlo (MC) simulation is an important technique that can help design advanced and challenging experimental setups. GATE (Geant4 application for tomographic emission) is a useful simulation toolkit for applications in nuclear medicine. Transarterial radioembolization is a treatment for liver cancer, where microspheres embedded with yttrium-90 (⁹⁰ Y) are administered intra-arterially to the tumor. Personalized dosimetry for this treatment may provide higher dosimetry accuracy compared to the conventional partition model (PM) calculation. However, incorporation of three-dimensional tomographic input data into MC simulation is an intricate process. In this article, 3D Slicer, free and open-source software, was utilized for the incorporation of patient tomographic images into GATE to demonstrate the feasibility of personalized dosimetry in hepatic radioembolization with ⁹⁰ Y.

METHODS

In this article, the steps involved in importing, segmenting, and registering tomographic images using 3D Slicer were thoroughly described, before importing them into GATE for MC simulation. The absorbed doses estimated using GATE were then compared with that of PM. SlicerRT, a 3D Slicer extension, was then used to visualize the isodose from the MC simulation.

RESULTS

A workflow diagram consisting of all the steps taken in the utilization of 3D Slicer for personalized dosimetry in ⁹⁰ Y radioembolization has been presented in this article. In comparison to the MC simulation, the absorbed doses to the tumor and normal liver were overestimated by PM by 105.55% and 20.23%, respectively, whereas for lungs, the absorbed dose estimated by PM was underestimated by 25.32%. These values were supported by the isodose distribution obtained via SlicerRT, suggesting the presence of beta particles outside the volumes of interest. These findings demonstrate the importance of personalized dosimetry for a more accurate absorbed dose estimation compared to PM.

CONCLUSION

The methodology provided in this study can assist users (especially students or researchers who are new to MC simulation) in navigating intricate steps required in the importation of tomographic data for MC simulation. These steps can also be utilized for other radiation therapy related applications, not necessarily limited to internal dosimetry.

Radiation exposure of microorganisms living in radioactive mineral springs

The TIRAMISU collaboration gathers expertise from biologists, physicists, radiochemists and geologists within the Zone-Atelier Territoires Uranifères (ZATU) in France to analyze the radiation exposure of microorganisms living in naturally radioactive mineral springs. These springs are small waterbodies that are extremely stable over geological time scales and display different physicochemical and radiological parameters compared to their surroundings. Water and sediment samples collected in 27 mineral springs of the volcanic Auvergne region (Massif Central, France) have been studied for their microbial biodiversity and their radionuclide content. Among the microorganisms present, microalgae (diatoms), widely used as environmental indicators of water quality, have shown to display an exceptional abundance of teratogenic forms in the most radioactive springs studied (radon activity up to 3700 Bq/L). The current work presents a first assessment of the dose received by the diatoms inhabiting these ecosystems. According to ERICA tool, microorganisms living in most of the sampled mineral springs were exposed to dose rates above 10 μGy/h due to the large concentration of radium in the sediments (up to 50 Bq/g). Radiological analyses of water and sediments were used as inputs to Monte Carlo simulations at micro-(GATE) and nano- (Geant4-DNA) scale in order to assess the direct and indirect damages on the diatom DNA.

Standardization and Validation of Brachytherapy Seeds' Modelling Using GATE and GGEMS Monte Carlo Toolkits

Simple Summary

This study used GATE and GGEMS simulation toolkits, to estimate dose distribution on Brachytherapy procedures. Specific guidelines were followed as defined by the American Association of Physicists in Medicine (AAPM) as well as by the European SocieTy for Radiotherapy and Oncology (ESTRO). Several types of brachytherapy seeds were modelled and simulated, namely Low-Dose-Rate (LDR), High-Dose-Rate (HDR), and Pulsed-Dose-Rate (PDR). The basic difference between GATE and GGEMS is that GGEMS incorporates GPU capabilities, which makes the use of Monte Carlo (MC) simulations more accessible in clinical routine, by minimizing the computational time to obtain a dose map. During the validation procedure of both codes with protocol data, differences as well as uncertainties were measured within the margins defined by the guidelines. The study concluded that MC simulations may be utilized in clinical practice, to optimize dose distribution in real time, as well as to evaluate therapeutic plans.

Abstract

This study aims to validate GATE and GGEMS simulation toolkits for brachytherapy applications and to provide accurate models for six commercial brachytherapy seeds, which will be freely available for research purposes. The AAPM TG-43 guidelines were used for the validation of two Low Dose Rate (LDR), three High Dose Rate (HDR), and one Pulsed Dose Rate (PDR) brachytherapy seeds. Each seed was represented as a 3D model and then simulated in GATE to produce one single Phase-Space (PHSP) per seed. To test the validity of the simulations’ outcome, referenced data (provided by the TG-43) was compared with GATE results. Next, validation of the GGEMS toolkit was achieved by comparing its outcome with the GATE MC simulations, incorporating clinical data. The simulation outcomes on the radial dose function (RDF), anisotropy function (AF), and dose rate constant (DRC) for the six commercial seeds were compared with TG-43 values. The statistical uncertainty was limited to 1% for RDF, to 6% (maximum) for AF, and to 2.7% (maximum) for the DRC. GGEMS provided a good agreement with GATE when compared in different situations: (a) Homogeneous water sphere, (b) heterogeneous CT phantom, and (c) a realistic clinical case. In addition, GGEMS has the advantage of very fast simulations. For the clinical case, where TG-186 guidelines were considered, GATE required 1 h for the simulation while GGEMS needed 162 s to reach the same statistical uncertainty. This study produced accurate models and simulations of their emitted spectrum of commonly used commercial brachytherapy seeds which are freely available to the scientific community. Furthermore, GGEMS was validated as an MC GPU based tool for brachytherapy. More research is deemed necessary for the expansion of brachytherapy seed modeling.

A deep-learning-based prediction model for the biodistribution of 90 Y microspheres in liver radioembolization

BACKGROUND

Radioembolization with ⁹⁰ Y microspheres is a treatment approach for liver cancer. Currently, employed dosimetric calculations exhibit low accuracy, lacking consideration of individual patient, and tissue characteristics.

PURPOSE

The purpose of the present study was to employ deep learning (DL) algorithms to differentiate patterns of pretreatment distribution of ^99m Tc-macroaggregated albumin on SPECT/CT and post-treatment distribution of ⁹⁰ Y microspheres on PET/CT and to accurately predict how the ⁹⁰ Y-microspheres will be distributed in the liver tissue by radioembolization therapy.

METHODS

Data for 19 patients with liver cancer (10 with hepatocellular carcinoma, 5 with intrahepatic cholangiocarcinoma, 4 with liver metastases) who underwent radioembolization with ⁹⁰ Y microspheres were used for the DL training. We developed a 3D voxel-based variation of the Pix2Pix model, which is a special type of conditional GANs designed to perform image-to-image translation. SPECT and CT scans along with the clinical target volume for each patient were used as inputs, as were their corresponding post-treatment PET scans. The real and predicted absorbed PET doses for the tumor and the whole liver area were compared. Our model was evaluated using the leave-one-out method, and the dose calculations were measured using a tissue-specific dose voxel kernel.

RESULTS

The comparison of the real and predicted PET/CT scans showed an average absorbed dose difference of 5.42% ± 19.31% and 0.44% ± 1.64% for the tumor and the liver area, respectively. The average absorbed dose differences were 7.98 ± 31.39 Gy and 0.03 ± 0.25 Gy for the tumor and the non-tumor liver parenchyma, respectively. Our model had a general tendency to underpredict the dosimetric results; the largest differences were noticed in one case, where the model underestimated the dose to the tumor area by 56.75% or 72.82 Gy.

CONCLUSIONS

The proposed deep-learning-based pretreatment planning method for liver radioembolization accurately predicted ⁹⁰ Y microsphere biodistribution. Its combination with a rapid and accurate 3D dosimetry method will render it clinically suitable and could improve patient-specific pretreatment planning.

Influence of dosimetry method on bone lesion absorbed dose estimates in PSMA therapy: application to mCRPC patients receiving Lu-177-PSMA-I&T

BACKGROUND

Patients with metastatic, castration-resistant prostate cancer (mCRPC) present with an increased tumor burden in the skeleton. For these patients, Lutetium-177 (Lu-177) radioligand therapy targeting the prostate-specific membrane antigen (PSMA) has gained increasing interest with promising outcome data. Patient-individualized dosimetry enables improvement of therapy success with the aim of minimizing absorbed dose to organs at risk while maximizing absorbed dose to tumors. Different dosimetric approaches with varying complexity and accuracy exist for this purpose. The Medical Internal Radiation Dose (MIRD) formalism applied to tumors assumes a homogeneous activity distribution in a sphere with unit density for derivation of tumor S values (TSV). Voxel S value (VSV) approaches can account for heterogeneous activities but are simulated for a specific tissue. Full patient-individual Monte Carlo (MC) absorbed dose simulation addresses both, heterogeneous activity and density distributions. Subsequent CT-based density weighting has the potential to overcome the assumption of homogeneous density in the MIRD formalism with TSV and VSV methods, which could be a major limitation for the application in bone metastases with heterogeneous density. The aim of this investigation is a comparison of these methods for bone lesion dosimetry in mCRPC patients receiving Lu-177-PSMA therapy.

RESULTS

In total, 289 bone lesions in 15 mCRPC patients were analyzed. Percentage difference (PD) of average absorbed dose per lesion compared to MC, averaged over all lesions, was + 14 ± 10% (min: - 21%; max: + 56%) for TSVs. With lesion-individual density weighting using Hounsfield Unit (HU)-to-density conversion on the patient's CT image, PD was reduced to - 8 ± 1% (min: - 10%; max: - 3%). PD on a voxel level for three-dimensional (3D) voxel-wise dosimetry methods, averaged per lesion, revealed large PDs of + 18 ± 11% (min: - 27%; max: + 58%) for a soft tissue VSV approach compared to MC; after voxel-wise density correction, this was reduced to - 5 ± 1% (min: - 12%; max: - 2%).

CONCLUSION

Patient-individual MC absorbed dose simulation is capable to account for heterogeneous densities in bone lesions. Since the computational effort prevents its routine clinical application, TSV or VSV dosimetry approaches are used. This study showed the necessity of lesion-individual density weighting for TSV or VSV in Lu-177-PSMA therapy bone lesion dosimetry.

Monte Carlo study of patient and medical staff radiation exposures during interventional cardiology

The aim of this study is to assess the radiation exposure of the patient and the medical staff during interventional cardiology procedures. Realistic exposure scenarios were developed using the adult reference anthropomorphic phantoms adopted by the International Commission on Radiological Protection (ICRP110_Male and ICRP110_Female), and the radiation transport code Geant4 (version 10.3). The calculated equivalent and effective doses were normalised by the simulated Kerma-Area Product (KAP), resulting in two conversion coefficients H_T/KAP and E/KAP. To properly evaluate the risk of exposure, several dose-dependent parameters have been investigated, namely: radiological parameters (tube kilovoltage peak (kVp), type of projection, field size (FOV)), and operator positions. Four projections (AP,PA,LAO25° and RAO25°) were simulated for three X-ray energy spectra (80,100 and 120 kVp) with four different values of FOV (15×15 cm²,20×20 cm²,25×25 cm² and 30×30 cm²). The results showed that the conversion coefficients values increase with increasing tube voltage as well as the FOV size. Recommended projection during the interventional cardiology procedures, whenever possible, should be the PA projection rather than AP projection. The most critical projection for the patient and the main operator is the RAO25° projection and the LAO25° projection respectively. The comparison of our results with the literature data showed good agreement allowing their use in the dosimetric characterization of interventional cardiology procedures.

Clinical Evaluation of a Three-Dimensional Internal Dosimetry Technique for Liver Radioembolization with 90Y Microspheres Using Dose Voxel Kernels

Background: The purpose of this study was to develop a rapid, reliable, and efficient tool for three-dimensional (3D) dosimetry treatment planning and post-treatment evaluation of liver radioembolization with ⁹⁰Y microspheres, using tissue-specific dose voxel kernels (DVKs) that can be used in everyday clinical practice. Materials and Methods: Two tissue-specific DVKs for ⁹⁰Y were calculated through Monte Carlo (MC) simulations. DVKs for the liver and lungs were generated, and the dose distribution was compared with direct MC simulations. A method was developed to produce a 3D dose map by convolving the calculated DVKs with the activity biodistribution derived from clinical single-photon emission computed tomography (SPECT) or positron emission tomography (PET) images. Image registration for the SPECT or PET images with the corresponding computed tomography scans was performed before dosimetry calculation. The authors first compared the DVK convolution dosimetry with a direct full MC simulation on an XCAT anthropomorphic phantom. They then tested it in 25 individual clinical cases of patients who underwent ⁹⁰Y therapy. All MC simulations were carried out using the GATE MC toolkit. Results: Comparison of the measured absorbed dose using tissue-specific DVKs and direct MC simulation on 25 patients revealed a mean difference of 1.07% ± 1.43% for the liver and 1.03% ± 1.21% for the tumor tissue, respectively. The largest difference between DVK convolution and full MC dosimetry was observed for the lung tissue (10.16% ± 1.20%). The DVK statistical uncertainty was <0.75% for both media. Conclusions: This semiautomatic algorithm is capable of performing rapid, accurate, and efficient 3D dosimetry. The proposed method considers tissue and activity heterogeneity using tissue-specific DVKs. Furthermore, this method provides results in <1 min, making it suitable for everyday clinical practice.

HOW MANY DOSEMETERS ARE NEEDED FOR CORRECT MEAN ORGAN DOSE ASSESSMENT WHEN PERFOMING PHANTOM DOSIMETRY? A PHANTOM STUDY EVALUATING LIVER ORGAN DOSE AND INVESTIGATING TLD NUMBERS AND WAYS OF DOSEMETER PLACEMENT

This study evaluated repeated mean organ dose measurements of the liver by phantom dosimetry and statistical modelling in order to find a way to reduce the number of dosemeters needed for precise organ dose measurements. Thermoluminescent dosemeters were used in an adult female phantom exposed to a biplanar x-ray source at three different axial phantom rotations. Generalised mixed linear effect modelling was used for statistical analysis. A subgroup of five to six organ-specific locations out of 28 yielded mean liver organ doses within 95% confidence intervals of measurements based on all 28 liver-specific dosemeter locations. No statistical difference of mean liver dose was observed with rotation of the phantom either 10° clockwise or counter-clockwise as opposed to the coronal plane. Phantom dosimetry handling time during organ dose measurements can be markedly reduced, in this case the liver, by 79% (22/28), while still providing precise mean organ dose measurements.

New calculation method for 3D dose distribution in tetrahedral-mesh phantoms in Geant4

The tetrahedral-mesh (TM) geometry, which is a very promising geometry for computational human phantoms, has a limitation in 3D dose distribution calculation for medical applications. Even though Geant4 provides the read-out geometry for calculating 3D dose distribution in the TM geometry, this method significantly slows down the computation speed. In the present study, we developed a new method, called Moving Voxel-based Dose-Distribution Calculator (MVDDC), to rapidly calculate a 3D dose distribution in a TM geometry. To evaluate the performance of the MVDDC method, a simple TM cubic phantom and a human phantom were implemented in Geant4. Subsequently, the phantoms were irradiated with proton spot beams under various conditions, and the obtained results were compared with those of the read-out geometry method. The results show that there is no significant difference between the dose distributions calculated using the new method and the read-out geometry method. With respect to the computational performance, the speeds of simulations using the MVDDC were approximately 1.4-2.7 times faster than those of the simulations using the read-out geometry method.

Size-specific dose estimations for pediatric chest, abdomen/pelvis and head CT scans with the use of GATE

PURPOSE

The purpose of this study is to create an organ dose database for pediatric individuals undergoing chest, abdomen/pelvis, and head computed tomography (CT) examinations, and to report the differences in absorbed organ doses, when anatomical differences exist for pediatric patients.

METHODS

The GATE Monte Carlo (MC) toolkit was used to model the GE BrightSpeed Elite CT model. The simulated scanner model was validated with the standard Computed Tomography Dose Index (CTDI) head phantom. Twelve computational models (2.1-14 years old) were used. First, contributions to effective dose and absorbed doses per CTDI_vol and per 100 mAs were estimated for all organs. Then, doses per CTDI_vol were correlated with patient model weight for the organs inside the scan range for chest and abdomen/pelvis protocols. Finally, effective doses per dose-length product (DLP) were estimated and compared with the conventional conversion k-factors.

RESULTS

The system was validated against experimental CTDI_w measurements. The doses per CTDI_vol and per 100 mAs for selected organs were estimated. The magnitude of the dependency between the dose and the anatomical characteristics was calculated with the coefficient of determination at 0.5-0.7 for the internal scan organs for chest and abdomen/pelvis protocols. Finally, effective doses per DLP were compared with already published data, showing discrepancies between 13 and 29% and were correlated strongly with the total weight (R² > 0.8) for the chest and abdomen protocols.

CONCLUSIONS

Big differences in absorbed doses are reported even for patients of similar age or same gender, when anatomical differences exist on internal organs of the body.

Focused very high-energy electron beams as a novel radiotherapy modality for producing high-dose volumetric elements

The increased inertia of very high-energy electrons (VHEEs) due to relativistic effects reduces scattering and enables irradiation of deep-seated tumours. However, entrance and exit doses are high for collimated or diverging beams. Here, we perform a study based on Monte Carlo simulations of focused VHEE beams in a water phantom, showing that dose can be concentrated into a small, well-defined volumetric element, which can be shaped or scanned to treat deep-seated tumours. The dose to surrounding tissue is distributed over a larger volume, which reduces peak surface and exit doses for a single beam by more than one order of magnitude compared with a collimated beam.

Influence of the SPECT calibration source position on the absorbed dose calculation for 131I-NaI therapy using GATE simulations

Many research groups have studied nuclear medicine image quantification to improve its accuracy in dose estimation. This work aims to evaluate the influence of the source calibration position for absorbed dose calculation for a ¹³¹I-NaI therapy using Monte Carlo (MC) simulations. The calibration approach consisted of a cylindrical phantom filled with water. A cylindrical ¹³¹I source with 361.1 ± 3.6 kBq ml^-1 was positioned at the center of the phantom and its outer part. Images were acquired with 150 00 counts per projection image acquired with SPECT detector (high counts density-HCD) and 3000 counts per projection (low counts density-LCD). MC simulations, performed with GATE code, were validated by comparing the S values of a water sphere uniformly filled with ¹³¹I, as from the sphere model of OLINDA/EXM 1.1. Calibration factors deviation between central and peripheral calibrations is more significant for HCD (18.3%) than for LCD images (3.7%). The 3D dose distribution map obtained from GATE resulted in a dose factor equal to 1.5 × 10^-3 mGy/(MBq.s). For both HCD and LCD images, the commonly used approach, which employs the central source calibration to obtain the dose from a peripheral source, resulted in dose overestimation. Results suggest that organ dose calculation can be improved considering the organ position in the field of view. Finally, patients' radiation protection in dosimetry studies could be improved considering the calibration source position, due to the superior accuracy in dose calculation.

A personalized, Monte Carlo-based method for internal dosimetric evaluation of radiopharmaceuticals in children

PURPOSE

Herein, we introduce a methodology for estimating the absorbed dose in organs at risk that is based on specified clinically derived radiopharmaceutical biodistributions and personalized anatomical characteristics.

METHODS

To evaluate the proposed methodology, we used realistic Monte Carlo (MC) simulations and computational pediatric models to calculate a parameter called in this work the "specific absorbed dose rate" (SADR). The SADR is a unique quantitative metric in that it is specific to a particular organ. It is defined as the absorbed dose rate in an organ when the biodistribution of radioactivity over the whole body is considered. Initially, we applied a validation procedure that calculated specific absorbed fractions (SAFs) from mono-energetic photon sources in the range of 10 keV-2 MeV and compared them with previously published data. We calculated the SADRs for five different radiopharmaceuticals (99m Tc-MDP, 123 I-mIBG, 131 I-MIBG, 131 I-NaI, and 153 Sm-EDTMP) based on their biodistributions at four or five different times; the biodistributions were derived from the clinical scintigraphic data of pediatric patients. We used six models representing male and female patients aged 5, 8, and 14 yr to investigate the absorbed dose variability due to anatomical variations. The GATE Monte Carlo toolkit was used to calculate absorbed doses per organ. Finally, we compared the SADR methodology to that of OLINDA/EXM 1.1 using rescaled masses according to the studied models. Four target organs were considered for calculating the absorbed doses.

RESULTS

The ratios of SAFs calculated with GATE simulations to those based on previously published data were between 0.9 and 2.2 when the liver was used as a source organ. Subsequently, we used GATE to calculate a dataset of SADRs for the six pediatric models. The SADRs for pediatric models whose total body weights ranged from 20 to 40 kg varied up to approximately 90%, whereas those for models of similar body masses varied less than 15%. Finally, we found absorbed dose discrepancies of approximately 10-150% between the SADR methodology and OLINDA for two different radiopharmaceuticals. Absorbed doses from SADRs and from individualized S-values in the same pediatric model differed approximately 1-50%.

CONCLUSIONS

Because pediatric radiopharmaceutical dosimetric estimates demonstrate large variation due to the patient's anatomical characteristics, personalized data should be considered. Using our SADR method in a larger population of phantoms and for a variety of radiopharmaceuticals could enhance the personalization of dosimetry in pediatric nuclear medicine. The proposed methodology provides the advantage of creating time-dependent organ dose rate curves.

1st European Congress of Medical Physics September 1-4, 2016; Medical Physics innovation and vision within Europe and beyond

Medical Physics is the scientific healthcare profession concerned with the application of the concepts and methods of physics in medicine. The European Federation of Organisations for Medical Physics (EFOMP) acts as the umbrella organization for European Medical Physics societies. Due to the rapid advancements in related scientific fields, medical physicists must have continuous education through workshops, training courses, conferences, and congresses during their professional life. The latest developments related to this increasingly significant medical speciality were presented during the 1st European Congress of Medical Physics 2016, held in Athens, September 1-4, 2016, organized by EFOMP, hosted by the Hellenic Association of Medical Physicists (HAMP), and summarized in the current volume.