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.

Physics and small-scale dosimetry of α $\alpha$ -emitters for targeted radionuclide therapy: The case of 211 At $

BACKGROUND: Monte Carlo simulations have been considered for a long time the gold standard for dose calculations in conventional radiotherapy and are currently being applied for the same purpose in innovative radiotherapy techniques such as targeted radionuclide therapy (TRT). PURPOSE: We present in this work a benchmarking study of the latest version of the Transport d'Ions Lourds Dans l'Aqua & Vivo (TILDA-V ) Monte Carlo track structure code, highlighting its capabilities for describing the full slowing down of α $\alpha$ -particles in water and the energy deposited in cells by α $\alpha$ -emitters in the context of TRT. METHODS: We performed radiation transport simulations of α $\alpha$ -particles (10 keV u - 1 ${\rm u}^{-1}$ -100 MeV u - 1 ${\rm u}^{-1}$ ) in water with TILDA-V and the Particle and Heavy Ion Transport code System (PHITS) version 3.33. We compared the predictions of each code in terms of track parameters (stopping power, range and radial dose profiles) and cellular S-values of the promising radionuclide astatine-211 ( 211 At $^{211}{\rm At}$ ). Additional comparisons were made with available data in the literature. RESULTS: The stopping power, range and radial dose profiles of α $\alpha$ -particles computed with TILDA-V were in excellent agreement with other calculations and available data. Overall, minor differences with PHITS were ascribed to phase effects, that is, related to the use of interaction cross sections computed for water vapor or liquid water. However, important discrepancies were observed in the radial dose profiles of monoenergetic α $\alpha$ -particles, for which PHITS results showed a large underestimation of the absorbed dose compared to other codes and experimental data. The cellular S-values of 211 At $^{211}{\rm At}$ computed with TILDA-V  agreed within 4% with the values predicted by PHITS and MIRDcell. CONCLUSIONS: The validation of the TILDA-V code presented in this work opens the possibility to use it as an accurate simulation tool for investigating the interaction of α $\alpha$ -particles in biological media down to the nanometer scale in the context of medical research. The code may help nuclear medicine physicians in their choice of α $\alpha$ -emitters for TRT. Further research will focus on the application of TILDA-V for quantifying radioinduced damage on the deoxyribonucleic acid (DNA) molecule.

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 ß-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 ß-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.

Intra-brain vascular models within the ICRP mesh-type adult reference phantoms for applications to internal dosimetry.

Objective. Phantoms of the International Commission on Radiological Protection provide a framework for standardized dosimetry. The modeling of internal blood vessels-essential to tracking circulating blood cells exposed during external beam radiotherapy and to account for radiopharmaceutical decays while still in blood circulation-is, however, limited to the major inter-organ arteries and veins. Intra-organ blood is accounted for only through the assignment of a homogeneous mixture of parenchyma and blood [single-region (SR) organs]. Our goal was to develop explicit dual-region (DR) models of intra-organ blood vasculature of the adult male brain (AMB) and adult female brain (AFB).Approach. A total of 4000 vessels were created amongst 26 vascular trees. The AMB and AFB models were then tetrahedralized for coupling to the PHITS radiation transport code. Absorbed fractions were computed for monoenergetic alpha particles, electrons, positrons, and photons for both decay sites within the blood vessels and for tissues outside these vessels. RadionuclideS-values were computed for 22 and 10 radionuclides commonly employed in radiopharmaceutical therapy and nuclear medicine diagnostic imaging, respectively.Main results. For radionuclide decays, values ofS(brain tissue ← brain blood) assessed in the traditional manner (SR) were higher than those computed using our DR models by factors of 1.92, 1.49, and 1.57 for therapeutic alpha-emitters, beta-emitters, and Auger electron-emitters, respectively in the AFB and by factors of 1.65, 1.37, and 1.42 for these same radionuclide categories in the AMB. Corresponding ratios of SR and DR values ofS(brain tissue ← brain blood) were 1.34 (AFB) and 1.26 (AMB) for four SPECT radionuclides, and were 1.32 (AFB) and 1.24 (AMB) for six common PET radionuclides.Significance. The methodology employed in this study can be explored in other organs of the body for proper accounting of blood self-dose for that fraction of the radiopharmaceutical still in general circulation.

S-values estimation of positron-emitting radionuclides in the ICRP voxel-based adult male organs using a new Geant4-based code DoseCalcs: validation study.

Identifying the organs and tissues at risk from internal radiation exposure caused by radiopharmaceuticals requires determining the absorbed dose. The absorbed dose for radiopharmaceuticals is calculated by multiplying cumulated activity in source organs by the S-value, a crucial quantity that connects the energy deposited in the target organ and the emitting source one. It is defined as the ratio of absorbed energy in the target organ per unit of mass and unit of nuclear transition in the source organ. In this study, we used a new Geant4-based code called DoseCalcs to estimate the S-values for four positron-emitting radionuclides ([Formula: see text]C, [Formula: see text]N, [Formula: see text]O, and [Formula: see text]F) using decay and energy data from International Commission on Radiological Protection (ICRP) Publication 107. Twenty-three regions were simulated as radiation sources in the ICRP voxelized adult model developed in ICRP Publication 110. The Livermore physics packages were tailored to radionuclide photon mono-energy and [Formula: see text]-mean energy. The estimated S-values based on [Formula: see text]-mean energy show good agreement with those in the OpenDose data whose values were calculated using the full [Formula: see text] spectrum. The results provide new S-values data for selected source regions; hence, they could be used for comparison and adult-patient dose estimation.

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, ß, α, and X-ray for any radioisotopes. METHODS: Mono-energetic VSV database for γ, ß, 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.

An analytic model to calculate voxel s-values for177Lu.

Objective.177Lu is one of the most employed isotopes in targeted radionuclide therapies and theranostics, and 3D internal dosimetry for such procedures has great importance. Voxel S-Values (VSVs) approach is widely used for this purpose, but VSVs are available for a limited number of voxel dimensions. The aim of this work is to develop an analytic model for the calculation of177Lu-VSVs in any cubic voxelized geometry of practical interest.Approach. Monte Carlo (MC) simulations were implemented with the toolkit GAMOS to evaluate VSVs in voxelized geometries of soft tissue from a source of177Lu homogeneously distributed in the central voxel. Nine geometric setups, containing 15 × 15 × 15 cubic voxels of sideslranging from 2 mm to 6 mm, in steps of 0.5 mm, were considered. For eachl, the VSVs computed as a function of the 'normalized radius',Rn= R/l(withR = distance from the center of the source voxel), were fitted with a parametric function. The dependencies of the parameters as a function oflwere then fitted with appropriate functions, in order to implement the model for deducing177Lu-VSVs for anylwithin the aforementioned range.Main results. The MC-derived VSVs were satisfactorily compared with literature data for validation, and the VSVs computed with the analytic model agree with the MC ones within 2% forRn≤ 2 and within 6% forRn> 2.Significance. The proposed model enables the easy and fast calculation, with a simple spreadsheet, of177Lu-VSVs in any cubic voxelized geometry of practical interest, avoiding the necessity of implementingad-hocMC simulations to estimate VSVs for specific voxel dimensions not available in literature data.

InterDosi Monte Carlo study of radiation exposure of a reference crab phantom due to radioactive wastewater deposited in marine environment following the Fukushima nuclear accident.

S-values are typically used to quantify internal doses of biota internally due to the incorporation of radionuclides. In this study, the InterDosi 1.0 Monte Carlo code was used to estimate S-values in five main organs of a crab phantom as well as in surrounding seawater for eleven radionuclides, namely 3H, 14C, 134Cs, 137Cs, 60Co, 125Sb, 90Sr, 129I, 99Tc, 106Ru, and 238Pu. After the Fukushima accident, these radionuclides have been detected in wastewater by the Japan Nuclear Regulatory Authority. In this work, S-values were calculated for all crab organs and the surrounding seawater. These values can be used in conjunction with any measured activities in water, to determine internal doses absorbed by crab organs. Furthermore, it is shown that for a self-absorption condition the studied radionuclides can be classified into five main categories, with 238Pu showing the highest S-values for any organ. Moreover, the results demonstrate that the obtained S-values decrease with increasing organ mass. In contrast, for a cross-absorption condition, the studied organs can be classified into seven main categories. In addition, by taking seawater as a source of irradiation, 238Pu had the highest cross-absorption S-values in two organs of particular biological relevance, the heart and gonads, when compared to the remaining radionuclides. It is concluded that due to the pre-calculated S-value database of a reference crab, it will become easier to use this organism as a bio indicator to study any radiation-induced effects on the marine environment.

Dosimetry assessment of theranostic Auger-emitting radionuclides in a micron-sized multicellular cluster model: A Monte Carlo study using Geant4-DNA simulations.

The present work is aimed at improving the multicellular dosimetry of several Auger radionuclides of interest for targeted cancer therapy, including 99mTc, 111In, 123I, 125I, and 201Tl. For this purpose, using the Geant4-DNA Monte Carlo code, a cluster of 13 similar spherical cells with a hexagonal packed arrangement was modeled, and the mean absorbed doses per unit cumulated activity (S-values) were calculated by considering two target←source configurations, cell←cell and nucleus←nucleus. The obtained ratios of cross-dose to self-dose S-value in terms of the distance between the source and target regions were evaluated and also compared to those estimated by the Medical Internal Radiation Dose (MIRD) method. Besides, the contribution of the Coster-Kronig, Auger and internal conversion electrons to the S-values was provided for each radionuclide. According to the results, it can be concluded that in contrast to self-absorption, the cross-absorption due to the Auger-emitters has not a significant role in the total energy deposition within a cell in the cluster.

A mesh-based model of liver vasculature: implications for improved radiation dosimetry to liver parenchyma for radiopharmaceuticals.

PURPOSE: To develop a model of the internal vasculature of the adult liver and demonstrate its application to the differentiation of radiopharmaceutical decay sites within liver parenchyma from those within organ blood. METHOD: Computer-generated models of hepatic arterial (HA), hepatic venous (HV), and hepatic portal venous (HPV) vascular trees were algorithmically created within individual lobes of the ICRP adult female and male livers (AFL/AML). For each iteration of the algorithm, pressure, blood flow, and vessel radii within each tree were updated as each new vessel was created and connected to a viable bifurcation site. The vascular networks created inside the AFL/AML were then tetrahedralized for coupling to the PHITS radiation transport code. Specific absorbed fractions (SAF) were computed for monoenergetic alpha particles, electrons, positrons, and photons. Dual-region liver models of the AFL/AML were proposed, and particle-specific SAF values were computed assuming radionuclide decays in blood within two locations: (1) sites within explicitly modeled hepatic vessels, and (2) sites within the hepatic blood pool residing outside these vessels to include the capillaries and blood sinuses. S values for 22 and 10 radionuclides commonly used in radiopharmaceutical therapy and imaging, respectively, were computed using the dual-region liver models and compared to those obtained in the existing single-region liver model. RESULTS: Liver models with virtual vasculatures of ~ 6000 non-intersecting straight cylinders representing the HA, HPV, and HV circulations were created for the ICRP reference. For alpha emitters and for beta and auger-electron emitters, S values using the single-region models were approximately 11% (AML) to 14% (AFL) and 11% (AML) to 13% (AFL) higher than the S values obtained using the dual-region models, respectively. CONCLUSIONS: The methodology employed in this study has shown improvements in organ parenchymal dosimetry through explicit consideration of blood self-dose for alpha particles (all energies) and for electrons at energies below ~ 100 keV.

Body-size-dependent Iodine-131Svalues.

In a recent epidemiologic risk assessment on late health effects of patients treated with radioactive iodine (RAI), organ/tissue doses of the patients were estimated based on iodine-131Svalues derived from the reference computational phantoms of the International Commission on Radiological Protection (ICRP). However, the use of theSvalues based on the reference phantoms may lead to significant biases in the estimated doses of patients whose body sizes (height and weight) are significantly different from the reference body sizes. To fill this critical gap, we established a comprehensive dataset of body-size-dependent iodine-131Svalues (rT← thyroid) for 30 radiosensitive target organs/tissues by performing Monte Carlo dose calculations coupled with a total of 212 adult male and female computational phantoms in different heights and weights. We observed that theSvalues tend to decrease with increasing body height; for example, theSvalue (gonads ← thyroid) of the 160 cm male phantom is about 3 times higher than that of the 190 cm male phantom at the 70 kg weight. We also observed that theSvalues tend to decrease with increasing body weight for some organs/tissues; for example, theSvalue (skin ← thyroid) of the 45 kg female phantom is about two times higher than that of the 130 kg female phantom at the 160 cm height. For other organs/tissues, which are relatively far from the thyroid, in contrast, theSvalues tend to increase with increasing body weight; for example, theSvalue (bladder ← thyroid) of the 45 kg female phantom is about 2 times lower than that of the 130 kg female phantom. Overall, the majority of the body-size-dependentSvalues deviated to within 25% from those of the reference phantoms. We believe that the use of body-size-dependentSvalues in dose reconstructions should help quantify the dosimetric uncertainty in epidemiologic investigations of RAI-treated patients.

Hemishell Zeolites Synthesized by Asymmetric Modification as Biphasic Nanoreactors with Tunable Amphiphilicity for Catalysis of Cascade Reactions.

It is strongly desired to design and synthesize amphiphilic nanoreactors with tunable compatibility, which are stable at the biphasic interface in both acidic and alkaline environments. Herein, a novel amphiphilic R1-ZSM-5-R2 nanoreactor with adjustable hydrophilic-lipophilic balance (solid) (HLB(S)) values has been successfully synthesized by hydrophilic/lipophilic asymmetric modification of the surface of hemishell zeolites. The hemishell zeolites obtained by alkali etching have different surfaces for this asymmetric modification. Owing to the unique hemishell structures and asymmetric modification, the R1-ZSM-5-R2 nanoreactors with an optimized type and amount of modified organosilanes show excellent stability and emulsifying properties under extreme environments, which is important for cascade reactions in a biphasic system. The modified amino groups on the surface of the nanoreactors not only enhance the hydrophilicity of the hemishell zeolites and stabilize ultrasmall Pt nanoparticles (1.90 nm) but also used for the catalytic synthesis of trans-cinnamaldehyde. The Pt@R1-ZSM-5-R2 amphiphilic catalysts fabricated through a one-step reduction of Pt nanoparticles present outstanding performances in the biphasic cascade synthesis of cinnamic acid, achieving a very high turnover frequency (TOF) of 978 h-1. The TOF values of the catalysts correspond well to the HLB(S) values of the R1-ZSM-5-R2 nanoreactors.

Cellular S values in spindle-shaped cells: a dosimetry study on more realistic cell geometries using Geant4-DNA Monte Carlo simulation toolkit.

OBJECTIVE: Cellular dosimetry plays a crucial role in radiobiology and evaluation of the relative merits of radiopharmaceuticals used for targeted radionuclide therapy. The present study aims to investigate the effects of various cell geometries on dosimetric characteristics of several Auger emitters distributed in different subcellular compartments using Monte Carlo simulation. METHODS: The Geant4-DNA extension of the Geant4 Monte Carlo simulation toolkit was employed to calculate the mean absorbed dose per unit cumulated activity (S value) for different subcellular distributions of several Auger electron-emitting theranostic radionuclides including 99mTc, 111In, 123I, 125I, and 201Tl. The simulations were carried out in various single-cell models of liquid water including spherical, ellipsoidal, spherical spindle, and ellipsoidal spindle cell models. The latter two models which are generalized from the first two models were inspired by the morphologies of spindle-shaped (fusiform) cells, and were developed to provide more realistic modeling of this common geometry observed in many healthy and cancerous cells. RESULTS: Evaluation of the S values calculated for the examined cell models reveals that the differences are small (less than 9%) for the cell ← cell, cell ← cell surface, and nucleus ← nucleus source-target combinations. However, moderate discrepancies are seen (up to 28%) when the nucleus is considered as the target, as well as the radioactivity is either internalized into the cytoplasm or bound to the cell membrane. CONCLUSIONS: The findings of the present work suggest that the assumption of spherical cell geometry may provide reasonably accurate estimates of the cellular/nuclear dose for the considered Auger emitters, even for spindle-shaped cells. Of course, this approximation should be used with caution for the nucleus ← cytoplasm and nucleus ← cell surface configurations, since the S-value sensitivity to the cell geometry is somewhat significant in these cases.

Comparison of MCNPX and MIRDcell in assessing self-dose and cross-dose delivered to cell nuclei and the development of a realistic geometric model.

Purpose: This study aims to provide a comparison between MCNPX and MIRDcell calculations for self-dose and cross-dose for three therapeutic isotopes used in internal radiotherapy (Lu-177, I-131 and Y-90) and to develop a multi-cellular geometric model to simulate an in vitro scenario.Materials and Methods: The self- and cross-dose to individual cell nuclei were assessed by Monte Carlo N-Particle eXtended (MCNPX). A close-packed cubic cell arrangement was assumed with the same amount of radioactivity per cell. Various cell sizes and subcellular distributions of radioactivity (nucleus, cytoplasm and cell membrane) were simulated. S values were obtained by MIRDcell for comparison. A Python 3.4 program was used to generate random cell coordinates in order to build a complex model that takes certain real conditions (cell size and cluster size) into account.Results: The relative differences of MCNPX versus MIRD S values (Sself) ranged from 2.88 to 10.10% for Lu-177; from 0 to 8.41% for I-131 and from 2.80 to 9.58% for Y-90. The relative differences of MCNPX versus MIRDcell cross-dose S values were 3.6%-15.7% for a sphere. The ratio of Scross max to Sself decreased for Lu-177 and I-131 with increasing cell size. The source localization within the cells had no significant impact on the cross-dosing. For single cells, the subcellular location of the source had an effect on Sself.Conclusions: MCNPX and MIRD cell-calculated S values showed good agreement. The model provided could be used to predict the biological effect caused by emitted radiation from therapeutic radionuclides at the cellular level.

Cellular dosimetry of [177Lu]Lu-DOTA-[Tyr3]octreotate radionuclide therapy: the impact of modeling assumptions on the correlation with in vitro cytotoxicity.

BACKGROUND: Survival and linear-quadratic model fitting parameters implemented in treatment planning for targeted radionuclide therapy depend on accurate cellular dosimetry. Therefore, we have built a refined cellular dosimetry model for [177Lu]Lu-DOTA-[Tyr3]octreotate (177Lu-DOTATATE) in vitro experiments, accounting for specific cell morphologies and sub-cellular radioactivity distributions. METHODS: Time activity curves were measured and modeled for medium, membrane-bound, and internalized activity fractions over 6 days. Clonogenic survival assays were performed at various added activities (0.1-2.5 MBq/ml). 3D microscopy images (stained for cytoplasm, nucleus, and Golgi) were used as reference for developing polygonal meshes (PM) in 3DsMax to accurately render the cellular and organelle geometry. Absorbed doses to the nucleus per decay (S values) were calculated for 3 cellular morphologies: spheres (MIRDcell), truncated cone-shaped constructive solid geometry (CSG within MCNP6.1), and realistic PM models, using Geant4-10.03. The geometrical set-up of the clonogenic survival assays was modeled, including dynamic changes in proliferation, proximity variations, and cell death. The absorbed dose to the nucleus by the radioactive source cell (self-dose) and surrounding source cells (cross-dose) was calculated applying the MIRD formalism. Finally, the correlation between absorbed dose and survival fraction was fitted using a linear dose-response curve (high α/ß or fast sub-lethal damage repair half-life) for different assumptions, related to cellular shape and localization of the internalized fraction of activity. RESULTS: The cross-dose, depending on cell proximity and colony formation, is a minor (15%) contributor to the total absorbed dose. Cellular volume (inverse exponential trend), shape modeling (up to 65%), and internalized source localization (up to + 149% comparing cytoplasm to Golgi) significantly influence the self-dose to nucleus. The absorbed dose delivered to the nucleus during a clonogenic survival assay is 3-fold higher with MIRDcell compared to the polygonal mesh structures. Our cellular dosimetry model indicates that 177Lu-DOTATATE treatment might be more effective than suggested by average spherical cell dosimetry, predicting a lower absorbed dose for the same cellular survival. Dose-rate effects and heterogeneous dose delivery might account for differences in dose-response compared to x-ray irradiation. CONCLUSION: Our results demonstrate that modeling of cellular and organelle geometry is crucial to perform accurate in vitro dosimetry.

Factors affecting accuracy of S values and determination of time-integrated activity in clinical Lu-177 dosimetry.

INTRODUCTION: In any radiotherapy, the absorbed dose needs to be estimated based on two factors, the time-integrated activity of the administered radiopharmaceutical and the patient-specific dose kernel. In this study, we consider the uncertainty with which such absorbed dose estimation can be achieved in a clinical environment. METHODS: To calculate the total error of dose estimation we considered the following aspects: The error resulting from computing the time-integrated activity, the difference between the S-value and the patient specific full Monte Carlo simulation, the error from segmenting the volume-of-interest (kidney) and the intrinsic error of the activimeter. RESULTS: The total relative error in dose estimation can amount to 25.0% and is composed of the error of the time-integrated activity 17.1%, the error of the S-value 16.7%, the segmentation error 5.4% and the activimeter accuracy 5.0%. CONCLUSION: Errors from estimating the time-integrated activity and approximations applied to dose kernel computations contribute about equally and represent the dominant contributions far exceeding the contributions from VOI segmentation and activimeter accuracy.

[Study of the Usefulness of the Constancy Tests in X-rays Image Processing Device by User].

The constancy test for quality control of X-ray image processing devices was evaluated using long-term and continuous constancy test results. Control was set so that under specific imaging conditions, variation in exposure dose is within 20% from the base line and variation in system sensitivity (S-value) obtained from images scanned with imaging plate is within 40%. Controlling the exposure dose enabled adjustments of tube currents and organized exchanges of the X-ray tube. Controlling the S-value allowed adjustments from increase due to age as well as organized exchanges of the photomultiplier tube foundation of the computed radiography scanner when control range is rapidly exceeded. Results from long-term and continuous constancy tests were of extreme value in comprehending the condition of the device for early detection of abnormalities, and thus indicate the significance of constancy tests for quality control of image processing device.

Monte Carlo single-cell dosimetry using Geant4-DNA: the effects of cell nucleus displacement and rotation on cellular S values.

Investigation of biological effects of low-dose ionizing radiation at the (sub-) cellular level, which is referred to as microdosimetry, remains a major challenge of today's radiobiology research. Monte Carlo simulation of radiation tracks can provide a detailed description of the physical processes involved in dimensions as small as the critical substructures of the cell. Hereby, in the present study, microdosimetric calculations of cellular S values for mono-energetic electrons and six Auger-emitting radionuclides were performed in single-cell models of liquid water using Geant4-DNA. The effects of displacement and rotation of the nucleus within the cell on the cellular S values were studied in spherical and ellipsoidal geometries. It was found that for the examined electron energies and radionuclides, in the case of nucleus cross-absorption where the radioactivity is either localized in the cytoplasm of the cell or distributed on the cell surface, rotation of the nucleus within the cell affects cellular S values less than displacement of the nucleus. Especially, the considerable differences observed in S(nucleus ← cell surface) values between an eccentric and a concentric cell-nucleus configuration in spherical and ellipsoidal geometries (up to 63% and up to 44%, respectively) suggests that the approximation of concentricity should be used with caution, at least for localized irradiation of the cell membrane by an Auger-emitter in targeted radionuclide cancer therapy. The obtained results, which are based on a more realistic modeling of the cell than was done before, provide more accurate information about nuclear dose. This can be useful for theranostic applications.

Differences in the S value between male and female murine model for diagnostic, therapeutic and theragnostic radionuclides.

The aim of this work was to calculate S values for 99mTc, 67Ga, 68Ga, 18F, 223Ra, 166Ho, 90Y, 161Tb 131I and 177Lu, using a mouse phantom (MOBY) standard and considering the anatomic sizes from males and females, the simulation of radiation transport was performed with GATE/Geant4 platform. This indicates that in the internal dosimetry the use of a customized geometry is relevant for each gender and a standard model is not a good choice.

Assessment of S values for I131 and Y90 based on the Digimouse voxel phantom.

INTRODUCTION: Most preclinical studies using radiopharmaceuticals have been carried out on mice. In nuclear medicine and radioimmunotherapy procedures, I131 and Y90 have been widely used. For better estimation of doses from these procedures, S value plays a vital role. In this study, we have evaluated S values for major source and target combinations of Digimouse voxel phantom. MATERIALS AND METHODS: We have used the Digimouse voxel phantom which was incorporated in Monte Carlo code FLUKA. Simulation studies were performed using Monte Carlo simulation code FLUKA. Latest publication of International Commission on Radiological Protection (ICRP) report 110 was used for assigning the values of organ compositions and densities. Photon and electron spectra for I131 and Y90 have been taken from the ICRP publication 107. For Digimouse voxel phantom, we have simulated 9 source regions with an assumed uniform distribution of I131 and Y90. RESULTS AND DISCUSSION: S values have been evaluated for I131 and Y90 for different source and target combinations of Digimouse voxel phantom and presented in tabular form. The S values can be applied prospectively or retrospectively to the calculation of radiation doses internally exposed to I131 and Y90, including nuclear medicine and radioimmunotherapy procedures. These S values have been very important for the calculation of absorbed doses for various organs similar in size to present study for mice. CONCLUSION: In this study S value for I131 and Y90 were evaluated for major organs of digimouse voxel phantom. These S values are very important for absorbed dose calculation for various organs of a mouse.