Specific Absorbed Fractions Research Articles

Re-evaluation of F-FDG absorbed and effective dose in adult and pediatric phantoms using DoseCalcs Monte Carlo platform: a validation study.

Positron emission tomography (PET) using F-FDG is a well-known modality for the diagnosis of various diseases in patients of different ages, sexes, and states of health, which implies that internal radiation dosimetry is highly desired for different phantom anatomies. In this study, we validate "DoseCalcs," a new Monte Carlo platform that combines personalized internal dosimetry calculations with Monte Carlo simulations. To achieve that, we used the specific absorbed fraction (SAF) calculated by DoseCalcs and those from ICRP publication 133 to estimate the absorbed dose per injected activity (AD/IA) and effective dose per injected activity (ED/IA) for F-FDG. The investigation focused on various voxelized phantoms representing different age groups, including adult male and female, and pediatric phantoms of various ages, from newborn to 15 years old. Using the DoseCalcs Monte Carlo platform, we have simulated the emission of F-FDG positrons based on the energy spectrum provided in ICRP publication 107. The results demonstrated the impact of anatomical differences and different organ/tissue compositions on radiation absorption, with significant variations in the AD/IA across different phantoms. Interestingly, organs/tissues near the emission source showed higher AD/IA, highlighting the anatomical dependence on the phantom. When our results were compared to established reference data, especially from ICRP128, most organs/tissues had good agreement. Still, some cases have shown differences. This shows how important it is to use accurate radionuclide data and biokinetic modeling in internal dosimetry calculations. Furthermore, we compared AD/IA and ED/IA values calculated in newborns by DoseCalcs with those derived from alternative codes, MCNP and EGSnrc. While the results generally exhibited consistency, subtle variations underscored the influence of biokinetics modeling choices and computational methodologies. Overall, this research contributes valuable insights into the precision of internal dosimetry calculations using "DoseCalcs-Gui" by providing one platform for Monte Carlo simulation and personalized internal dosimetry in nuclear medicine. The DoseCalcs platform is free for research and available for download at www.github.com/TarikEl/DoseCalcs-Gui .

Enhanced internal dosimetry for alimentary tract organs in nuclear medicine based on the ICRP mesh-type reference phantoms

This study introduces a refined approach for more accurately estimating radiation doses to alimentary tract organs in nuclear medicine, by utilizing the ICRP pediatric and adult mesh-type reference computational phantoms (MRCPs) that improved the anatomical representation of these organs. Our initial step involved compiling a comprehensive dataset of electron Specific Absorbed Fractions (SAFs) for all source-target pairs of alimentary tract organs in both adult and pediatric phantoms, calculating SAFs for all cases in the present study only except those computed in the previous study for certain pediatric phantom cases. Subsequently, we determined S values for 1252 radionuclides, facilitating dosimetry applications. The consistency of target and source masses for alimentary tract organs in the MRCPs with the reference values in ICRP Publication 89 led to noticeable differences in SAF, S values, and consequently, absorbed dose coefficients when compared to the stylized models in ICRP Publication 100. Notably, the S value ratios (MRCP/stylized) for selected radionuclides—11C, 18F, 68Ga, and 131I—ranged from 0.41 to 7.60. Particularly for therapeutic 131I-iodide in thyroid cancer, the use of MRCPs resulted in up to 1.49 times higher absorbed dose coefficients for the colon than those derived from stylized models, while the stomach dose coefficients decreased by a factor of 0.72. The application of our findings promises enhanced, more realistic dosimetry for alimentary tract organs, especially beneficial for radiopharmaceuticals likely to accumulate within these organs.

Evaluation of the InterDosi code in estimating S-values with the digimouse voxelized mouse phantom

This study delves into the realm of internal dosimetry in small animals, focusing on the crucial role of S-values (absorbed dose rate per unit activity). The primary objective is to enhance our understanding of ionizing radiation distribution and optimize treatment strategies involving multidisciplinary approach which integrates dosimetry data, computational modeling, clinical expertise, and patient-specific factors to deliver safe, effective, and personalized cancer treatments, paving the way for more effective and tailored radiotherapeutic interventions and diagnostic cancer research. The research specifically targets the assessment of radiation doses absorbed in target organs and adjacent tissues during targeted radionuclide therapy with iodine-131 and yttrium-90. Executed on the ERSN (Radiations and Nuclear Systems Laboratory) computing grid, the study employed optimized calculations using 20 CPU (Central Processing Unit) cores for each of the 103 SAF (Specific Absorbed Fractions) grid sizes of photons and electrons. The InterDosi code, a user-friendly tool, streamlined S-value estimation within the Digimouse phantom. Nine source organs and the entire murine anatomy were comprehensively evaluated. The G4EMLiverMorePhysics package ensured high-confidence dosimetric coefficient simulations. S-values revealed insights into absorbed dose influences, including organ mass and inter-organ distance. Comparative analyses were conducted with reference data, which involved a simplified S-value calculation. The results aligned well with the reference data, especially with similar SAF grid sizes for electrons and photons. Accuracy improved with larger grid sizes. This study confirms InterDosi's effectiveness, highlighting its adaptability and precision in estimating S-values for various radionuclides, making it a valuable tool for evaluating radiation doses in murine anatomy.

Development and validation of a Medaka fish voxel-based model for internal ionizing radiation dosimetry

In this paper, we present a voxel-based phantom of Medaka fish that can be used to assess the internal radiation doses that would be absorbed by different organs of this fish species if exposed to radioactive wastewater released into the ocean. The geometric model for fish was generated based on available Wavefront Object files for smooth-bodied Medaka fish organs, whereas due to the lack of Medaka fish material specification, the material model was constructed using material data appropriate to ICRP 110 adult male voxel-based phantom. Absorbed Fractions (AFs) and Specific Absorbed Fractions (SAFs) were calculated for eight organs of major interest as sources and for each organ as target at a set of discrete photon, electron, alpha and neutron energies. To validate the present model the calculated AFs in the studied organs were compared to ones obtained in similar organs in a voxel-based phantom of another teleost fish species called Limanda limanda. The results presented are consistent with the reference dosimetric data. We concluded that the Medaka model can be used in radioecology research to improve marine radiation protection.

Specific Absorbed Fractions for Reference Paediatric Individuals.

The calculation of doses to organs and tissues of interest due to internally emitting radionuclides requires knowledge of the time-dependent distribution of the radionuclide, its physical decay properties, and the fraction of emitted energy absorbed per mass of the target. The latter property is quantified as the specific absorbed fraction (SAF). This publication provides photon, electron, alpha particle, and neutron (for nuclides undergoing spontaneous fission) SAF values for the suite of reference individuals. The reference individuals are defined largely by information provided in ICRP Publication 89. Some improvements and additional data are provided in this publication which define the reference individual's source and target region masses used in the Occupational Intake of Radionuclides (OIR) and Dose Coefficients for Intakes of Radionuclides by Members of the Public series of publications. The set of reference individuals includes males and females at 0 (newborn), 1, 5, 10, 15, and 20 (adult) years of age. The reference adult masses and SAFs provided in this publication are identical to those in ICRP Publication 133 and those used in the OIR series of publications. Computation of SAF values involves simulating radiation transport in computational models which represent the geometry of the reference individuals. The reference voxel phantoms of ICRP Publication 143 are used for photon and neutron transport, and most electron transport. Alpha particle transport is not necessary for large tissue regions as the short range allows for an assumption of full energy absorption (absorbed fraction of unity) for self-irradiation geometries. Additional computational models are needed for charged particles in small, overlapping, or interlaced geometries. Stylised models are described and used for electrons and alpha particles in the alimentary and respiratory tract regions. Image-based models are used to compute SAFs for charged particles within the skeleton. This publication is accompanied by an electronic supplement which includes files containing SAFs for each radiation type in each reference individual. The supplement also includes source and target region masses for each reference individual, as well as skeletal dose-response functions for photons incident upon the skeleton.© 2024 ICRP. Published by SAGE.

Monte Carlo simulations for targeted beta therapy: An optimization for liver lesions and comparison of dose distributions in other organs

ObjectiveTo optimize targeted beta therapy for liver lesions in adult male phantom by comparing the efficacy and safety profiles of five different beta-emitting radionuclides: 90Y, 166Ho, 153Sm, 47Sc, and 177Lu. MethodsThis study includes Monte Carlo simulations of the behavioral characteristics of five different beta emitters that have current or potential use in targeted beta therapy. The energy loss of beta particles moving within the material through ionization or chemical processes, the energy transferred to the material, the energy lost by beta particles along the distance traveled within the tissue, and consequently, the stopping power are calculated using the Bethe-Bloch formula. The CSDA (continuous slowing-down approximation) range of beta particles within the tissue is examined using ESTAR and GEANT codes, while the stopping power of the tissue is investigated using FLUKA, ESTAR, and GEANT codes. Tissue dose calculations for the target organ are obtained using the IDAC-Dose2.1 and MIRDcalc simulation programs, using parameters such as absorbed dose per accumulated activity (S-factor) and specific absorbed fraction (SAF). Additionally, dose and flux values are obtained using the PHITS program. ResultsThe behaviors and dose contribution of beta particles in liver tissue have been addressed in various ways. 90Y, which has the highest average beta energy, was observed to provide a higher absorbed dose value in the liver compared to other beta-emitting isotopes, while the lowest absorbed dose was observed with 177Lu. In other organs, it has been observed that 90Y and 47Sc contribute to a higher absorbed dose compared to other beta-emitting isotopes. ConclusionsThis study emphasizes the complexity and significance of targeted beta therapy optimization.

Construction of voxel-based Portunus haanii phantom and its absorbed fractions and specific absorbed fractions calculation based on Monte Carlo simulations

ObjectiveTo build the database of the absorbed fractions (AFs) and specific absorbed fractions (SAFs), in order to accurately assess the internal radiation dose in non-human biota. MethodsA voxel-based Portunus haanii phantom was established based on the computed tomography (CT) images. A set of AFs and SAFs were calculated with Monte Carlo toolkit Geant4 for the emission of monoenergetic photons and electrons with energies ranging from 10 ​keV to 5 ​MeV. ResultsThe mass of the voxel-based Portunus haanii phantom (392.2 ​g) was in agreement with the actual mass (389.2 ​g), indicating the reliability of the phantom. The calculated AFs and SAFs, based on the voxel-based Portunus haanii phantom, provided precise and reliable data for conducting internal radiation dose calculations specifically tailored to the Chinese Red Swimming Crab (Portunus haanii). The results indicated that the self-AFs and self-SAFs were affected by both the radiation energy and the mass of the source/target organ. Moreover, the AFs and SAFs for cross irradiation, were not only dependent on the energy and the mass of the target organ, but also on the relative position of the source and target organs. ConclusionThese results serve as a valuable resource for accurately evaluating the internal radiation exposure of this species.

Organ dose calculator for diagnostic nuclear medicine patients based on the ICRP reference voxel phantoms and biokinetic models

The exponential growth in the use of nuclear medicine procedures represents a general radiation safety concern and stresses the need to monitor exposure levels and radiation-related long term health effects in NM patients. In the current study, following our previous work on NCINM version 1 based on the UF/NCI hybrid phantom series, we calculated a comprehensive library of S values using the ICRP reference pediatric and adult voxel phantoms and established a library of biokinetic data from multiple ICRP Publications, which were then implemented into NCINM version 2. We calculated S values in two steps: calculation of specific absorbed fraction (SAF) using a Monte Carlo radiation transport code combined with the twelve ICRP pediatric and adult voxel phantoms for a number of combinations of source and target region pairs; derivation of S values from the SAFs using the ICRP nuclear decay data. We also adjusted the biokinetic data of 105 radiopharmaceuticals from multiple ICRP publications to match the anatomical description of the ICRP voxel phantoms. Finally, we integrated the ICRP phantom-based S values and adjusted biokinetic data into NCINM version 2. The ratios of cross-fire SAFs from NCINM 2 to NCINM 1 for the adult phantoms varied widely from 0.26 to 5.94 (mean = 1.24, IQR = 0.77–1.55) whereas the ratios for the pediatric phantoms ranged from 0.64 to 1.47 (mean = 1.01, IQR = 0.98–1.03). The ratios of absorbed dose coefficients from NCINM 2 over those from ICRP publications widely varied from 0.43 (colon for 99mTc-ECD) to 2.57 (active marrow for 99mTc-MAG3). NCINM 2.0 should be useful for dosimetrists and medical physicists to more accurately estimate organ doses for various nuclear medicine procedures.

Photon-specific absorbed fraction estimates in stylized ORNL and voxelized ICRP adult male phantoms using a new developed Geant4-based code "DoseCalcs": a validation study.

When a radiotracer is injected into a patient's body as part of a nuclear medicine investigation, the entire body is exposed to the ionizing radiation emitted, which can cause biological damage. Therefore, it is important to predict the internal radiation dose to properly balance the advantages of radiological examinations. Currently, various Monte Carlo tools, such as MCNP, Geant4, and GATE, are available to estimate internal radiation dosimetry-related quantities, such as S values (S) and specific absorbed fractions (SAF). Such codes make physics easier for physicists who are experienced with computer programming; however, programming and/or simulation inputs remain a time-consuming and intensive task. In this study, we present a newly developed Geant4-based code for internal dosimetry calculations, namely "DoseCalcs". To assess the performance of the geometrical methods and computational capabilities of our developed tool, we used the GDML, TEXT, STL, and C++ methods to model the ORNL adult phantom, and a voxel-based structure to construct the ICRP adult male. SAFs in the ORNL and ICRP adult male phantoms for eight discrete mono-energetic photons with energies ranging from 0.01 to 2 MeV are calculated with DoseCalcs and compared to ORNL and OpenDose reference data. The two phantoms showed good agreement with both references, which indicates the accuracy of DoseCalcs for subsequent use in estimating internal dosimetry quantities using a variety of geometrical methods.

Specific Absorbed Fractions for Spontaneous Fission Neutron Emitters in the ICRP Reference Pediatric Voxel Phantom Series.

Specific absorbed fractions (SAFs) are key components in the workflow of internal exposure assessment following the intake of a radionuclide, allowing quick conversion of particle energy released in a source region to the expected absorbed dose in target regions throughout the body. For data completeness, SAFs for spontaneous fission neutron emitters are currently needed for the recently adopted ICRP reference pediatric voxel phantom series. With 77 source regions within each reference individual and 28 radionuclides decaying via spontaneous fission, full Monte Carlo simulation requires significant computation time. In order to reduce this burden, a novel method for neutron SAF estimation was undertaken. The Monte Carlo N-Particle version 6.1 (MCNP6) simulation package was chosen to simulate the 252 Cf Watt fission neutron spectrum originating from 15 source regions in each phantom; dose estimation within 41 target tissues allowed for assessment of the SAF value for each source-target pair. For the remaining source regions, chord length distributions were computed using MATLAB code to determine the separation between the source-target pairs within the pediatric phantom series. These distance distributions were used in conjunction with a 252 Cf neutron dose point kernel calculated in soft tissue, which was modified to account for the source region's depth from the surface of the body. Lastly, the 252 Cf SAF dataset was extended to the other 27 spontaneous fission neutron emitters based on differences in the Watt fission spectrum parameters of each radionuclide. This methodology has been shown to accurately estimate spontaneous fission neutron SAFs to within 20% of the Monte Carlo estimated value for most source-target pairs in the ICRP reference pediatric series.

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

PurposeTo 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.MethodComputer-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.ResultsLiver 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.ConclusionsThe 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.

Estimation of electron-specific absorbed fractions with the InterDosi code using ICRP adult female voxel-based phantom

The International Commission on Radiological Protection (ICRP) through its publications recommends the estimation of Specific Absorbed Fractions (SAFs) using voxelized phantoms in order to assess the doses internally absorbed by organs exposed to ionizing radiation. In the present work, we report a large set of SAFs calculated using the ICRP Adult Female (ICRP-AF) phantom. The new Geant4-based code called InterDosi version 1.0 was used to simulate monoenergetic electrons of 20 different energies, ranging from 0.005 to 10 MeV, emitted uniformly from 18 different source organs. In order to estimate SAFs in 169 target organs/regions, 360 Monte Carlo multithreaded simulations were run on 32 CPUs of the HPC-MARWAN-CNRST computing grid. The calculated SAFs were compared to the recent results obtained using GATE 8.1 code and published by the OpenDose collaboration. It is shown that the obtained results are in well-agreement with the reference values, with absolute discrepancies less than 0.6% for a self-absorption condition and less than 5% in almost all energies for a cross-absorption condition. We concluded that InterDosi code might be used for dosimetry of internal electron emitters.

Radiation shielding properties of gallium alloys

Consideration of the photon interaction in the target medium is not sufficient for the estimation of absorbed dose in the different organs from a source of photons. For the accurate absorbed dose calculation specific absorbed fraction of energy is required. It is defined as the ratio of the energy absorbed by the target to the energy emitted by the source. The calculation of specific absorbed fraction of energy depends on the buildup factor values and the interaction of secondary electrons. X-rays and gamma rays undergo scattering on entering a medium, thus the energy gets degraded, due to which secondary radiation are produced. These secondary radiations can be calculated using buildup factor. Gallium alloys are less toxic and cost effective material compared to lead. Due to the importance of Gallium alloys in radiation shielding, the study of an interaction of X-rays and gamma radiation in these alloys becomes important.In the present work we have investigated the energy exposure buildup factors (EBF) and specific absorbed fraction (SAF) of energy for some gallium alloys such as Gallium alloy [Al-50%, Ga-50%], Galfenol [Fe-30%, Ga-70%] and Galinstan [Ga-68.5%, In-21.5%, Sn-10%]. With the increase in the penetration depth, the value of buildup factors increases. It is observed that the values of exposure buildup factors and Specific absorbed fraction of energy are larger for the gallium alloy Galinstan than the other studied gallium alloys. From this work it is clear that among the studied gallium alloys, in Galinstan absorption of X-rays and gamma radiations more compared to other studied alloys. Hence, we can conclude that the gallium alloy Galinstan is a good absorber of X-rays and gamma radiation among the studied gallium alloys. This work is useful in the shielding of radiation.

Specific absorbed fraction of energy and relative dose in some polymers

We, in this work investigated the specific absorbed fraction (SAF) of energy, energy absorption buildup factors (ABFs) and relative dose (RD) in the energy range 15 keV–15 MeV for some polymers such as Zylon (C14H6O2N2), Technora (C34H24O5N4), Mylar (C10H8O4), Twaron (C14H10O2N2), Bakalite (C14H12O2) and Orlon (C3H3N) up to the penetration depth (PD) of 40 mfp using GP fitting method. With the increase in the penetration depth, buildup factors increase. It is observed that the values of shielding parameters like effective atomic number (EAN), buildup factor (BF) and mass attenuation coefficient (MAC) are larger for the polymer Technora (C34H24O5N4) than the other studied polymers. SAF of energy is maximum for the polymer Technora (C34H24O5N4. Hence, we can conclude that the polymer Technora (C34H24O5N4) is a good absorber of X-rays, neutrons and gamma among the investigated polymers. This work finds its usefulness in the shielding of radiation.

InterDosi simulations of photon and alpha specific absorbed fractions in zubal voxelized phantom

In this work we used the InterDosi code to estimate photon specific absorbed fractions (SAFs) for some organs of the Zubal adult male voxelized phantom. Chemical compositions and densities of ICRP 110 adult male organs were attributed to those of the studied voxelized phantom. The SAFs of monoenergetic photons with energies ranging from 0.01 to 2 MeV, were calculated for three target regions, namely kidneys, liver, and spleen, which were the radiation source regions too. The obtained SAFs were compared to recent results obtained with the GATE code. In the GATE study, chemical compositions and densities of different organs were obtained from the ICRU report number 44. The inter-comparisons between the two studies show reasonably similar results, as 80% of the calculated SAFs are consistent within 2.5% discrepancy. This demonstrates the usefulness and applicability of the InterDosi code for internal dose calculations in a voxel-based phantom. We completed this work by studying the alpha SAFs in some organs for energies emitted by 213Bi used in targeted alpha-therapy and an analytical formula was derived for rapid alpha self-irradiation calculation in soft tissues.

Radiopharmacokinetic modelling and radiation dose assessment of 223Ra used for treatment of metastatic castration-resistant prostate cancer

PurposeRa-223 dichloride (223Ra, Xofigo®) is used for treatment of patients suffering from castration-resistant metastatic prostate cancer. The objective of this work was to apply the most recent biokinetic model for radium and its progeny to show their radiopharmacokinetic behaviour. Organ absorbed doses after intravenous injection of 223Ra were estimated and compared to clinical data and data of an earlier modelling study.MethodsThe most recent systemic biokinetic model of 223Ra and its progeny, developed by the International Commission on Radiological Protection (ICRP), as well as the ICRP human alimentary tract model were applied for the radiopharmacokinetic modelling of Xofigo® biodistribution in patients after bolus administration. Independent kinetics were assumed for the progeny of 223Ra. The time activity curves for 223Ra were modelled and the time integrated activity coefficients, overset{sim }{a}left({r}_S,{T}_Dright), in the source regions for each progeny were determined. For estimating the organ absorbed doses, the Specific Absorbed Fractions (SAF) and dosimetric framework of ICRP were used together with the aforementioned overset{sim }{a}left({r}_S,{T}_Dright) values.ResultsThe distribution of 223Ra after injection showed a rapid plasma clearance and a low urinary excretion. Main elimination was via faeces. Bone retention was found to be about 30% at 4 h post-injection. Similar tendencies were observed in clinical trials of other authors. The highest absorbed dose coefficients were found for bone endosteum, liver and red marrow, followed by kidneys and colon.ConclusionThe biokinetic modelling of 223Ra and its progeny may help to predict their distributions in patients after administration of Xofigo®. The organ dose coefficients of this work showed some variation to the values reported from clinical studies and an earlier compartmental modelling study. The dose to the bone endosteum was found to be lower by a factor of ca. 3 than previously estimated.

Specific absorbed fractions for a revised series of the UF/NCI pediatric reference phantoms: internal photon sources

Assessment of radiation absorbed dose to internal organs of the body from the intake of radionuclides, or in the medical setting through the injection of radiopharmaceuticals, is generally performed based upon reference biokinetic models or patient imaging data, respectively. Biokinetic models estimate the time course of activity localized to source organs. The time-integration of these organ activity profiles are then scaled by the radionuclide S-value, which defines the absorbed dose to a target tissue per nuclear transformation in various source tissues. S-values are computed using established nuclear decay information (particle energies and yields), and a parameter termed the specific absorbed fraction (SAF). The SAF is the ratio of the absorbed fraction—fraction of particle energy emitted in the source tissue that is deposited in the target tissue—and the target organ mass. While values of the SAF may be computed using patient-specific or individual-specific anatomic models, they have been more widely available through the use of computational reference phantoms. In this study, we report on an extensive series of photon SAFs computed in a revised series of the University of Florida and the National Cancer Institute pediatric reference phantoms which have been modified to conform to the specifications embodied in the ICRP reference adult phantoms of Publication 110 (e.g. organs modeled, organ ID numbers, blood contribution to elemental compositions). Following phantom anatomical revisions, photon radiation transport simulations were performed using MCNPX v2.7 in each of the ten phantoms of the series—male and female newborn, 1 year old, 5 year old, 10 year old, and 15 year old—for 60 different tissues serving as source and/or target regions. A total of 25 photon energies were considered from 10 keV to 10 MeV along a logarithm energy grid. Detailed analyses were conducted of the relative statistical errors in the Monte Carlo target tissue energy deposition tallies at low photon energies and over all energies for source–target combinations at large intra-organ separation distances. Based on these analyses, various data smoothing algorithms were employed, including multi-point weighted data smoothing, and log–log interpolation at low energies (1 keV and 5 keV) using limiting SAF values based upon target organ mass to bound the interpolation interval. The final dataset is provided in a series of ten electronic supplemental files in MS Excel format. The results of this study were further used as the basis for assessing the radiative component of internal electron source SAFs as described in our companion paper (Schwarz et al 2021) for this same pediatric phantom series.

Monte Carlo calculation of photon specific absorbed fractions in digimouse voxelized phantom using InterDosi code

Development of new radiopharmaceuticals is carried out on a small animal such as a mouse. With the aim to well-assess internal absorbed dose reaching various organs of a mouse, physical quantities defined by MIRD methodology known as specific absorbed fractions (SAFs) were evaluated using a voxelized mouse phantom. In our study, in order to provide more accurate photon SAFs, we used recent chemical compositions and densities of human organs taken from ICRP publication number 110. A new computational Geant4-based code called InterDosi has been developed to calculate SAFs for source-target organ pairs in Digimouse phantom irradiated by discrete monoenergetic photons of energies ranging from 0.015 to 4 MeV. Results show that, photon self-irradiation SAFs decrease with increasing energy or mass. For a cross-irradiation case, the photon SAFs were found to depend on distance source-to-target and organs close to the source have the most important photon SAFs. A comparison between our results and those reported in an earlier study with similar values of voxel sizes, masses, densities and chemical compositions, generally shows a significant agreement between them. Therefore, for a given configuration (energy, source), a smooth variation was found in the case of a self-irradiation. Contrariwise, medium discrepancy (less than 24%) was found in the case of a cross-irradiation, which can be attributed to the strong impact of the implementation of physics of the simulation in the two different codes and also the large statistical uncertainties associated to the calculation of SAFs in distant organs.

Specific absorbed fractions for a revised series of the UF/NCI pediatric reference phantoms: internal electron sources

In both the International Commission on Radiological Protection (ICRP) and Medical Internal Radiation Dose (MIRD) schemata of internal dosimetry, the S-value is defined as the absorbed dose to a target organ per nuclear decay of the radionuclide in a source organ. Its computation requires data on the energies and yields of all radiation emissions from radionuclide decay, the mass of the target organ, and the value of the absorbed fraction—the fraction of particle energy emitted in the source organ that is deposited in the target organ. The specific absorbed fraction (SAF) is given as the ratio of the absorbed fraction and the target mass. Historically, in the early development of both schemata, computational simplifications were made to the absorbed fraction in considering both organ self-dose () and organ cross-dose (). In particular, the value of the absorbed fraction was set to unity for all ‘non-penetrating’ particle emissions (electrons and alpha particles) such that they contributed only to organ self-dose. As radiation transport codes for charged particles became more widely available, it became increasingly possible to abandon this distinction and to explicitly consider the transport of internally emitted electrons in a manner analogous to that for photons. In this present study, we report on an extensive series of electron SAFs computed in a revised series of the UF/NCI pediatric phantoms. A total of 28 electron energies—0–10 MeV—along a logarithmic energy grid are provided in electronic annexes, where 0 keV is associated with limiting values of the SAF. Electron SAFs were computed independently for collisional energy losses (SAFCEL) and radiation energy losses (SAFREL) to the target organ. A methodology was employed in which values of SAFREL were compiled by first assembling organ-specific and electron energy-specific bremsstrahlung x-ray spectra, and then using these x-ray spectra to re-weight a previously established monoenergetic database of photon SAFs for all phantoms and source–target combinations. Age-dependent trends in the electron SAF were demonstrated for the majority of the source–target organ pairs, and were consistent to values given for the ICRP adult phantoms. In selected cases, however, anticipated age-dependent trends were not seen, and were attributed to anatomical differences in relative organ positioning at specific phantom ages. Both the electron SAFs of this study, and the photon SAFs from our companion study, are presently being used by ICRP Committee 2 in its upcoming pediatric extension to ICRP Publication 133.

Specific absorbed fractions and radionuclide S-values for tumors of varying size and composition

Accurate estimates of tumor absorbed dose are essential for the evaluation of treatment efficacy in radiopharmaceutical cancer therapy. Although tumor dosimetry via the MIRD schema has been previously investigated, prior studies have been limited to the consideration of soft-tissue tumors. In the present study, specific absorbed fractions (SAFs) for monoenergetic photons, electrons, and alpha particles in tumors of varying compositions were computed using Monte Carlo simulations in MCNPX after which self-irradiation S-values for 22 radionuclides (along with 14 additional alpha-emitter progeny) were generated for tumors of both varying size and tissue composition. The tumors were modeled as spheres with radii ranging from 0.10 cm to 6.0 cm and with compositions varying from 100% soft tissue (ST) to 100% mineral bone (MB). The energies of the photons and electrons were varied on a logarithm energy grid from 10 keV to 10 MeV. The energies of alpha particles were varied along a linear energy grid from 0.5 MeV to 12 MeV. In all cases, a homogenous activity distribution was assumed throughout the tumor volume. Furthermore, to assess the effect of tumor shape, several ellipsoidal tumors of different compositions were modeled and absorbed fractions were computed for monoenergetic electrons and photons. S-values were then generated using detailed decay data from the 2008 MIRD Monograph on Radionuclide Data and Decay Schemes. Our study results demonstrate that a soft-tissue model yields relative errors of 25% and 71% in the absorbed fraction assigned to uniform sources of 1.5 MeV electrons and 100 keV photons, respectively, localized within a 1 cm diameter tumor of MB. The data further show that absorbed fractions for moderate ellipsoids can be well approximated by a spherical shape of equal mass within a relative error of < 8%. S-values for 22 radionuclides (and their daughter progeny) were computed with results demonstrating how relative errors in SAFs could propagate to relative errors in tumor dose estimates as high as 86%. A comprehensive data set of radionuclide S-values by tumor size and tissue composition is provided for application of the MIRD schema for tumor dosimetry in radiopharmaceutical therapy.