Electron Specific Absorbed Fractions Research Articles

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.

Evaluation of Doses Absorbed to a Bone Marrow Stem Cell Layer from Short-lived Radionuclides in the Blood Vessels and from Long-lived Radionuclides in the Cortical Bone

The International Commission on Radiological Protection (ICRP) internal dose assessment model, currently adopted in Japanese regulation, assumes uniform distribution of radionuclides in bone marrow blood (ICRP Publication 60). Recent studies have revealed a localization of hematopoietic stem cells (HSCs) and immune cells in the perivascular region of the bone marrow sinusoids, suggesting a need to consider nonuniform distributions of the blood source and HSCs. To evaluate energy transfer to HSCs, a simplified model of cervical vertebrae with bone tissues and blood vessels was built using data from the adult Japanese male phantom. Doses absorbed by HSCs from blood and hard bone sources were calculated using a Monte Carlo simulation, and absorbed fractions (AFs) and specific absorbed fractions (SAFs) from electrons were compared with those in the ICRP 1990 model. In the cervical vascular model, electron SAFs from sinusoidal blood in the red bone marrow (RBM) to the target perivascular region were 1.2 to 6.9 times higher than the SAF in the ICRP 1990 model, suggesting an underestimation of the RBM dose. Electrons from the cortical bone source to the perisinusoidal target exhibited energy transfer. The ICRP 1990 model underestimates electron SAFs from radionuclides in sinusoidal blood and cortical bones. A more elaborate model is needed to examine doses for the RBM and effects on hematopoietic and immune functions.

S VALUES FOR NEUROIMAGING PROCEDURES ON KOREAN PEDIATRIC AND ADULT HEAD COMPUTATIONAL PHANTOMS.

Over the past decades, the application of single-photon emission computed tomography and positron emission tomography in neuroimaging has markedly increased. In the current study, we used a series of Korean computational head phantoms with detailed cranial structures for 6-, 9-, 12-, 15-y-old children and adult and a Monte Carlo transport code, MCNPX, to calculate age-dependent specific absorbed fraction (SAF) for mono-energetic electrons ranging from 0.01 to 4 MeV and S values for seven radionuclides widely used in nuclear medicine neuroimaging for the combination of ten source and target regions. Compared to the adult phantom, the 6-y phantom showed up to 1.7-fold greater SAF (cerebellum < cerebellum) and up to 1.4-fold greater S values (vitreous body < lens) for 123I. The electron SAF data, combined with our previous photon SAF data, will facilitate absorbed dose calculations for various cranial structures in patients undergoing neuroimaging procedures.

KOREAN PEDIATRIC AND ADULT HEAD COMPUTATIONAL PHANTOMS AND APPLICATION TO PHOTON SPECIFIC ABSORBED FRACTIONS CALCULATIONS.

In recent decades, applications of single photon emission computed tomography and positron emission tomography in clinical neuroimaging have markedly increased. In this study, we developed a series of Korean computational head phantoms with detailed cranial substructures for 6-, 9-, 12- and 15-year-old children and adult by non-uniformly adjusting a template head phantom to match the Korean standard head dimensions. The Korean head phantoms were coupled with a Monte Carlo transport code to calculate age-dependent specific absorbed fraction (SAF) for the combination of 10 source and target regions and mono-energetic photons ranging from 0.01 to 4 MeV. Compared to the adult phantom, the 6-y phantom showed up to 1.4-fold greater self-absorption SAF (cerebellum) and up to 1.8-fold greater cross-irradiation SAF (cerebellum < eye balls). With addition of electron SAFs in the future, our photon SAF data will facilitate dose calculations for various cranial substructures in patients undergoing cranial neuroimaging procedures.

ICRP Publication 133: The ICRP computational framework for internal dose assessment for reference adults: specific absorbed fractions.

Dose coefficients for assessment of internal exposures to radionuclides are radiological protection quantities giving either the organ equivalent dose or effective dose per intake of radionuclide following ingestion or inhalation. In the International Commission on Radiological Protection’s (ICRP) Occupational Intakes of Radionuclides (OIR) publication series, new biokinetic models for distribution of internalised radionuclides in the human body are presented as needed for establishing time-integrated activity within organs of deposition (source regions). This series of publications replaces Publications 30 and 68 (ICRP, 1979, 1980, 1981, 1988, 1994b). In addition, other fundamental data needed for computation of the dose coefficients are radionuclide decay data (energies and yields of emitted radiations), which are given in Publication 107 (ICRP, 2008), and specific absorbed fraction (SAF) values – defined as the fraction of the particle energy emitted in a source tissue region that is deposited in a target tissue region per mass of target tissue. This publication provides the technical basis for SAFs relevant to internalised radionuclide activity in the organs of Reference Adult Male and Reference Adult Female as defined in Publications 89 and 110 (ICRP, 2002, 2009). SAFs are given for uniform distributions of mono-energetic photons, electrons, alpha particles, and fission-spectrum neutrons over a range of relevant energies. Electron SAFs include both collision and radiative components of energy deposition. SAF data are matched to source and target organs of the biokinetic models of the OIR publication series, as well as the Publication 100 (ICRP, 2006) Human Alimentary Tract Model and the Publication 66 (ICRP, 1994a) Human Respiratory Tract Model, the latter as revised within Publication 130 (ICRP, 2015). This publication further outlines the computational methodology and nomenclature for assessment of internal dose in a manner consistent with that used for nuclear medicine applications. Numerical data for particle-specific and energy-dependent SAFs are given in electronic format for numerical coupling to the respiratory tract, alimentary tract, and systemic biokinetic models of the OIR publication series.

Two Realistic Beagle Models for Dose Assessment.

Previously, the authors developed a series of eight realistic digital mouse and rat whole body phantoms based on NURBS technology to facilitate internal and external dose calculations in various species of rodents. In this paper, two body phantoms of adult beagles are described based on voxel images converted to NURBS models. Specific absorbed fractions for activity in 24 organs are presented in these models. CT images were acquired of an adult male and female beagle. The images were segmented, and the organs and structures were modeled using NURBS surfaces and polygon meshes. Each model was voxelized at a resolution of 0.75 × 0.75 × 2 mm. The voxel versions were implemented in GEANT4 radiation transport codes to calculate specific absorbed fractions (SAFs) using internal photon and electron sources. Photon and electron SAFs were then calculated for relevant organs in both models. The SAFs for photons and electrons were compatible with results observed by others. Absorbed fractions for electrons for organ self-irradiation were significantly less than 1.0 at energies above 0.5 MeV, as expected for many of these small-sized organs, and measurable cross irradiation was observed for many organ pairs for high-energy electrons (as would be emitted by nuclides like 32P, 90Y, or 188Re). The SAFs were used with standardized decay data to develop dose factors (DFs) for radiation dose calculations using the RADAR Method. These two new realistic models of male and female beagle dogs will be useful in radiation dosimetry calculations for external or internal simulated sources.

Electron specific absorbed fractions for the adult male and female ICRP/ICRU reference computational phantoms

The calculation of radiation dose from internally incorporated radionuclides is based on so-called absorbed fractions (AFs) and specific absorbed fractions (SAFs). SAFs for monoenergetic electrons were calculated for 63 source regions and 67 target regions using the new male and female adult reference computational phantoms adopted by the ICRP and ICRU and the Monte Carlo radiation transport programme package EGSnrc. The SAF values for electrons are opposed to the simplifying assumptions of ICRP Publication 30. The previously applied assumption of electrons being fully absorbed in the source organ itself is not always true at electron energies above approximately 300–500 keV. High-energy electrons have the ability to leave the source organ and, consequently, the electron SAFs for neighbouring organs can reach the same magnitude as those for photons for electron energies above 1 MeV. The reciprocity principle known for photons can be extended to electron SAFs as well, thus making cross-fire electron SAFs mass-independent. To quantify the impact of the improved electron dosimetry in comparison to the dosimetry using the simple assumptions of ICRP Publication 30, absorbed doses per administered activity of three radiopharmaceuticals were evaluated with and without explicit electron transport. The organ absorbed doses per administered activity for the two evaluation methods agree within 2%–3% for most organs for radionuclides with decay spectra having electron energies below a few hundred keV and within approximately 20% if higher electron energies are involved. An important exception is the urinary bladder wall, where the dose is overestimated by 60–150% using the simplified ICRP 30 approach for the radiopharmaceuticals of this study.

Internal photon and electron dosimetry of the newborn patient---a hybrid computational phantom study

Estimates of radiation absorbed dose to organs of the nuclear medicine patient are a requirement for administered activity optimization and for stochastic risk assessment. Pediatric patients, and in particular the newborn child, represent that portion of the patient population where such optimization studies are most crucial owing to the enhanced tissue radiosensitivities and longer life expectancies of this patient subpopulation. In cases where whole-body CT imaging is not available, phantom-based calculations of radionuclide S values—absorbed dose to a target tissue per nuclear transformation in a source tissue—are required for dose and risk evaluation. In this study, a comprehensive model of electron and photon dosimetry of the reference newborn child is presented based on a high-resolution hybrid-voxel phantom from the University of Florida (UF) patient model series. Values of photon specific absorbed fraction (SAF) were assembled for both the reference male and female newborn using the radiation transport code MCNPX v2.6. Values of electron SAF were assembled in a unique and time-efficient manner whereby the collisional and radiative components of organ dose-–for both self- and cross-dose terms—were computed separately. Dose to the newborn skeletal tissues were assessed via fluence-to-dose response functions reported for the first time in this study. Values of photon and electron SAFs were used to assemble a complete set of S values for some 16 radionuclides commonly associated with molecular imaging of the newborn. These values were then compared to those available in the OLINDA/EXM software. S value ratios for organ self-dose ranged from 0.46 to 1.42, while similar ratios for organ cross-dose varied from a low of 0.04 to a high of 3.49. These large discrepancies are due in large part to the simplistic organ modeling in the stylized newborn model used in the OLINDA/EXM software. A comprehensive model of internal dosimetry is presented in this study for the newborn nuclear medicine patient based upon the UF hybrid computational phantom. Photon dose response functions, photon and electron SAFs, and tables of radionuclide S values for the newborn child-–both male and female-–are given in a series of four electronic annexes available at stacks.iop.org/pmb/57/1433/mmedia. These values can be applied to optimization studies of image quality and stochastic risk for this most vulnerable class of pediatric patients.

IMPACT ON 141Ce, 144Ce, 95Zr, AND 90Sr BETA EMITTER DOSE COEFFICIENTS OF PHOTON AND ELECTRON SAFS CALCULATED WITH ICRP/ICRU REFERENCE ADULT VOXEL COMPUTATIONAL PHANTOMS

The current dose coefficients for internal dose assessment of occupationally exposed persons and the general public were derived using the methodology of the International Commission on Radiological Protection (ICRP), which is similar to the Medical Internal Radiation Dose (MIRD)-type methodology. One component of this methodology is the mathematical representation of the human body (so-called MIRD-type phantoms) developed at the Oak Ridge National Laboratory for calculations of photon specific absorbed fractions (SAFs). Concerning the beta emissions, it is assumed in general that they irradiate only the organ where the radionuclide resides, whereas for walled organs, a fixed fraction of the emitted energy is absorbed within the wall. For the active marrow and bone surface targets, absorbed fractions were explicitly provided in ICRP Publication 30. The ICRP Publications 66 and 100 contain further detailed energy-dependent absorbed fraction data for the airways and the segments of the alimentary tract. In the present work, the voxel phantoms representing the reference male and female adults, recently developed at the Helmholtz Zentrum München-German Research Center for Environmental Health (HMGU) in collaboration with the Task Group DOCAL of ICRP Committee 2, were used for the Monte Carlo computation of photon as well as electron SAFs. These voxel phantoms, being constructed from computed tomography (CT) scans of individuals, are more realistic in shape and location of organs in the body than the mathematical phantoms; therefore, they provide photon SAFs that are more precise than those stemming from mathematical phantoms. In addition, electron SAFs for solid and walled organs as well as tissues in the alimentary tract, the respiratory tract, and the skeleton were calculated with Monte Carlo methods using these phantoms to complement the data of ICRP Publications 66 and 100 that are confined to self-irradiation. The SAFs derived for photons and electrons are then used to calculate the dose coefficients of the beta emitters 141Ce, 144Ce, 95Zr, and 90Sr. It is found that the differences of the dose coefficients due to the revised SAFs are much larger for injection and ingestion than for inhalation. The equivalent doses for colon and ingestion with the new voxel-based SAFs are significantly smaller than the values with the MIRD-type photon SAFs and simplifying assumptions for electrons. For lungs and inhalation, no significant difference was observed for the equivalent doses, whereas for injection and ingestion, an increase of the new values is observed.

Application of the ICRP/ICRU reference computational phantoms to internal dosimetry: calculation of specific absorbed fractions of energy for photons and electrons

The emission of radiation from a contaminated body region is connected with the dose received by radiosensitive tissue through the specific absorbed fractions (SAFs) of emitted energy, which is therefore an essential quantity for internal dose assessment. A set of SAFs were calculated using the new adult reference computational phantoms, released by the International Commission on Radiological Protection (ICRP) together with the International Commission on Radiation Units and Measurements (ICRU). Part of these results has been recently published in ICRP Publication 110 (2009 Adult reference computational phantoms (Oxford: Elsevier)). In this paper, we mainly discuss the results and also present them in numeric form. The emission of monoenergetic photons and electrons with energies ranging from 10 keV to 10 MeV was simulated for three source organs: lungs, thyroid and liver. SAFs were calculated for four target regions in the body: lungs, colon wall, breasts and stomach wall. For quality assurance purposes, the simulations were performed simultaneously at the Helmholtz Zentrum München (HMGU, Germany) and at the Institute for Radiological Protection and Nuclear Safety (IRSN, France), using the Monte Carlo transport codes EGSnrc and MCNPX, respectively. The comparison of results shows overall agreement for photons and high-energy electrons with differences lower than 8%. Nevertheless, significant differences were found for electrons at lower energy for distant source/target organ pairs. Finally, the results for photons were compared to the SAF values derived using mathematical phantoms. Significant variations that can amount to 200% were found. The main reason for these differences is the change of geometry in the more realistic voxel body models. For electrons, no SAFs have been computed with the mathematical phantoms; instead, approximate formulae have been used by both the Medical Internal Radiation Dose committee (MIRD) and the ICRP due to the limitations imposed by the computing power available at this time. These approximations are mainly based on the assumption that electrons are absorbed locally in the source organ itself. When electron SAFs are calculated explicitly, discrepancies with this simplifying assumption are notable, especially at high energies and for neighboring organs where the differences can reach the same order of magnitude as for photon SAFs.

RADAR Realistic Animal Model Series for Dose Assessment

Rodent species are widely used in the testing and approval of new radiopharmaceuticals, necessitating murine phantom models. As more therapy applications are being tested in animal models, calculating accurate dose estimates for the animals themselves becomes important to explain and control potential radiation toxicity or treatment efficacy. Historically, stylized and mathematically based models have been used for establishing doses to small animals. Recently, a series of anatomically realistic human phantoms was developed using body models based on nonuniform rational B-spline. Realistic digital mouse whole-body (MOBY) and rat whole-body (ROBY) phantoms were developed on the basis of the same NURBS technology and were used in this study to facilitate dose calculations in various species of rodents. Voxel-based versions of scaled MOBY and ROBY models were used with the Vanderbilt multinode computing network (Advanced Computing Center for Research and Education), using geometry and tracking radiation transport codes to calculate specific absorbed fractions (SAFs) with internal photon and electron sources. Photon and electron SAFs were then calculated for relevant organs in all models. The SAF results were compared with values from similar studies found in reference literature. Also, the SAFs were used with standardized decay data to develop dose factors to be used in radiation dose calculations. Representative plots were made of photon electron SAFs, evaluating the traditional assumption that all electron energy is absorbed in the source organs. The organ masses in the MOBY and ROBY models are in reasonable agreement with models presented by other investigators noting that considerable variation can occur between reported masses. Results consistent with those found by other investigators show that absorbed fractions for electrons for organ self-irradiation were significantly less than 1.0 at energies above 0.5 MeV, as expected for many of these small-sized organs, and measurable cross-irradiation was observed for many organ pairs for high-energy electrons (as would be emitted by nuclides such as (32)P, (90)Y, or (188)Re).

MO-E-AUD B-07: SAF Values for Internal Electron Emitters Calculated for the RPI-P Pregnant-Female Models Using Monte Carlo Methods

Purpose: to calculate specific absorbed fraction (SAF) values for internal electron emitters based on more realistic RPI‐P serial pregnant female models. Method and Materials: The RPI‐P series pregnant‐female models developed by Xu and coworkers were used for Monte Carlo simulation. Those models are based on boundary‐representation method for organ delineation. The image sources are from clinical CTimage, VIP‐Man image, and public domain images. The pregnant woman models, RPI‐P3, RPI‐P6, and RPI‐P9, were implemented into a previously developed Monte Carlo user code, EGS4‐VLSI. In this study, internal electron emitters were considered for the following energies: 10, 15, 20, 30, 50, 100, 200, 500, 1000, 1500, 2000, and 4000 keV. SAF values to the fetus were calculated for each of these energies involving 35 source organs.Results and Discussion: SAF factors from source organs to the fetus have been calculated for all the three pregnant female models. Results show that electron SAF values follow linear relationship as equation: log (SAF(fetus ← source) = A ⋅ log (E) + B , where E is the electron energy, A and B are coefficients. A and B coefficients were calculated. R2 coefficient, the determination for the linear relationship, is ranging from 0.90∼1.00 except source organ=heart for RPI‐P3 model. It means the linear relationship between log(SAF) and log(E) is fitting well. Conclusion: SAF values have been derived based on a new developed RPI‐P series pregnant‐female models using Monte Carlo method. For electron emitters ranging from 10 keV to 4000 keV, the log(SAF) and log(energy) relationship can be approximated by linear function.

SU‐FF‐I‐77: Photon and Electron Specific Absorbed Fractions From the UF Pediatric Tomographic Phantoms

Purpose: To calculate photon and electron specific absorbed fractions (SAF) by utilizing series of realistic tomographic phantoms of pediatric patients. Method and Material: The series of UF tomographic phantoms, 9‐month male, 4‐year female, 8‐year female, 11‐year male, and 14‐year male, developed at University of Florida, were imported into the EGSnrc Monte Carlo code for this study. Monoenergetic photons and electrons were simulated as homogeneously distributed in various source organs. All possible source‐target organ pairs were considered to calculate absorbed fractions at the target organs from both internal electron and photon emitters localized in source organs. Results: The new sets of photon and electron SAFs were tabulated for the particle energies from 0.01 MeV to 4 MeV. The photon SAFs were compared with those from the ORNL phantoms. It was shown that ORNL phantoms failed to correctly represent the proximities of certain organ pairs and caused significant discrepancies from the UF phantom in terms of photon SAF values. The electron self‐absorbed fractions calculated from the UF phantoms were compared to that given in the ICRP Publication 30 assumptions. For higher electron energies, the assumption of 100% self‐absorption in the ICRP schema was shown to be incorrect, especially for the smaller phantoms and smaller organs with large surface‐to‐volume ratios. For example, the thyroid self absorption was only 21% for 4 MeV electrons in the UF 9‐month phantom. Conclusion: Photon and electron SAFs were calculated for various ages of pediatric patients by utilizing realistic tomographic computational phantoms. It was shown that the tomographic phantoms accurately represent the shapes of various internal organs and their proximities to surrounding organs. The explicit consideration of electron transport demonstrates that the traditional assumption of full energy deposition by energetic beta‐particles can be in error, especially for the younger and smaller patients of this age series.