Specific Absorbed Fractions Research Articles

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.

Estimate of S-values for children due to six positron emitting radionuclides used in PET examinations

Positron emission tomography (PET) has revolutionized the diagnosis of cancer since its conception. When combined with computed tomography (CT), PET/CT performed in children produces highly accurate diagnoses from images of regions affected by malignant tumors. Considering the high risk to children when exposed to ionizing radiation, a dosimetric study for PET/CT procedures is necessary. Specific absorbed fractions (SAF) were determined for monoenergetic photons and positrons, as well as the S-values for six positron emitting radionuclides (11C, 13N, 18F, 68Ga, 82Rb, 15O), and 22 source organs. The study was performed for six pediatric anthropomorphic hybrid models, including the newborn and 1 year hermaphrodite, 5 and 10-year-old male and female, using the Monte Carlo N-Particle eXtended code (MCNPX, version 2.7.0). The results of the SAF in source organs and S-values for all organs showed to be inversely related to the age of the phantoms, which includes the variation of body weight. The results also showed that radionuclides with higher energy peak emission produces larger auto absorbed S-values due to local dose deposition by positron decay. The S-values for the source organs are considerably larger due to the interaction of tissue with non-penetrating particles (electrons and positrons) and present a linear relationship with the phantom body masses. The results of the S-values determined for positron-emitting radionuclides can be used to assess the radiation dose delivered to pediatric patients subjected to PET examination in clinical settings. The novelty of this work is associated with the determination of auto absorbed S-values, in six new pediatric virtual anthropomorphic phantoms, for six emitting positrons, commonly employed in PET exams.

Inclusion of thin target and source regions in alimentary and respiratory tract systems of mesh-type ICRP adult reference phantoms

It is not feasible to define very small or complex organs and tissues in the current voxel-type adult reference computational phantoms of the International Commission on Radiological Protection (ICRP), which limit dose coefficients for weakly penetrating radiations. To address the problem, the ICRP is converting the voxel-type reference phantoms into mesh-type phantoms. In the present study, as a part of the conversion project, the micrometer-thick target and source regions in the alimentary and respiratory tract systems as described in ICRP Publications 100 and 66 were included in the mesh-type ICRP reference adult male and female phantoms. In addition, realistic lung airway models were simulated to represent the bronchial (BB) and bronchiolar (bb) regions. The electron specific absorbed fraction (SAF) values for the alimentary and respiratory tract systems were then calculated and compared with the values calculated with the stylized models of ICRP Publications 100 and 66. The comparisons show generally good agreement for the oral cavity, oesophagus, and BB, whereas for the stomach, small intestine, large intestine, extrathoracic region, and bb, there are some differences (e.g. up to ~9 times in the large intestine). The difference is mainly due to anatomical difference in these organs between the realistic mesh-type phantoms and the simplified stylized models. The new alimentary and respiratory tract models in the mesh-type ICRP reference phantoms preserve the topology and dimensions of the voxel-type ICRP phantoms and provide more reliable SAF values than the simplified models adopted in previous ICRP Publications.

RODES software for dose assessment of rats and mice contaminated with radionuclides

In order to support animal experiments of chronic radionuclides intake with realistic dosimetry, voxel-based three-dimensional computer models of mice and rats of both sexes and three ages were built from magnetic resonance imaging. Radiation transport of mono-energetic photons of 11 energies and electrons of 7 energies was simulated with MCNPX 2.6c to assess specific absorbed fractions (SAFs) of energy emitted from 13 source regions and absorbed in 28 target regions. RODES software was developed to combine SAF with radiation emission spectra and user-supplied biokinetic data to calculate organ absorbed doses per nuclear transformation of radionuclides in source regions (S-factors) and for specific animal experiments with radionuclides. This article presents the design of RODES software including the simulation of the particles in the created rodent voxel phantoms. SAF and S-factor values were compared favourably with published results from similar studies. The results are discussed for rodents of different ages and sexes.

Development of a mouse computational model for MCNPx based on Digimouse (r) images and dosimetric assays

The aim of this study was to create and test a new mice 3D-voxel phantom named DM_BRA for mice and human first-estimation radiopharmaceutical dosimetry. Previously, the article reviews the state-of-art in animal model development. Images from Digimouse CT database were used in the segmentation and on the generation of the voxelized phantom. Simulations for validation of the DM_BRA model was performed at 0.015, 0.1, 0.5, 1 and 4 MeV photons with heart-source. Specific Absorbed Fractions (SAF) data were compared with literature data. The organ masses of DM_BRA correlated well with existing models based on the same dataset; however, few small organ masses hold significant variations. The SAF data in most simulated cases were statistically equal to a significant level of 0.01 to the reference data.

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.

Construction of Taiwanese Adult Reference Phantoms for Internal Dose Evaluation.

In the internal dose evaluation, the specific absorbed fraction (SAF) and S-value are calculated from the reference phantom based on Caucasian data. The differences in height and weight between Caucasian and Asian may lead to inaccurate dose estimation. In this study, we developed the Taiwanese reference phantoms. 40 volunteers were recruited. Magnetic resonance images (MRI) were obtained, and the contours of 15 organs were drawn. The Taiwanese reference man (TRM) and Taiwanese reference woman (TRW) were constructed. For the SAF calculation, the differences in the self-absorption SAF (self-SAF) between the TRM, TRW, and Oak Ridge National Laboratory (ORNL) adult phantom were less than 10% when the difference in organ mass was less than 20%. The average SAF from liver to pancreas of TRM was 38% larger than that of the ORNL adult phantom, and the result of TRW was 2.02 times higher than that of the ORNL adult phantom. For the S-value calculation, the ratios of TRW and ORNL adult phantom ranged from 0.91 to 1.57, and the ratios of TRM and ORNL adult phantom ranged from 1.04 to 2.29. The SAF and S-value results were dominantly affected by the height, weight, organ mass, and geometric relationship between organs. By using the TRM and TRW, the accuracy of internal dose evaluation can be increased for radiation protection and nuclear medicine.

In vivo measurements for biokinetic and monte carlo simulations of absorbed dose in paediatric patients using radiopharmaceuticals

Introduction The use of radiopharmaceuticals implies calculation of absorbed doses according to MIRD methodology. However, particularly in children, other parameters are thought to be important and determinant for dose calculations. Amongst these are the pharmacokinetics characteristics of each radiotracer and their relationship with organ physiology and pathology, and the patients’ anatomical characteristics. Therefore, there is a need to improve dosimetric calculations based on other algorithms taking into account those variables. Purpose Improve 99mTc-DMSA and 99mTc-MAG3 calculated absorbed doses in children after intravenous administration for diagnostic purposes. Gamma camera and in vivo measurements were used to assess the biokinetic data. The specific absorbed fraction (SAF) values are estimated via Monte Carlo simulations and scaled VOXEL phantoms. Materials and methods In vivo measurements were mathematically analysed in order to obtain a value for the activity as function of time. The simulations were performed using MCNPX and two paediatric VOXEL phantoms (BABY and CHILD) which were scaled to fit several anatomical parameters, such as body weight, height and kidney mass, amongst others. The biokinetic curves were compared with reference data. Multiple SAF values were obtained to determine absorbed doses in these children. Results Deviations from the reference values were found, mainly related to children specific pathology. Furthermore we found SAF values to be strongly dependent on body and organ mass, kidney dimensions and velocity of excretion amongst other anatomical and physiological parameters. Conclusion The data strongly suggests that other parameters rather than age and body weight are paramount in calculating absorbed doses in paediatric populations.

Estimation of Photon and Electron Specific Absorbed Fractions for Selected Organs of a Human Voxelizedphantom Using GATE Monte Carlo Package

Background: The specific absorbed fraction (SAF) of energy is an essential element of internal dose assessment. Objectives: Here we report a set of SAFs calculated for selected organs of ahuman computational phantom. Materials and Methods: The Monte Carlo transport code GATE version 6.1 was used to simulate monoenergetic photons and elec- trons with energies ranging from 10 keV to 2 MeV. The particles were emitted from three source organs: the kidneys, liver, and spleen. SAFs were calculated for three target regions in the body (kidneys, liver, and spleen) and compared with the results obtained using MCNP4B and GATE/GEANT4 Monte Carlo codes. For most photon energies, the self-irradiation was higher and cross-irradiation was lower in the results obtained using GATE than those obtained usingMCNP4B. Results: The results showed generally good agreement for photons and high-energy electrons, with discrepancies within -2 3%. Nevertheless, significant dierences were found for cross-irradiation of photons of lower energy and electrons of higher energy due to statistical uncertainties larger than 10%. The comparisons of SAF values for the human voxel phantom did not show significant dierences; furthermore, the results demonstrated the usefulness and applicability of the GATE Monte Carlo package for voxel level dose calculations in non-uniform media. Conclusions: The present SAF calculation for the Zubal voxel phantom is validated by comparison with the results obtained using other Monte Carlo codes.

WE‐H‐207A‐07: Image‐Based Versus Atlas‐Based Internal Dosimetry

Purpose:Monte Carlo (MC) simulation is known as the gold standard method for internal dosimetry. It requires radionuclide distribution from PET or SPECT and body structure from CT for accurate dose calculation. The manual or semi‐automatic segmentation of organs from CT images is a major obstacle. The aim of this study is to compare the dosimetry results based on patient's own CT and a digital humanoid phantom as an atlas with pre‐specified organs.Methods:SPECT‐CT images of a 50 year old woman who underwent bone pain palliation with Samarium‐153 EDTMP for osseous metastases from breast cancer were used. The anatomical date and attenuation map were extracted from SPECT/CT and three XCAT digital phantoms with different BMIs (i.e. matched (38.8) and unmatched (35.5 and 36.7) with patient's BMI that was 38.3). Segmentation of patient's organs in CT image was performed using itk‐SNAP software. GATE MC Simulator was used for dose calculation. Specific absorbed fractions (SAFs) and S‐values were calculated for the segmented organs.Results:The differences between SAFs and S‐values are high using different anatomical data and range from −13% to 39% for SAF values and −109% to 79% for S‐values in different organs. In the spine, the clinically important target organ for Samarium Therapy, the differences in the S‐values and SAF values are higher between XCAT phantom and CT when the phantom with identical BMI is employed (53.8% relative difference in S‐value and 26.8% difference in SAF). However, the whole body dose values were the same between the calculations based on the CT and XCAT with different BMIs.Conclusion:The results indicated that atlas‐based dosimetry using XCAT phantom even with matched BMI for patient leads to considerable errors as compared to image‐based dosimetry that uses the patient's own CT Patient‐specific dosimetry using CT image is essential for accurate results.

Practical Nuclear Medicine and Utility of Phantoms for Internal Dosimetry: XCAT Compared with Zubal.

The absorbed doses for two radioisotopes, 99mTc and 131I, between previously validated Zubal phantom and the recently developed XCAT phantom were compared. GATE Monte Carlo code was used to simulate the statistical process of radiation. A XCAT phantom with voxel and matrix sizes similar to a standard Zubal phantom was generated. According to Medical International Radiation Dose formalism, specific absorbed fraction (SAF) values for photons and S-factors for beta particles were tabulated. The amounts of absorbed doses were calculated and compared in different organs. The differences of gamma radiation doses, SAFs, between Zubal and XCAT are >50% in adrenal from adrenal, pancreas from pancreas and thyroid from thyroid, in lung from kidney, kidneys from lungs and in kidneys from thyroid and thyroid from kidneys. The beta radiation doses differences between Zubal and XCAT are >50% in thyroid from thyroid, bladder from bladder, kidney from kidney, liver from bladder, thyroid from bladder and kidney from thyroid. The size and distances of the organs were different between XCAT and Zubal phantoms. Denoted differences of SAF and S-factors correspond to the different organ geometries in phantoms. The results of absorbed doses in Zubal and XCAT phantoms are different. The variations prohibit easy comparison or interchangeability of dosimetry between these phantoms.

DOSIS DEBIDO A LA INCORPORACIÓN POR INGESTIÓN DE EMISORES BETA PUROS

El cálculo de dosis debido a la incorporación de radionúclidos se realiza en función de los coeficientes de dosis equivalente comprometida h(50) y dosis efectiva comprometida e(50). En este trabajo se calculan estos coeficientes de dosis para la ingestión única de 3H, 14C, 32P, 35S utilizando los valores de las fracciones de absorción específicas (SAF) del phantom computacional masculino de referencia de la ICRP(International Commission on Radiological Protection). Se usó además el modelo del tracto alimentario humano, HATM, para la determinación del número total de desintegraciones (Us) y la metodología de cálculo de las nuevas recomendaciones del ICRP dadas en la publicación 103. En general se encontró que los coeficientes de dosis equivalente comprometida para la mayoría de órganos presentan relativamente pocas diferencias, con excepción de algunos órganos como las paredes del estómago, el colon y la vejiga, además se encontró que existen diferencias significativas en las dosis de la medula ósea (RBM) y la superficie ósea (BS) para el caso particular del 32P.

Application of GATE/Geant4 for internal dosimetry using male ICRP reference voxel phantom by specific absorbed fractions calculations for photon irradiation

Targeted internal radiation is a type of radiation therapy in nuclear medicine (NM) that is efficient in small and disseminated tumours specially in metastatic disease. In any medical procedure that involves radiation exposure the radiation dosimetry is required and the dose distribution in NM is of great interest to estimate absorbed doses in organs of risk and tumours. Specific absorbed fraction (SAF) is an essential parameter that connects the dose received by a tissue and the emitting volume and, currently, SAF calculations using Monte Carlo (MC) simulations may be considered one of the most appropriate methods for individualized dosimetry. In this work, SAF values for International Commission on Radiological Protection adult male voxelized phantom were calculated using GATE/Geant4 Standard and Livermore packages and compared with MCNPX and EGSnrc reference data. Photon irradiations with energies ranging from 10 keV to 10 MeV were simulated in three different source organs: liver, lungs and thyroid. SAF values of four target organs (breasts, lungs, colon wall and stomach wall) were calculated and compared with reference data. Obtained results agreed with the reference data with an average accuracy better than 3% for energies above 50 keV and up to 3.3% for the full range of energies. Results suggest that, although good agreement was found for both GATE/Geant4 packages with MCNPX and EGSnrc, Livermore package shows more stability and it is slightly more accurate than Standard package, mostly when its results are compared with EGSnrc. However, the agreement of both GATE/Geant4 packages with the MCNPX and EGSnrc codes might depend on the pair TO/SO. Finally, results indicate that GATE might be used for internal dosimetry of gamma emitters and also encourage the development of a GATE SAF database using reference phantoms.

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.

Specific absorbed fractions of electrons and photons for Rad-HUMAN phantom using Monte Carlo method**Supported by Strategic Priority Research Program of Chinese Academy of Sciences (XDA03040000), National Natural Science Foundation of China (910266004, 11305205, 11305203) and National Special Program for ITER (2014GB112001)

The specific absorbed fractions (SAF) for self- and cross-irradiation are effective tools for the internal dose estimation of inhalation and ingestion intakes of radionuclides. A set of SAFs of photons and electrons were calculated using the Rad-HUMAN phantom, which is a computational voxel phantom of a Chinese adult female that was created using the color photographic image of the Chinese Visible Human (CVH) data set by the FDS Team. The model can represent most Chinese adult female anatomical characteristics and can be taken as an individual phantom to investigate the difference of internal dose with Caucasians. In this study, the emission of mono-energetic photons and electrons of 10 keV to 4 MeV energy were calculated using the Monte Carlo particle transport calculation code MCNP. Results were compared with the values from ICRP reference and ORNL models. The results showed that SAF from the Rad-HUMAN have similar trends but are larger than those from the other two models. The differences were due to the racial and anatomical differences in organ mass and inter-organ distance. The SAFs based on the Rad-HUMAN phantom provide an accurate and reliable data for internal radiation dose calculations for Chinese females.

SU‐E‐T‐507: Internal Dosimetry in Nuclear Medicine Using GATE and XCAT Phantom: A Simulation Study

PurposeMonte Carlo simulations are routinely used for internal dosimetry studies. These studies are conducted with humanoid phantoms such as the XCAT phantom. In this abstract we present the absorbed doses for various pairs of source and target organs using three common radiotracers in nuclear medicine.MethodsThe GATE software package is used for the Monte Carlo simulations. A typical female XCAT phantom is used as the input. Three radiotracers 153Sm, 131I and 99mTc are studied. The Specific Absorbed Fraction (SAF) for gamma rays (99mTc, 153Sm and 131I) and Specific Fraction (SF) for beta particles (153Sm and 131I) are calculated for all 100 pairs of source target organs including brain, liver, lung, pancreas, kidney, adrenal, spleen, rib bone, bladder and ovaries.ResultsThe source organs themselves gain the highest absorbed dose as compared to other organs. The dose is found to be inversely proportional to distance from the source organ. In SAF results of 153Sm, when the source organ is lung, the rib bone, gain 0.0730 (Kg‐1) that is more than lung itself.ConclusionThe absorbed dose for various organs was studied in terms of SAF and SF. Such studies hold importance for future therapeutic procedures and optimization of induced radiotracer.

Effect of respiratory motion on internal radiation dosimetry.

Estimation of the radiation dose to internal organs is essential for the assessment of radiation risks and benefits to patients undergoing diagnostic and therapeutic nuclear medicine procedures including PET. Respiratory motion induces notable internal organ displacement, which influences the absorbed dose for external exposure to radiation. However, to their knowledge, the effect of respiratory motion on internal radiation dosimetry has never been reported before. Thirteen computational models representing the adult male at different respiratory phases corresponding to the normal respiratory cycle were generated from the 4D dynamic XCAT phantom. Monte Carlo calculations were performed using the mcnp transport code to estimate the specific absorbed fractions (SAFs) of monoenergetic photons/electrons, the S-values of common positron-emitting radionuclides (C-11, N-13, O-15, F-18, Cu-64, Ga-68, Rb-82, Y-86, and I-124), and the absorbed dose of (18)F-fluorodeoxyglucose ((18)F-FDG) in 28 target regions for both the static (average of dynamic frames) and dynamic phantoms. The self-absorbed dose for most organs/tissues is only slightly influenced by respiratory motion. However, for the lung, the self-absorbed SAF is about 11.5% higher at the peak exhale phase than the peak inhale phase for photon energies above 50 keV. The cross-absorbed dose is obviously affected by respiratory motion for many combinations of source-target pairs. The cross-absorbed S-values for the heart contents irradiating the lung are about 7.5% higher in the peak exhale phase than the peak inhale phase for different positron-emitting radionuclides. For (18)F-FDG, organ absorbed doses are less influenced by respiratory motion. Respiration-induced volume variations of the lungs and the repositioning of internal organs affect the self-absorbed dose of the lungs and cross-absorbed dose between organs in internal radiation dosimetry. The dynamic anatomical model provides more accurate internal radiation dosimetry estimates for the lungs and abdominal organs based on realistic modeling of respiratory motion. This work also contributes to a better understanding of model-induced uncertainties in internal radiation dosimetry.

SU‐E‐T‐345: Validation of a Patient‐Specific Monte Carlo Targeted Radionuclide Therapy Dosimetry Platform

Purpose:There is a compelling need for personalized dosimetry in targeted radionuclide therapy given that conventional dose calculation methods fail to accurately predict dose response relationships. To address this need, we have developed a Geant4‐based Monte Carlo patient‐specific 3D dosimetry platform for TRT. This platform calculates patient‐specific dose distributions based on serial CT/PET or CT/SPECT images acquired after injection of the TRT agent. In this work, S‐values and specific absorbed fractions (SAFs) were calculated using this platform and benchmarked against reference values.Methods:S‐values for 1, 10, 100, and 1000g spherical tumors with uniform activity distributions of I‐124, I‐125, I‐131, F‐18, and Ra‐223 were calculated and compared to OLINDA/EXM reference values. SAFs for monoenergetic photons of 0.01, 0.1, and 1 MeV and S factors for monoenergetic electrons of 0.935 MeV were calculated for the liver, kidneys, lungs, pancreas, spleen, and adrenals in the Zubal Phantom and compared with previously published values. Sufficient particles were simulated to keep the voxel statistical uncertainty below 5%.Results:The calculated spherical S‐values agreed within a few percent of reference data from OLINDA/EXM for each radionuclide and sphere size. The comparison of photon SAFs and electron S‐values with previously published values showed good agreement with the previously published values. The S‐values and SAFs of the source organs agreed within 1%.Conclusion:Our platform has been benchmarked against reference values for a variety of radionuclides and over a wide range of energies and tumor sizes. Therefore, this platform could be used to provide accurate patientspecific dosimetry for use in radiopharmaceutical clinical trials.

Improved estimates of the radiation absorbed dose to the urinary bladder wall

Specific absorbed fractions (SAFs) have been calculated as a function of the content in the urinary bladder in order to allow more realistic calculations of the absorbed dose to the bladder wall. The SAFs were calculated using the urinary bladder anatomy from the ICRP male and female adult reference computational phantoms. The urinary bladder and its content were approximated by a sphere with a wall of constant mass, where the thickness of the wall depended on the amount of urine in the bladder. SAFs were calculated for males and females with 17 different urinary bladder volumes from 10 to 800 mL, using the Monte Carlo computer program MCNP5, at 25 energies of mono-energetic photons and electrons ranging from 10 KeV to 10 MeV. The decay was assumed to be homogeneously distributed in the urinary bladder content and the urinary bladder wall, and the mean absorbed dose to the urinary bladder wall was calculated. The Monte Carlo simulations were validated against measurements made with thermoluminescent dosimeters. The SAFs obtained for a urine volume of 200 mL were compared to the values calculated for the urinary bladder wall using the adult reference computational phantoms. The mean absorbed dose to the urinary wall from 18F-FDG was found to be 77 µGy/MBq formales and 86 µGy/MBq for females, while for 99mTc-DTPA the mean absorbed doses were 80 µGy/MBq for males and 86 µGy/MBq for females. Compared to calculations using a constant value of the SAF from the adult reference computational phantoms, the mean absorbed doses to the bladder wall were 60% higher for 18F-FDG and 30% higher for 99mTc-DTPA using the new SAFs.

Comparison of internal doses calculated using the specific absorbed fractions of the average adult Japanese male phantom with those of the reference computational phantom-adult male of ICRP publication 110

In order to study the effects of body sizes and masses of organs and tissues on internal dose assessment, the values corresponding to effective dose coefficients for intakes of radionuclides were calculated using the specific absorbed fractions (SAFs) of two phantoms: the average adult Japanese male phantom (JM-103) and the reference computational phantom-adult male (RCP-AM) of the International Commission on Radiological Protection. SAFs were evaluated using the phantoms and Monte Carlo radiation transport code MCNPX or were taken from published data. As a result of a comparison for 2894 cases of 923 radionuclides, the maximum discrepancy in the effective dose coefficients between the JM-103 and RCP-AM was about 40%. However, the discrepancies were smaller than 10% in 97% of all cases.