Specific Absorbed Fractions Research Articles

NCINM: organ dose calculator for patients undergoing nuclear medicine procedures

Nuclear medicine is the second largest source of medical radiation exposure to the general population after computed tomography imaging. Informed decisions regarding the use of nuclear medicine procedures require a better understanding of the magnitude of radiation dose and associated health risks. However, existing model-based organ dose estimation tools rely on simplified human anatomy models or commercial programs. Therefore, we developed a publicly-available dose calculation tool based on more sophisticated human anatomy models. We calculated a comprehensive library of photon and electron specific absorbed fractions (SAF) for multiple combinations of source and target regions within a series of pediatric and adult computational human phantoms matching the International Commission on Radiological Protection (ICRP)’s reference data, combined with a Monte Carlo radiation transport code. Then, we derived a library of S values from these SAFs and the nuclear decay data from ICRP Publication 107. Finally, we created a graphical user interface, named National Cancer Institute Dosimetry System for Nuclear Medicine (NCINM), to facilitate the dosimetry process. Approximately 13 million S values were derived from 2 million SAFs computed in this work. Comprehensive comparisons were conducted at different steps of the dosimetry chain with data available in software OLINDA/EXM 1.0 and IDAC 2.1. For instance, median ratios of photon self-absorption SAFs available from OLINDA/EXM 1.0 and IDAC 2.1 to those calculated in this study were 1.3 (interquartile range = 1.1–1.6) and 1.0 (interquartile range = 0.98–1.0), respectively. SAF differences between NCINM and OLINDA/EXM 1.0 were explained by the large inter-phantom anatomical variability. Our results illustrate the importance of realistic human anatomy models for use in dosimetry software. More phantoms and radionuclides, as well as a biokinetic module, will soon be added. Applications of the NCINM program include computation of absorbed doses for use in radiation epidemiologic studies and patient dose monitoring in nuclear medicine.

OpenDose: Open-Access Resource for Nuclear Medicine Dosimetry.

Radiopharmaceutical dosimetry depends on the localization in space and time of radioactive sources and requires the estimation of the amount of energy emitted by the sources deposited within targets. In particular, when computing resources are not accessible, this task can be performed using precomputed tables of specific absorbed fractions (SAFs) or S values based on dosimetric models. The aim of the OpenDose collaboration is to generate and make freely available a range of dosimetric data and tools. Methods: OpenDose brings together resources and expertise from 18 international teams to produce and compare traceable dosimetric data using 6 of the most popular Monte Carlo codes in radiation transport (EGSnrc/EGS++, FLUKA, GATE, Geant4, MCNP/MCNPX, and PENELOPE). SAFs are uploaded, together with their associated statistical uncertainties, in a relational database. S values are then calculated from monoenergetic SAFs on the basis of the radioisotope decay data presented in International Commission on Radiological Protection Publication 107. Results: The OpenDose collaboration produced SAFs for all source region and target combinations of the 2 International Commission on Radiological Protection Publication 110 adult reference models. SAFs computed from the different Monte Carlo codes were in good agreement at all energies, with SDs below individual statistical uncertainties. Calculated S values were in good agreement with OLINDA/EXM 2.0 (commercial) and IDAC-Dose 2.1 (free) software. A dedicated website (www.opendose.org) has been developed to provide easy and open access to all data. Conclusion: The OpenDose website allows the display and downloading of SAFs and the corresponding S values for 1,252 radionuclides. The OpenDose collaboration, open to new research teams, will extend data production to other dosimetric models and implement new free features, such as online dosimetric tools and patient-specific absorbed dose calculation software, together with educational resources.

Geant4/GATE Comparison of Geometry Optimization Algorithms for Internal Dosimetry Using Voxelized Phantoms

Geant4 Application for Tomographic Emission (GATE) is a Monte Carlo code, including the algorithms integrated inside Geant4 and other specific tools dedicated to tomography. Though, the detailed physical modeling of Geant4 is computationally demanding in order to simulate photon interactions and transport, particularly using voxelized phantoms. To circumvent the relatively slow simulation of voxelized phantoms for radiotherapy applications, GATE offers some relevant optimization methods to minimize the time consumption. In this study, specific absorbed fractions (SAFs) in Golem voxelized phantom using GATE Monte Carlo code for three optimization algorithms: Nested Parametrized Volume, Regular navigation algorithm and Compressed voxels methods have been used to calculate SAFs and compared to the literature data. The computation time has been also compared and discussed for the three methods. Compressed voxels method is more than 16 times faster than the two other parameterization methods for internal dosimetry field.

Development of a nonhuman primate computational phantom for radiation dosimetry.

The nonhuman primate (NHP) is an important animal model for evaluating the response of the human body to radiation exposure owing to similarities between its organ structure, genome, life span, and metabolism. However, there is a lack of radiation dosimetry estimations for NHPs. The aim of this work is to construct a computational phantom of NHPs and estimate absorbed fractions and specific absorbed fractions for internal radiation dosimetry. A female rhesus monkey was frozen and sectioned using a cryomacrotome. The transaxial sectioned images were imported into Adobe Photoshop for segmentation of internal organs. A total of 31 organs/tissues were identified and labeled. The segmented voxel phantom was then converted to a boundary representation (BREP) phantom to enable easy alteration of the phantom to mimic monkeys of different stature. The BREP model was then voxelized and imported into the MCNPX Monte Carlo radiation transport code for electron and photon dosimetry calculations. To estimate the appropriateness of using human phantoms as surrogate models for NHPs, absorbed fractions (AFs) and specific absorbed fractions (SAFs) of monoenergetic electrons and photons were calculated and compared with the ICRP reference newborn female phantom and a 1-yr-old female phantom. Considerable differences were observed for both self-absorbed and cross-absorbed doses for some organs between the NHP phantom and newborn phantom. For example, the ratios of the self-absorbed SAF(stomach wall) Monkey to SAF(stomach wall) Newborn ranged from 0.06 at 10keV to 0.29 at 3MeV for photons while the corresponding ratios to cross-absorbed SAF(liver→kidney) Monkey to SAF(liver→kidney) Newborn ranged from 0.78 at 50keV to 5.78 at 10keV for photons. Conversely, values of self-absorbed SAF were much higher (ratios of 2.39-4.19) for the brain and much lower for the uterus (0.51-0.61) in the monkey model compared to the newborn phantom. These dose differences can be attributed to the disparities between organ masses, shapes, and positions between the two phantoms. The developed NHP model can be exploited for the assessment of radiation dose to NHPs in preclinical radiation dosimetry studies and radiation therapy research.

Assessment of self- and cross-absorbed SAF values for HDRK-man using Geant4 code: internal photon and electron emitters

The ultimate need to account for the partial amount of energy deposited in target tissue/organ resulting from internal inhalation, ingestion, and injection intakes of radionuclides, defined by the Medical Internal Radiation Dosimetry committee as the specific absorbed fraction (SAF), has become obvious. In this study, we assessed the SAF values for self- and cross-absorption, which were calculated for a uniform distribution of monoenergetic photon and electron emitters with energies ranging from 15 keV to 3 MeV. The voxelized human phantom “High-Definition Reference Korean-man” (HDRK-man), which was implemented using the Monte Carlo simulation code Geant4 (version 10.1), was used for several combinations of target–source organs. The results were compared to those of the International Commission on Radiological Protection Reference (ICRP133) and Zubal phantoms. It was found that the SAF values of the three models have a similar trend. However, the SAF values for the HDRK-man phantom were higher than those of the other two models, with a relatively good agreement with those for the ICRP133 phantom (differences of 13.9 ± 2.8 and 12.1 ± 3.2 for photon and electron emitters, respectively). To analyze the differences in SAF values, we calculated the chord length distributions (CLDs) for selected target–source combinations. The parameters of organ mass (or volume) and CLDs, in addition to the adopted computational procedures, mainly cause such discrepancies. For realistic radionuclide emission spectra, an overall overestimation was observed when computing the S values for three radiopharmaceuticals studied (I-131, In-111, and Lu-177) and for liver–spleen intra- and inter-organ absorption when compared with published data. The new arrangement of S and SAF values is expected to add value for multidisciplinary research and clinical communities.

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.

Evaluation of Electron Specific Absorbed Fractions in Organs of Digimouse Voxel Phantom Using Monte Carlo Simulation Code FLUKA

Background: For preclinical evaluations of radiopharmaceuticals, most studies are carried out on mice. Values of electron specific absorbed fractions (SAF) have had vital role in the assessment of absorbed dose. In past studies, electron specific absorbed fractions were given for limited source target pairs using older reports of human organ compositions.Objective: Electron specific absorbed fraction values for monoenergetic electrons of energies 15, 50, 100, 500, 1000 and 4000 keV were evaluated for the Digimouse voxel phantom incorporated in Monte Carlo code FLUKA. The organ sources considered in this study were lungs, skeleton, heart, bladder, testis, stomach, spleen, pancreas, liver, kidney, adrenal, eye and brain. The considered target organs were lungs, skeleton, heart, bladder, testis, stomach, spleen, pancreas, liver, kidney, adrenal and brain. Eye and brain were considered as target organs only for eye and brain as source organs. From the latest report (International Commission on Radiological Protection ICRP) publication number 110, organ compositions and densities were adopted.Results: The electron specific absorbed fraction values for self-irradiation decreases with increasing electron energy. The electron specific absorbed fraction values for cross-irradiation are also found to be dependent on the electron energy and the geometries of source and target. Organ masses and electron specific absorbed fraction values are presented in tabular form. Conclusion: The results of this study will be useful in evaluating the absorbed dose to various organs of mice similar in size to the present study.

Evaluation of Electron Specific Absorbed Fractions in Organs of Digimouse Voxel Phantom Using Monte Carlo Simulation Code FLUKA

For preclinical evaluations of radiopharmaceuticals, most studies are carried out on mice. Electron specific absorbed fractions (SAF) values have had vital role in the assessment of absorbed dose. In past studies, electron SAFs were given for limited source target pairs using older reports of human organ compositions. Electron specific absorbed fraction values for monoenergetic electrons of energies 15, 50, 100, 500, 1000 & 4000 keV were evaluated for the Digimouse voxel phantom incorporated in Monte Carlo code FLUKA. From the latest report (International Commission on Radiological Protection ICRP) 110, organ compositions and densities were adopted. We have used the Digimouse voxel phantom which was incorporated in Monte Carlo code FLUKA. Simulation studies were performed using FLUKA. The organ sources considered in this study were lungs, skeleton, heart, bladder, testis, stomach, spleen, pancreas, liver, kidney, adrenal, eye and brain. The considered target organs were lungs, skeleton, heart, bladder, testis, stomach, spleen, pancreas, liver, kidney, adrenal and brain. Eye and brain were considered as target organs only for eye and brain as source organs. The electron SAF values for self-irradiation decreases with increasing electron energy. The electron SAF values for cross-irradiation are also found to be dependent on the electron energy and the geometries of source and target. Organ masses and electron SAF values are presented in tabular form. The results of this study will be useful in evaluating the absorbed dose to various organs of mice similar in size to the present study.

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.

Absorbed dose evaluation at different organs after 131I oral contamination of two Wistar rat models

Iodine-131 (131I) is one of the most frequently used radionuclides for diagnosis and therapy of thyroid diseases. It is administered orally in the treatment of cancer to eliminate the residual postoperative microscopic tumor foci, and the residual normal thyroid tissue for early detection of recurrence [1]. The comparative behavior of 131I concentration into two animalmodels with total and partial thyroid has been investigated in our previous work [2]. The accumulated activities have been measured in fourteen organs. In this study, the mean absorbed doses resulting from 131I accumulated in all organs have been evaluated using RODES software [3, 4]. With this software, mean absorbed doses were calculatedfor selected organs (thyroid, lungs, heart, liver, kidneys, stomach, spleen, large and small intestine, testes, urinary bladder wall) by combining the specific absorbed fractions (SAF) of energy with radiation emission spectra and biokinetic data determined from our previous experimental study [2]. Calculations were based on the 131I photon and electron emissions reported by [5] and SAFs previously calculated by Monte Carlo simulation in the voxel phantom of an adult male rate [3, 4]. The obtained results show high absorbed dosesdeliveredto stomach and lungs for both models compared to other organs. The dose received by the testes and salivary glands is found to be higher in the case of the rat model without thyroid. Conversely, the spleen and bladder wall received lower doses in this latter model compared to those received by the rat model with thyroid. One can also note that the difference in mean absorbed dose received by liver, lungs, heart, and walls of the stomach is not significant between the two rat models.

Development of Prototype Iranian male pelvic phantom for internal dosimetry

Introduction: Existing phantoms have been constructed based on Caucasian, non-Caucasian and race-specific datasets. According to previous studies made efforts to present Korean- specific phantoms and Chinese female phantom based on CVH dataset due to compare the resulting internal dosimetry with the Caucasian based data showed possible racial difference in human anatomy between Caucasian and non- Caucasian populations. These phantoms have been used for internal radionuclide dosimetry and other nuclear-medicine applications. Specific absorbed fractions (SAF) are tools for internal dose estimation of radionuclide intake. The differences were due to the racial and anatomical differences in organ mass and inter-organ distance. Many studies have been tried to provide an accurate and reliable data for internal radiation dose calculations. Therefor we construct an Iranian computational phantom to evaluate radiation dose organs within male pelvic region based on Iranian datasets. Internal dosimetry on organ is getting more important from the viewpoint of radiation protection of organ at risk. Materials and Methods: The first version of computational Iranian male pelvis phantom(PIPM) is provided by 5 MRI datasets of Iranian male for precise organ dose calculation, at first organs within pelvic region are segmented by enhancing their margins in each slice, determination of organs volume then, prototype Iranian male pelvis phantom was developed by modifying the equation of internal organs of the ORNL adult phantom finally, this phantom was imported to general purpose Monte Carlo code to simulate photon transport from internal radiation source. Results: The results showed that the individual comparable phantom can be calculated with acceptable accuracy using geometric registration. Comparison of dose calculation for Iranian male pelvic, MIRD and RADAR phantoms showed that differences of self-SAF were affected by organ volume differences. The average of the percent SAF deviation was calculated in testes organ -7.73%, -5.53% for PIMP phantom into MIRD and RADAR phantoms respectively. The largest deviation was observed in urinary bladder organ follow as 37.3%, 52.3% into MIRD and RADAR phantoms and it is shown -23.5% variations in self-SAF prostate organ into MIRD phantom. This phantom could be used for absorbed dose calculation of Iranian male pelvic region with acceptable accuracy in nuclear medicine. Conclusion: A prototype computational Iranian adult male pelvis phantom was represented from 5 Iranian adult males; the results showed that comparable phantom can be calculated internal absorbed dose with significant differences. Accurate voxel phantom is needed for dosimetric simulation in nuclear medicine for malignant tumors in male pelvic region. However, most of the existing voxel phantoms are constructed on the basis of Caucasian or non-Chinese population. It might be provided first Iranian male reference volume organ dataset for future researches.

P089] Calculations of specific absorbed fractions in organs of human body in nuclear medicine due application of 81MKr

Monte Carlo simulations were performed to assess the dose in the treatment of radiopharmaceuticals 81mKr. This radiopharmaceutical is used in treatments in nuclear medicine as an indication for cardiovascular and pulmonary diseases. The aim of this paper was to evaluate the specific absorbed fraction (SAF) when this radiopharmaceutical is incorporated in the lungs. For this purpose, we developed a voxel phantom (thorax) and was compared to the ORNL phantom. All calculations and simulations are done using the MCNP5/X code. Purpose Nuclear medicine uses radioactive isotopes and compounds for diagnosis and therapy. The use of different isotopes depending on the procedure applied to the patient dose distribution. In order to produce the least damage to the patient in therapy and provide good results for diagnostic purposes, the isotope must have a short half-life. Methods In this paper, two types of phantom were used - ORNL mathematical phantoms and voxel phantom. In order to calculate absorbed doses and SAF, a voxel phantom of the thorax was developed. The construction of voxel phantom depends on the quality of digital images of patients obtained during CT or MRI examination. In this paper, a voxel phantom (Thorax model) was obtained using a DICOM set of 108 CT images of the female patient under approved standard protocols. Results The SAF for the selected organ is calculated by the following formula: SAF = E d E i m Where is: the E d is mean energy is deposited throughout the body, E i the primary energy emitted by the source and m is the mass organ/tissue. Calculated SAF values of both phantom models are normalized in relation to the lungs, which provide a comparison of this two models. Conclusion The obtained results of the evaluation of SAF in different organs/tissues, during the incorporation of 81mKr in the lungs, are shown during the process in the scintigraphic examination. The difference in the results obtained by the use of the voxel model and the ORNL mathematical phantom model is between 4.47% and 19.7%.

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

Herein, we introduce a methodology for estimating the absorbed dose in organs at risk that is based on specified clinically derived radiopharmaceutical biodistributions and personalized anatomical characteristics. To evaluate the proposed methodology, we used realistic Monte Carlo (MC) simulations and computational pediatric models to calculate a parameter called in this work the "specific absorbed dose rate" (SADR). The SADR is a unique quantitative metric in that it is specific to a particular organ. It is defined as the absorbed dose rate in an organ when the biodistribution of radioactivity over the whole body is considered. Initially, we applied a validation procedure that calculated specific absorbed fractions (SAFs) from mono-energetic photon sources in the range of 10keV-2MeV and compared them with previously published data. We calculated the SADRs for five different radiopharmaceuticals (99m Tc-MDP, 123 I-mIBG, 131 I-MIBG, 131 I-NaI, and 153 Sm-EDTMP) based on their biodistributions at four or five different times; the biodistributions were derived from the clinical scintigraphic data of pediatric patients. We used six models representing male and female patients aged 5, 8, and 14yr to investigate the absorbed dose variability due to anatomical variations. The GATE Monte Carlo toolkit was used to calculate absorbed doses per organ. Finally, we compared the SADR methodology to that of OLINDA/EXM 1.1 using rescaled masses according to the studied models. Four target organs were considered for calculating the absorbed doses. The ratios of SAFs calculated with GATE simulations to those based on previously published data were between 0.9 and 2.2 when the liver was used as a source organ. Subsequently, we used GATE to calculate a dataset of SADRs for the six pediatric models. The SADRs for pediatric models whose total body weights ranged from 20 to 40kg varied up to approximately 90%, whereas those for models of similar body masses varied less than 15%. Finally, we found absorbed dose discrepancies of approximately 10-150% between the SADR methodology and OLINDA for two different radiopharmaceuticals. Absorbed doses from SADRs and from individualized S-values in the same pediatric model differed approximately 1-50%. Because pediatric radiopharmaceutical dosimetric estimates demonstrate large variation due to the patient's anatomical characteristics, personalized data should be considered. Using our SADR method in a larger population of phantoms and for a variety of radiopharmaceuticals could enhance the personalization of dosimetry in pediatric nuclear medicine. The proposed methodology provides the advantage of creating time-dependent organ dose rate curves.

Development and Validation of RAPID: A Patient-Specific Monte Carlo Three-Dimensional Internal Dosimetry Platform.

This work describes the development and validation of a patient-specific Monte Carlo internal dosimetry platform called RAPID (Radiopharmaceutical Assessment Platform for Internal Dosimetry). RAPID utilizes serial PET/CT or SPECT/CT images to calculate voxelized three-dimensional (3D) internal dose distributions with the Monte Carlo code Geant4. RAPID's dosimetry calculations were benchmarked against previously published S-values and specific absorbed fractions (SAFs) calculated for monoenergetic photon and electron sources within the Zubal phantom and for S-values calculated for a variety of radionuclides within spherical tumor phantoms with sizes ranging from 1 to 1000 g. The majority of the S-values and SAFs calculated in the Zubal Phantom were within 5% of the previously published values with the exception of a few 10 keV photon SAFs that agreed within 10%, and one value within 16%. The S-values calculated in the spherical tumor phantoms agreed within 2% for 177Lu, 131I, 125I, 18F, and 64Cu, within 3.5% for 211At and 213Bi, within 6.5% for 153Sm, 111In, 89Zr, and 223Ra, and within 9% for 90Y, 68Ga, and 124I. In conclusion, RAPID is capable of calculating accurate internal dosimetry at the voxel-level for a wide variety of radionuclides and could be a useful tool for calculating patient-specific 3D dose distributions.

Individualized adjustments to reference phantom internal organ dosimetry—scaling factors given knowledge of patient external anatomy

Internal radiation dose estimates for diagnostic nuclear medicine procedures are typically calculated for a reference individual. Resultantly, there is uncertainty when determining the organ doses to patients who are not at 50th percentile on either height or weight. This study aims to better personalize internal radiation dose estimates for individual patients by modifying the dose estimates calculated for reference individuals based on easily obtainable morphometric characteristics of the patient. Phantoms of different sitting heights and waist circumferences were constructed based on computational reference phantoms for the newborn, 10 year-old, and adult. Monoenergetic photons and electrons were then simulated separately at 15 energies. Photon and electron specific absorbed fractions (SAFs) were computed for the newly constructed non-reference phantoms and compared to SAFs previously generated for the age-matched reference phantoms. Differences in SAFs were correlated to changes in sitting height and waist circumference to develop scaling factors that could be applied to reference SAFs as morphometry corrections. A further set of arbitrary non-reference phantoms were then constructed and used in validation studies for the SAF scaling factors. Both photon and electron dose scaling methods were found to increase average accuracy when sitting height was used as the scaling parameter (~11%). Photon waist circumference-based scaling factors showed modest increases in average accuracy (~7%) for underweight individuals, but not for overweight individuals. Electron waist circumference-based scaling factors did not show increases in average accuracy. When sitting height and waist circumference scaling factors were combined, modest average gains in accuracy were observed for photons (~6%), but not for electrons. Both photon and electron absorbed doses are more reliably scaled using scaling factors computed in this study. They can be effectively scaled using sitting height alone as patient-specific morphometric parameter.

Validation of GATE for bone and bone marrow with calculation specific absorbed fraction for photons.

GATE/GEANT is a Monte Carlo code dedicated to nuclear medicine that allows calculation of the dose to organs (bone and bone marrow) of voxel phantoms. On the other hand, Medical Internal Radiation Dose (MIRD) is a well-developed system for estimation of the dose to human organs. In this study, results obtained from GATE/GEANT using leg of Snyder phantom is compared to published MIRD data. For this, the mathematical leg of Snyder phantom was discretized and converted to a digital phantom of 100 × 100 × 200 voxels. The activity was considered uniformly distributed within bone and bone marrow. The GATE/GEANT Monte Carlo code was used to calculate the dose to the bone and bone marrow of the leg phantom from mono-energetic photons of 10, 15, 20, 30, 50, 100, 200, 500, and 1000 keV. The dose was converted into a specific absorbed fraction (SAF) and the results were compared to the corresponding published MIRD data. On average, there was a good correlation between the two series of data for self-absorption (r 2 = 0.99) and for cross-irradiation (r 2 = 0.99). However, the GATE/GEANT data were on average 1.01 ± 0.79% higher than the corresponding MIRD data for self-absorption. As for cross-irradiation, the GATE/GEANT data were on average 8.11 ± 7.95% higher than the MIRD data. In this study, the SAF values derived from GATE/GEANT and the corresponding MIRD published data were compared. On average, the SAF values derived with GATE/GEANT showed an acceptable correlation and agreement with the MIRD data for the photon energies of 50-1000 keV. For photons of 10-30 keV, there was an only poor agreement between the GATE/GEANT results and MIRD data.

Geant4/GATE Monte Carlo Code for Internal Dosimetry Using Voxelized Phantom

It is of great interest to estimate absorbed doses in organs before radiation therapy, especially in nuclear medicine field. In this regard, the internal dose distribution is required. According to the MIRD formalism, Specific Absorbed Fraction (SAF) is an essential parameter for internal dosimetry. In the present work, SAF values for the voxelized phantom (Golem) of the GSF-National Research Center for Environment and Health were calculated using Geant4/GATE with Standard packages and compared with GSF Monte Carlo reference data. Photon irradiations of 30, 100 keV and 1 MeV energy were simulated in eleven different sources and target organs: liver, kidneys, lungs, brain, pancreas, spleen, colon, Red bone marrow (RBM), stomach, thyroid and adrenals. The SAF for self-absorption and for cross-irradiation to other organs were calculated and compared with literature. The results agree with published data, with an average relative difference less than 3%, for the self-absorption of 100 keV and 1 MeV photon energies. The agreement of Geant4/GATE and GSF code might depend on the distance between target and source, the target mass and the photons energy. Generally, the present results indicate that GATE might be used with gamma emitters for internal dosimetry in regard to our prospective works.

Abstract ID: 155 OpenDose: A collaborative effort to produce reference dosimetric data with Monte Carlo simulation software

Radiopharmaceutical dosimetry for diagnostic and therapy uses reference data describing energy deposition from a source to target tissues. Reference data are usually expressed as S-values, i.e. mean absorbed dose in a target from a source decay (Gy.Bq-1.s-1). The OpenDose project aims at providing an open database of robust reference S-values generated from different Monte Carlo (MC) software, through an international collaboration. The ICRP 110 reference adult female [1] was chosen as the first model, with 140 segmented tissues-organs and a voxel size of 1.775 × 1.775 × 4.84 mm3. Specific Absorbed Fractions (SAFs) are being computed for different sources, with mono-energetic photons and electrons. S-values will be generated by interpolating these mono-energetic SAFs for isotope decay schemes [2] . Producing data with different software using various physics lists will allow a cross-verification of the results and their associated uncertainties. The OpenDose project already includes 12 research teams and 5 of the most popular MC software used in radiopharmaceutical dosimetry: Geant4/GATE, Fluka, EGSnrc/EGS++, MCNP/MCNPX and Penelope. A total of 46 source tissues-organs have so far been simulated with GATE for mono-energetic particles (photons + electrons) from 5 keV to 10 MeV. Each simulation requires ∼ 1 day of computation on a single CPU for 108 primary particles and produces a 3D map (voxel-based) of absorbed doses and uncertainties. When available, the data from the other software will be analysed and compared in terms of SAFs for different targets and energies. Next steps include the integration of SAFs to provide S-values and build the database to allow Nuclear Medicine centres to easily access and use the data. Through a collaboration of research teams using different MC techniques, we are developing a freely accessible database of reference dosimetric data. A preliminary study using the ICRP adult female model is ongoing. The project is open to new research teams to increase the variety of software and will expand to more computational models and studies.

Feasibility of reducing differences in estimated doses in nuclear medicine between a patient-specific and a reference phantom

The feasibility of reducing the differences between patient-specific internal doses and doses estimated using reference phantoms was evaluated. Relatively simple adjustments to a polygon-surface ICRP adult male reference phantom were applied to fit selected individual dimensions using the software Rhinoceros®4.0. We tested this approach on two patient-specific phantoms: the biggest and the smallest phantoms from the Helmholtz Zentrum München library. These phantoms have unrelated anatomy and large differences in body-mass-index. Three models approximating each patient’s anatomy were considered: the voxel and the polygon-surface ICRP adult male reference phantoms and the adjusted polygon-surface reference phantom. The Specific Absorbed Fractions (SAFs) for internal photon and electron sources were calculated with the Monte Carlo code EGSnrc. Employing the time-integrated activity coefficients of a radiopharmaceutical (S)-4-(3-18F-fluoropropyl)-l-glutamic acid and the calculated SAFs, organ absorbed-dose coefficients were computed following the formalism promulgated by the Committee on Medical Internal Radiation Dose. We compared the absorbed-dose coefficients between each patient-specific phantom and other models considered with emphasis on the cross-fire component. The corresponding differences for most organs were notably lower for the adjusted reference models compared to the case when reference models were employed. Overall, the proposed approach provided reliable dose estimates for both tested patient-specific models despite the pronounced differences in their anatomy. To capture the full range of inter-individual anatomic variability more patient-specific phantoms are required. The results of this test study suggest a feasibility of estimating patient-specific doses within a relative uncertainty of 25% or less using adjusted reference models, when only simple phantom scaling is applied.

Specific Absorbed Fractions of Internal Photon and Electron Emitters in a Human Voxel-based Phantom: A Monte Carlo Study.

The specific absorbed fraction (SAF) of energy is an essential element of internal dose assessment. Here reported a set of SAFs calculated for selected organs of a human voxel-based phantom. The Monte Carlo transport code GATE version 6.1 was used to simulate monoenergetic photons and electrons with energies ranging from 10 keV to 2 MeV. The particles were emitted from three source organs: 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 the MCNP4B and GATE/GEANT4 Monte Carlo codes. For most photon energies, the self-irradiation is higher, and the cross-irradiation is lower in the GATE results compared to the MCNP4B. The results show generally good agreement for photons and high-energy electrons with discrepancies within − 2% ±3%. Nevertheless, significant differences 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 the SAF values for the human voxel phantom do not show significant differences, and the results also demonstrated the usefulness and applicability of GATE Monte Carlo package for voxel level dose calculations in nonuniform media. The present SAFs calculation for the Zubal voxel phantom is validated by the intercomparison of the results obtained by other Monte Carlo codes.