Specific absorbed fractions for internal photon emitters calculated for a tomographic model of a pregnant woman

Simple variance reduction in Monte Carlo calculations of specific absorbed fractions: Russian roulette and splitting at the source organ

PURPOSE

To investigate the capabilities of several variance reduction techniques in the calculation of specific absorbed fractions in cases where the source and the target organs are far away and/or the target organs have a small volume.

METHODS

The specific absorbed fractions have been calculated by using the Monte Carlo code PENELOPE and by assuming the thyroid gland as the source organ and the testicles, the urinary bladder, the uterus, and the ovaries as the target ones. A mathematical anthropomorphic phantom, similar to the MIRD-type phantoms, has been considered. Photons with initial energies of 50, 100 and 500 keV were emitted isotropically from the volume of the source organ. Simulations have been carried out by implementing the variance reduction techniques of splitting and Russian roulette at the source organ only and the interaction forcing at the target organs. The influence of the implementation details of those techniques have been investigated and optimal parameters have been determined. All simulations were run with a CPU time of 1.5 · 10⁵ s.

RESULTS

Specific absorbed fractions with relative uncertainties well below 10% have been obtained in most cases, agreeing with those used as reference. The best value for the factor defining the application of the Russian roulette technique was r = 0.3. The best value for the splitting number was between s = 3 and s = 10, depending on the specific energies and target organs.

CONCLUSIONS

The proposed strategy provides an effective method for computing specific absorbed fractions for the most unfavorable situations, with a computing effort that is considerably reduced with respect to other methodologies.

Specific absorbed fractions in thyroid diagnostics and treatment: Monte Carlo calculation with PENELOPE

In nuclear medicine, diagnostic and therapy procedures in which a certain radiopharmaceutical is administered to a patient are performed. An important point is the determination of the dose absorbed by the important organs of the patient due to these procedures. This dose depends on the particular radionuclide used and the so-called specific absorbed fractions. In this work, by means of Monte Carlo (MC) simulation, the specific absorbed fractions in case the thyroid gland acts as a source organ and for photon energies between 30 keV and 2 MeV have been determined. The computer code PENELOPE has been used as well as the adult male mathematical phantom provided with the distribution of this code. Three different simulation types were carried out. In one of them, only photon transport was considered. In the other two, electron transport was included, doing a detailed and a mixed simulation. In general, the fractions were estimated with uncertainties <9 %, for the mixed and detailed simulations, and <3 %, for the simulation in which only photons are included. For some target organs and, especially for energies <100 keV, the uncertainties found were larger. The results obtained here have been compared with those obtained by other authors using other MC codes. A good agreement has been found in 80 % of the cases.

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.

Specific absorbed fractions for internal electron emitters derived for a set of anatomically realistic reference pregnant female models

The specific absorbed fraction (Phi), defined by the Medical Internal Radiation Dose Committee, is generally applied to evaluate the average absorbed dose in a target organ as a result of radioactive materials deposited in a source organ. This paper reports a new set of Phi values for internal electron emitters ranging from 10 keV to 4 MeV from various internal organs of the mother to the fetus based on three newly developed pregnant female tomographic models, called RPI-P3, RPI-P6 and RPI-P9. The results show a linear log relationship between Phi values and electron energy. The linear log coefficients have been derived and reported. The relationship between Phi values and mean distances between source organs and the fetus were also determined to allow for individual dosimetry. Since the RPI-P models have finer details of human anatomy and more realistic organ volumes and geometries, which follow the latest ICRP reference values, the newly derived Phi values could be used as reference values in determination of the dose to the fetus from internal electron emitters.

SAF values for internal photon emitters calculated for the RPI-P pregnant-female models using Monte Carlo methods

Estimates of radiation absorbed doses from radionuclides internally deposited in a pregnant woman and her fetus are very important due to elevated fetal radiosensitivity. This paper reports a set of specific absorbed fractions (SAFs) for use with the dosimetry schema developed by the Society of Nuclear Medicine's Medical Internal Radiation Dose (MIRD) Committee. The calculations were based on three newly constructed pregnant female anatomic models, called RPI-P3, RPI-P6, and RPI-P9, that represent adult females at 3-, 6-, and 9-month gestational periods, respectively. Advanced Boundary REPresentation (BREP) surface-geometry modeling methods were used to create anatomically realistic geometries and organ volumes that were carefully adjusted to agree with the latest ICRP reference values. A Monte Carlo user code, EGS4-VLSI, was used to simulate internal photon emitters ranging from 10 keV to 4 MeV. SAF values were calculated and compared with previous data derived from stylized models of simplified geometries and with a model of a 7.5-month pregnant female developed previously from partial-body CT images. The results show considerable differences between these models for low energy photons, but generally good agreement at higher energies. These differences are caused mainly by different organ shapes and positions. Other factors, such as the organ mass, the source-to-target-organ centroid distance, and the Monte Carlo code used in each study, played lesser roles in the observed differences in these. Since the SAF values reported in this study are based on models that are anatomically more realistic than previous models, these data are recommended for future applications as standard reference values in internal dosimetry involving pregnant females.

A feasibility study to calculate unshielded fetal doses to pregnant patients in 6-MV photon treatments using Monte Carlo methods and anatomically realistic phantoms

A Monte Carlo-based procedure to assess fetal doses from 6-MV external photon beam radiation treatments has been developed to improve upon existing techniques that are based on AAPM Task Group Report 36 published in 1995 [M. Stovall et al., Med. Phys. 22, 63-82 (1995)]. Anatomically realistic models of the pregnant patient representing 3-, 6-, and 9-month gestational stages were implemented into the MCNPX code together with a detailed accelerator model that is capable of simulating scattered and leakage radiation from the accelerator head. Absorbed doses to the fetus were calculated for six different treatment plans for sites above the fetus and one treatment plan for fibrosarcoma in the knee. For treatment plans above the fetus, the fetal doses tended to increase with increasing stage of gestation. This was due to the decrease in distance between the fetal body and field edge with increasing stage of gestation. For the treatment field below the fetus, the absorbed doses tended to decrease with increasing gestational stage of the pregnant patient, due to the increasing size of the fetus and relative constant distance between the field edge and fetal body for each stage. The absorbed doses to the fetus for all treatment plans ranged from a maximum of 30.9 cGy to the 9-month fetus to 1.53 cGy to the 3-month fetus. The study demonstrates the feasibility to accurately determine the absorbed organ doses in the mother and fetus as part of the treatment planning and eventually in risk management.

Computational Anthropomorphic Models of the Human Anatomy: The Path to Realistic Monte Carlo Modeling in Radiological Sciences

The widespread availability of high-performance computing and popularity of simulations stimulated the development of computational anthropomorphic models of the human anatomy for medical imaging modalities and dosimetry calculations. The widespread interest in molecular imaging spurred the development of more realistic three- to five-dimensional computational models based on the actual anatomy and physiology of individual humans and small animals. These can be defined by either mathematical (analytical) functions or digital (voxel-based) volume arrays (or a combination of both), thus allowing the simulation of medical imaging data that are ever closer to actual patient data. The paradigm shift away from the stylized human models is imminent with the development of more than 30 voxel-based tomographic models in recent years based on anatomical medical images. We review the fundamental and technical challenges of designing computational models of the human anatomy, and focus particularly on the latest developments and future directions of their application in the simulation of radiological imaging systems and dosimetry calculations.