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

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

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

InterDosi Monte Carlo simulations of photon and electron specific absorbed fractions in a voxel-based crab phantom

InterDosi is a new in-house Monte Carlo code that aims at facilitating the use of the Geant4 toolkit for internal dosimetry using voxel-based phantoms. In the present work the dosimetric capabilities of this code are assessed by calculating self-irradiation specific absorbed fractions (SI-SAFs) in a voxel-based crab phantom. Recent standard human organ compositions and densities taken from ICRP Publication 110 have been used for material specifications of the four organs of a crab, namely, the heart, hepatopancreas, gills, and gonads, whereas the material assigned to the crab shell has been modeled based on literature values. The SI-SAFs were calculated for mono-energetic photons of energies between 10 and 4000 keV, and for mono-energetic electrons of energies between 100 and 4000 keV. The statistical errors corresponding to the calculated SI-SAFs were all less than 0.01%. The results obtained demonstrate that the simulated masses and volumes of the crab organs are in good agreement with those presented in the literature. In addition, the dosimetric results show that the calculated SI-SAFs are generally consistent with those reported in the literature, with some moderate differences due to differences in material specification. It is concluded that the InterDosi code can be successfully employed in internal dose estimations in small organisms, and it is suggested that material specifications specifically relating to crab tissues should be developed to provide more precise SI-SAFs.

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.

TSH and Thyrotropic Agonists: Key Actors in Thyroid Homeostasis

This paper provides the reader with an overview of our current knowledge of hypothalamic-pituitary-thyroid feedback from a cybernetic standpoint. Over the past decades we have gained a plethora of information from biochemical, clinical, and epidemiological investigation, especially on the role of TSH and other thyrotropic agonists as critical components of this complex relationship. Integrating these data into a systems perspective delivers new insights into static and dynamic behaviour of thyroid homeostasis. Explicit usage of this information with mathematical methods promises to deliver a better understanding of thyrotropic feedback control and new options for personalised diagnosis of thyroid dysfunction and targeted therapy, also by permitting a new perspective on the conundrum of the TSH reference range.