Comparison of photon organ and effective dose coefficients for PIMAL stylized phantom in bent positions in standard irradiation geometries

Computational phantoms with articulated arms and legs have been constructed to enable the estimation of radiation dose in different postures. Through a graphical user interface, the Phantom wIth Moving Arms and Legs (PIMAL) version 4.1.0 software can be employed to articulate the posture of a phantom and generate a corresponding input deck for the Monte Carlo N-Particle (MCNP) radiation transport code. In this work, photon fluence-to-dose coefficients were computed using PIMAL to compare organ and effective doses for a stylized phantom in the standard upright position with those for phantoms in realistic work postures. The articulated phantoms represent working positions including fully and half bent torsos with extended arms for both the male and female reference adults. Dose coefficients are compared for both the upright and bent positions across monoenergetic photon energies: 0.05, 0.1, 0.5, 1.0, and 5.0 MeV. Additionally, the organ doses are compared across the International Commission on Radiological Protection’s standard external radiation exposure geometries: antero-posterior, postero-anterior, left and right lateral, and isotropic (AP, PA, LLAT, RLAT, and ISO). For the AP and PA irradiation geometries, differences in organ doses compared to the upright phantom become more profound with increasing bending angles and have doses largely overestimated for all organs except the brain in AP and bladder in PA. In LLAT and RLAT irradiation geometries, energy deposition for organs is more likely to be underestimated compared to the upright phantom, with no overall change despite increased bending angle. The ISO source geometry did not cause a significant difference in absorbed organ dose between the different phantoms, regardless of position. Organ and effective fluence-to-dose coefficients are tabulated. In the AP geometry, the effective dose at the 45° bent position is overestimated compared to the upright phantom below 1 MeV by as much as 27% and 82% in the 90° position. The effective dose in the 45° bent position was comparable to that in the 90° bent position for the LLAT and RLAT irradiation geometries. However, the upright phantom underestimates the effective dose to PIMAL in the LLAT and RLAT geometries by as much as 30% at 50 keV.     

Calculation of dose components in head phantom for boron neutron capture therapy.

Application of neutrons to cancer treatment has been a subject of considerable clinical and research interest since the discovery of the neutron by Chadwick in 1932 (3). Boron neutron capture therapy (BNCT) is a technique of radiation oncology which is used in treating brain cancer (glioblastoma multiform) or melanoma and that consists of preferentially loading a compound containing 10B into the tumor location, followed by the irradiation of the patient with a beam of neutron. Dose distribution for BNCT is mainly based on Monte Carlo simulations. In this work, the absorbed dose spatial distribution resultant from an idealized neutron beam incident upon ahead phantom is investigated using the Monte Carlo N-particles code, MCNP 4B. The phantom model used is based on the geometry of a circular cylinder on which sits an elliptical cylinder capped by half an ellipsoid representing the neck and head, both filled with tissue-equivalent material. The neutron flux and the contribution of individual absorbed dose components, as a function of depths and of radial distance from the beam axis (dose profiles) in phantom model, is presented and discussed. For the studied beam the maximum thermal neutron flux is at a depth of 2 cm and the maximum gamma dose at a depth of 4 cm.     

Organ and effective dose rate coefficients for submersion exposure in occupational settings

External dose coefficients for environmental exposure scenarios are often computed using assumption on infinite or semi-infinite radiation sources. For example, in the case of a person standing on contaminated ground, the source is assumed to be distributed at a given depth (or between various depths) and extending outwards to an essentially infinite distance. In the case of exposure to contaminated air, the person is modeled as standing within a cloud of infinite, or semi-infinite, source distribution. However, these scenarios do not mimic common workplace environments where scatter off walls and ceilings may significantly alter the energy spectrum and dose coefficients. In this paper, dose rate coefficients were calculated using the International Commission on Radiological Protection (ICRP) reference voxel phantoms positioned in rooms of three sizes representing an office, laboratory, and warehouse. For each room size calculations using the reference phantoms were performed for photons, electrons, and positrons as the source particles to derive mono-energetic dose rate coefficients. Since the voxel phantoms lack the resolution to perform dose calculations at the sensitive depth for the skin, a mathematical phantom was developed and calculations were performed in each room size with the three source particle types. Coefficients for the noble gas radionuclides of ICRP Publication 107 (e.g., Ne, Ar, Kr, Xe, and Rn) were generated by folding the corresponding photon, electron, and positron emissions over the mono-energetic dose rate coefficients. Results indicate that the smaller room sizes have a significant impact on the dose rate per unit air concentration compared to the semi-infinite cloud case. For example, for Kr-85 the warehouse dose rate coefficient is 7% higher than the office dose rate coefficient while it is 71% higher for Xe-133.     

The concept of the effective dose a proposal for the combination of organ doses

Irradiation of the human body by external or internal sources leads mostly to a simultaneous exposure of several organs. However, so far no clear and consistent recommendations for the combination of organ doses and the assessment of an exposure limit under such irradiation conditions are available. Following a proposal described in ICRP-publication 14 one possible concept for the combination of organ doses is discussed in this paper. This concept is based on the assumption that at low doses the total radiation detriment to the exposed person is given by the sum of radiation detriments to the single organs. Taking into account a linear dose-risk relationship, the sum of weighted organ doses leads to the definition of an "Effective Dose". The applicability and consequences of this "Effective Dose Concept" are discussed especially with regard to the assessment of the maximum permissible intake of radionuclides into the human body and the combination of external and internal exposure.     

External exposure of deciduous tooth enamel to photons: dose conversion coefficients for standard radiation fields

Dose conversion coefficients for teeth of children were computed for external photon sources by means of Monte Carlo methods using a modified MIRD-type mathematical phantom of a 5-year-old child. The tooth region is separated into eight smaller regions that represent incisors, canines, first and second molars. Each of these sub-regions is separated into enamel and dentin parts. Dose conversion coefficients were computed as ratio of absorbed dose in the enamel and air kerma. They are given for unidirectional (AP, PA, RLAT, LLAT), rotational (ROT) and isotropic (ISO) photon sources in the energy range from 10 keV to 10 MeV. All computations were performed with the MCNP4 code including coupled electron-photon transport. The computed coefficients demonstrate a significant non-linearity versus photon energy, which is more pronounced than that observed for adult phantoms. Due to this non-linearity, use of the EPR-measured doses in human teeth requires information on the incident photon fluence spectra. The data presented can be used for assessment of public exposure.     

Development of a phantom for dose distribution verification in Stereotactic Radiosurgery

A geometric acrylic phantom was designed and built for dose distribution verification in Stereotactic Radiosurgery. Acrylic objects representing the tumor tissue, (target volume (TV)), and the organ at risk (OAR), the brainstem, were inserted inside this phantom. The TV is represented by two semi-spheres of acrylic with a diameter of 13.0 mm, both having a central cavity for accommodation of a TLD-100 detector and a small radiochromic Gafchromic EBT film. The OAR is represented by the two parts of a 38.0 mm length acrylic cylinder with a diameter 18.0 mm and cavities along the cylinder central axis able to accommodate 5 TLD – 100 detectors and another of EBT film between the two cylinder parts. This experimental setup was submitted to a radiosurgical treatment, after which the TL dosimeters were evaluated and their responses were compared with the planned dose values. The radiochromic EBT films showed the dose distributions. The linear accelerator used was a Varian 2300 C/D, generating a 6 MV photon beam. The investigated phantom system was able to check the accuracy of dose delivery to predetermined points and the dose distribution due to stereotactic radiosurgery treatments and proved to be a good tool for quality control in these situations.     

An EGS4 based Monte Carlo code for the calculation of organ equivalent dose to a modified Yale voxel phantom.

Organ or tissue equivalent dose, the most important quantity in radiation protection, cannot be measured directly. Therefore it became common practice to calculate the quantity of interest with Monte Carlo methods applied to so-called human phantoms, which are virtual representations of the human body. The Monte Carlo computer code determines conversion coefficients, which are ratios between organ or tissue equivalent dose and measurable quantities. Conversion coefficients have been published by the ICRP (Report No. 74) for various types of radiation, energies and fields, which have been calculated, among others, with the mathematical phantoms ADAM and EVA. Since then progress of image processing, and of clock speed and memory capacity of computers made it possible to create so-called voxel phantoms, which are a far more realistic representation of the human body. Voxel (Volume pixel) phantoms are built from segmented CT and/or MRI images of real persons. A complete set of such images can be joined to a 3-dimensional representation of the human body, which can be linked to a Monte Carlo code allowing for particle transport calculations. A modified version of the VOX_TISS8 human voxel phantom (Yale University) has been connected to the EGS4 Monte Carlo code. The paper explains the modifications, which have been made, the method of coupling the voxel phantom with the code, and presents results as conversion coefficients between organ equivalent dose and kerma in air for external photon radiation. A comparison of the results with published data shows good agreement.     

NUNDO: a numerical model of a human torso phantom and its application to effective dose equivalent calculations for astronauts at the ISS

The health effects of cosmic radiation on astronauts need to be precisely quantified and controlled. This task is important not only in perspective of the increasing human presence at the International Space Station (ISS), but also for the preparation of safe human missions beyond low earth orbit. From a radiation protection point of view, the baseline quantity for radiation risk assessment in space is the effective dose equivalent. The present work reports the first successful attempt of the experimental determination of the effective dose equivalent in space, both for extra-vehicular activity (EVA) and intra-vehicular activity (IVA). This was achieved using the anthropomorphic torso phantom RANDO^® equipped with more than 6,000 passive thermoluminescent detectors and plastic nuclear track detectors, which have been exposed to cosmic radiation inside the European Space Agency MATROSHKA facility both outside and inside the ISS. In order to calculate the effective dose equivalent, a numerical model of the RANDO^® phantom, based on computer tomography scans of the actual phantom, was developed. It was found that the effective dose equivalent rate during an EVA approaches 700 μSv/d, while during an IVA about 20 % lower values were observed. It is shown that the individual dose based on a personal dosimeter reading for an astronaut during IVA results in an overestimate of the effective dose equivalent of about 15 %, whereas under an EVA conditions the overestimate is more than 200 %. A personal dosemeter can therefore deliver quite good exposure records during IVA, but may overestimate the effective dose equivalent received during an EVA considerably.     

A review of experimental measurements of effective diffusive permeabilities and effective diffusion coefficients in biofilms

Experimental measurements of effective diffusive permeabilities and effective diffusion coefficients in biofilms are reviewed. Effective diffusive permeabilities, the parameter appropriate to the analysis of reaction-diffusion interactions, depend on solute type and biofilm density. Three categories of solute physical chemistry with distinct diffusive properties were distinguished by the present analysis. In order of descending mean relative effective diffusive permeability (De/Daq) these were inorganic anions or cations (0.56), nonpolar solutes with molecular weights of 44 or less (0.43), and organic solutes of molecular weight greater than 44 (0.29). Effective diffusive permeabilities decrease sharply with increasing biomass volume fraction suggesting a serial resistance model of diffusion in biofilms as proposed by Hinson and Kocher (1996). A conceptual model of biofilm structure is proposed in which each cell is surrounded by a restricted permeability envelope. Effective diffusion coefficients, which are appropriate to the analysis of transient penetration of nonreactive solutes, are generally similar to effective diffusive permeabilities in biofilms of similar composition. In three studies that examine diffusion of very large molecular weight solutes (>5000) in biofilms, the average ratio of the relative effective diffusion coefficient of the large solute to the relative effective diffusion coefficient of either sucrose or fluorescein was 0.64, 0.61, and 0.36. It is proposed that large solutes are effectively excluded from microbial cells, that small solutes partition into and diffuse within cells, and that ionic solutes are excluded from cells but exhibit increased diffusive permeability (but decreased effective diffusion coefficients) due to sorption to the biofilm matrix.     

Red marrow dose measurement in phantom for tele-irradiation of the nasopharynx by cobalt-60 unit

This investigation is to study red marrow dose measurement for the 60Co gamma-ray treatment of nasopharyngeal carcinoma with the aid of Rando phantom. The energy of scattered radiation in space and in tissue was investigated by means of half-value layers with thermoluminescent dosimeters. Other related factors such as field size, SSD, and orientation of primary beams were also investigated. In the non-metastases cases, the red marrow dose is 26 +/- 6 rads which is about 0.40% tumor dose excluding the marrow at the NPC site. For the metastases cases, the red marrow dose is still about 0.40% tumor dose. If the red marrow at the NPC site is included, the red marrow dose is about 14.5% tumor dose for both non-metastases and metastases cases.     

Solid cancer risk coefficient for fast neutrons in terms of effective dose

Cancer mortality risk coefficients for neutrons have recently been assessed by a procedure that postulates for the neutrons a linear dose dependence, invokes the excess risk of the A-bomb survivors at a gamma-ray dose D(1) of 1 Gy, and assumes a neutron RBE as a function of D(1) between 20 and 50. The excess relative risk (ERR) of 0.008/mGy has been obtained for R(1) = 20 and 0.016/mGy for R(1) = 50. To compare these results to the current ICRP nominal risk coefficient for solid cancer mortality (0.045/Sv for a population of all ages; 0.036/Sv for a working population), the ERR is translated into lifetime attributable risk and is then related to effective dose. The conversion is not trivial, because the neutron effective dose has been defined by ICRP not as a weighted genuine neutron dose (neutron kerma), but as a weighted dose that includes the dose from gamma rays that are induced by neutrons in the body. If this is accounted for, the solid cancer mortality risk for a working population is found to agree with the ICRP nominal risk coefficient for neutrons in their most effective energy range, 0.2 MeV to 0.5 MeV. In radiation protection practice, there is an added level of safety, because the effective dose, E, is-for monitoring purposes-assessed in terms of the operational quantity H*, which overestimates E substantially for neutrons between 0.01 MeV and 2 MeV.     

Determination of glucose and ethanol effective diffusion coefficients in Ca-alginate gel.

Glucose and ethanol diffusion coefficients in 2% Ca-alginate gel were measured using the experimental technique based on solute diffusion into or out of gel beads in a well-stirred solution. The aim of the study was to make the measurements under typical conditions found in alcoholic fermentations, such as the concentrations of glucose (100 g l-1) and ethanol (50 g l-1), the simultaneous counter-diffusion of glucose and ethanol, and the presence of cells in the gel beads at a level of 10(9) cells g-1 of beads. Previously, an evaluation of the error associated with the methodology used indicated how the experimental procedure would minimize the error. The individual measurement of glucose and ethanol coefficients in 2% Ca-alginate with no cells gave values of 5.1 and 9.6 x 10(-6) cm2 s-1, respectively, which are lower than those in water. When the effect of counter-diffusion was investigated, both coefficients decreased: glucose by 14% and ethanol by 28%. When cells were incorporated into the beads, only the ethanol coefficient decreased significantly, while the glucose coefficient apparently increased its value to 6.9 10(-6) cm2 s-1.     

Comparison of the reversibility of locipet23 andlys2 after UV irradiation in the standard and UV-sensitive strains ofSaccharomyces cerevisiae

Reversibility of the respiration-deficient locuspet23 and auxotrophic locuslys2 was followed in the standard (RAD1) and UV sensitive (rad1–2) strains ofSaccharomyces cerevisiae, both after identical doses of UV radiation and at identical survival. When comparing the reversibility after the treatment with identical doses of UV radiation a much higher reversibility of both loci in strainrad1–2 could be detected. When comparing the reversibility of the loci in question at identical survival of both strains it could be found that the reversibility of thepet23 locus is again much higher in strainrad1–2, whereas the reversibility of thelys2 locus is roughly identical in the two strains. Thus, the function of geneRAD1 in repair processes is apparently associated with the “error-free” repair, both at low and high doses of ultraviolet radiation.     

Comparison of translation loads for standard and alternative genetic codes

Background  

The (almost) universality of the genetic code is one of the most intriguing properties of cellular life. Nevertheless, several variants of the standard genetic code have been observed, which differ in one or several of 64 codon assignments and occur mainly in mitochondrial genomes and in nuclear genomes of some bacterial and eukaryotic parasites. These variants are usually considered to be the result of non-adaptive evolution. It has been shown that the standard genetic code is preferential to randomly assembled codes for its ability to reduce the effects of errors in protein translation.     

Effects of microorganisms on effective oxygen diffusion coefficients and solubilities in fermentation media

Effective oxygen diffusion coefficients and solubilities were measured for submerged cultures of Saccharomyces cerevisiae, Escherichia coli, and Penicillium chrysogenum. Both effective oxygen diffusion coefficients and solubilities were found to decrease with increasing cell concentrations in the fermentation media. Comparison of the experimental results of effective oxygen diffusion coefficients in fermentation media with values theoretically predicted on the assumption of unpenetrable microbial cells indicates that oxygen molecules diffuse through the cells during the diffusion process. Within the cell concentration range of typical submerged fermentations, the effective oxygen diffusion coefficient of the fermentation media can be described as D(e) = A(1)f + A(2)f(2). In this equation, fis the cell volume fraction and both A(1) and A(2) are functions of the shape of the cells and the ratio of effective oxygen diffusion coefficient in microbial cells to that in the medium.