Voxel model of a rabbit: assessment of absorbed doses in organs after CT examination performed by two different protocols.

The objective of this work was to assess absorbed doses in organs and tissues of a rabbit, following computed tomography (CT) examinations, using a dedicated 3D voxel model. Absorbed doses in relevant organs were calculated using the MCNP5 Monte Carlo software. Calculations were perfomed for two standard CT protocols, using tube voltages of 110 kVp and 130 kVp. Absorbed doses were calculated in 11 organs and tissues, i.e., skin, bones, brain, muscles, heart, lungs, liver, spleen, kidney, testicles, and fat tissue. The doses ranged from 15.3 to 28.3 mGy, and from 40.2 to 74.3 mGy, in the two investigated protocols. The organs that received the highest dose were bones and kidneys. In contrast, brain and spleen were organs that received the smallest doses. Doses in organs which are stretched along the body did not change significantly with distance. On the other hand, doses in organs which are localized in the body showed maximums and minimums. Using the voxel model, it is possible to calculate the dose distribution in the rabbit's body after CT scans, and study the potential biological effects of CT doses in certain organs. The voxel model presented in this work can be used to calculated doses in all radiation experiments in which rabbits are used as experimental animals.

Monte Carlo investigation of electron specific energy distribution in a single cell model.

Knowledge of microdosimetric quantities of certain radionuclides is important in radio immune cancer therapies. Specific energy distribution of radionuclides, which are bound to the cell, is the microdosimetric quantity essential in the process of radionuclide selection for patient tumour treatment. The aim of this paper is to establish an applicable method to determine microdosimetric quantities for various radionuclides. The established method is based on knowledge of microdosimetric quantities of monoenergetic electrons. In this paper these quantities are determined for the single-cell model for a range of electron energies up to [Formula: see text], using the Monte Carlo transport code PENELOPE. The results show that using monoenergetic specific energies, reconstruction of the specific energy of beta-emitting radionuclides can be successfully done with very high accuracy. Microdosimetric quantities share information about the physical processes involved and give insight about energy depositions, which is of use in the procedure of radionuclide selection for a given type of therapy.

Simulation of H p (10) and effective dose received by the medical staff in interventional radiology procedures.

Interventional radiology and cardiology are widespread employed techniques for diagnosis and treatment of several pathologies because they avoid the majority of the side-effects associated with surgical treatments, but are known to increase the radiation exposure to patient and operators. In recent years many studies treated the exposure of the operators performing cardiological procedures. The aim of this work is to study the exposure condition of the medical staff in some selected interventional radiology procedures. The Monte Carlo simulations have been employed with anthropomorphic mathematical phantoms reproducing the irradiation scenario of the medical staff with two operators and the patient. A personal dosemeter, put on apron, was modelled for comparison with measurements performed in hospitals, done with electronic dosemeters, in a reduced number of interventional radiology practices. Within the limits associated to the use of numerical anthropomorphic models to mimic a complex interventional procedure, the personal dose equivalent, H p (10), was evaluated and normalised to the simulated Kerma-Area Product, KAP, value, indeed the effective dose has been calculated. The H p (10)/KAPvalue of the first operator is about 10 µSv/Gy.cm2, when ceiling shielding is not used. This value is calculated on the trunk and it varies of +/-30% moving the dosemeter to the waist or to the neck. The effective dose, normalised to the KAP value, varies between 0.03 and 0.4 µSv/Gy.cm2. Considering all the unavoidable approximation of this kind of investigations, the comparisons with hospital measurement and literature data showed a good agreement allowing to use of the present results for dosimetric characterisation of interventional radiology procedures.

Doses from radon progeny as a source of external beta and gamma radiation.

Great deal of work has been devoted to determine doses from alpha particles emitted by (222)Rn and its progeny. In contrast, contribution of beta particles and following gamma radiation to total dose has mostly been neglected so far. The present work describes a study of the detriment of (222)Rn progeny for humans due to external exposure. Doses and dose conversion factors (DCFs) were determined for beta and gamma radiation in main organs and remainder tissue of the Oak Ridge National Laboratory phantom, taking into account (222)Rn progeny (214)Pb and (214)Bi distributed in the middle of a standard or typical room with dimensions 4 m × 5 m × 2.8 m. The DCF was found to be 7.37 µSv/WLM. Skin and muscle tissue from remainder tissue receives largest dose. Beta and gamma radiation doses from external exposure were compared with alpha, beta, and gamma doses from internal exposure where the source of radioactivity was the lungs. Total doses received in all main organs and remainder tissues were obtained by summing up the doses from external and internal exposure and the corresponding DCF was found to be 20.67 µSv/WLM.

Doses from beta radiation in sensitive layers of human lung and dose conversion factors due to 222Rn/220Rn progeny.

Great deal of work has been devoted to determine doses from alpha particles emitted by (222)Rn and (220)Rn progeny. In contrast, contribution of beta particles to total dose has been neglected by most of the authors. The present work describes a study of the detriment of (222)Rn and (220)Rn progeny to the human lung due to beta particles. The dose conversion factor (DCF) was introduced to relate effective dose and exposure to radon progeny; it is defined as effective dose per unit exposure to inhaled radon or thoron progeny. Doses and DCFs were determined for beta radiation in sensitive layers of bronchi (BB) and bronchioles (bb), taking into account inhaled (222)Rn and (220)Rn progeny deposited in mucus and cilia layer. The nuclei columnar secretory and short basal cells were considered to be sensitive target layers. For dose calculation, electron-absorbed fractions (AFs) in the sensitive layers of the BB and bb regions were used. Activities in the fast and slow mucus of the BB and bb regions were obtained using the LUNGDOSE software developed earlier. Calculated DCFs due to beta radiation were 0.21 mSv/WLM for (222)Rn and 0.06 mSv/WLM for (220)Rn progeny. In addition, the influence of Jacobi room parameters on DCFs was investigated, and it was shown that DCFs vary with these parameters by up to 50%.

EFFECT OF BUILDUP FACTORS ON INDOOR GAMMA DOSE RATE.

The effect of buildup factors on absorbed dose rate in air and the effective dose from gamma rays of primordial radionuclides in building materials, was investigated in the article. Specific absorbed dose rates were calculated for the standard concrete room, as well as, for rooms where brick and covering building materials were used. For all room models the Harima (G-P) buildup factors were applied, while for the standard room the Berger's and Taylor's buildup factors were used, too. The contribution of the radiation buildup to absorbed dose rate and effective dose was determined as large as 41%.

Multiple Stressor Effects of Radon and Phthalates in Children: Background Information and Future Research.

The present paper reviews available background information for studying multiple stressor effects of radon (222Rn) and phthalates in children and provides insights on future directions. In realistic situations, living organisms are collectively subjected to many environmental stressors, with the resultant effects being referred to as multiple stressor effects. Radon is a naturally occurring radioactive gas that can lead to lung cancers. On the other hand, phthalates are semi-volatile organic compounds widely applied as plasticizers to provide flexibility to plastic in consumer products. Links of phthalates to various health effects have been reported, including allergy and asthma. In the present review, the focus on indoor contaminants was due to their higher concentrations and to the higher indoor occupancy factor, while the focus on the pediatric population was due to their inherent sensitivity and their spending more time close to the floor. Two main future directions in studying multiple stressor effects of radon and phthalates in children were proposed. The first one was on computational modeling and micro-dosimetric studies, and the second one was on biological studies. In particular, dose-response relationship and effect-specific models for combined exposures to radon and phthalates would be necessary. The ideas and methodology behind such proposed research work are also applicable to studies on multiple stressor effects of collective exposures to other significant airborne contaminants, and to population groups other than children.

A theoretical approach to indoor radon and thoron distribution.

A model based on the Finite Element Method was developed to simulate indoor behavior of radon ((222)Rn), thoron ((220)Rn) and their progeny, as well as, to calculate their spatial distributions. Since complex physical processes govern the distribution several simplifications were made in the presented model. Different locations of possible radon/thoron sources, diffusion of these gases, their radioactive decay, etc were taken into account. Influences of different parameters on thoron/radon as well as indoor distribution of their progeny, such as the geometry and room dimension, the presence of aerosols and their size distribution expressed through the diffusion coefficient, different kinds of ventilation, etc, were investigated. It has been found that radon is distributed homogeneously, while the thoron concentration is rather inhomogeneous and decreases exponentially with the distance from the source. Regardless of the source distribution, the distribution of radon was homogeneous, except at places near an air inlet and outlet. However, the distribution of thoron depends on the source distribution. If thoron emanates from walls or the floor, its concentration decreases with the distance from the wall. Moreover, the concentration gradient is much larger near walls. This suggests that the actual selection of the site effect should be taken into account when obtaining a representative value of indoor (220)Rn and their progeny for dose assessment. The simulation results of activities and their distribution were in accordance with the results of other studies and experiments.

MCNPX CALCULATIONS OF SPECIFIC ABSORBED FRACTIONS IN SOME ORGANS OF THE HUMAN BODY DUE TO APPLICATION OF 133Xe, 99mTc and 81mKr RADIONUCLIDES.

Monte Carlo simulations were performed to evaluate treatment doses with wide spread used radionuclides 133Xe, 99mTc and 81mKr. These different radionuclides are used in perfusion or ventilation examinations in nuclear medicine and as indicators for cardiovascular and pulmonary diseases. The objective of this work was to estimate the specific absorbed fractions in surrounding organs and tissues, when these radionuclides are incorporated in the lungs. For this purpose a voxel thorax model has been developed and compared with the ORNL phantom. All calculations and simulations were performed by means of the MCNP5/X code.

Monte Carlo studies on photon interactions in radiobiological experiments.

X-ray and γ-ray photons have been widely used for studying radiobiological effects of ionizing radiations. Photons are indirectly ionizing radiations so they need to set in motion electrons (which are a directly ionizing radiation) to perform the ionizations. When the photon dose decreases to below a certain limit, the number of electrons set in motion will become so small that not all cells in an "exposed" cell population can get at least one electron hit. When some cells in a cell population are not hit by a directly ionizing radiation (in other words not irradiated), there will be rescue effect between the irradiated cells and non-irradiated cells, and the resultant radiobiological effect observed for the "exposed" cell population will be different. In the present paper, the mechanisms underlying photon interactions in radiobiological experiments were studied using our developed NRUphoton computer code, which was benchmarked against the MCNP5 code by comparing the photon dose delivered to the cell layer underneath the water medium. The following conclusions were reached: (1) The interaction fractions decreased in the following order: 16O > 12C > 14N > 1H. Bulges in the interaction fractions (versus water medium thickness) were observed, which reflected changes in the energies of the propagating photons due to traversals of different amount of water medium as well as changes in the energy-dependent photon interaction cross-sections. (2) Photoelectric interaction and incoherent scattering dominated for lower-energy (10 keV) and high-energy (100 keV and 1 MeV) incident photons. (3) The fractions of electron ejection from different nuclei were mainly governed by the photoelectric effect cross-sections, and the fractions from the 1s subshell were the largest. (4) The penetration fractions in general decreased with increasing medium thickness, and increased with increasing incident photon energy, the latter being explained by the corresponding reduction in interaction cross-sections. (5) The areas under the angular distribution curves of photons exiting the medium layer and subsequently undergoing interactions within the cell layer became smaller for larger incident photon energies. (6) The number of cells suffering at least one electron hit increased with the administered dose. For larger incident photon energies, the numbers of cells suffering at least one electron hit became smaller, which was attributed to the reduction in the photon interaction cross-section. These results highlighted the importance of the administered dose in radiobiological experiments. In particular, the threshold administered doses at which all cells in the exposed cell array suffered at least one electron hit might provide hints on explaining the intriguing observation that radiation-induced cancers can be statistically detected only above the threshold value of ~100 mSv, and thus on reconciling controversies over the linear no-threshold model.

Derivation of V function for LR 115 SSNTD from its partial sensitivity to 222Rn and its short-lived progeny.

Solid-state nuclear track detectors (SSNTDs) have been widely applied for measurements of environmental concentrations of 222Rn and its progeny. The V function for an SSNTD is important for understanding the track development in the SSNTD as well as for real life applications. The partial sensitivity rhoi of the LR 115 detector applied in the bare mode to 222Rn and its short-lived progeny is related to the equilibrium factor F through the proxy equilibrium factor Fp. On the other hand, rhoi is also dependent on the removed active layer thickness during chemical etching, which is related to the V function for the LR 115 detector. In the present paper, the experimentally obtained rhoi values of the LR 115 detector for different removed active layer thickness are used to derive the V function for the LR 115 SSNTD, which took the form of the Durrani--Green's function, i.e., [formula: see text] , with the best-fitted constants as a1=14.23; a2=0.48; a3=5.9 and a4=0.077 (a5=1).

Derivation of V function for LR 115 SSNTD from its sensitivity to 220Rn in a diffusion chamber.

The sensitivity of the LR 115 detector inside a diffusion chamber to (220)Rn gas concentration is dependent on the removed active layer thickness during chemical etching. This dependence is related to the V function for the LR 115 detector (where V is the ratio between the track etch velocity V(t) and the bulk etch velocity V(b)) and the geometry of the diffusion chamber. The present paper presents the experimentally determined relationship between the sensitivity of the LR 115 detector inside a Karlsruhe diffusion chamber (determined from the number of etched tracks completely penetrating the active cellulose nitrate layer) and the removed active layer thickness. These data were used to derive the V function for the LR 115 detector, which took the functional form of the Durrani-Green's function, i.e., V=1+((a1e-)(a2R+a3e(-a)4R))(1(-e)(-a5R)), with the best-fitted constants as a(1)=14.50, a(2)=0.50, a(3)=3.9 and a(4)=0.066.

Room model with three modal distributions of attached 220Rn progeny and dose conversion factor.

Jacobi parametric room model was extended to the three modal distribution of aerosols, and applied to (220)Rn progeny. The computer program was developed to calculate ratios of progeny activity concentrations in different modes to (220)Rn concentration. The ratios are relatively small and they are given as functions on ventilation rate. Dose conversion factor (DCF) for (220)Rn progeny was calculated as 4.5 mSv WLM(-1), which is smaller by over three times than that for (222)Rn progeny.

Conversion coefficients for determination of dispersed photon dose during radiotherapy: NRUrad input code for MCNP.

Radiotherapy is a common cancer treatment module, where a certain amount of dose will be delivered to the targeted organ. This is achieved usually by photons generated by linear accelerator units. However, radiation scattering within the patient's body and the surrounding environment will lead to dose dispersion to healthy tissues which are not targets of the primary radiation. Determination of the dispersed dose would be important for assessing the risk and biological consequences in different organs or tissues. In the present work, the concept of conversion coefficient (F) of the dispersed dose was developed, in which F = (Dd/Dt), where Dd was the dispersed dose in a non-targeted tissue and Dt is the absorbed dose in the targeted tissue. To quantify Dd and Dt, a comprehensive model was developed using the Monte Carlo N-Particle (MCNP) package to simulate the linear accelerator head, the human phantom, the treatment couch and the radiotherapy treatment room. The present work also demonstrated the feasibility and power of parallel computing through the use of the Message Passing Interface (MPI) version of MCNP5.

Monte Carlo studies on neutron interactions in radiobiological experiments.

Monte Carlo method was used to study the characteristics of neutron interactions with cells underneath a water medium layer with varying thickness. The following results were obtained. (1) The fractions of neutron interaction with 1H, 12C, 14N and 16O nuclei in the cell layer were studied. The fraction with 1H increased with increasing medium thickness, while decreased for 12C, 14N and 16O nuclei. The bulges in the interaction fractions with 12C, 14N and 16O nuclei were explained by the resonance spikes in the interaction cross-section data. The interaction fraction decreased in the order: 1H > 16O > 12C > 14N. (2) In general, as the medium thickness increased, the number of "interacting neutrons" which exited the medium and then further interacted with the cell layer increased. (3) The area under the angular distributions for "interacting neutrons" decreased with increasing incident neutron energy. Such results would be useful for deciphering the reasons behind discrepancies among existing results in the literature.

Assessment of environmental radon hazard using human respiratory tract models.

Radon is a natural radioactive gas derived from geological materials. It has been estimated that about half of the total effective dose received by human beings from all sources of ionizing radiation is attributed to 222Rn and its short-lived progeny. In this paper, the use of human respiratory tract models to assess the health hazard from environmental radon is reviewed. A short history of dosimetric models for the human respiratory tract from the International Commission on Radiological Protection (ICRP) is first presented. The most important features of the newest model published by ICRP in 1994 (as ICRP Publication 66) are then described, including the morphometric model, physiological parameters, radiation biology, deposition of aerosols, clearance model and dose weighting. Comparison between different morphometric models and comparison between different deposition models are then given. Finally, the significance of various parameters in the lung model is discussed, including aerosol parameters, subject related parameters, target and cell related parameters, and parameters that define the absorption of radon from the lungs to blood. Dosimetric calculations gave a dose conversion coefficient of 15 mSv/WLM, which is higher than the value 5 mSv/WLM derived from epidemiological studies. ICRP stated that dosimetric models should only be used for comparison of doses in the human lungs resulted from different exposure conditions.

External doses to humans from 137Cs in soil.

Calculations of absorbed doses in organs of the human body and the total effective dose due to Cs in soil as a source of external exposure are presented in this work. Calculations were done using the MCNP-4B software package. The assumption was made that photons with an energy of 662 keV are emitted in a cylindrical volumetric source in soil up to the depth of 20 cm. Depth distributions of Cs at 19 locations around Kragujevac (a city in central Serbia) were measured by a HPGe detector. An ORNL phantom of an adult human standing on the soil above the center of a cylindrical radioactive source was used to calculate the conversion coefficients, i.e., absorbed doses in an organ per unit specific activity. The conversion coefficients in organs are given as a function of the source depth in soil. The largest absorbed dose was found in skin. The annual effective dose in humans was estimated from these calculations and the measured activity depth profile of Cs in soil. The average effective dose was found to be 3.17 microSv y. This value was rather small in comparison with other sources of natural ionizing radiation. One may conclude that Cs was a negligible source of external exposure in the area around the city.

Absorbed dose in target cell nuclei and dose conversion coefficient of radon progeny in the human lung.

To calculate the absorbed dose in the human lung due to inhaled radon progeny, ICRP focussed on the layers containing the target cells, i.e., the basal and secretory cells. Such an approach did not consider details of the sensitive cells in the layers. The present work uses the microdosimetric approach and determines the absorbed alpha-particle energy in non-spherical nuclei of target cells (basal and secretory cells). The absorbed energy for alpha particles emitted by radon progeny in the human respiratory tract was calculated in basal- and secretory-cell nuclei, assuming conical and ellipsoidal forms for these cells. Distributions of specific energy for different combinations of alpha-particle sources, energies and targets are calculated and shown. The dose conversion coefficient for radon progeny is reduced for about 2mSv/WLM when conical and ellipsoidal cell nuclei are considered instead of the layers. While changes in the geometry of secretory-cell nuclei do not have significant effects on their absorbed dose, changes from spherical to conical basal-cell nuclei have significantly reduced their absorbed dose from approximately 4 to approximately 3mGy/WLM. This is expected because basal cells are situated close to the end of the range of 6MeV alpha particles. This also underlines the significance of better and more precise information on targets in the T-B tree. A further change in the dose conversion coefficient can be achieved if a different weighting scheme is adopted for the doses for the cells. The results demonstrate the necessity for better information on the target cells for more accurate dosimetry for radon progeny.

Killing of target cells due to radon progeny in the human lung.

The dose conversion coefficient (DCC) is used to assess the risk due to inhaled radon progeny in the human lung. The present work uses the microdosimetric approach and determines the linear energy transfer in the target cell nuclei. Killing of target cells was also taken into account through an effect-specific track length model. To focus on the relevant part of the absorbed dose in the cell nuclei, the absorbed dose, which causes cell-killing is discarded in the final calculations of the DCC. Following this approach, the calculated DCC has become 3.4 mSv WLM(-1) which is very close to the epidemiologically derived value of approximately 4 mSv WLM(-1).

Alpha-particle radiobiological experiments using thin CR-39 detectors.

The present paper studied the feasibility of applying comet assay to evaluate the DNA damage in individual HeLa cervix cancer cells after alpha-particle irradiation. We prepared thin CR-39 detectors (<20 microm) as cell-culture substrates, with UV irradiation to shorten the track formation time. After irradiation of the HeLa cells by alpha particles, the tracks on the underside of the CR-39 detector were developed by chemical etching in (while floating on) a 14 N KOH solution at 37 degrees C. Comet assay was then applied. Diffusion of DNA out of the cells could be generally observed from the images of stained DNA. The alpha-particle tracks corresponding to the comets developed on the underside of the CR-39 detectors could also be observed by just changing the focal plane of the confocal microscope.