Absorbed dose estimations of 131I for critical organs using the GEANT4 Monte Carlo simulation code

The aim of this study is to compare the absorbed doses of critical organs of ¹³¹I using the MIRD (Medical Internal Radiation Dose) with the corresponding predictions made by GEANT4 simulations. S-values (mean absorbed dose rate per unit activity) and energy deposition per decay for critical organs of ¹³¹I for various ages, using standard cylindrical phantom comprising water and ICRP soft-tissue material, have also been estimated. In this study the effect of volume reduction of thyroid, during radiation therapy, on the calculation of absorbed dose is also being estimated using GEANT4. Photon specific energy deposition in the other organs of the neck, due to ¹³¹I decay in the thyroid organ, has also been estimated. The maximum relative difference of MIRD with the GEANT4 simulated results is 5.64% for an adult's critical organs of ¹³¹I. Excellent agreement was found between the results of water and ICRP soft tissue using the cylindrical model. S-values are tabulated for critical organs of ¹³¹I, using 1, 5, 10, 15 and 18 years (adults) individuals. S-values for a cylindrical thyroid of different sizes, having 3.07% relative differences of GEANT4 with Siegel & Stabin results. Comparison of the experimentally measured values at 0.5 and 1 m away from neck of the ionization chamber with GEANT4 based Monte Carlo simulations results show good agreement. This study shows that GEANT4 code is an important tool for the internal dosimetry calculations.     

Neutron dose distribution from 12C induced reactions on Ti and Ag using proton recoil scintillator

We measured the double differential neutron yield at 0°, 30°, 60° and 90° from ¹²C⁵⁺ induced reactions on thick targets of Ti and Ag at 12 MeV/amu at the Cyclotron at National Institute of Radiological Sciences, Inage, Japan, with 5″ × 5″ proton recoil scintillation detectors BC-501. The measured neutron spectra were unfolded using pulse height unfolding algorithm and energy and angular distribution obtained. Energy distribution of neutron ambient dose equivalent, H*(10) and absorbed dose, D at different angles was determined from double differential neutron spectra using ICRP recommended fluence to dose conversion coefficients.     

Influence of Bonner sphere response functions above 20 MeV on unfolded neutron spectra and doses

Monte Carlo (MC) codes for neutron transport calculations such as MCNP, MCNPX, FLUKA, PHITS, and GEANT4, crucially rely on cross sections that describe the interaction of neutrons with nuclei. For neutron energies below 20 MeV, evaluated cross sections are available that are validated against experimental data. In contrast, for high energies (above 20 MeV) experimental data are scarce and, for this reason, every neutron transport code is based on theoretical nuclear models to describe interactions of neutrons with nuclei in matter. Here we report on the calculation of a complete set of response functions for a Bonner spheres spectrometer (BSS), by means of GEANT4 using the Bertini and Binary Intranuclear Cascade (INC) models for energies above 20 MeV. The recent results were compared with those calculated by MCNP/LAHET and MCNP/HADRON MC codes. It turns out that, whatever code used, the response functions were rather similar for neutron energies below 20 MeV, for all 16 detector/moderator combinations of the considered BSS system. For higher energies, however, differences of more than a factor of 2 were observed, depending on neutron energy, detector/moderator combination, MC code, and nuclear model used. These differences are discussed in terms of neutron fluence rates measured at the environmental research station (UFS), “Schneefernerhaus”, (Zugspitze mountain, Germany, 2650 m a.s.l.) for energies below 0.4 eV (thermal neutrons), between 0.4 eV and 100 keV (epithermal neutrons), between 100 keV and 20 MeV (evaporation neutrons), and above 20 MeV (cascade neutrons). In terms of total neutron fluence rates, relative differences of up to 4% were obtained when compared to the standard MCNP/LAHET results, while in terms of total ambient dose equivalent, relative differences of up to 8% were obtained. Both the GEANT4 Binary INC and Bertini INC gave somewhat larger fluence and dose rates in the epithermal region. Most relevant for dose, however, those response functions calculated with the GEANT4 Bertini INC model provided about 18% less neutrons in the cascade region, which led to a roughly 13% smaller contribution of these neutrons to ambient dose equivalent. It is concluded that doses from secondary neutrons from cosmic radiation as deduced from BSS measurements are uncertain by about 10%, simply because of some differences in nuclear models used by various neutron transport codes.     

Organ point dose measurements in clinical multi slice computed tomography (MSCT) examinations with the MOSkin™ radiation dosimeter

This study reports on the application of the MOSkin™ dosimeter in MSCT imaging for the real-time measurement of absorbed organ point doses in a tissue-equivalent female anthropomorphic phantom. MOSkin™ dosimeters were placed within the phantom to measure absorbed point organ doses for 2 commonly applied clinical scan protocols, namely the renal calculus scan and the pulmonary embolus scan. Measured organ doses in the imaged field of view were found to be in the dose range 4.7–9.5 mGy and 16.2–27.4 mGy for the renal calculus scan and pulmonary scan protocols respectively. For the derivation of effective dose, using the more recent ICRP 103 tissue weighting factors (w_T) compared to that of the ICRP 60 w_T resulted in a difference in the derived effective dose by up to 0.8 mSv (−20%) in the renal calculus protocol and up to 1.8 mSv (18%) in the pulmonary embolus protocol. This difference is attributed to the reduced radiosensitivity of the gonads and the increased radiosensitivity of breast tissue in the latest ICRP 103 assigned w_T. The results of this study show that the MOSkin™ dosimeter is a useful real-time tool for the direct assessment of organ doses in clinical MSCT examinations.     

Evaluation of the dosimetric characteristics of a radiophotoluminescent glass dosimeter for high-energy photon and electron beams in the field of radiotherapy

We aimed to evaluate the suitability of a glass dosimeter (GD) for high-energy photon and electron beams in experimental and clinical use, especially for radiation therapy. We examined the expanded dosimetric characteristics of GDs including dose linearity up to 500 Gy, uniformity among GD lots and for individual GDs, the angular dependence, and energy dependence of 4 therapeutic x-ray qualities. In addition, we measured the dosimetric features (dose linearity, uniformity, angular dependence, and energy dependence) of the GD for electron beams of 10 different electron energy qualities. All measurements with the exception of dose linearity for photon beam were performed in a water phantom. For high-energy photon beams, dose linearity has a linear relationship for a dose ranging from 1 to 500 Gy with the coefficient of determination; R² of 0.998. The uniformity of each GD of dose measurements was within ±0.5% for four GD lots and within ±1.2% for 80 GDs. In terms of the effects of photon beam angle, lower absorbed doses of within 1.0% were observed between 60° and 105° than at 90°. The GD energy dependence of 4 photon beam energy qualities was within ±2.0%. On the other hand, the result of the dose linearity for high-energy electron beams showed well fitted regression line with the coefficient of determination; R² of 0.999 between 6 and 20 MeV. The uniformity of GDs exposed to the nominal electron energies 6, 9, 12, 16, and 20 MeV was ±1.2%. In terms of the angular dependence to electron beams, absorbed doses were within 2.0% between 60° and 105° than at 90°. In evaluation of the energy dependence of the GD at nominal electron energies between 5 and 20 MeV, we obtained responses between 1.1% and 3.5% lower than that for a cobalt-60 beam. Our results show that GDs can be used as a detector for determining doses when a high-energy photon beam is used, and that it also has considerable potential for dose measurement of high-energy electron beam.     

A preliminary study of the performance of a novel design of multi-element tissue equivalent proportional counter for neutron monitoring

In this study we investigate the dependence of the sensitivity of a TEPC upon its surface area and demonstrate that a compact multi-element tissue equivalent proportional counter (METEPC) has a counting sensitivity comparable to a commercially available 12.7 cm (5 inch) diameter spherical TEPC. The METEPC incorporates 61 cylindrical counting volumes of internal diameter of 0.5 cm and height 5 cm, machined in a single block of tissue equivalent plastic. It is the simplest design available in the multi-element geometrical configuration and is approximately nine times smaller in volume than that of a conventional 12.7 cm spherical TEPC. The neutron sensitivity of commercially available TEPCs and the METEPC simulating a 2 μm tissue site size was examined experimentally using the McMaster University 1.25 MV double stage Tandetron accelerator, which produces low energy neutrons via the ⁷Li(p, n)⁷Be reaction. The mean energy of the neutron beams produced ranged from 34 keV to 354 keV. The results presented in this study suggests that the compact METEPC is able to produce measurements in low dose rate radiation environments with the same precision in a given length of time as could be obtained with a 12.7 cm diameter spherical TEPC.     

Electron–hole transition energy for a spherical quantum dot confined in a nano-cylindrical wire

A single-band constant confining potential is applied to InAs spherical quantum dot confined in a GaAs cylindrical nano-wire to determine the electronic structure. The energy eigenvalues and transition energies are numerically calculated as a function of the dot radius. The calculations were performed within the effective mass approximation, using the finite element method. The effect of both spherical and cylindrical confinement, the size dependence of the ground and first excited state energies for electron and heavy hole and transition energies are reported and compared with experimental and theoretical results in relevant conditions.     

Simulation and experimental study of an indigenously designed and constructed THGEM-based microdosimeter for dose-equivalent measurement

Most of the GEM/THGEM-based microdosimetric detectors presented in the literature simulate 2 μm of tissue which results in a flat neutron dose-equivalent response in the MeV region. The objective of this work was to introduce a neutron microdosimeter with a more extended flat response. In this regard, a THGEM-based microdosimeter with plexiglas walls, simulating 1 μm of tissue was designed and constructed. Its performance was investigated by both simulation and experimentation to determine the microdosimetric quantity of “lineal energy”.In the simulation study, lineal energy distribution, mean quality factor and dose-equivalent response of the microdosimeter for eleven neutron energies from 10 keV to 14 MeV, along with the energy spectrum of ²⁴¹Am-Be neutrons, were calculated by the Geant4 simulation toolkit. Obtained lineal energy distributions were compatible with the distributions determined by a Rossi counter. Also, the mean quality factors agreed well with the values reported by the ICRU report 40 which confirmed tissue equivalent behavior of the microdosimeter. They were different from the effective quality factor values within 15% between 20 keV and 14 MeV. This led to a flat dose-equivalent response with 20% difference from a median value of 0.82 in the above energy range which was an improvement compared with other THGEM-based detectors, simulating 2 μm of tissue. In spite of the satisfactory determination of the dose-equivalent, the microdosimeter had low detection sensitivity.In the experimental study, the measured lineal energy distribution of ²⁴¹Am-Be neutrons was in agreement with the simulated distribution. Further, the measured mean quality factor and dose-equivalent differed by 1.5% and 3.5%, respectively, from the calculated values. Finally, it could be concluded that the investigated microdosimeter reliably determined the desired dose-equivalent value of each neutron field with every energy spectrum lying between 20 keV and 14 MeV.     

OSL and TL retrospective dosimetry with a fluorapatite glass-ceramic used for dental restorations

Optically Stimulated Luminescence (OSL) and Thermoluminescence (TL) properties of a fluorapatite glass-ceramic have been investigated, with a view to developing a dose assessment technique for medical triage following unplanned exposures of individuals to ionizing radiation. The ceramic is an innovative material used in dental prostheses and restorations. It is strongly sensitive to radiation and the intensity of both the OSL and TL signals are proportional to the absorbed radiation dose. We focused on the optimization of the measuring procedure and investigated characteristics such as reproducibility, fading, minimum detectable dose (MDD), dose response and photon energy response of TL and OSL signals. The dental ceramic exhibited very good reproducibility (<5% at 2σ level) when measured and a linear dose response for a wide range of doses (50 mGy–20 Gy). The MDD values for the samples investigated were ∼5 mGy. The material is not tissue equivalent and the OSL and TL signals are strongly dependent on incident photon energy. Both the luminescence signals exhibited significant fading during the first few hours after irradiation. Its rate was dependent on the parameters of measurement. The results indicate that the material can be used for the purposes of accident dosimetry, however, the fading and photon energy response have to be properly corrected for a reliable dose assessment.     

Radial distribution of dose within heavy charged particle tracks – Models and experimental verification using LiF:Mg,Cu,P TL detectors

A new method of experimental verification of radial dose distribution models using solid state thermoluminescent (TL) detectors LiF:Mg,Cu,P has been recently proposed. In this work the method was applied to verify the spatial distribution of energy deposition within a single ¹³¹Xe ion track. Detectors were irradiated at the Department of Physics of the University of Jyväskylä, Finland. The obtained results have been compared with theoretical data, calculated according to the Zhang et al., Cucinotta et al. and Geiss et al. radial dose distribution (RDD) models. At the lowest dose range the Zhang et al. RDD model exhibited the best agreement as compared to experimental data. In the intermediate dose range, up to 10⁴ Gy, the best agreement was found for the RDD model of Cucinotta et al. The probability of occurrence of doses higher than 10⁴ Gy within a single ¹³¹Xe ion track was found to be lower than predicted by all the studied RDD models. This may be a result of diffusion of the charge, which is then captured by TL-related trapping sites, at the distances up to dozens of nanometers from the ionization site.     

Analysis of radiation dose variations measured by passive dosimeters onboard the International Space Station during the solar quiet period (2007–2008)

The average absorbed dose and dose equivalent rates from space radiation were observed using passive dosimeters with same material and configuration at the same location onboard the International Space Station (ISS) over four different occasions (I–IV) between 2007 and 2008. The passive dosimeters consisted of a combination of thermoluminescent detectors (TLDs) and plastic nuclear track detectors (PNTDs). Total average absorbed dose rate increased by 68 ± 9% over two years. The observed increase was due to the incremental increase in the altitude of the ISS over the course of the experiment and the corresponding increase in trapped proton flux encountered during passage of the ISS through the SAA (South Atlantic Anomaly), which was confirmed with the results monitored by DB-8 active dosimeter on the ISS. The PNTD data showed that the average absorbed dose and dose equivalent rates from particles of LET_∞H₂O ≥ 100 keV/μm were 28 ± 2% and 51 ± 3% of ≥10 keV/μm during Periods I–III, while the dose contributions of particles ≥100 keV/μm during Period IV were 36 ± 5% and 59 ± 10%, respectively. The integral dose equivalent distribution during Period IV shows significant enhancement from particles ≥100 keV/μm. These facts suggest that a significant fraction of the high LET component is due to short-range recoil nuclei produced in target fragmentation reactions between primary protons and the nuclei of the passive dosimeters and surrounding materials.     

Dose uncertainties for large solar particle events: input spectra variability and human geometry approximations.

The true uncertainties in estimates of body organ absorbed dose and dose equivalent, from exposures of interplanetary astronauts to large solar particle events (SPEs), are essentially unknown. Variations in models used to parameterize SPE proton spectra for input into space radiation transport and shielding computer codes can result in uncertainty about the reliability of dose predictions for these events. Also, different radiation transport codes and their input databases can yield significant differences in dose predictions, even for the same input spectra. Different results may also be obtained for the same input spectra and transport codes if different spacecraft and body self-shielding distributions are assumed. Heretofore there have been no systematic investigations of the variations in dose and dose equivalent resulting from these assumptions and models. In this work we present a study of the variability in predictions of organ dose and dose equivalent arising from the use of different parameters to represent the same incident SPE proton data and from the use of equivalent sphere approximations to represent human body geometry. The study uses the BRYNTRN space radiation transport code to calculate dose and dose equivalent for the skin, ocular lens and bone marrow using the October 1989 SPE as a model event. Comparisons of organ dose and dose equivalent, obtained with a realistic human geometry model and with the oft-used equivalent sphere approximation, are also made. It is demonstrated that variations of 30-40% in organ dose and dose equivalent are obtained for slight variations in spectral fitting parameters obtained when various data points are included or excluded from the fitting procedure. It is further demonstrated that extrapolating spectra from low energy (< or = 30 MeV) proton fluence measurements, rather than using fluence data extending out to 100 MeV results in dose and dose equivalent predictions that are underestimated by factors as large as 2-3. Finally, it is also demonstrated that the use of equivalent sphere approximations to represent body organ self-shielding distributions results in organ doses and dose equivalent predictions that are 2-3 times larger than values obtained with anthropomorphic shielding configurations.     

Specific absorbed fractions of electrons and photons for Rad-HUMAN phantom using Monte Carlo method

The specific absorbed fractions(SAF) for self- and cross-irradiation are effective tools for the internal dose estimation of inhalation and ingestion intakes of radionuclides. A set of SAFs of photons and electrons were calculated using the Rad-HUMAN phantom, which is a computational voxel phantom of a Chinese adult female that was created using the color photographic image of the Chinese Visible Human(CVH) data set by the FDS Team. The model can represent most Chinese adult female anatomical characteristics and can be taken as an individual phantom to investigate the difference of internal dose with Caucasians. In this study, the emission of mono-energetic photons and electrons of 10 ke V to 4 Me V energy were calculated using the Monte Carlo particle transport calculation code MCNP. Results were compared with the values from ICRP reference and ORNL models. The results showed that SAF from the Rad-HUMAN have similar trends but are larger than those from the other two models. The differences were due to the racial and anatomical differences in organ mass and inter-organ distance. The SAFs based on the Rad-HUMAN phantom provide an accurate and reliable data for internal radiation dose calculations for Chinese females.     

Measurement of target fragments produced by 160 MeV proton beam in aluminum and polyethylene with CR-39 plastic nuclear track detectors

Production of target fragments from reactions of 160 MeV proton beams in aluminum and polyethylene was measured with CR-39 plastic nuclear track detectors (PNTD). Due to the detection limit of PNTD, primary protons cannot be detected; only low-energy short-range target fragments are registered. As a feasibility study, a so called “two step etching method” was employed to get the linear energy transfer (LET) spectra, absorbed dose, and dose equivalent. This method is discussed in this paper, together with the measured results.     

Hartree-Fock and shell model calculations for giant nuclei

The properties of giant nuclear systems like Uranium/Uranium etc. (A?476) have been studied in statical, spherical Hartree-Fock approximation. Various effective forces of Skyrme type were used to calculate binding energies, density distributions and single particle spectra. A shell model of Woods-Saxon type is shown to be more or less equivalent with respect to the shell structure derived from Skyrme models and easy generizable to slightly deformed giant nuclei. Strong magic shells could be found even in this region of the periodic table, but it seems that they cannot contribute significantly to a stabilization of these systems. The total binding energy is in good agreement with liquid drop model extrapolations. Only very briefly we discuss possibilities and problems for calculating deformation energies.     

Evaluation of target photon dose mixed in mono-energetic neutron fields using 7Li(p,n)7Be reaction

Target photons mixed in the 144, 250 and 565 keV mono-energetic neutron calibration fields were measured using a cylindrical NaI(Tl) detector with 7.62 cm both in diameter and in length. The ambient dose equivalent H*(10) of the photons was evaluated by applying the “G(E) function” to the measured pulse height spectrum. Neutrons induce photons by nuclear reactions in the NaI(Tl) detector and affect the pulse height spectrum. In order to eliminate the influence of these neutron events, the time-of-flight technique was applied with operating the accelerator in the pulse mode. The ratios by the ambient dose equivalent H*(10) of the photons to the 144, 250 and 565 keV neutrons were evaluated to be 3.3%, 4.7% and 0.9%, respectively. Although high energy photons ranging from 6 to 7 MeV are emitted by the ¹⁹F(p,αγ)¹⁶O reactions, the dose of the target photons is low enough to calibrate neutron dosemeters except for ones with high sensitivity to the photons.     

Characterisation of a real-time fibre-coupled beryllium oxide (BeO) luminescence dosimeter in X-ray beams

For the first time the feasibility of using beryllium oxide (BeO) ceramics as a fibre-coupled radioluminescent dosimeter is investigated. BeO ceramic exhibits both radioluminescence (RL) and optically stimulated luminescence (OSL), and has the potential to be a near tissue equivalent alternative to Al₂O₃:C. A BeO fibre-coupled radioluminescence dosimeter is demonstrated and characterised for 6 MV X-rays and superficial X-ray energies, 150 kVp and 120 kVp. Based on the results, we demonstrate the capability of the RL BeO FOD for accurate and reproducible dose measurements with a linear dose rate and dose response. It has also been found that the percentage depth dose curves for 6 MV agreed with ion chamber measurements to within 2%, except in the build up region. For the 150 kVp and 120 kVp photon beams, the depth dose measurements agreed with ion chamber measurements to within 2.5% and 4%, respectively.     

Evaluation of neutron production in new accelerators for radiotherapy

Ambient dose equivalent, H*(10), and personal dose equivalent, Hp(10), were calculated in different points located inside two different treatment rooms. 15-MV Varian and 15-MV Elekta accelerators were used in these studies. The geometry of both accelerators heads and treatment rooms were built up to perform the Monte Carlo simulations. The patient was also simulated using an ICRU phantom. Calculations were done using the MCNPX code. Ambient dose equivalents rates from neutrons range between 1.2 and 419 mSv/h in the Elekta treatment room and between 0.96 and 1140 mSv/h in the Varian treatment room, depending on the location. These values suggest a larger neutron production in the Varian than in the Elekta accelerator.     

Publicly available database of measurements with the silicon spectrometer Liulin onboard aircraft

Aircrew members are exposed to ionizing radiation due to their work onboard aircraft. ICRP recommended the monitoring of their effective doses because they regularly exceed the limit of 1 mSv per year for the public exposure. The effective doses are routinely calculated by computer codes that take into account flight parameters like altitude, geographic position, and solar activity. This approach was preferred against personal dosimeters method because the effective dose cannot be evaluated experimentally. However, it is generally accepted, that these calculations should be periodically verified by measurements of H*(10) which is frequently used as a surrogate for effective dose. This report refers about the database (available online http://hroch.ujf.cas.cz/∼aircraft/) of long-term measurements with the silicon spectrometer Liulin onboard aircraft. The measurements have been performed since March 2001; so up to date, the database covers a period of 11-years (with a few interruptions) which is usually the duration of the whole solar cycle. The database comprises more than 10⁵ individual records of energy deposition spectra, absorbed dose rates, and ambient dose equivalent rates. Each record contains also the information on all flight parameters needed for calculation of dosimetric quantities by the computer codes, and thus the database represent an useful tool for verification of the routine dosimetry of aircraft crews.     

Secondary doses delivered to an anthropomorphic male phantom under prostate irradiation with proton and carbon ion beams

Secondary radiation exposure of patients undergoing radiation therapy with light ions is of great concern due to possible tissue damage and risk of induction of secondary cancers.Secondary particles such as neutrons, protons and heavier ions are produced when the primary ions interact through nuclear inelastic reactions with the beam-line components, and with the tissues of the patient.Evaluations of secondary doses delivered to an anthropomorphic male phantom under prostate irradiation with ¹H and ¹²C ion beams with energies 172 MeV and 330 MeV/u, respectively, have been performed with the Monte Carlo code SHIELD-HIT.Fluences of secondary particles with atomic mass A = 1–7 and energies up to 200–600 MeV/u are observed in organs even at larger distances (40–50 cm) from the irradiated volume. The secondary absorbed doses in selected organs are discussed taking into account the dose contribution from secondary neutrons, and the contribution from charged fragments that are not the products of neutron interactions. For ¹²C ion irradiation, a substantial contribution to the absorbed organ dose is due to charged fragments. This contribution decreases from 81% in the organs close to the irradiated volume to 35–40% in the organs at larger distances.