Monte Carlo Investigation Research Articles

Monte Carlo investigation of the effect of skin tissue optical properties on detected Cherenkov emission.

In this study, we use Monte Carlo modelling to investigate the effect of tissue optical properties on Cherenkov emission detected from tissue surface. MC simulations are performed for wavelength between 400-1000nm and the values of absorption coefficient at each wavelength are determined based on the molar extinction coefficients of oxy- and deoxy-hemoglobin, with varying total haemoglobin concentration and tissue oxygen saturation of 70%. Tissue reduced scattering coefficient is approximated using μs'(λ) = Aλ-0.838. A range of clinically relevant tissue optical properties was investigated, with absorption coefficient between 0.1 and 1 cm-1 and reduced scattering coefficient between 5 and 40 cm-1 at 665nm. The angular distribution, depth of origins and the effect of tissue optical properties on Cherenkov emission on tissue surface are evaluated.

A Monte Carlo investigation of the dose distribution for new I-125 Low Dose Rate brachytherapy source in water and in different media

Abstract Permanent and temporary implantation of I-125 brachytherapy sources has become an official method for the treatment of different cancers. In this technique, it is essential to determine dose distribution around the brachytherapy source to choose the optimal treatment plan. In this study, the dosimetric parameters for a new interstitial brachytherapy source I-125 (IrSeed-125) were calculated with GATE/GEANT4 Monte Carlo code. Dose rate constant, radial dose function and 2D anisotropy function were calculated inside a water phantom (based on the recommendations of TG-43U1 protocol), and inside several tissue phantoms around the IrSeed-125 capsule. Acquired results were compared with MCNP simulation and experimental data. The dose rate constant of IrSeed-125 in the water phantom was about 1.038 cGy·h−1U−1 that shows good consistency with the experimental data. The radial dose function at 0.5, 0.9, 1.8, 3 and 7 cm radial distances were obtained as 1.095, 1.019, 0.826, 0.605, and 0.188, respectively. The results of the IrSeed-125 is not only in good agreement with those calculated by other simulation with MCNP code but also are closer to the experimental results. Discrepancies in the estimation of dose around IrSeed-125 capsule in the muscle and fat tissue phantoms are greater than the breast and lung phantoms in comparison with the water phantom. Results show that GATE/GEANT4 Monte Carlo code produces accurate results for dosimetric parameters of the IrSeed-125 LDR brachytherapy source with choosing the appropriate physics list. There are some differences in the dose calculation in the tissue phantoms in comparison with water phantom, especially in long distances from the source center, which may cause errors in the estimation of dose around brachytherapy sources that are not taken account by the TG43-U1 formalism.

Magnetization reversal of hard/soft magnetic bilayer

We present a model of hard/soft magnetic bilayer for the Monte Carlo investigation under the external magnetic field. The influence of the exchange interaction constants of soft magnetic layer Jsoft on the magnetic bilayer magnetization reversal processes is explored for the first time. Behavior of the magnetic bilayer under the external magnetic field is shown to agree with theoretical predictions well enough.

Monte Carlo investigation of the effect of gold nanoparticles’ distribution on cellular dose enhancement

PurposeIn recent decades‎, ‎radiotherapy has became one of the most useful and cost effective tools in cancer treatment‎. Having a number of unique characteristics made gold nanoparticles (GNPs) as promising candidates in achieving dose enhancement in tumor and improving the radiation treatment outcome‎. ‎Considering the widely documented tendency of GNPs to localize around the nucleus‎, ‎the distribution model of GNPs in the medium needs to be investigated in detail and to be well quantified‎. MethodsThe MCNPX Monte Carlo code was used to simulate particle transport in cells‎. ‎A single cell‎, ‎including an ellipsoidal cytoplasm with a centrally located sphere surrounded by a cube as the medium was modeled‎. ‎The physical dose due to the irradiation of 10–100 keV monoenergetic photons was calculated in the nucleus‎. To investigate the effect of the approach used for distribution of GNPs in the medium on dose enhancement factor‎ (‎DEF)‎, three models have been proposed‎: ‎filling the medium and the cytoplasm with either a gold-water mixture or a homogeneously distributed GNPs‎, ‎and the heterogeneous model by simulating spheres of GNPs localized in the cytoplasm‎. ResultsThe results show an overestimation of dose enhancement caused by gold-water mixture rather than that of homogeneous distribution‎. The DEFs for the heterogeneous model‎, ‎while follow the same behavior of two other models against various primary energies‎, ‎quantitatively demonstrate a significant distinction‎. ‎This model was also examined by studying the effect of concentration (the ratio of the mass of GNPs to the mass of the medium)‎, ‎size and density of‎ GNPs on nucleus dose enhancement‎. ‎It was found that though the values calculated for DEF strongly depend on concentration of GNPs‎, ‎for a given concentration‎, ‎they would not considerably be affected by changing the GNPs' size‎. As a criterion to assess their density around the nucleus‎, ‎the distance between two typical GNPs was found to be a key determinant factor to predict and to control dose enhancement in the target volume‎. For the primary photons of 30 keV‎, ‎our results showed a negligible difference between the DEFs calculated due to the cell orientation in the medium‎. ‎By extending the heterogeneous model for ten different locations of nucleus in the cell‎, ‎the energy dependence behavior of DEF was observed‎. ConclusionsThe results obtained emphasize the importance of accurate modeling of the distribution of GNPs in cell to demonstrate the more precise potential of gold in improving radiotherapy treatments‎.

Hysteresis Cycle and Magnetization Behaviors of a Mixed-Spin (7/2, 3/2) Ferrimagnetic Ising Model: Monte Carlo Investigation

The effects of single-ion anisotropies and an external magnetic field on the magnetization of the mixed-spin (7/2, 3/2) ferrimagnetic Ising system are investigated within Monte Carlo simulation. Under certain values of the physical parameters, multiple hysteresis loop behaviors such as double, triple, and quintuple hysteresis cycles have been observed. Particularly, the superparamagnetic phase has been shown. The ground-state phase diagrams are presented.

Monte Carlo investigation on precise dosimetry of HDR breast brachytherapy with Accuboost

Introduction: Accuboost is a HDR brachytherapy system in early stages breast cancer treatment. This device provides a completely noninvasive procedure with parallel-opposed radiation from two immobilizing peripheral applicators that caused it a preferred option of modalities to choose. In most commercial treatment planning systems, tissues are considered as a simple water phantom. Inaccuracies arised from geometry and compositions can lead to deviations in interested dose distribution. This study intends to simulate HDR breast brachytherapy with Accuboost device in order to consider requirement corrections to have an accurate treatment dosimetry. Materials and Methods: MCNP5 simulation is employed to model phantoms in dosimetric measurements. We tried to take geometrical corrections into account in three phantoms. Dose distributions as dose volume histogram parameters and isodose plots obtained from these phantoms are compared to those of water. Corrections are included using real breast tissue with ICRU-44 breast composition, chest defined with an air cavity as lung and a curved surface to introduce a more realistic contour for breast skin. Accuboost is simulated with radiations from eighteen HDR 192-Ir sources on each applicator with appropriate tungsten shields. Results: Differences in most regions with less than 1% to water and maximum value about 5 % have been observed in phantom with ICRU-44 breast material compared to the water. Lung presence have increased deviations a few percent but it has not significantly differed. In phantom with all corrections maximum differences are almost the same as both earlier and deviations with about 5-7% are found near the chest and around the breast skin not in contact to applicators. Conclusion: Based on the results, although discrepancies between corrected phantom and water phantom are in areas near the skin the main differences are in clinical accepted uncertainties and other points of tissue have shown accommodatian. Therefore essential correction may be not needed.

Monte Carlo investigation of noise and high-order harmonic extraction in graphene

High-order harmonic generation is an interesting option to reach the THz range for generation and detection applications. In this work the time-domain and spectral behavior of carrier velocity fluctuations in free-standing graphene under the action of a high frequency alternating electric field are studied by means of an ensemble Monte Carlo simulator. The distortion of the velocity response is explained in terms of the carrier wavevector distribution, average energies and scattering activity. The intensity of the harmonics is analyzed for several frequencies and electric field amplitudes. Noise and regular contributions to the power spectral density of the velocity response are discerned, and the harmonic modes available for several bandwidths are discussed. Results show that it is possible to reach the THz range with conditions comparable to those found for III–V materials, at the expense of narrow bandwidths.

Monte Carlo investigation on the effect of air gap under bolus in post-mastectomy radiotherapy

ObjectivesTo investigate the dosimetric effect of air gaps under bolus on skin dose for left-sided post-mastectomy radiotherapy with loco regional involvement. MethodsEight patients were planned retrospectively with volume modulated arc therapy (VMAT) and conventional static Field-in-Field (FinF) methods. Three different setups were applied for the 5-mm bolus over the chest wall having 0, 5 or 10 mm air gap under the bolus. The dose calculation was performed using Monte Carlo (MC) simulation. In addition, Analytic Anisotropic Algorithm (AAA) was used to demonstrate the differences observed in clinical setting. ResultsThe investigated air gaps under the bolus had minimal effect on surface dose for FinF plans (relative difference ≤ 2.6%), whereas for VMAT plans the surface dose decreased 13.6% when compared to the case with no air gap. In both FinF and VMAT, the largest differences between AAA and MC were seen at the surface where AAA underestimated the dose by 1.5 Gy (p < 0.05) on average; while the dose in the target volume excluding the surface was relatively similar being on average 0.3 Gy (p > 0.05) larger with AAA than with MC calculations. ConclusionsThe surface dose was significantly lower with VMAT technique than with FinF technique. Possible air gaps under the bolus reduced the surface dose significantly further for VMAT but not for FinF treatments, which may have clinical impact on recurrence rate. AAA was shown to underestimate the surface dose when compared to MC calculation.

Secondary Neutron Dose From a Dynamic Collimation System During Intracranial Pencil Beam Scanning Proton Therapy: A Monte Carlo Investigation

Patients receiving pencil beam scanning (PBS) proton therapy with the addition of a dynamic collimation system (DCS) are potentially subject to an additional neutron dose from interactions between the incident proton beam and the trimmer blades. This study investigates the secondary neutron dose rates for both single-field uniform dose (SFUD) and intensity modulated proton therapy treatments. Secondary neutron dose distributions were calculated for both a dynamically collimated and an uncollimated, dual-field chordoma treatment plan and compared with previously published neutron dose rates from other contemporary scanning treatment modalities. Monte Carlo N-Particle transport code was used to track all primary and secondary particles generated from nuclear reactions within the DCS during treatment through a model of the patient geometry acquired from the computed tomography planning data set. Secondary neutron ambient dose equivalent distributions were calculated throughout the patient using a meshgrid with a tally resolution equivalent to that of the treatment planning computed tomography. The median healthy-brain neutron ambient dose equivalent for a dynamically collimated intracranial chordoma treatment plan using a DCS was found to be 0.97mSv/Gy for the right lateral SFUD field, 1.37mSv/Gy for the apex SFUD field, and 1.24mSv/Gy for the composite intensity modulated proton therapy distribution from 2 fields. These results were at least 55% lower than what has been reported for uniform scanning modalities with brass apertures. However, they still reflect an increase in the excess relative risk of secondary cancer incidence compared with an uncollimated PBS treatment using only a graphite range shifter. Regardless, the secondary neutron dose expected from the DCS for these PBS proton therapy treatments appears to be on the order of, or below, what is expected for alternative collimated proton therapy techniques.

OA212] A Monte Carlo investigation of dose width product (DWP) as a dose index for CBCT dosimetry

Purpose Dose-length-product (DLP) plays a vital role in assessing the total energy delivered to patients from CT scans. Moreover, DLP along with conversion coefficients are employed in evaluating effective doses from CT scans. The aim of this study is to propose an alternative dose index for cone-beam CT (CBCT) dosimetry analogue to DLP, which shows limitations for scans acquired with wide beams. Methods Monte Carlo simulations based on on-board-imager (OBI) kV system were run to investigate the alternative dose index called dose-width-product (DWP), which has a form similar to that of DLP, where width of the scan beam (W) along the z-axis is multiplied by CTDIw. This was proposed for CBCT scans and requires measurement of CTDI100 for a reference beam (≤40 mm) in the standard CTDI PMMA phantoms and measuring CTDI of the reference beam and a wide beam of interest free-in-air. In order to study suitability of DWP, its efficiency was estimated as a ratio of DWP to DWP∞, and compared to DLP/DLP∞. Head and body phantoms with diameters of 16 and 32 cm, respectively, and a length of 900 mm were simulated to assess DWP∞ and DLP∞, whereas the standard length (150 mm) was used to assess DWP and DLP. The efficiency was estimated using tube potentials of (80, 100, 120, 140) kV and beams of width ranged from 20 – 300 mm. Results The tube potential and width of the scan beam played a minimal role in the efficiency. Values of DLP efficiency were 80.1 ± 0.5% for head and 72.4 ± 2.4% for body under the different conditions studied. DWP efficiency values for head and body phantoms were all within 1.9% of those for DLP, which gives an indication for suitability of using DWP for CBCT scans. Conclusions DWP efficiency values were comparable to those for DLP. The dose index DWP is a modified form or an extension to the DLP concept. It indicates possibility of estimation of the total energy resulted from a given CBCT scan with a level of accuracy matching that of DLP. DWP can also be utilized to derive conversion coefficients used to assess effective dose.

Monte Carlo investigation of mixed spin Ising 2D-nanoparticles

The magnetic properties of mixed spin-1/2 and spin-1 Ising 2D-nanoparticles (circle and square) with core–shell structure are examined by the use of the Monte Carlo simulation. The phase diagrams are investigated and show qualitatively interesting features. In particular, three and four physically different phases are identified for the square and circle nanoparticles, respectively. These phases are separated by different transitions that could be called ordinary, core and shell transitions according to the strengths of the exchange interactions.

Direct Simulation Monte Carlo Shock Simulation of Saturn Entry Probe Conditions

This work describes the initial efforts of a direct simulation Monte Carlo investigation of bow shock physics for Saturn entry probe scenarios and the influence of nonequilibrium phenomena on Saturn entry conditions. Therefore, the purpose of this paper is to employ models that are sufficient for a typical hypersonic entry direct simulation Monte Carlo simulation and identify the areas where improvement is required. The Computation of Hypersonic Ionizing Particles in Shocks (CHIPS) direct simulation Monte Carlo code is used to simulate two rarefied hypersonic shock tube experiments on a hydrogen–helium mixture performed in the Electric Arc Shock Tube (EAST) at NASA Ames Research Center. The CHIPS simulations are postprocessed through the NEQAIR line-by-line radiation code to compare directly to the experimental results. Improved collision cross-sections, inelastic collision parameters, and reaction rates are determined for a high-temperature direct simulation Monte Carlo simulation of a seven-species hydrogen–helium mixture, and an electronic excitation model is implemented in the CHIPS code. Simulation results for 27.8 and shock waves are obtained at 0.2 and 0.1 Torr, respectively, and compared to measured spectra in the vacuum ultraviolet, ultraviolet, visible, and infrared ranges. These results confirm the persistence of nonequilibrium for several centimeters behind the shock and the diffusion of atomic hydrogen upstream of the shock wave. Although the magnitude of the radiance does not match experiments and an ionization inductance period is not observed in the simulations, the discrepancies indicate where improvements are needed in the CHIPS and NEQAIR models.

Liquid xenon in nuclear medicine: state-of-the-art and the PETALO approach

Liquid xenon has several attractive features, which make it suitable for applications to nuclear medicine, such as high scintillation yield and fast scintillation decay time, better than currently used crystals. Since the '90s, several attempts have been made to build Positron Emission Tomography scanners based on liquid xenon, which can be divided into two different approaches: on one hand, the detection of the ionization charge in TPCs, and, on the other one, the detection of scintillation light with photomultipliers. PETALO (Positron Emission Tof Apparatus with Liquid xenOn) is a novel concept, which combines liquid xenon scintillating cells and silicon photomultipliers for the readout. A first Monte Carlo investigation has pointed out that this technology would provide an excellent intrinsic time resolution, which makes it possible to measure the Time-Of-Flight with high efficiency. Also, the transparency of liquid xenon to UV and blue wavelengths opens the possibility of exploiting both scintillation and Cherenkov light for a high-sensitivity TOF-PET.

A study of R2 measure under the accelerated failure time models

ABSTRACTFor right-censored data, the accelerated failure time (AFT) model is an alternative to the commonly used proportional hazards regression model. It is a linear model for the (log-transformed) outcome of interest, and is particularly useful for censored outcomes that are not time-to-event, such as laboratory measurements. We provide a general and easily computable definition of the R2 measure of explained variation under the AFT model for right-censored data. We study its behavior under different censoring scenarios and under different error distributions; in particular, we also study its robustness when the parametric error distribution is misspecified. Based on Monte Carlo investigation results, we recommend the log-normal distribution as a robust error distribution to be used in practice for the parametric AFT model, when the R2 measure is of interest. We apply our methodology to an alcohol consumption during pregnancy data set from Ukraine.

Abstract ID: 230 Experimental and Monte Carlo investigation of the depth-dependent fluence perturbation of parallel-plate chambers in clinical electron beams

The electron fluence inside a parallel-plate ionization chamber positioned in a water phantom and exposed to a clinical electron beam deviates from the unperturbed fluence in water in absence of the chamber. One reason for the fluence perturbation is the "in-scattering effect" as described in ICRU 35. Aim of this work is a detailed experimental and with Monte Carlo simulations investigation of this effect. The experiments and Monte Carlo Simulations, with EGSnrc [1], were performed in a solid-water phantom (RW3) sized 30 × 30 × 30 cm3. One of the RW3 plates of height h = 0.5 cm had a hole with radius r = 1 cm which acts as the air cavity in the phantom. The phantom was placed below an electron applicator of field size 10 × 10 cm2 and irradiated with 6 MeV electrons from an Elekta Synergy accelerator. The air cavity was positioned at depths of 1.0, 1.5, 2.0 and 2.5 cm in the phantom. For the exerpiments the spatial resolved dose directly below the cavity was scored with GAFCHROMIC ETB3 films. The films were scanned with an Epson 10000xl-scanner (resolution 600 dpi, color depth 48 bit); a dose response calibration curve for the red channel was applied. At 1.0 cm depth the relative dose profile shows the well known dose increase at the air/RW3 boundary of about 4% relative to the dose at the center of the cavity and a de-crease of about 8% outside the air cavity within the RW3-phantom. The amplitude of this "dose oscillation" decreases with increasing depth of the cavity within the phantom and disappears at about 2 cm, which is less than the half value thickness of the electron beam. The results are in agreement with already published Monte Carlo results [2]. The results are partly in contradiction to the ideas summarized in ICRU 35 but confirm recent Monte Carlo simulations from Zink et al.

Monte Carlo Investigation of Photon Beam Characteristics and its Variation with Incident Electron Beam Parameters for Indigenous Medical Linear Accelerator.

Purpose:A Monte Carlo model of a 6 MV medical linear accelerator (linac) unit built indigenously was developed using the BEAMnrc user code of the EGSnrc code system. The model was benchmarked against the measurements. Monte Carlo simulations were carried out for different incident electron beam parameters in the study.Materials and Methods:Simulation of indigenously developed linac unit has been carried out using the Monte Carlo based BEAMnrc user-code of the EGSnrc code system. Using the model, percentage depth dose (PDD), and lateral dose profiles were studied using the DOSXYZnrc user code. To identify appropriate electron parameters, three different distributions of electron beam intensity were investigated. For each case, the kinetic energy of the incident electron was varied from 6 to 6.5 MeV (0.1 MeV increment). The calculated dose data were compared against the measurements using the PTW, Germany make RFA dosimetric system (water tank MP3-M and 0.125 cm3 ion chamber).Results:The best fit of incident electron beam parameter was found for the combination of beam energy of 6.2 MeV and circular Gaussian distributed source in X and Y with FWHM of 1.0 mm. PDD and beam profiles (along both X and Y directions) were calculated for the field sizes from 5 cm × 5 cm to 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 2%.Conclusions:A Monte Carlo model of indigenous linac (6 MV) has been developed and benchmarked against the measured data.

A Combined Density Functional Theory and Monte Carlo Investigation of the Competitive Adsorption of Atomic Oxygen and Chlorine to the Ni (111) Surface

Designing the next generation of corrosion resistant alloys will rely, in part, on developing a better understanding of how to favor the formation of protective surface oxide films in chemically aggressive environments. In this work, we examined the early stages of oxygen adsorption on Ni (111) in the presence of chloride, a known corrosive agent. The analysis was accomplished from first principles via the construction of a database of density functional theory (DFT) calculations which was then used to fit a cluster expansion model Hamiltonian. DFT calculations showed that the face centered cubic (fcc) sites were most stable for adsorption of both chloride and oxygen in agreement with the literature. Hence, only adsorption to the fcc sites was considered in the cluster expansion and the DFT database. The potential function was constructed with the aid of a genetic algorithm (GA) which helped with the selection of appropriate clusters from a candidate pool of figures. The potential function that was developed contained a total of 50 coupled (i.e. Cl-O) and non-coupled (i.e. Cl-Cl and O-O) clusters and was reasonably good at predicting DFT results with a leave-one-out cross validation score of 28.6 meV/site. The potential function was then used to perform a systematic and efficient calculation of the adsorption isotherms of oxygen and chloride at 300 K. It was observed that relatively large chloride chemical potentials were required to displace adsorbed oxygen. Furthermore, the chloride adsorption isotherms indicated a rapid shift between 0.6 ML and 1 ML. This transition was explained by the need for a large chloride chemical potential to overcome the repulsive forces between the chloride adatoms.

Monte Carlo investigation of the MBE growth of GaAs on the surfaces with different crystallographic orientations

MBE growth of GaAs on the surfaces with different orientations is studied by kinetic Monte Carlo simulations. The model allows taking into account the most relevant processes and reconstructions on the surfaces. The simulations show that islands grown on the (111)A surface have a chaotic shape whereas those on the (001) and (110) surfaces have more ordered structure due to the features of the surface structure. The diffusion length of Ga adatoms is observed to be dependent on the growth rate and V/III flux ratio. This influence is stronger for the (111) and (110) surfaces demonstrating that diffusion is weaker on the (001) surface. However, the diffusion length reduces with increasing growth rate and As2/Ga flux ratio tending to become equal for all surfaces under consideration.

Caution on emergent continuous symmetry: A Monte Carlo investigation of the transverse-field frustrated Ising model on the triangular and honeycomb lattices

Continuous symmetries are believed to emerge at many quantum critical points in frustrated magnets. In this work, we study two candidates of this paradigm: the transverse-field frustrated Ising model (TFFIM) on the triangle and the honeycomb lattices. The former is the prototypical example of this paradigm, and the latter has recently been proposed as another realization. Our large-scale Monte Carlo simulation confirms that the quantum phase transition (QPT) in the triangle lattice TFFIM indeed hosts an emergent O(2) symmetry, but that in the honeycomb lattice TFFIM is a first-order QPT and does not have an emergent continuous symmetry. Furthermore, our analysis of the order parameter histogram reveals that such different behavior originates from the irrelevance and relevance of anisotropic terms near the QPT in the low-energy effective theory of the two models. The comparison between theoretical analysis and numerical simulation in this work paves the way for scrutinizing investigation of emergent continuous symmetry at classical and quantum phase transitions.

A Monte Carlo approach for the bouncer model

A Monte Carlo investigation is made in a dissipative bouncer model to describe some statistical properties for chaotic dynamics as a function of the control parameters. The dynamics of the system is described via a two dimensional mapping for the variables velocity of the particle and phase of the moving wall at the instant of the impact. A small stochastic noise is introduced in the time of flight of the particle as an attempt to investigate the evolution of the system without the need to solve transcendental equations. We show that average values along the chaotic dynamics do not strongly depend on the noise size. It allows us to propose a Monte Carlo like simulation that lead to calculate average values for the observables with great accuracy and fast simulations.