Photon Transport Code Research Articles

Electron contribution to photon transport in coupled photon‐electron problems: inner‐shell impact ionization correction to XRF

The Monte Carlo code PENELOPE (coupled electron‐photon Monte Carlo) has been used to compute the effect of the secondary electrons on the X‐ray fluorescence characteristic lines. The mechanism that produces this contribution is the inner‐shell impact ionization. The ad hoc code KERNEL (which calls the PENELOPE library) has been used to simulate a forced first collision at the origin of coordinates. The electron correction (produced by the secondary electrons and their multiple scattering) has been studied in terms of angle, space and energy. The energy dependence has been quantified in the interval 1–150 keV, for all the emission lines (K, L and M) of the elements with atomic numbers Z = 11–92. For each characteristic line, the energy dependence is described by simple parametric expressions corresponding to the five energy regions delimited by the K, L1, L2 and L3 absorption edges. It has been introduced a new photon kernel comprising the correction due to inner‐shell impact ionization. The new kernel is suitable to be adopted in photon transport codes (either deterministic or Monte Carlo) with a minimal effort. Finally, the new kernel has been studied for different elements and lines to trace a general behavior. Copyright © 2013 John Wiley & Sons, Ltd.

三维中子-光子输运的蒙特卡罗程序MCMG

三维中子-光子输运的蒙特卡罗程序MCMG

Detailed calculation of inner-shell impact ionization to use in photon transport codes

Secondary electrons can modify the intensity of the XRF characteristic lines by means of a mechanism known as inner-shell impact ionization (ISII). The ad-hoc code KERNEL (which calls the PENELOPE package) has been used to characterize the electron correction in terms of angular, spatial and energy distributions. It is demonstrated that the angular distribution of the characteristic photons due to ISII can be safely considered as isotropic, and that the source of photons from electron interactions is well represented as a point source. The energy dependence of the correction is described using an analytical model in the energy range 1–150keV, for all the emission lines (K, L and M) of the elements with atomic numbers Z=11–92. It is introduced a new photon kernel comprising the correction due to ISII, suitable to be adopted in photon transport codes (deterministic or Monte Carlo) with a minimal effort. The impact of the correction is discussed for the most intense K (Kα1,Kα2,Kβ1) and L (Lα1,Lα2) lines.

SU-D-218-06: Acceleration of Optical Photon Monte Carlo Simulations Using the Macro Monte Carlo Method.

The purpose of this work is to demonstrate that optical photon Monte Carlo simulations via a photon transport code called MCML can be accelerated using macro Monte Carlo (MMC) techniques. MCML was modified to incorporate the macro Monte Carlo radiation transport method. The original MCML uses scattering, absorption, reflection and refraction physics to transport optical photons through multi-layered geometries. The code determines transmission, reflection and absorption for the layered geometry specified by the user. To make the code run faster, the MMC version of MCML uses large, multi-interaction steps in regions that are homogeneous. These large steps are pre-computed and stored in a database with many step sizes, materials and photon energies. The MMC version of MCML determines whether large MMC steps or traditional Monte Carlo should be used depending on the photon's current location inthe geometry. The MMC version of MCML was tested against the original MCML code for a number of simple test geometries. It was also tested in anatomical geometries that are often uses in optical photon modeling. This includes skin and skull geometries. In each case, the reflection and transmission results from each code differed by less than 0.5%. The absorption data produced by each code also differed by less than 0.5% in most cases, and never differed by more than 2%. The MMC version of MCML runs between 1-3 times as many particles per unit time comparedto MCML, depending on the geometry. Applying Macro Monte Carlo methods to MCML produces a faster code without compromising accuracy. The speed-gains are greatest in geometries thathave regions that are large relative to the mean scattering length for photons in that region. This work has the potential to accelerate light modeling for both photodynamic therapy and near-infrared spectroscopic imaging.

Development of parallel monte carlo electron and photon transport (PMCEPT) code III: Applications to medical radiation physics

Minimizing the differences between dose distributions calculated at the treatment planning stage and those delivered to the patient is an essential requirement for successful radiotheraphy. Accurate calculation of dose distributions in the treatment planning process is important and can be done only by using a Monte Carlo calculation of particle transport. In this paper, we perform a further validation of our previously developed parallel Monte Carlo electron and photon transport (PMCEPT) code [Kum and Lee, J. Korean Phys. Soc. 47, 716 (2005) and Kim and Kum, J. Korean Phys. Soc. 49, 1640 (2006)] for applications to clinical radiation problems. A linear accelerator, Siemens’ Primus 6 MV, was modeled and commissioned. A thorough validation includes both small fields, closely related to the intensity modulated radiation treatment (IMRT), and large fields. Two-dimensional comparisons with film measurements were also performed. The PMCEPT results, in general, agreed well with the measured data within a maximum error of about 2%. However, considering the experimental errors, the PMCEPT results can provide the gold standard of dose distributions for radiotherapy. The computing time was also much faster, compared to that needed for experiments, although it is still a bottleneck for direct applications to the daily routine treatment planning procedure.

Metal–silicate partitioning experiments in the diamond anvil cell: A comment on potential analytical errors

We have performed simulations of the electron microbeam analysis of metal–silicate partitioning experiments in which a small (∼50μm) ball of metal is embedded in a silicate matrix. This geometry approximates that found in many high pressure diamond-cell experiments aimed at determining the partition coefficients (Di) of elements between metal and silicate. The simulations involved using the (publicly available) Monte Carlo simulation package PENEPMA which is based on the general-purpose electron–photon transport code PENELOPE. The code allows complex geometrical structures to be defined by quadric surfaces, which enclose homogeneous materials specified by the user. The package simulates both electron and photon transport, keeps track of the X-ray production mechanism and includes the continuum contribution.Our principal results came from simulating the effects of secondary fluorescence on the apparent metal–silicate partition coefficient of Ni for the case of an Ni-rich metal (26.8% Ni, 73.2% Fe) embedded in a silicate matrix. If, as is frequently the case, analysis of the silicate is performed 5–10μm from the silicate–metal interface then secondary fluorescence of Ni in the metal will increase the apparent NiO concentration in the silicate by ∼1000ppm. In the case considered, this is 25% of the Ni content of the silicate and the fluorescence effect decreases the apparent DNi from 110 to 84. For this type of experiment, therefore, the fluorescence effect reduces the apparent partition coefficient.We performed similar simulations of the analysis of the centre of a 10μm diameter metal ball embedded in the silicate matrix to determine apparent Si and O concentrations in the metal. In this case, because the electrons do not penetrate so far into the metal, the fluorescence effect is very small and can be neglected.Finally, we show that focused ion beam (FIB) sections, while providing the potential physically to separate metal from silicate present additional, but not insurmountable, problems of standardisation. The best results would be obtained with a section >3μm in thickness.We recommend that, in general, experimentalists consider simulating analysis of a typical experimental product before publication of results which may be affected by secondary fluorescence.

Nuclear spectroscopy for in situ soil elemental analysis: Monte Carlo simulations

We developed a model to simulate a novel inelastic neutron scattering (INS) system for in situ non-destructive analysis of soil using standard Monte Carlo Neutron Photon (MCNP5a) transport code. The volumes from which 90%, 95%, and 99% of the total signal are detected were estimated to be 0.23m3, 0.37m3, and 0.79m3, respectively. Similarly, we assessed the instrument's sampling footprint and depths. In addition we discuss the impact of the carbon's depth distribution on sampled depth.

SU‐GG‐T‐408: Validation of a Novel Dose Calculation Approach for Heterogeneous Voxelized Phantoms in a Parallel Computation Environment Using Electron Dose Kernels for Radiotherapy

Purpose Here we present a rapid whole body dose estimation approach applied to a clinical water phantom and whole body CT‐voxelized phantom, using electron dose kernels coupled with deterministic photon transport. This method yields fast, accurate whole body doses. Method and Materials A novel dose calculation methodology called EDK‐SN, or “Electron Dose Kernel‐Discrete Ordinates” rapidly estimates organ doses in a voxelized human phantom, accounts for in‐ and out‐of‐field doses using external photon beam therapy. We begin by solving the complete photon transport problem using parallel computing with the 3‐D discrete ordinates (SN) photon transport code PENTRAN. We then project pre‐computed (via Monte Carlo) voxel‐based Electron Dose Kernels (EDKs), mapping them to surrounding voxels via quaternion rotation, scaled by the magnitude of photon fluence from the SN calculation. An 8 MV flat‐weighted beam is incident on an 11×11×11 cm3 water phantom, and on a 15 year old human phantom, down‐sampled to 1×1×1 cm3 (60×27×167 voxels); a 6 MV Philips Elekta Linac photon spectrum has also been simulated. The percent depth dose was compared to clinical CC04 chamber measurement results; comparison of doses using the EDK‐Sn method and clinical treatment planning system (in field dose) will also be presented. Results The EDK‐SN technique has demonstrated independent agreement with Monte Carlo photon‐electron transport calculations for whole body dose. The EDK‐SN method yields a speedup of ∼8 (30 minutes versus 4+ hours) over the traditional parallel Monte Carlo, with <7% difference in different organs (smaller given stochastic uncertainties). The Monte Carlo simulated percent depth dose and clinical chamber PDD measurement agree within 10% among different field sizes. Conclusion The EDK‐SN method for high energy photon external beam dose calculations has been validated based on clinical external therapy beam calculations. This method will help to determine both in‐field and out‐of‐field radiation dose for radiotherapy.

Assessment of Primordial Radionuclides in Pakistani Red Bricks and Associated Radiation Doses

Specific activity of primordial radionuclides and associated radiation hazards due to 40K, 226Ra, and 232Th have been measured in backed red brick samples, collected from five highly populated areas of the North West Frontier Province of Pakistan. For the detection, analysis and data acquisition, a high purity germanium detector was used. Associated external doses were calculated using a Monte Carlo neutron photon transport code. A theoretical model to determine the gamma dose rate at 1 m height from the floor, made of bricks, was employed for the calculation of mass attenuation coefficient and self-absorption in the floor for the gamma energies of these radionuclides and their progeny. Monte Carlo simulation shows that in this study the floor, having more than an effective thickness of 15 cm, contributes very little to the external gamma dose rate. The values of the external dose rate and annual effective dose are found to be much lower than the world average as well as from other countries of the world.

Validation of IRBURN calculation code system through burnup benchmark analysis

Assessment of the reactor fuel composition during the irradiation time, fuel management and criticality safety analysis require the utilization of a validated burnup calculation code system. In this work a newly developed burnup calculation code system, IRBURN, is introduced for the estimation and analysis of the fuel burnup in LWR reactors. IRBURN provides the full capabilities of the Monte Carlo neutron and photon transport code MCNP4C as well as the versatile code for calculating the buildup and decay of nuclides in nuclear materials, ORIGEN2.1, along with other data processing and linking subroutines. This code has the capability of using different depletion calculation schemes. The accuracy and precision of the implemented algorithms to estimate the eigenvalue and spent fuel isotope concentrations are demonstrated by validation against reliable benchmark problem analyses. A comparison of IRBURN results with experimental data demonstrates that the code predicts the spent fuel concentrations within 10% accuracy. Furthermore, standard deviations of the average values for isotopic concentrations including IRBURN data decreases considerably in comparison with the same parameter excluding IRBURN results, except for a few sets of isotopes. The eigenvalue comparison between our results and the benchmark problems shows a good prediction of the k-inf values during the entire burnup history with the maximum difference of 1% at 100 MWd/kgU.

Generation and Application of Bremsstrahlung Production Data Calculated by EGS4 Code

Bremsstrahlung radiation (hereinafter referred to as bremsstrahlung) production data are needed in the calculation of buildup factors, including the contribution of secondary photons by the photon transport codes, which do not handle electron transport. The emission of bremsstrahlung is treated as exactly as possible by the introduction of EGS4 results. The bremsstrahlung production data by pair-created electrons per pair creation reaction and Compton scattered electrons per Compton scattering are evaluated for 26 elements from hydrogen to uranium and four compounds and mixtures of water, concrete, air, and lead glass. The error estimation of bremsstrahlung contribution to buildup factors by the invariant embedding (IE) method coupled with these bremsstrahlung data is coincident with fully transported results by the EGS4 code within ˜5%. By the introduction of bremsstrahlung production data into IE methods, we can calculate buildup factors included by the contribution of those with good accuracy up to deep penetration. By the interpolation and mixture of bremsstrahlung production data for each element, we can evaluate the data of the element or mixture whose data are not evaluated by the EGS4 code.

Semantic Web-based PMCEPT Monte Carlo Code Simulations in Medical Physics

PMCEPT is a three-dimensional parallel Monte Carlo electron and photon transport code based on the standard message passing interface. It has been intensively validated for use in diverse medical physics applications such as radiotherapy, dosimetry, nuclear medicine, X-ray diagnostics, and radiation protection calculations. Despite extensive validation, the Monte Carlo code may not be an easy tool to pick up for a user working in medical areas because it requires some advanced knowledge of information technologies. To overcome this diculty, we developed a function web based on recent semantic web technology. Recent developments of the semantic web make it possible to understand and satisfy various requests of people and machines to intelligently use web contents or even control other machines. This paper describes a specific functional web (including shell scripts) to run the PMCEPT code through a network and verifies the overall dosimetric accuracy of PMCEPT physics algorithms with a 6-MV beam model of the Siemens Primus linear accelerator. A medical professional with relatively little information technology experience can take advantage of the Monte Carlo simulation via the functional web running on our dedicated server with his or her PC (client). We compare calculations with measurements in water and poly methyl methacrylate (PMMA) phantoms for 6-MV Siemens Primus photon beams. The target geometry of a PMMA head and neck phantom was derived from the CT data (DICOM format). Maximum discrepancies were lesser than 2 % in the tail parts of the lateral dose distributions. This study also suggests that our functional web may have a wide range of scientific applications because it is able to control any computer code running on physically separate machines via the Internet.

Monte Carlo simulations of the broad-band spectra of Sagittarius A* through the use of general relativistic magnetohydrodynamics

We present results of simulations of the spectrum of the accretion flow onto the supermassive black hole in our Galactic Centre, Sagittarius A*, generated with a coupling of Monte-Carlo (MC) radiation and general relativistic magnetohydrodynamic (GRMHD) codes. In our modeling, we use the 2D HARM GRMHD code to first model the physical parameters of the disk, then feed its results into our 2D MC photon transport code. We will discuss results obtained which fit radio, IR, and Chandra-obtained flaring or quiescent x-ray data points, as well as the validity of the amount of scaling of input parameters (density, temperature, and magnetic field) required to fit these points. HARM output will be used to suggest whether the scaling is within reasonable limits.

Radiophotoluminescent Observation of X-ray Microbeam Track in Silver-Activated Phosphate Glass

X-ray microbeam transport in radiophotoluminescent (RPL) glass was examined by fluorescence microscopy. A silver-activated phosphate glass dosimeter plate was irradiated with an X-ray microbeam emitted from a 50 kV micro focus X-ray tube equipped with a glass capillary. A RPL region (wavelength; ∼650 nm) was observed with UV light excitation along the X-ray microbeam track in the glass dosimeter plate. The divergence of the RPL track was small owing to the X-ray energy dependence of the divergence of the microbeam, which was determined from the critical angle of the X-ray reflection on the inner wall of the glass capillary. The image of the RPL track was analyzed using photon transport and light trace simulation codes. It was confirmed by comparison between the RPL observation and simulation results that the RPL image in the silver-activated phosphate glass accurately projected the energy-deposited track along the X-ray microbeam.

Patient-Specific Monte Carlo Dose Calculations for High-Dose-Rate Endorectal Brachytherapy With Shielded Intracavitary Applicator

An integrated software platform was developed to perform a patient-specific dosimetric study on high-dose-rate (192)Ir endorectal brachytherapy. Monte Carlo techniques were used to examine the perturbation effects of an eight-channel intracavitary applicator with shielding and a liquid-inflatable balloon. Such effects are ignored in conventional treatment planning systems that assume water-equivalent geometries. A total of 40 Task Group 43-based rectal patient plans were calculated using the PTRAN_CT Monte Carlo photon transport code. The silicone applicator, tungsten or lead shielding, contrast solution-filled balloon, and patient anatomy were included in the simulations. The dose to water and dose to medium were scored separately. The effects of heterogeneities and uncertainties in source positioning were examined. A superposition calculation method using pregenerated Monte Carlo dose distributions about the shielded applicator in water was developed and validated for efficient treatment planning purposes. On average, metal shielding decreases the mean dose to the contralateral normal tissues by 24% and reduces the target volume covered by the prescribed dose from 97% to 94%. Tissue heterogeneities contribute to dose differences of <1% relative to the prescribed dose. The differences in the dose volume indices between dose to water and dose to medium-based calculations were <1% for soft tissues, <2% for bone marrow, and >20% for cortical bone. A longitudinal shift of +/-2.5 mm and a rotational shift of +/-15 degrees in applicator insertion reduced the target volume receiving the prescribed dose by </=4%. The shielded applicator improved dose conformity and normal tissue sparing; however, Task Group 43-based treatment planning might compromise target coverage by not accounting for shielding.

Calculation of the reactor neutron time of flight spectrum by convolution technique

It is a very complex and time-consuming process to simulate the nuclear reactor neutron spectrum from the reactor core to the export channel by applying a Monte Carlo program. This paper presents a new method to calculate the neutron spectrum by using the convolution technique which considers the channel transportation as a linear system and the transportation scattering as the response function. It also applies Monte Carlo Neutron and Photon Transport Code (MCNP) to simulate the response function numerically. With the application of convolution technique to calculate the spectrum distribution from the core to the channel, the process is then much more convenient only with the simple numerical integral numeration. This saves computer time and reduces some trouble in re-writing of the MCNP program.

Sci-Sat AM(2): Brachy-08: Monte Carlo calculations of 192Ir high dose rate brachytherapy treatment plans using CT and cone beam CT images.

The feasibility of using cone beam computed tomography (CBCT) images for Monte Carlo (MC) brachytherapy dose calculations has been investigated. To evaluate the effects of tissue heterogeneities and finite patient dimensions for 192Ir high dose rate treatment, CT-based MC calculations for breast and head and neck cases were first performed using the PTRAN_CT photon transport code. PTRAN_CT is an accelerated MC code specifically designed for patient-specific dose calculations. Muscles and adipose tissues, which are nearly indistinguishable in CBCT images, are found to cause minimal dose perturbations at 192Ir energies compared to water. The proximity of the tumor to the skin, however, will have an observable impact on the dose up to a few percent. Therefore, for CBCT calculations, a reasonable assignment of material and density values to the patient voxel geometry, with a good delineation of the skin and bony structures, will suffice for MC dose calculations. A CBCT-based calculation for an actual treatment plan with the tumor close to the cheek was performed. The results were compared to TG43 calculations to quantify the dose differences in the target and critical structures. Since the dose delivered to the tumor is mostly primary dose, deviations are found mostly in the organs at risk where scatter contribution becomes more significant. This study shows that for HDR brachytherapy applications, CBCT-based MC calculations is a feasible option despite inferior image quality and larger uncertainties in the Hounsfield Units compared to CT images. Research supported by Nucletron BV.

TH-D-351-06: Comparison Between 2D Monte Carlo Modeled and Experimental Cone-Beam CT X-Ray Projections

Purpose: Fast and accurate modeling of cone-beam CT (CBCT) x-ray projection data can improve cone-beam CT (CBCT) image quality either by conditioning projection data prior to image reconstruction or by supporting rigorous comparative simulation studies of competing image reconstruction and processing algorithms. In this study, we compare Monte Carlo- computed x-ray projections with projections experimentally acquired from our Varian Trilogy CBCT imaging system for phantoms of known design. Method and Materials: Our recently developed Monte Carlo photon-transport code, PTRAN, was used to compute primary and scatter projections for cylindrical phantoms of known diameter (CatPhan and NA model 76-410) with and without bow-tie filter and antiscatter grid for both full- and half-fan geometries. The simulations were based upon measured 120 kVp spectra, beam profiles, and flat-panel detector (4030CB) point-spread functions. The beam-stop array method was used to acquire scatter and SPR distributions from the OBI images. The biasing of scatter measurements due to the long detector PSF tails was corrected either by a lead mask or by deconvolution. Computed projections were compared to flat- and dark-field corrected 4030CB images. Results: The simulated primary profiles agree with experiment within 3%, while the simulated scatter profiles agree within 8–10%. Both PSF measurements and mask measurements indicate that scatter radiation values can be biased by as much as 7% detector PSF tails. Conclusion: In agreement with the literature, the difference between simulated and measured projection data is of the order of 6–8%. Higher accuracy can be achieved mainly by improving the beam modeling and correcting the non linearities induced by the detector PSF. This project was supported in part by grants from Varian Medical Systems and NCI (R01 CA 75371).

Evaluation of 192Ir high-dose-rate rectal brachytherapy treatment plans using a fast Monte Carlo photon transport code

Purpose: To retrospectively evaluate the treatment plans of a rectal cancer patient cohort who received 192Ir high-dose-rate (HDR) brachytherapy with a metal-shielded intracavitary mould applicator. Using fast Monte Carlo (MC) techniques, dose perturbations due to applicator and anatomical heterogeneities and their effects on the target dose coverage were examined.

BrachyGUI: an adjunct to an accelerated Monte Carlo photon transport code for patient-specific brachytherapy dose calculations and analysis

A number of accelerated Monte Carlo (MC) codes have been developed in recent years for brachytherapy applications, one of which is PTRAN_CT. Developed as an extension to the well-benchmarked PTRAN code, PTRAN_CT can be used to perform efficient patient-specific dose calculations. The code can explicitly account for the patient geometry converted from computed-tomography (CT) images, as well as perturbations due to the brachytherapy applicator and seeds. We have developed a software tool called BrachyGUI that provides an integrated environment for preparing patient and treatment plan-specific input data files for PTRAN_CT. It also comes with dose calculation, analysis, and treatment planning capabilities. In this article, we will describe the interface of BrachyGUI with PTRAN_CT for CT-based calculations, and examine the calculation efficiency of PTRAN_CT. We conclude that it is now feasible to use PTRAN_CT for high dose rate brachytherapy treatment planning on a routine clinical basis.