Photon Transport Simulation Research Articles

Monte Carlo calculation of kerma to a point in the vicinity of media interfaces

Recent work has demonstrated that Monte Carlo photon transport (MCPT) simulation is a practical and accurate dosimetry tool for characterizing brachytherapy dose distributions in the presence of complex three-dimensional (3D) geometries. Because of the steep dose gradients encountered in this application, the choice of estimation algorithm, i.e., the algorithm for extracting dose estimates from individual photon histories, is critical to ensure adequate spatial resolution and computational efficiency. While conventional MCPT dose estimation techniques are sufficiently accurate in many cases, they inevitably fail with the point of interest is located near media interfaces, since they require the point of interest to be contained by a region of homogeneous medium of several millimetres in diameter. This severely limits the domain of application of MCPT in brachytherapy dosimetry and prevents dose estimation at points such as those immediately behind lead shields contained in gynaecological applicators, or simulation of small intricately structured detector response. We have successfully adapted to an MCPT code a neutron-flux estimator, the one-more-collided flux estimator (OMCFE), which overcomes this problem. OMCFE is derived from the familiar next-flight estimation algorithm, and, through the use of a photon trajectory resampling scheme, does not require homogeneous media at the point of interest. it is therefore a true point-dose estimator. OMCFE was tested against a variety of one- and three-dimensional benchmark problems. These benchmarks included mono-energetic point sources of 30 and 600 keV, as well as a 3M model 6702 125I seed, a 137Cs tube, and an 192Ir seed in geometries approximating common brachytherapy dosimetric problems. OMCFE agrees closely with conventional MCPT dose estimation algorithms in solving these benchmark problems, while in many cases being much more efficient.

Photon contribution to tumor dose from considerations of 131I radiolabeled antibody uptake in liver, spleen, and whole body.

The contribution of penetrating photon energy to tumor dose is usually ignored because of the difficulty in calculating absorbed fractions and because it is frequently assumed to represent a small proportion of the total energy. The MABDOSE software--written explicitly to simulate photon transport for the calculation of penetrating radiation absorbed fractions--was used to simulate the 131I photon spectrum originating from the liver, spleen, and whole body source organs. Specific absorbed fractions were calculated for tumors of radius 1.0, 1.5, and 2.0 cm placed near the liver and spleen in the Reference Man geometry. Cumulated activities were estimated using values reported from the literature. Dosimetry estimates from the combined cumulated activity and specific absorbed fractions indicate that neglecting the photon contribution underestimates the tumor dose by 10%-25%.

Solar Radiative Fluxes for Broken Cloud Fields above Reflecting Surfaces

Abstract A statistical bidirectional method for including the effects of underlying reflecting surfaces in Monte Carlo simulations of atmospheric photon transport is presented. It is illustrated for the idealized Lambertian surface and a general particulate surface. It is shown that the simple Lambertian surface should be an adequate approximation (to mildly anisotropic surfaces) for calculation of overall fluxes for broken cloud fields. The problem of incorporating multiple internal reflections of solar radiation between cloud and surface is reviewed, and cloud field reflectances to upwelling radiation rk as a function of the number of internal reflections k are calculated. Unlike plane-parallel and regular arrays of cloud, all rk are generally not equal for more realistic scaling cloud fields. Strictly speaking, this prohibits use of the familiar geometric sum formulas for flux calculation in a multiple reflecting system. Fortunately, though fortuitously, the geometric sum formulas can be used in such c...

Calculation of dose and contrast for two mammographic grids

To aid in selecting optimal conditions for screening mammography practice in Sweden, the performance of two mammographic grids (one moving and one stationary) has been investigated. Monte Carlo techniques were used to simulate photon transport in the breast. Transport through the breast support, grid covers, grid and image receptor (33.9 mg cm-2 Gd2O2S) was treated analytically. The contrast of a 100 mu m calcification has been evaluated for three tissue compositions (adipose, glandular, 50 : 50 fractions by weight of adipose and glandular tissue) as a function of breast thickness (2-8 cm) and potential difference (25-30 kV, Mo anode). Contrast for a 5 cm 'average' breast at 28 kV was improved by 40% using the moving grid and by 30% using the stationary one; the corresponding increases in breast absorbed dose, keeping the energy imparted to the image receptor constant, were 90% and 150%, respectively. The superior properties of the moving grid were examined.

MCPT: A Monte Carlo code for simulation of photon transport in tomographic scanners

MCPT is a special-purpose Monte Carlo code designed to simulate photon transport in tomographic scanners. Variance reduction schemes and sampling games present in MCPT were selected to characterize features common to most tomographic scanners. Combined splitting and biasing (CSB) games are used to systematically sample important detection pathways. An efficient splitting game is used to tally particle energy deposition in detection zones. The pulse height distribution of each detector can be found by convolving the calculated energy deposition distribution with the detector's resolution function. A general geometric modelling package, HERMETOR, is used to describe the geometry of the tomographic scanners and provide MCPT information needed for particle tracking. MCPT's modelling capabilities are discribed and preliminary experimental validation is presented.

Determination of flow profiles in porous media using shifts in gamma spectra

AbstractA new technique was developed to determine the tracer location and fluid velocity in porous media nonintrusively. This technique exploits the competitive effects between the photoelectric interaction and the Compton scattering phenomenon to determine the distance between a radioactive tracer in a porous medium and an externally positioned detector. The photon energy distribution shifts toward lower photon energies as the tracer moves away from the detector. The shift in the energy distribution can be quantified by the ratio of the scattered photon intensity to the full energy photon intensity. A convective‐dispersion model was used to determine the spatial distribution of the radioactive tracer. An analog Monte Carlo method was developed to simulate photon transport in porous media. Comparison between experimental data and the model shows that in‐situ tracer velocities can be accurately predicted.

Effects of gold and silver backings on the dose rate around an 125I seed

Measurements of the effect of either gold or silver backing on the dose rate around an 125I seed were performed using a Therados RFA7 dosimetry system and a small diode detector which was 2.5 mm in diameter and 0.06 mm thick. It was found that the presence of the gold or silver backing modifies the diode response on the side of the 125I seed away from the backing. The effect depends on the backing material and the distance from the seed. There is a small increase close to the gold backing but a decrease further away. This decrease at distances greater than 10 mm from the seed is uniformly 10%, the same as found when the seed is backed by air. There is an increase of up to 25% observed with silver backing the seed and this increase remains significant more than 30 mm from the seed. When the response increases, the results are hard to interpret quantitatively because of variations in the diode response per unit dose with photon energy and extreme sensitivity to geometric changes. Nonetheless, except for the increase at close distances with the gold, the results are in agreement with EGS4 Monte Carlo photon transport simulations which are for a simplified geometry and account for x-ray fluorescence from the K-shell. Furthermore, the increase in the gold-backed case is qualitatively explained by Williamson's Monte Carlo calculations which take into account the L-shell fluorescent x-rays from gold.

Development and validation of a Monte Carlo simulation of photon transport in an Anger camera

The geometric component of the point spread function (PSF) of a gamma camera collimator can be determined analytically, and the penetration component can be calculated readily by numerical ray-tracing. A Monte Carlo simulation of photon transport which includes collimator scatter is developed. The simulation was implemented with an array processor which propagates up to 1024 photons in parallel, allowing accurate estimates of the total radial PSF in less than a day. The simulation was tested by imaging monoenergetic point sources of Tc-99m, Cr-51, and Sr-85 (140, 320, and 514 keV, respectively) on a General Electric Star Cam with low-energy, general-purpose, and medium-energy collimators. Comparisons of measured and simulated PSFs demonstrate the validity of the model and the significance of collimator scatter in the degradation of image quality.

On the possibility of ‘real-time’ Monte Carlo calculations for the estimation of absorbed dose in radioimmunotherapy

Dosimetry calculations of monoclonal antibodies (MABs) are made difficult because the focus of radioactivity is targeted for a nonstandard volume in a nonstandard geometry, precluding straighforward application of the MIRD formalism. The MABDOS software addresses this shortcoming by interactive placement of a spherical perturbation into the Standard Man geometry for each tumor focus. S tables are calculated by a Monte Carlo Simulation of photon transport for each organ system (including tumor) that localizes activity. Performance benchmarks are reported that measure the time required to simulate 60 000 photons for each penetrating radiation in the spectrum of 99mTc and 131I using the kidney as source organ. Results indicate that calculation times are probably prohibitive on current microcomputer platforms. Mini and supercomputers offer a realistic platform for MABDOS patient dosimetry estimates.

Monte Carlo computer simulation of the XRF intensity dependence on the propagation plane inclination

When a multicomponent sample is irradiated with a collimated beam of X-rays, the absorption by the photoelectric effect give place to a characteristic X-ray emission of each atom class, which is known as X-ray fluorescence (XRF). If this outgoing radiation is detected by collimating into a sharp direction, a strong geometrical dependence on the incident and take-off angles (measured in relation to the normal vector to the irradiated surface) is found. A particular parameterization [1] of both angles which relates them to the so called propagation plane (the plane that contains both X-ray collimated beams or, equivalently, both the incidence and the take-off directions of the X-rays), made it possible for every class of atom to separate the radiation they emitted as consequence of the absorption of the incident excitation spectrum (primary process) from which they produced by absorption of characteristic radiation emitted by neighbor atoms of different classes, whose X-rays have enough energy to be absorbed by the observed atoms (secondary process). Both types of XRF emission, despite of their different origins, have exactly the same energy and can not be individuated by other means. When detection occurs, the whole intensity is accumulated at every energy. The above mentioned parameterization was found following a theoretical model [1], which permitted us to predict that the calculated intensity due to the secondary process should vanish while the primary remains unchanged, when the α angle of the propagation plane tilt increases toward 1 2 π. For the limit case α = 1 2 π this property make it possible to separate the radiation emitted by the different effects, because then only the primary remains. Computer simulation was necessary to have a first principles checking of what was predicted theoretically. Monte Carlo simulation confirmed the previous results and helped us to understand why this happened. By dividing the sample into thin slabs and looking at the accumulated intensity in every one of them, it is possible to explain the invariance of the primary emission and the vanishing of the secondary one [2]. Careful application of variance reduction techniques renders feasible to implement the photon transport simulation on a single user microcomputer powered with a floating-point numerical coprocessor, while the simulation times are quite lower than the reported CPU time of a VAX/780 timeshared computer.

Polarized light from circumstellar dust: observations and models

AbstractTwo models of a biconical dust nebula with an opaque torus in the symmetry plane are presented: a model containing dust in the whole biconical space (A) and an “empty” model with the dust layer on the surface of this empty space (B). Surface brightness distribution and polarization of the emitted light are computed using the Monte Carlo simulation of photon transport in the nebula. The results have features similar to those observed in several compact reflection nebulae.

A Monte Carlo program for photon transport using analogue sampling of scattering angle in coherent and incoherent scattering processes

A computer program was developed for the Monte Carlo simulation of photon transport. The program was designed for photon transport simulation in geometries occurring in diagnostic radiology and especially for the investigation of scattered radiation. A method is described for the analogue sampling of scattering angle in coherent and incoherent scattering processes. The two scattering processes are treated separately, and the influence of coherent scattering, an often neglected process, can be estimated quantitatively. The program can also be used for the calculation of the energy imparted to water slabs and fluorescent screens.

Multiple electron scattering in gamma ray imaging detectors

The effect of converter thickness on spatial resolution and detection efficiency of gamma ray imaging detectors was investigated by Monte Carlo simulation of photon and electron transport in thin slabs. The simulations were carried out by employing a multiple electron scattering theory to simulate successive changes in electron trajectory during propagation through the converter and detector materials. For comparison, results were also obtained using the approximate method of assuming straight-line paths for the electrons. The straight-line path simulations indicate that both spatial resolution and detection efficiency can be improved by the use of thicker converters, while the multiple electron scattering simulations indicate that, without the use of pulse height selection, detection efficiency can be improved only at the expense of spatial resolution. The calculated spatial resolution and detection efficiency for 0.662 MeV photons incident on a 0.18 cm thick mylar converter in juxtaposition with a 1.27 cm thick argon slab detector (NTP) are 2.1 cm and 0.39% and 1.3 cm and 0.64% for the multiple electron scattering and straight-line path analyses, respectively.

A model simulating photon transport in a finite medium

A method for computing absorbed dose due to ionizing 60Co radiation in finite two-dimensional absorbers is presented in which only parameters describing the geometry of the source-target configuration are used as input. The solution of the model is shown to be the limit of a monotone sequence. Application to radiation dosimetry for radiotherapy treatment planning is pointed out.