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Biomedical subjects

I Kawrakow

Publications and source records attributed to I Kawrakow.

8 recordsLinked to original sources
PubMed2000

Investigation of variance reduction techniques for Monte Carlo photon dose calculation using XVMC.

Several variance reduction techniques, such as photon splitting, electron history repetition, Russian roulette and the use of quasi-random numbers are investigated and shown to significantly improve the efficiency of the recently developed XVMC Monte Carlo code for photon beams in radiation therapy. It is demonstrated that it is possible to further improve the efficiency by optimizing transpon parameters such as electron energy cut-off, maximum electron energy step size, photon energy cut-off and a cut-off for kerma approximation, without loss of calculation accuracy. These methods increase the efficiency by a factor of up to 10 compared with the initial XVMC ray-tracing technique or a factor of 50 to 80 compared with EGS4/PRESTA. Therefore, a common treatment plan (6 MV photons, 10 x 10 cm2 field size, 5 mm voxel resolution, 1% statistical uncertainty) can be calculated within 7 min using a single CPU 500 MHz personal computer. If the requirement on the statistical uncertainty is relaxed to 2%, the calculation time will be less than 2 min. In addition, a technique is presented which allows for the quantitative comparison of Monte Carlo calculated dose distributions and the separation of systematic and statistical errors. Employing this technique it is shown that XVMC calculations agree with EGSnrc on a sub-per cent level for simulations in the energy and material range of interest for radiation therapy.

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PubMed2000

Monte Carlo study of correction factors for Spencer-Attix cavity theory at photon energies at or above 100 keV.

To develop a primary standard for 192Ir sources, the basic science on which this standard is based, i.e., Spencer-Attix cavity theory, must be established. In the present study Monte Carlo techniques are used to investigate the accuracy of this cavity theory for photons in the energy range from 20 to 1300 keV, since it is usually not applied at energies below that of 137Cs. Ma and Nahum [Phys. Med. Biol. 36, 413-428 (1991)] found that in low-energy photon beams the contribution from electrons caused by photons interacting in the cavity is substantial. For the average energy of the 192Ir spectrum they found a departure from Bragg-Gray conditions of up to 3% caused by photon interactions in the cavity. When Monte Carlo is used to calculate the response of a graphite ion chamber to an encapsulated 192Ir source it is found that it differs by less than 0.3% from the value predicted by Spencer-Attix cavity theory. Based on these Monte Carlo calculations, for cavities in graphite it is concluded that the Spencer-Attix cavity theory with delta = 10 keV is applicable within 0.5% for photon energies at 300 keV or above despite the breakdown of the assumption that there is no interaction of photons within the cavity. This means that it is possible to use a graphite ion chamber and Spencer-Attix cavity theory to calibrate an 192Ir source. It is also found that the use of delta related to the mean chord length instead of delta = 10 keV improves the agreement with Spencer-Attix cavity theory at 60Co from 0.2% to within 0.1% of unity. This is at the level of accuracy of which the Monte Carlo code EGSnrc calculates ion chamber responses. In addition, it is shown that the effects of other materials, e.g., insulators and holders, have a substantial effect on the ion chamber response and should be included in the correction factors for a primary standard of air kerma.

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PubMed2000

Accurate condensed history Monte Carlo simulation of electron transport. I. EGSnrc, the new EGS4 version.

In this report a new EGS4 version, called EGSnrc to reflect the substantial changes made to the original code is reported, which incorporates a new any-angle multiple elastic scattering theory, an improved electron-step algorithm, a correct implementation of the fictitious cross section method for sampling distances between discrete interactions, a more accurate evaluation of energy loss, as well as an exact boundary crossing algorithm. It is demonstrated that EGSnrc allows for an artifact free Monte Carlo simulation of ion chamber response and backscattering, situations that have been considered in the past as the two of the most stringent tests of condensed history Monte Carlo codes. A detailed discussion of the effect of the various components of the condensed history simulation of electron transport on the simulated ion chamber response is given in the accompanying paper.

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PubMed2000

Accurate condensed history Monte Carlo simulation of electron transport. II. Application to ion chamber response simulations.

In this report the condensed history Monte Carlo simulation of electron transport and its application to the calculation of ion chamber response is discussed. It is shown that the strong step-size dependencies and lack of convergence to the correct answer previously observed are the combined effect of the following artifacts caused by the EGS4/PRESTA implementation of the condensed history technique: dose underprediction due to PRESTA'S pathlength correction and lateral correlation algorithm; dose overprediction due to the boundary crossing algorithm; dose overprediction due to the breakdown of the fictitious cross section method for sampling distances between discrete interaction and the inaccurate evaluation of energy-dependent quantities. These artifacts are now understood quantitatively and analytical expressions for their effect are given.

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PubMed1997

Electron beam dose calculations with the VMC algorithm and the verification data of the NCI working group.

The accuracy of the Monte Carlo algorithm for fast electron dose calculation, VMC, is demonstrated by comparing calculations with measurements performed by a working group of the National Cancer Institute (NCI) of the USA. For both energies investigated, 9 and 20 MeV, the measurements in water are taken to determine the energy spectra of the Varian Clinac 1800 accelerator. For the majority of the experiments a good agreement is found between measurements and VMC calculations. However, in some cases deviations have been observed, which could be explained by the incompletely known geometry on the one hand and by inconsistent data on the other hand. As a reference, dose distributions calculated by the MDAH pencil-beam algorithm are also presented. It is shown that, especially near low- or high-density inhomogeneities, large dose overestimations and underestimations are calculated by using a pencil-beam approach, whereas VMC is able to reproduce the correct doses for these cases also.

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PubMed1997

Improved modeling of multiple scattering in the Voxel Monte Carlo model.

The implementation of an improved model for multiple scattering into the-Voxel Monte Carlo (VMC) algorithm for fast electron dose calculation in radiation therapy is presented. The model takes into account path-length corrections (PLC) and lateral displacement in the individual electron steps. The extraction of the scattering power from the available computed-tomography images is discussed. It is shown that with the improved modeling of multiple electron scattering, the VMC algorithm is comparable in accuracy with PRESTA, the electron transport algorithm of EGS4. The problem of double counting of contributions from atomic electrons to the scattering power is considered and a simple solution is found.

Algorithms↗
PubMed1996

3D electron dose calculation using a Voxel based Monte Carlo algorithm (VMC).

A new model for calculating electron beam dose has been developed. The algorithm is based on a two- or three-dimensional geometry defined by computerized tomography (CT) images. The Monte Carlo technique was used to solve the electron transport equation. However, in contrast to conventional Monte Carlo models (EGS4) several approximations and simplifications in the description of elementary electron processes were introduced reducing in this manner the computational time by a factor of about 35 without significant loss in accuracy. The Monte Carlo computer program does not need any precalculated data. The random access memory required is about 16 Mbytes for a 128(2) X 50 matrix, depending on the resolution of the CT cube. The Voxel Monte Carlo model (VMC) was tested in comparison to calculations by EGS4 and the "Hogstrom algorithm" (MDAH) using several fictive phantoms. In all cases a good coincidence has been found between EGS4 and VMC, especially near tissue inhomogeneities, whereas the MDAH algorithm has produced dose underestimations of up to 40%.

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