Biomedical subjects

Iwan Kawrakow

Publications and source records attributed to Iwan Kawrakow.

6 recordsLinked to original sources

PubMed2006

On the effective point of measurement in megavoltage photon beams.

This paper presents a numerical investigation of the effective point of measurement of thimble ionization chambers in megavoltage photon beams using Monte Carlo simulations with the EGSNRC system. It is shown that the effective point of measurement for relative photon beam dosimetry depends on every detail of the chamber design, including the cavity length, the mass density of the wall material, and the size of the central electrode, in addition to the cavity radius. Moreover, the effective point of measurement also depends on the beam quality and the field size. The paper therefore argues that the upstream shift of 0.6 times the cavity radius, recommended in current dosimetry protocols, is inadequate for accurate relative photon beam dosimetry, particularly in the build-up region. On the other hand, once the effective point of measurement is selected appropriately, measured depth-ionization curves can be equated to measured depth-dose curves for all depths within +/- 0.5%.

Electrodes↗

PubMed2006

Efficient x-ray tube simulations.

This article describes an efficiency study of directional bremsstrahlung splitting (DBS) for x-ray tube modeling. DBS is shown to be up to five or six orders of magnitude more efficient at 50 or 135 kV tube potential than a simulation without splitting, and 60 times more efficient compared to uniform bremsstrahlung splitting. A methodology is presented to determine the optimum splitting number for a given situation using a second degree polynomial expression derived from theoretical considerations. Very large optimum splitting numbers are found for small fields (5 mm radius) at 1 m from the x-ray source, which are relevant for half-value layer (HVL) calculations and for simulations related to primary air kerma standards. Two approaches for the calculation of kerma at a plane and inside a volume using track-length estimation are implemented in BEAMnrc, a user-code from the EGSnrc Monte Carlo simulation system for photon and electron transport. A practical application of DBS to HVL calculations for a Comet MXR-320 x-ray tube is reported. The agreement with measured HVLs at different constant tube potentials is found to be better than 2.3%.

Computer Simulation↗

PubMed2005

Calculation of photon energy deposition kernels and electron dose point kernels in water.

Effects of changes in the physics of EGSnrc compared to EGS4/PRESTA on energy deposition kernels for monoenergetic photons and on dose point kernels for beta sources in water are investigated. In the diagnostic energy range, Compton binding corrections were found to increase the primary energy fraction up to 4.5% at 30 keV with a corresponding reduction of the scatter component of the kernels. Rayleigh scattered photons significantly increase the scatter component of the kernels and reduce the primary energy fraction with a maximum 12% reduction also at 30 keV where the Rayleigh cross section in water has its maximum value. Sampling the photo-electron angular distribution produces a redistribution of the energy deposited by primaries around the interaction site causing differences of up to 2.7 times in the backscattered energy fraction at 20 keV. Above the pair production threshold, the dose distribution versus angle of the primary dose component is significantly different from the EGS4 results. This is related to the more accurate angular sampling of the electron-positron pair direction in EGSnrc as opposed to using a fixed angle approximation in default EGS4. Total energy fractions for photon beams obtained with EGSnrc and EGS4 are almost the same within 0.2%. This fact suggests that the estimate of the total dose at a given point inside an infinite homogeneous water phantom irradiated by broad beams of photons will be very similar for kernels calculated with both codes. However, at interfaces or near boundaries results can be very different especially in the diagnostic energy range. EGSnrc calculated kernels for monoenergetic electrons (50 keV, 100 keV, and 1 MeV) and beta spectra (32P and 90Y) are in excellent agreement with reported EGS4 values except at 1 MeV where inclusion of spin effects in EGSnrc produces an increase of the effective range of electrons. Comparison at 1 MeV with an ETRAN calculation of the electron dose point kernel shows excellent agreement.

Algorithms↗

PubMed2005

On the efficiency of photon beam treatment head simulations.

The paper presents a theoretical derivation of the photon fluence simulation efficiency in photon beam treatment head simulations without and with bremsstrahlung splitting. The expressions obtained provide a rigorous explanation of the efficiency dependence on scoring zone size and splitting number and explain the larger efficiency of the directional bremsstrahlung splitting technique compared to uniform bremsstrahlung splitting. They are also very useful for determining the optimum splitting number in practical treatment head simulations and for understanding the variation of simulation efficiency with beam energy and field size.

Body Burden↗

PubMed2003

Calculations for plane-parallel ion chambers in 60Co beams using the EGSnrc Monte Carlo code.

The EGSnrc Monte Carlo simulation system is used to obtain, for 10 plane-parallel ionization chambers in 60Co beams, the correction factors Kcomp and Pwall that account for the nonequivalence of the chamber wall material to the buildup cap and the phantom material, respectively. A more robust calculation method has been used compared to that used in previous works. A minor conceptual error related to the axial nonuniformity correction factor, Kan, has been identified and shown to have an effect of about 0.2%. The assumption that Pwall in-phantom is numerically equal to Kcomp calculated for a water buildup cap is shown to be accurate to better than 0.06%, thereby justifying the use of Kcomp calculations which are much more efficient. The effect on the calculated dose to the air in the cavity of the particle production threshold and transport energies used in the simulations is studied. Uncertainties in the calculated correction factors due to uncertainties in the photon and electron cross-section data are studied. They are 0.14% and 0.24%, respectively (1 standard deviation), for Kcomp factors. The uncertainties on Kwall factors are 0.03% from photon cross-section uncertainties and negligible from electron cross-section uncertainties. A comparison with previous EGS4/PRESTA calculations shows that present results are systematically higher by an average of 0.8%, ranging from 0.4% up to 1.4%. The present results are in better agreement with reported experimental values.

Algorithms↗

PubMed2003

Monte Carlo calculated correction factors for primary standards of air kerma.

Many laboratories with cavity chambers as primary standards for air kerma are considering using additional Monte Carlo calculated correction factors, in particular the correction for attenuation and scatter in the walls, Kwall, and possibly the correction for point of measurement, Kan. Standards labs also use Monte Carlo calculated Spencer-Attix stopping-power ratios for graphite to air. The purpose of this article is to investigate the sensitivity of these calculations to their details and to assign uncertainties to the calculated values. We also investigate the correction needed for the Canadian primary standard to account for a polystyrene insulator, Kcomp and find that it is quite large (1.0046 +/- 0.0017). The article shows that the values of correction factors are very robust and insensitive to most details of the calculations except the values of the underlying electron stopping powers which have a significant effect on the stopping-power ratio and on Kcomp. The 1% uncertainties on the photon cross-sections have a negligible effect on these correction factors except for Kcomp. As a result of these investigations, with no change in the stopping power data used, the Canadian primary standard of air kerma in a 60Co beam needs to be increased by 0.54%.

Air↗