Biomedical subjects

Ernesto Mainegra-Hing

Publications and source records attributed to Ernesto Mainegra-Hing.

4 recordsLinked to original sources

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↗

PubMed2006

On the accuracy of techniques for obtaining the calibration coefficient N(K) of 192Ir HDR brachytherapy sources.

The accuracy of interpolation or averaging procedures for obtaining the calibration coefficient N(K) for 192Ir high-dose-rate brachytherapy sources has been investigated using the EGSnrc Monte Carlo simulation system. It is shown that the widely used two-point averaging procedure of Goetsch et al. [Med. Phys. 18, 462 (1991)] has some conceptual problems. Most importantly, they recommended, as did the IAEA, averaging A(wall)N(K) values whereas one should average 1/N(K) values. In practice this and other issues are shown to have little effect except for Goetsch et al.'s methods for determining A(wall) values. Their method of generalizing the A(wall) values measured in one geometry to other geometries is incorrect by up to 2%. However, these errors in A(wall) values cause systematic errors of only 0.3% in 192Ir calibration coefficients. It is shown that A(wall) values need not be included in the averaging technique at all, thereby simplifying the technique considerably. It is demonstrated that as long as ion chambers with a flat response are used and/or very heavily filtered 250 kV (or higher) beams of x rays are used in the averaging, then almost all techniques can provide adequate accuracy.

Brachytherapy↗

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↗

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↗