Biomedical subjects

B Faddegon

Publications and source records attributed to B Faddegon.

4 recordsLinked to original sources

PubMed2005

Monte Carlo simulation of large electron fields.

Accurate simulation of large electron fields may lead to improved accuracy in Monte Carlo treatment planning while simplifying the commissioning procedure. We have used measurements made with wide-open jaws and no electron applicator to adjust simulation parameters. Central axis depth dose curves and profiles of 6-21 MeV electron beams measured in this geometry were used to estimate source and geometry parameters, including those that affect beam symmetry: incident beam direction and offset of the secondary scattering foil and monitor chamber from the beam axis. Parameter estimation relied on a comprehensive analysis of the sensitivity of the measured quantities, in the large field, to source and geometry parameters. Results demonstrate that the EGS4 Monte Carlo system is capable of matching dose distributions in the largest electron field to the least restrictive of 1 cGy or 1 mm, with D(max) of 100 cGy, over the full energy range. This match results in an underestimation of the bremsstrahlung dose of 10-20% at 15-21 MeV, exceeding the combined experimental and calculational uncertainty in this quantity of 3%. The simulation of electron scattering at energies of 15-21 MeV in EGS4 may be in error. The recently released EGSnrc/BEAMnrc system may provide a better match to measurement.

Computer Simulation↗

PubMed2004

Comparison of beam characteristics of a gold x-ray target and a tungsten replacement target.

A new W-Cu target was designed to replace the existing Au target on a linear accelerator model in common use in radiotherapy. This work shows that targets of different material composition can be designed to produce beams with the same dosimetric character over a wide range of beam energies without adjusting the beam energy. The target design objective was to improve mechanical robustness, replacing water in the beam path with a Cu heat sink, without altering the beam properties for the nominal clinical energy range of 4-25 MV. The W-Cu could then be installed in place of the Au target without recommissioning. The effect of the target swap was measured in the test cells for 11 different beams ranging in nominal energy from 4 to 25 MV, with focus on open field dose distributions, including diagonal profiles taken for the largest (40x40 cm) field, measured at 4 different gantry angles. Depth dose curves agreed to 0.4% or better, profiles to 1.2% or better. Monte Carlo simulations of the treatment head were done for representative energies of 6 and 18 MV. Calculated and measured dose distributions generally matched within 1%, although dose measured in the build-up region of large fields was significantly more than in the simulations. Calculated spectral distributions on the central axis and angular distributions of energy fluence matched for the two targets, whereas angular distributions of fluence were significantly different. Matching energy fluence gave a more favorable match of dose profiles than matching fluence. The target was further tested on several machines operating in a radiotherapy clinic. Measurements were made for a wide range of open field sizes and with selected wedges and blocks. Dose distributions for the two targets agreed to 1.4% or better, including the dose in wedged fields. Wedge factors changed by no more than 0.5%, transmission through a 4.4 HVL block no more than 1.5%. The response of the monitor chamber was found to change, generally by 1%-2%. Therefore, when the W-Cu target was used to replace the Au target, the output of the machine was measured and adjusted appropriately, but there was no requirement for recommissioning.

Gold↗

PubMed2001

Description and dosimetric verification of the PEREGRINE Monte Carlo dose calculation system for photon beams incident on a water phantom.

PEREGRINE is a three-dimensional Monte Carlo dose calculation system written specifically for radiotherapy. This paper describes the implementation and overall dosimetric accuracy of PEREGRINE physics algorithms, beam model, and beam commissioning procedure. Particle-interaction data, tracking geometries, scoring, variance reduction, and statistical analysis are described. The BEAM code system is used to model the treatment-independent accelerator head, resulting in the identification of primary and scattered photon sources and an electron contaminant source. The magnitude of the electron source is increased to improve agreement with measurements in the buildup region in the largest fields. Published measurements provide an estimate of backscatter on monitor chamber response. Commissioning consists of selecting the electron beam energy, determining the scale factor that defines dose per monitor unit, and describing treatment-dependent beam modifiers. We compare calculations with measurements in a water phantom for open fields, wedges, blocks, and a multileaf collimator for 6 and 18 MV Varian Clinac 2100C photon beams. All calculations are reported as dose per monitor unit. Aside from backscatter estimates, no additional, field-specific normalization is included in comparisons with measurements. Maximum discrepancies were less than either 2% of the maximum dose or 1.2 mm in isodose position for all field sizes and beam modifiers.

Algorithms↗

PubMed1984

The effect of misonidazole as a hypoxic radiosensitizer at low dose.

Using an automated low dose survival assay, the radiosensitizing effectiveness of misonidazole at low radiation dose (0-6 Gy) was measured in cultured mammalian cells. Also measured was its effectiveness at high doses of radiation (0-35 Gy) using the conventional survival assay. In both cases, several concentrations of the drug from 0 to 5 mM were used. The data at low doses were analyzed by a two-parameter mathematical equation with linear and quadratic dose terms, S = e-alpha D-beta D2, which proved to be a good fit to the experimental data at all misonidazole concentrations. It is shown that whereas the coefficient of the quadratic dose term, beta, increases significantly with increasing misonidazole concentration, the drug does not significantly affect the coefficient of the linear term, alpha. The enhancement ratio (ER) of misonidazole is shown to be decreased at lower doses. The clinical implications of this result are discussed.

Animals↗