RESUMO
A Monte Carlo study of dosimetry for eye plaque brachytherapy was performed. BrachyDose, an EGSnrc user-code which makes use of Yegin's multi-geometry package, was used to fully model Iodine-125 (model 6711) and Palladium-103 (model 200) brachytherapy seeds and the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). Three-dimensional dose distributions in the eye region were obtained. In general, dose to water was scored, however the implications of replacing water with eye materials was explored. The effect of the gold alloy (Modulay) backing was investigated and the dose was found to be sensitive to the elemental composition of the backing. The presence of the silicon polymer (Silastic) seed carrier resulted in substantial dose decreases relative to water, particularly for Pd-103. For the Modulay backing and Silastic insert combination in a 20 mm plaque, the dose decrease relative to water is of the order of 12% for I-125 and 20% for Pd-103 at a distance of 1 cm from the central seed along the plaque's central axis. For the configurations of seeds used in COMS plaques, interseed attenuation is a small effect within the eye region. The introduction of an air interface results in a dose reduction in its vicinity which depends on the plaque's position within the eye and the source type. The dose distributions in the eye for the two different sources were compared and, for the same prescription dose, Pd-103 generally offers a lower dose to critical normal structures.
RESUMO
The replacement correction factor (Prepl ) in ion chamber dosimetry accounts for the effects of the medium being replaced by the air cavity of the chamber. In TG-21, Prepl was conceptually separated into two components: fluence correction, Pfl , and gradient correction, Pgr . In TG-51, for electron beams, the calibration is at dref where Pgr is required for cylindrical chambers and Pfl is unknown and assumed to be the same as that for a beam having the same mean electron energy at dmax . For cylindrical chambers in high-energy photon beams, Prepl also represents a major uncertainty in current dosimetry protocols. In this study, Prepl is calculated with high precision (<0.1%) by the Monte Carlo method as the ratio of the dose in a phantom to the dose scored in water-walled cylindrical cavities of various radii (with the center of the cavity being the point of measurement) in both high energy photon and electron beams. It is found that, for electron beams, the mean electron energy at depth is a good beam quality specifier for Pfl ; and TG-51's adoption of Pfl at dmax with the same mean electron energy for use at dref is proven to be accurate. For Farmer chambers in photon beams, there is essentially no beam quality dependence for Prepl values. In a Co photon beam, the calculated Prepl is about 0.4-0.6% higher than the TG-21 value, indicating TG-21 (and TG-51) used incorrect values of Prepl for cylindrical chambers.
RESUMO
In a typical x-ray tube, off-focal radiation is mainly generated by the backscattered electrons that re-enter the anode outside the focal spot. In an earlier study, the EGSnrc/BEAMnrc system was modified to be able to properly transport the anode backscattered electrons, and to tally their subsequent generation of off-focal x-rays. In the current study, a diagnostic system and a recent digital mammography system are simulated using the modified BEAMnrc code, and the simulation results are compared with experimental measurements from the literature. Simulation results show excellent agreement with experimental measurements for the spectral shape of both the primary and the off-focal components, and also for the integral off-focal-to-primary ratio. The spectrum of the off-focal component at the patient plane is softer than the primary, which causes a slight softening in the overall spectrum. For a given configuration, the off-focal component increases with tube voltage because of the increased probability for off-focal x-rays to escape the anode self-filtration and the total filtration. This study validates our earlier implementation of off-focal radiation in EGSnrc/BEAMnrc, and provides a well-benchmarked tool that simulates x-ray tubes more realistically. The macro to add this feature to BEAMnrc is available from the authors.
RESUMO
The EGSnrc Monte Carlo code was evaluated for its ability to calculate the relative response of a variety of ion chambers to Co-60 beams as a means of justifying the use of this code in future investigations of cavity theory. EGSnrc calculations were compared to measurements with four separate ion chambers, which were each configured with several wall materials (ranging from plastic to lead) and cavity sizes (or cavity air pressures). The experimental results included measurements by Nilsson et al. in 1992, and experiments by Whyte, Attix et al. and Cormack and Johns in the mid-to-late 1950's designed to evaluate Spencer-Attix cavity theory. Experiments by Whyte involved measurements of the response per unit mass as a function of cavity air pressure for a large cylindrical chamber, whereas the other experiments consisted of measurements of the response per unit mass (or ionization current) as a function of the distance between the front and back wall (cavity height) of a plane-parallel chamber. EGSnrc calculations, which could account for the change in response associated with changes in wall material in most cases, were generally within 1-3% of experimental values, even for experimental data that required calculations of unreported wall corrections determined using experimental techniques. The ability of EGSnrc to accurately model these experiments, which showed variations up to 300%, confirms its suitability for detailed Monte Carlo studies of cavity theory.
