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Biomedical subjects

B L Werner

Publications and source records attributed to B L Werner.

At least 19 recordsLinked to original sources
PubMed2001

The biological functions of glutathione revisited in arabidopsis transgenic plants with altered glutathione levels.

A functional analysis of the role of glutathione in protecting plants from environmental stress was undertaken by studying Arabidopsis that had been genetically modified to have altered glutathione levels. The steady-state glutathione concentration in Arabidopsis plants was modified by expressing the cDNA for gamma-glutamyl-cysteine synthetase (GSH1) in both the sense and antisense orientation. The resulting plants had glutathione levels that ranged between 3% and 200% of the level in wild-type plants. Arabidopsis plants with low glutathione levels were hypersensitive to Cd due to the limited capacity of these plants to make phytochelatins. Plants with the lowest levels of reduced glutathione (10% of wild type) were sensitive to as little as 5 microM Cd, whereas those with 50% wild-type levels required higher Cd concentrations to inhibit growth. Elevating glutathione levels did not increase metal resistance. It is interesting that the plants with low glutathione levels were also less able to accumulate anthocyanins supporting a role for glutathione S-transferases for anthocyanin formation or for the vacuolar localization and therefore accumulation of these compounds. Plants with less than 5% of wild-type glutathione levels were smaller and more sensitive to environmental stress but otherwise grew normally.

Anthocyanins↗
PubMed1998

The FE-lspd model for electron beam dosimetry.

The FE-lspd model is a two-component electron beam model that distinguishes between electrons that can be described by small-angle transport theory and electrons that are too widely scattered for small-angle transport theory to be applicable. The two components are called the primary beam and the laterally scattered primary distribution (lspd). The primary beam component incorporates a simple version of the Fermi-Eyges model and dominates dose calculations at therapeutic depths. The lspd component corrects erros in the lateral spreading of the primary beam component, thereby improving the accuracy by which the FE-lspd model calculates dose distribution in blocked fields. Comparisons were made between dose profiles and central-axis depth dose distributions in small fields calculated by the FE-lspd, Fermi-Eyges and EGS4 Monte Carlo models for a 10 MeV beam in a homogeneous water phantom. The maximum difference between the dose calculated using the FE-lspd model and EGS4 Monte Carlo is about 6% at a field diameter of about 1 cm, and less than 2% for field sizes greater than 3 cm diameter. The maximum difference between the Fermi-Eyges and Monte Carlo calculations is about 18% at a field diameter of about 2.5 cm. A comparison was made with the central-axis depth dose distribution measured in water for a 3 cm diameter field in a 10 MeV clinical electron beam. The errors in the dose distribution were found to be less than 2% using the FE-lspd model but almost 18% using the Fermi-Eyges model. A comparison was also made with pencil beam profiles calculated using the second-order Fermi-Eyges transport model.

Calibration↗
PubMed1997

Choosing $FUDGEMS in EGS4 Monte Carlo.

Li and Rogers describe a small but significant problems in the EGS4 Monte Carlo program. EGS4 normally overestimates the electron scattering power by redundantly taking atomic orbital electron scattering into account in two different ways. This paper shows that the two scattering models EGS4 uses can be combined consistently by setting the parameter $FUDGEMS to a fractional value. For 10 MeV electron beams in water, inconsistent use of the scattering power can result in differences of about +/-1.5% in the broad beam maximum dose.

Electrons↗
PubMed1992

Photon beam dosimetry at a blocked beam edge using diffusion approximation.

A simple analytical model is presented for the transport of secondary electrons at a photon beam edge using the energy averaged solution of the Boltzmann equation, originally developed for beta-ray dosimetry at a plane interface. Dose at a point under a block is assumed to be due to secondary electrons and the scattered photons generated from the primary photon beam. The diffusion approximation is used for the secondary electron transport at a virtual plane interface created by the block. The dose from the scattered photon component is treated as decaying exponentially with distance from the beam edge. Comparisons made with the model and measurements are in general agreement for high energy accelerator beams.

Diffusion↗
PubMed1991

Border separation for adjacent orthogonal fields.

Field border separations for adjacent orthogonal fields can be calculated geometrically, given the validity of some important assumptions such as beam alignment and field uniformity. Thermoluminescent dosimetry (TLD) measurements were used to investigate dose uniformity across field junctions as a function of field separation and, in particular, to review the CCSG recommendation for the treatment of medulloblastoma with separate head and spine fields.

Cerebellar Neoplasms↗
PubMed1983

Comparison of broad beam central axis depth dose curves from different accelerators using the universal depth dose curve model.

Electron beam central axis depth dose distribution can be transformed to fluence distributions with geometric depth replaced by a measure of the state of angular dispersion of the beam. A transformed central axis depth dose distribution was called a 'fluence curve.' The transformation was applied to a set of central axis depth dose curves calculated by Monte Carlo code for broad electron beams of energies ranging from 1 to 60 MeV in homogeneous phantoms of water, aluminium, and copper. For the energies and compositions likely to be encountered in external beam radiation therapy, the resulting fluence curves, were found to belong to a single parameter family. A collimator scatter parameter was introduced to take into account the initial angular dispersion produced by the collimator of an accelerator. Given the energy of the beam, the medium in which the beam is passing through, and the collimator scatter parameter, the fluence curve associated with the beam can easily be transformed back to a calculated depth dose curve. The collimator scatter parameter necessary to fit the depth dose curves measured on different accelerators was investigated. The results for the Clinac 18, LMR-13, Mevatron XII, Mevatron 80, Microtron, Sagittaire, Siemens Betatron, and the Therac 20 are presented.

Computers↗
PubMedDate not supplied

Lead shielding for electrons.

Using a 13 MeV electron beam as an example, transmission curves for various thicknesses of lead were measured. The data indicated that the choice of shielding thickness depends greatly on the depth at which the measurements are made. The importance of this reference depth and criteria for shield design when shields of minimum thickness are required is discussed.

Electrons↗
PubMedDate not supplied

A model for calculating electron beam scattering in treatment planning.

The Fermi-Eyges theory of electron scattering overestimates the scattering of electron beams used in radiation therapy. The reason for this overestimate is the neglect of the loss of electrons which are scattered into highly oblique paths and removed from the beam at relatively shallow depths. A modification of Eyges' solution to Fermi's equation is presented to take this loss of electrons into account. Equations for the calculation of isodose distributions for any medium using pencil beams are developed. Experimental confirmation is presented for electron beams of 13 and 18 MeV in homogeneous water, polystyrene, Lucite, and aluminum phantoms.

Electrons↗
PubMedDate not supplied

Model for calculating depth dose distributions for broad electron beams.

Central axis electron beam depth dose distributions can be transformed by replacing dose by fluence and depth by a measure of angular dispersion. This transformation was applied to a set of broad beam central axis depth dose distributions calculated by Monte Carlo code for beams with initial energies ranging from 1 to 60 MeV in homogeneous media of water, aluminum, and copper. The resulting fluence distributions belong to a family of curves that can be parametrized by a single-valued function of initial beam energy and medium and can be used to calculate fluence distributions for accelerators. Fluence curves can be easily transformed to depth dose curves.

Models, Theoretical↗
PubMedDate not supplied

The perturbation of electron beam dose distributions at medium interfaces.

The perturbation of electron beam dose distributions in the vicinity of medium interfaces is calculated. Two different solutions to the Boltzmann equation, one an energy averaged solution and the other a diffusion approximation solution, are presented for the calculation of the electron distribution in the vicinity of interfaces. An extension of the energy averaged formalism is presented for media with high atomic number. A new effective depth approximation is introduced to calculate the beam conditions beneath the interface. The transformation from electron distribution to dose distribution is made and dose calculations are compared with published electron beam dose measurements.

Diffusion↗
PubMedDate not supplied

The production of secondary electrons in an electron beam.

Convenient methods for calculating the ratio of restricted to unrestricted collision stopping power in water over a wide range of initial and cutoff energies, the rate of production of secondary electrons, and the primary electron dose distribution in an electron beam, are presented.

Electrons↗
PubMedDate not supplied

Dose perturbations at interfaces in photon beams.

A model based on an approximation called the partial fluence approximation is presented for the calculation of dose distributions in the vicinity of medium interfaces in photon beams. The predictions of the model are compared with dose distributions measured in layered phantoms consisting of aluminum and polystyrene, for photon beams ranging in energy from 60Co to 24 MV.

Humans↗
PubMedDate not supplied

Dose distributions in regions containing beta sources: large spherical source regions in a homogeneous medium.

The energy averaged Boltzmann equation model is applied to the determination of dose distributions in infinite, homogeneous media with uniform, monoenergetic, isotropic source distributions in spherical regions of radius larger than the electron range. The generalization to the case of spherically symmetric source distributions is made. Comparisons with dose distributions calculated by the integration of dose point kernels derived from Monte Carlo calculations are presented.

Beta Particles↗
PubMedDate not supplied

Dose perturbations at interfaces in photon beams: secondary electron transport.

An improved, quantitative version of the partial fluence model [Med. Phys. 14, 585 (1987)] for the calculation of dose perturbations at media interfaces in photon beams is presented and compared with measurements made at interfaces between polystyrene and materials ranging in atomic number from aluminum to lead, for photon beams ranging in energy from 60Co to 24 MV.

Electrons↗