Biomedical subjects

M S Weinhous

Publications and source records attributed to M S Weinhous.

13 recordsLinked to original sources

PubMed1992

The selection of portal aperture using interactively displayed Beam's Eye Sections.

Portal apertures are often defined by wax-pencil lines drawn on simulator films. The simulator-film images suffer degradation due to X ray absorption and scatter in tissues proximal to and distal to the intended target volume. We describe a method for avoiding the degradation by using a 3-D dataset to produce high-contrast section images with a user-defined orientation. The calculation of these Beam's Eye Section (BES) images requires a set of medium-to high-resolution transverse section data (the 3-D dataset), as well as a medium- to high-performance computer workstation. These CT-like section images, providing more contrast and detail than a projection (that is, a simulator or Digitally Reconstructed Radiograph [DRR]) image, should allow for more accurate selection of portal apertures. The BES image plane is oriented perpendicular to the central axis of a user-selected beam. Once the user has created a beam, the system allows the user to step along that beam with a new BES image produced at each step. Contours manually drawn on these BES images are ultimately projected onto a DRR image. The outermost segments of these sometimes crisscrossing contours (the union of all projected areas) accurately defines the portal aperture needed to encompass the target at all levels.

Computer Simulation↗

PubMed1991

Study of treatment variation in the radiotherapy of head and neck tumors using a fiber-optic on-line radiotherapy imaging system.

On-line radiotherapy imaging systems allow convenient daily acquisition of portal images for treatment verification. The information can also be used to study treatment variability. Using a prototype fiber-optic imaging system, we have measured the treatment variation of 17 head and neck patients. Daily digital portal images were acquired for the on-cord left and right lateral fields. Treatment variations were quantified using the Cumulative Verification Image Analysis (CVIA) method developed at our institute. In the CVIA method, daily portal images were aligned according to three anatomical points predefined on a digitized simulation, or prescription, image. After each image alignment, the block position was cumulated in a bit-map and superimposed on the prescription image to give a cumulative verification summary image. Iso-frequency distributions, or contours, of the block overlap were calculated and examined with respect to the prescription treatment area. The range of the treatment variation was large for the 17 patients. On average, separation of the 0% to 100% block overlap contours was about 10 mm, and the 20% to 80%, 5 mm. The block overlap contours were also used to calculate the frequency with which the prescription area as defined on the simulation film had been treated. The fraction of the prescription area treated depended on the accuracy of the treatment setup and patient repositioning, as expected. At best, approximately 95% of the prescribed area was irradiated 100% of the time during the entire course of radiotherapy. At worst, approximately 70% of the prescribed area was irradiated 100% of the time. These results demonstrate that despite immobilization, large setup variation can still occur. Presenting treatment variation data as population averages does not reflect on the large variation that may be observed in the individual patient.

Adolescent↗

PubMed1990

Treatment verification using a computer workstation.

The outcome of radiation therapy is clearly dependent upon the accuracy with which dose is delivered to the target volume. With the ever increasing use of oblique and even non-coplaner beams, verification issues have become more significant. Presently, radiation oncologists verify the accuracy with which treatments are delivered by comparing a reference ("prescription," usually simulator) on-film image to a portal on-film image on a lightbox. This process is fraught with difficulties as the images are usually rendered with different magnifications, orientations, and contrast. Using a computer workstation, a system has been created that allows a physician to enhance, register, and transparently overlay the portal image on the prescription image. As the prescription and portal images are rendered in shades-of-yellow and shades-of-gray, respectively, a physician can interactively adjust the images and can easily detect set-up errors and/or beam placement errors. Thus greater treatment accuracy should be achievable, especially for unfamiliar beam orientations.

Humans↗

PubMed1986

Collection efficiency of an ionisation chamber in a pulsed swept beam: chamber size effects.

Boag's theory for the collection efficiency of a small ionisation chamber in a pulsed swept beam is generalised by taking chamber size into account. The collection efficiency is given in terms of the chamber radius, the Gaussian scale constant of the stationary beam, and the maximum distance between beam and chamber centres. It is shown that, for cases of practical interest, collection efficiency is independent of chamber size.

Humans↗

PubMed1986

Collection efficiency of an ionisation chamber in a pulsed swept beam: collimator scatter effects.

The expressions for calculating the collection efficiency of an ionisation chamber in a pulsed swept beam, as derived by Boag, explicitly assume that the stationary beam has a Gaussian radial intensity distribution and implicitly assumes that this distribution, upon being swept, is unperturbed by extra-phantom scatter. Consequently a hyperbolic pulse-size distribution is expected on the central axis. Measurements of pulse-size distributions at the isocentre of a Sagittaire accelerator for small collimator settings yield hyperbolic distributions in accord with this model. However, measurements of large-field pulse-size distributions yield markedly non-hyperbolic distributions for electron energies from 7 to 32 MeV. It is shown that application of the model in such cases might result in significant errors depending on the inherent collection efficiency of the chamber.

Electrons↗

PubMed1985

Convection currents in a water calorimeter.

A flexible, temperature-regulated water calorimeter has been constructed containing two pairs of thermistor sensors at depths of 6.23 and 10.0 cm. It may be irradiated by vertical or horizontal beams, and operated at temperatures in the range from 3 to 40 degrees C. When irradiated at 30 degrees C with a vertically downward 19 MeV electron beam, the responses of the proximal and midline thermistors were in accordance with the depth-dose curve. When irradiated horizontally, the initial patterns of temperature rise were the same, but after about 30 s (4 Gy) the rate of temperature rise decreased at the proximal thermistors and increased at the midline thermistors. Shortly after irradiation, the temperature curve and increased at the midline thermistors. Shortly after irradiation, the temperature curve of the midline thermistors crossed that for the proximal thermistors, a pattern that suggested the presence of convection currents. To test this hypothesis, the calorimeter was operated at 4 degrees C. The temperature patterns for horizontal irradiation became the same as those obtained with vertical beams, thus demonstrating the production of convection currents in water at a temperature of 30 degrees C for temperature gradients as small as 10(-3) degrees C cm-1.

Calorimetry↗

PubMedDate not supplied

An improved electron energy-loss straggling algorithm for Monte Carlo transport codes.

The commonly used Blunck and Leisegang electron energy-loss distribution falls off too rapidly with increasing energy loss. Also, for large thicknesses and/or low-Z media, where their distribution should approach Landau's, it normalizes to 0.92 rather than 1.0, it overestimates the number of very small energy-loss events, and its peak is shifted from lambda = -0.225 to 0.1. Because of these shortcomings, calculations made using this distribution yield a mean straggled energy loss which is lower than the value predicted by the continuous slowing down approximation (CSDA). An improved version of the Blunck-Leisegang distribution, which exhibits better normalization and falloff, has been developed. Further, an algorithm was created which (depending on the CSDA energy loss, Z,A, electron energy, and transport step size) samples the electron's straggled energy loss from the more accurate of the available distribution functions.

Electrons↗

PubMedDate not supplied

Determining Pion, the correction factor for recombination losses in an ionization chamber.

The 1983 AAPM protocol for the determination of absorbed dose from high-energy photon and electron beams recommends using Pion (the reciprocal of collection efficiency), as determined by the two-voltage technique, to correct for recombination losses in ionization chambers. Methods and data for the determination of ionization chamber collection efficiencies are scattered throughout the literature. The present work consolidates the available information, rectifies certain omissions, and provides several convenient and readily implemented methods for determining Pion. Computer programs, quadratic approximations, and data tables are presented to facilitate the determination of Pion for continuous, pulsed, and pulsed-swept beams.

Radiometry↗

PubMedDate not supplied

Enhancement of electron beam dose distributions by longitudinal magnetic fields: Monte Carlo simulations and magnet system optimization.

A Monte Carlo electron-photon transport code was developed in order to determine the effects of static, longitudinal, magnetic fields on dose distributions produced by high-energy electron beams, and to optimize the design of a superconducting magnet system. As a result of these simulations, a 20-cm-i.d., 30-cm-o.d., 15-cm-tall, single-coil, magnet system was designed that could be incorporated into a mobile treatment table for use with a standard radiation therapy accelerator. Operating at a current density of 18 kA/cm2, the magnet would produce field strengths of 1-4 T in the phantom and 0.01 T at the accelerator exit window. Magnetically enhanced dose distributions, calculated for 20- and 30-MeV electron beams, show a pronounced Bragg peak, steeper gradients to the sides and rear, and a roughly fourfold increase in the peak dose to entrance dose ratios relative to those similarly calculated without a magnetic field. These magnetically enhanced dose distributions have the potential for sparing intervening tissue when high-energy electrons are used for the treatment of deep-seated tumors.

Electrons↗

PubMedDate not supplied

Calorimetric determination of the cavity-gas calibration factor Ngas.

In the recently published AAPM protocol for the dosimetry of high-energy photons and electrons, the response of an ionization chamber is defined as the dose to the gas in the chamber per unit electrometer reading, Ngas. Using a graphite calorimeter, Ngas has been determined for a Farmer-type ionization chamber using 4- and 25-MV x rays. The procedure was to measure the dose to graphite using the calorimeter, and then obtain the response of the chamber at the same depth in a graphite phantom. Equation (9) of the AAPM protocol was then used to calculate Ngas. The values of Ngas determined with the calorimeter are within 1% of Ngas calculated according to the AAPM protocol, using the 60Co exposure-calibration factor.

Calorimetry↗

PubMedDate not supplied

The direct determination of dose-to-water using a water calorimeter.

A flexible, temperature-regulated, water calorimeter has been constructed which consists of three nested cylinders. The innermost "core" is a 10 X 10 cm right cylinder made of glass, the contents of which are isolated from the environment. It has two Teflon-washered glass valves for filling, and two thermistors are supported at the center by glass capillary tubes. Surrounding the core is a "jacket" that provides approximately 2 cm of air insulation between the core and the "shield." The shield surrounds the jacket with a 2.5-cm layer of temperature-regulated water flowing at 51/min. The core is filled with highly purified water the gas content of which is established prior to filling. Convection currents, which may be induced by dose gradients or thermistor power dissipation, are eliminated by operating the calorimeter at 4 degrees C. Depending upon the power level of the thermistors, 15-200 microW, and the insulation provided by the glass capillary tubing, the temperature of the thermistors is higher than that of the surrounding water. To minimize potential errors caused by differences between calibration curves obtained at finite power levels, the zero-power-level calibration curve obtained by extrapolation is employed. Also the calorimeter response is corrected for the change in power level, and therefore thermistor temperature, that follows the resistance change caused by irradiation. The response of the calorimeter to 4-MV x rays has been compared to that of an ionization chamber irradiated in an identical geometry.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetry↗

PubMedDate not supplied

The variability of clinical thermoluminescent dosimetry systems: a multi-institutional study.

Thirty-two radiotherapy centers in the USA and Canada cooperated in a study of the variability of clinical thermoluminescent dosimetry (TLD) systems. The primary purpose of the survey was to ascertain the accuracy of TLD for the determination of in vivo dose measurements. Each participating institution provided two TLD packets for irradiation on a Clinac 4, at a prearranged time. Two batch irradiations were made. Thirty-two TLD packets, one from each institution, were uniformly irradiated to a dose of 22.35 cGy (known by us, but not by the participants). A second group of 32 packets were likewise irradiated to a dose of 179.0 cGy. Participants were told only that their TLD's would be irradiated to doses between 10 and 50 cGy, and 100 to 200 cGy. TLD's were then returned to the institutions of origin for readout, and the doses reported to us for analysis. Calibration factors, readout and annealing procedures, etc., were all established independently by each participant. Although these procedures varied widely between institutions, the mean values of the reported doses were within 5% and 3% of the expected values for the low and high doses, respectively. Standard deviations in the reported doses were 10% and 5%. Also of interest, however, is the finding that 22% (i.e., 14 out of 64) of the dose reportings were in error by more than 10%. The implications of these findings vis à vis radiotherapy are discussed.

Evaluation Studies as Topic↗

PubMedDate not supplied

Testing of a medical linear accelerator's computer-control system.

In August of 1987, the Radiation Oncology Center at the Mallinckrodt Institute of Radiology became the first academic-medical-center user of Varian's computer-controlled therapy accelerator, a prototype version of the 2100C. Installation was accomplished by the retrofit of a computer system to our two-year-old Clinac 1800. Being well aware of the malfunctions that occurred in other computer-controlled accelerators, we took extraordinary measures to assure proper operation of the new, computerized, system. Our acceptance procedure included tests of (i) mechanical systems (isocentricity, digital readouts, etc.); (ii) radiation parameters (flatness, symmetry, output, etc.); (iii) manual safety systems (emergency off switches, etc.); (iv) the computer console systems (communications integrity, state integrity, etc.); and (v) the interlock systems (some 50 electrical, mechanical, and/or computer-controlled interlocks). As items (i), (ii), and (iii) do not differ significantly from the tests for a non-computer-controlled machine, they will not be discussed here. Rather, this report will concentrate on the methods that were devised to test the computer-control and interlock systems.

Computer Systems↗