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Biomedical subjects

J W Andrew

Publications and source records attributed to J W Andrew.

16 recordsLinked to original sources
PubMed1992

The surface doses from orthovoltage x-ray treatments.

Measurements have been made of the surface dose with treatment cones and lead shielding used with orthovoltage x-ray systems. Both devices significantly increase the surface dose, probably by the generation of photoelectrons. The surface dose depends upon the shape and material of the cone and the x-ray energy. The surface dose can be 500% higher than the dose measured with a Baldwin-Farmer chamber. The significance of these doses is discussed in the light of recent information on the thickness of the epithelium. It is suggested that every effort be made to reduce the surface dose by using low atomic number material between the lead and the patient surface.

Humans↗
PubMed1990

The use of digitized port films in assessing new treatment modalities.

An inexpensive computer-based imaging system has been used to compare simulator and portal radiographs. The films were digitized, normalized, and then overlaid for measurement. In a test group of patients, a new treatment protocol for pelvis irradiation was evaluated using this technique. For 90% of the patients, the average error was found to be smaller than 5 mm, and there were no field placement errors greater than 1 cm.

Female↗
PubMed1984

Homogeneous irradiation of the "short-necked" laryngeal cancer patient.

A technique for homogeneous irradiation of the "short-necked" laryngeal cancer patient is presented. The method is similar to a previously described technique in that inferiorly angled opposed lateral beams are used with tissue compensators and beam wedges. The advantages of the technique presented here are that the patient is treated supine rather than sitting and therapy simulation is more easily carried out. Experimental verification of the calculated radiation distributions was carried out in a water phantom having the same shape as the patient. These results show the extent of dose homogeneity and in addition show that neglecting tissue inhomogeneity, the measured and calculated dose distributions agree within 2%.

Glottis↗
PubMed1981

Measurement of x-ray exposures using mailed thermoluminescent dosimeters.

Thermoluminescent dosimeters (TLD) have been sent by mail to all hospitals in Nova Scotia to survey the radiographic and fluoroscopic x-ray exposures to patients, For each radiographic system a TLD was placed in the x-ray field during a routine abdominal radiographic procedure on an average size patient (20 cm AP). The TLD was not visible on the film and in no way effected the examination. Fluoroscopic exposure rates were measured using TLD and a 20 cm water phantom. The dosimeter was exposed for one minute using technique factors for an abdomen of average size. The average abdominal exposure to a 20 cm thick patient on 137 different radiographic units was .74 R. The average fluoroscopic exposure rat to a 20 cm water phantom using 54 different fluoroscopic systems was 5.4 R/min.

Fluoroscopy↗
PubMed1977

Signal to noise enhancement in dark field electron micrographs of vasopressin: filtering of arrays of images in reciprocal space.

A technique for filtering arrays of images of dispersed molecules is presented which takes advantage of the crystallographic properties of regular arrays and the rapidity of optical treatment. A filter in reciprocal space consisting of perforations in a square or rectangular lattice, as determined by the image arrrangement, reduces noise and averages the images simultaneously by transmitting only regularly recurring image structure. The filter is universal for all image contents and introduces no additional biases. The signal to noise improvement approaches square root N for N independent images of the same molecule in the same configuration. The procedure which is simple, rapid and inexpensive, is demonstrated with the aid of dark field electron micrographs of the protein vasopressin and combined with iodination of that molecule to elucidate its structure.

Crystallography↗
PubMedDate not supplied

Effect of small inhomogeneities on dose in a cobalt-60 beam.

The effect of small ring-shaped cavities on the dose delivered by a cobalt-60 beam to a homogeneous medium was studied experimentally and theoretically. Changes as small as 1 x 10(-4) of the total dose were measured. Experimental results show that, depending on the position of the cavity, replacing water with a small cavity can either increase or decrease scatter dose to a point in the medium. The increase in scatter dose was not anticipated and to our knowledge is not predicted by any of the presently available inhomogeneity dose correction algorithms used in treatment planning. Calculations were based on the perturbations of first and second scatter dose contributions and show that the presence of a cavity in the medium introduces three processes that decrease scatter dose and five that increase it. The calculated net effect is in good agreement with experiment. Additional calculations show that the effect of a single small inhomogeneity cannot be easily extrapolated to larger inhomogeneities and that multiple inhomogeneities do not act independently. Results from this study demonstrate two constraints that must be satisfied by future dose calculation algorithms: (i) they must correctly determine dose in a homogeneous non-unit density material, and (ii) they must account for the change in dose due to small inhomogeneities in the medium.

Cobalt Radioisotopes↗
PubMedDate not supplied

A microcomputer-based system for radiotherapy beam compensator design and patient contour plotting.

A microcomputer-based system for radiotherapy beam compensator design has been constructed. A patient surface measuring device which projects a grid pattern on the patient is attached to the collimator of a therapy treatment simulator. A linear motion potentiometer, pivoted at one end on a ball joint attached to the collimator, is extended to touch each grid point in turn. The extension of the potentiometer is monitored by a microcomputer which calculates the coordinates of the patient's surface as each grid point is entered. A template for compensator construction is subsequently printed on the microcomputer printer. The patient surface profile along any grid line can also be plotted and used in conjunction with the compensator data to produce an isodose distribution for the compensated field.

Computers↗
PubMedDate not supplied

Characteristics of the photon beam from a new 25-MV linear accelerator.

The Therac 25 is a relatively compact therapy machine, the heart of which is a double-pass electron linear accelerator. The electron beam is injected into the accelerator at the treatment head end of the machine and is accelerated back down the arm to an energy of 13 MeV. At this end of the machine a magnet system reflects the beam back into the structure where it gains up to an additional 12 MeV of energy. After leaving the linear accelerator the beam is bent by an achromatic head magnet through 270 degrees to the treatment head. The machine produces eight electron beams and a 25-MV photon beam. In this work only the parameters of the photon beam are addressed based on measurements at the first two clinical sites. Percentage depth doses, tissue phantom ratios, and beam symmetry and stability are presented and discussed.

Humans↗
PubMedDate not supplied

Characteristics of electron beams from a new 25-MeV linear accelerator.

Clinically useful electron fields are produced on the Atomic Energy of Canada, Limited Therac 25 linear accelerator by computer-controlled scanning of the electron beam. Measurements were made to determine the properties of these electron fields. Central axis percentage depth dose and bremsstrahlung background were compared for these fields and for the fields from selected machines that use scattering foils. Dose calibrations were made in both water and polystyrene using the American Association of Physicists in Medicine Task Group 21 protocol. Measurements were made to determine the relative output factors, virtual source position, and the attenuation of the electron fields by lead.

Electrons↗
PubMedDate not supplied

Radiation protection aspects of a new high-energy linear accelerator.

The Therac-25 is a new 25-MeV linear accelerator manufactured by Atomic Energy of Canada, Ltd. The first two units have recently been installed in Toronto, Ontario and Halifax, Nova Scotia. Calculations and measurements of primary and secondary radiation levels were made. Neutron dose-equivalent rates were measured inside and outside the room. The maximum leakage rate at 1 m from the accelerator target was 0.4% Sv per peak photon Gy. The tenth value layer for neutrons from the Therac-25, at the entrance to a one-legged maze was found to be 5.5 cm of polyethylene. Measurements were done to estimate daily technologist exposure due to induced activity in the treatment room.

Electrons↗
PubMedDate not supplied

A video-based patient contour acquisition system for the design of radiotherapy compensators.

Recent advances in computer and video technology have enabled significant advances in the field of surface topography measurement. This paper will describe a digital video patient contour acquisition system for the design of radiotherapy beam compensators. The system is mounted on a tray which slides into the collimator of a radiotherapy simulator. The tray holds a source which projects a 1-mm wide fan beam of light along the simulator beam axis onto the patient. The tray also holds a small solid state video camera which views the projected line of light on the patient surface. Video images are digitized and processed on an 80386 based IBM PC-compatible computer. Transverse contours are obtained from one digitized video image or, in case of opposed fields, from two or more image frames taken at different gantry angles. Entire surfaces are measured by scanning the bed longitudinally and acquiring a number of transverse contours. Posterior surfaces are obtained by scanning a mold. A printer produces a template for making the compensator from sheet lead.

Algorithms↗
PubMedDate not supplied

Optimal design of radiation compensators.

A method to achieve the optimal design of radiation therapy compensating filters in multibeam situations is proposed. Dose distribution is optimized such that it approaches the prescribed value in the target zone, is minimized everywhere else, and does not exceed designated limits at vulnerable regions. Field shapes, sizes, and weights, together with filter thicknesses that take into account missing tissues and body heterogeneities, are calculated as a result of this process. The dose distribution, after the application of the calculated filters and beams, is also predicted. The method is rigorous, and its accuracy depends only upon the accuracy of the applied dose calculation method. Examples are discussed and verified using data obtained from Theraplan 300L treatment planning system.

Equipment Design↗
PubMedDate not supplied

Self-adhesive lead for compensator production and radiation shielding.

Sheet lead is a commonly used material for the manufacture of compensators and for radiation shielding. However, it suffers from such problems as nonuniformity of thickness, difficulties in accurate cutting and alignment, and toxicity. A double-sided adhesive lead sheet has been developed which overcomes these problems.

Adhesives↗