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Biomedical subjects

G S Ibbott

Publications and source records attributed to G S Ibbott.

7 recordsLinked to original sources
PubMed1990

Preoperative versus postoperative irradiation in the prophylaxis of heterotopic bone formation in rats.

Irradiation treatment, commenced within 1-5 days post-surgery, reliably prophylaxes heterotopic bone formation but is painful and impairs desirable postop immobilization. To compare pre- versus post-op radiation, we bilaterally implanted bone matrix pellets into the thighs of 111 30-day-old Long Evans rats. Rats were randomized to time of radiation initiation (2 days pre-op, 1 hr pre-op, or 2 days post-op) and dose (300, 800, 1800, 2400, or 3000 cGy in 1 fraction). Pellets were removed on post-op day 16 or 48 and evaluated histologically and radiologically. Histologic analysis showed dose-related suppression in bone formation, that is, 40%, 27%, 6.8%, 2.5%, 6.4%, and 0.0% bone formed among sites receiving 0, 300, 800, 1800, 2400, and 3000 cGy, respectively. The difference in bone formation between control and irradiated implant sites was significant at every dose level in all treatment groups (p less than or equal to .03). There was no statistically significant difference in overall bone formation between post-op (7.1%) and 1 hr pre-op (5.3%) groups, whereas 2 day pre-op rats formed significantly more bone (12.6%). Stratified by dose, however, there were no significant differences between treatment groups except at 800 cGy. At this dose, 2 day pre-op rats formed more bone (10.6%) than 1 hr pre-op (6.6%) or post-op (3.3%) groups. Results suggest that a) radiation given shortly prior to a stimulus inducing proliferation among multipotential cells may inhibit subsequent proliferation and/or differentiation, and b) clinical formation of heterotopic bone may be preventable via modest doses of irradiation delivered shortly prior to surgery.

Animals↗
PubMed1988

Hip prostheses during pelvic irradiation: effects and corrections.

Treatment of pelvic malignancies frequently includes the use of lateral, arc, or rotational fields. The presence of hip prostheses in these treatment fields will perturb the dose distribution. Correction factors for metal-based alloys used in artificial hips have not previously been reported. Prostheses constructed from frequently used alloys were obtained and measurements were made of the transmission of 4MV and 10MV photons. These measured data were compared with computed correction factors. The computer uses the "ratio of tissue-maximum ratios (TMR's)" method of heterogeneity correction. The computer was provided with both the physical density and the relative electron density of each prosthesis for comparison purposes, since electron densities for hip prostheses are not widely known. Correction factors determined from electron densities demonstrated better agreement with measured data. The "ratio of TMR's" correction algorithm does not consider the contribution of scattered radiation in the dose computations. Consequently, a small adjustment to the relative electron density of the prosthetic hip was required at lower X ray beam energies. Agreement was satisfactory for higher energy X rays, and thus no adjustment was necessary. Relative electron densities and adjusted electron densities for alloys used in artificial hips are provided for computer-aided treatment planning. Recommendations for incorporating the hip prosthesis into the treatment planning process are also provided.

Electrons↗
PubMed1975

Cross-sectional anatomic images by gamma ray transmission scanning.

60Co gamma-ray transmission data were measured along linear scan paths at a number of angular orientations with respect to the patient and submitted to a computer software program. Reconstructed images are displayed as digital density printouts and as isodensity contours on an x-y plotter or oscilloscope screen. Image resolution is limited primarily by factors such as collimation and the amount of transmission data collected; with the rather rudimentary apparatus at disposal, a spatial resolution better than 5 mm has been achieved.

Computers↗
PubMedDate not supplied

Stem corrections for ionization chambers.

Ionization chambers often exhibit a stem effect, caused by interactions of radiation with air near the chamber end, or with dielectric in the chamber stem or cable. These interactions contribute to the apparent measured exposure. To determine the stem efffect for several common ionization chamber systems, exposures were measured with TLD capsules placed at the center of 60Co fields of various sizes. These exposure measurements then were repeated with various ionization chamber systems, including two Victoreen R meters (25- and 100-R chambers), a Capintec 192 dosimeter with a Farmer 0.6-cm3 probe, a PTW transit dose probe, and an EG and G IC-18 probe with a Keithley 610-B electrometer. From a comparison of TLD and ionization chamber measurements of the variation in exposure rate with field size, stem corrections for the different systems were determined within 1%.

Cobalt Radioisotopes↗
PubMedDate not supplied

Computational analysis and dosimetric evaluation of a commercial irregular-fields computer program.

The proper evaluation of the accuracy of a new computer program for radiation-therapy dosimetry requires consideration of both the mathematical algorithm used in the program and the performance required in clinical applications. As an example, our evaluation of the irregular-fields dosimetry program currently marketed by SHM for their Rad-8 system is described. The evaluation begins with an explanation of the mathematical computation described. The evaluation begins with an explanation of the mathematical computation method, with emphasis on the points where the calculation differs from previous methods. Next, the procedure for setting up the beam data file is discussed. Finally, a step-by-step procedure is described in which calculated doses are compared with measured doses, using a Varian Clinac-4 with lead flattening filter, and the limits within which a +/- 5% accuracy is attainable are estimated. Some sources of error and areas for possible improvements are mentioned.

Computers↗
PubMedDate not supplied

Computerized patient contours using the scanning arm of compound B-scanner.

Full utilization of the precision of newer radiation therapy devices requires patient contours drawn with greater accuracy than is possible with the conventional lead wire technique. Polaroid photographs can introduce large errors due to distortion and small image size. Techniques including electromechanical or optical devices and CT scans offer improved accuracy, but often at added expense. A method for obtaining contours has been developed which utilizes a treatment planning minicomputer (equipped with an analog-to-digital converter and plotter) and a commercially available ultrasound B-scanning arm. Voltages corresponding to the X-Y position of the tip of the scanning arm are fed from the scanner to the A/D interface, smoothed, scaled, and plotted. The resulting drawing is a full scale external patient contour. The accuracy of this method is compared to alternative techniques.

Analog-Digital Conversion↗