Biomedical subjects

S K Jani

Publications and source records attributed to S K Jani.

25 records · Page 2Linked to original sources

PubMedDate not supplied

Gamma vs beta irradiation: which is superior?

BACKGROUND: The debate over which radioisotope, gamma-emitting or beta-emitting, is ideal for vascular brachytherapy, is as old as the field itself. Gamma isotopes such as Ir-192 have been successfully used in large-scale clinical trials. However, radiation protection and safety remains a practical issue when high energy gamma rays are used in interventional suites. METHODS: In this report, we have evaluated dosimetric properties of currently available radiation sources, gamma and beta, in terms of their utility in vascular brachytherapy. Published data have been analyzed in comparing the depth doses of these two radiation modalities. RESULTS AND CONCLUSIONS: Our evaluation shows that significant dosimetric differences do exist between gamma and beta sources. The superiority of one over the other in clinical utility will more likely be determined by the outcome of human trials.

Beta Particles↗

PubMedDate not supplied

Evaluation of optically projected x-ray images from an elastomer-based ionographic system.

In ionographic imaging, the ions produced in a high-pressure, high-Z gas are normally collected on an insulating foil. If a deformable dielectric gel is used as a collecting electrode, then the electrostatic image can be visualized by optically projecting the gel deformation onto a screen. In the present work, we have evaluated the imaging performance of such an ionographic system in terms of its sensitivity and resolution capabilities. X-ray images of a sharp lead edge formed on a thin silicone elastomer were optically projected and recorded on x-ray films. The edge profiles were obtained at different values of x-ray exposure, elastomer thickness, gas pressure, and collecting voltage across the chamber. These profiles were processed to obtain the sensitivity and resolution of the system at different operating conditions. Images of some test objects are shown using this system in order to demonstrate its clinical potential.

Chlorofluorocarbons, Methane↗

PubMedDate not supplied

Dosimetric aspects of a rotating beam splitter used in tangential field breast treatment.

A rotating beam splitter was designed and fabricated for use in treating tangential breast fields on an AECL Theratron-80 cobalt teletherapy unit. Its dosimetric properties were studied using a 0.6-cm3 Baldwin-Farmer ionization chamber with Keithley electrometer and a Scanditronix RFA-3 three-dimensional water phantom scanner with semiconductor detector. An aluminum plate, which held the semicircular rotating 5-HVL (half-value layer) lead block, extended to the phantom surface (80-cm source-surface distance). The beam was blocked directly along the central axis and also at distances up to 7.5 mm off-axis, corresponding to the projected extent of the 1.5-cm-diam source. The penumbra at the central ray and at each off-axis point was measured at dmax and at 5-cm depth in water. A reduction in the penumbra from 8 to about 2 mm for 20 X 20 cm2 beam was observed regardless of the off-axis distance of the block. Isodose distributions obtained for various field sizes indicated that the percent depth doses of the split fields agree well with the equivalent squares of the irradiated field sizes. Output measurements in water and in air indicated that scatter from the aluminum plate more than compensates for the reduction in backscatter factor, due to the decrease in irradiated area when the beam splitter is used. Isodose curves in various planes were obtained at clinically useful rotational angles of the beam splitter. Computer generated isodose curves have been obtained that match the measured curves to be used in treatment planning.

Breast Neoplasms↗

PubMedDate not supplied

Leakage radiation from electron applicators on a medical accelerator.

The leakage characteristics of electron applicators on our Clinac 2500 linear accelerator have been measured. The leakage radiation in the patient plane and at the surface of the electron applicators has been measured for applicator sizes from 6 cm X 6 cm to 25 cm X 25 cm and beam energies from 6 to 22 MeV. For certain applicator/energy combinations the leakage radiation was significant. The leakage radiation, relative to the central axis dose, was found to be up to 7% in the patient plane and up to 39% at the applicator surface. Reducing the collimator setting or adding lead at select locations on the applicator surface was effective in reducing the magnitude of the radiation leakage.

Biophysical Phenomena↗

PubMedDate not supplied

Dose anisotropy around an Au-198 seed source.

For interstitial implants with radioactive Au-198 seeds, the dosimetric calculations usually ignore the finite source size and employ a point source approximation, resulting in a computed isotropic dose distribution. However, the measured radiation fluence from Au-198 seeds is reported to be anisotropic, suggesting some amount of dose anisotropy in tissue. We have measured this dose anisotropy around Au-198 seeds (2.5 X 0.8 mm2) using Kodak X-OMAT XV-2 films placed in contact with individual seeds in a phantom. Autoradiographs obtained for various exposure times were digitized and studied with an image analysis computer network. The network's overall spatial resolution was about 0.01 cm. The optical isodensity contours around the seed were obtained. The useful range of optical density (OD) for the system was found to be 1 to 2 OD units. Within this range, the shape of an optical isodensity contour would be identical to an isodose contour. Proximal to the source, the contours were elliptical in shape, elongated along the seed axis. However, further away, the elongation was in the direction normal to the seed axis. This was in agreement with the reported data on the radiation fluence around Au-198 seeds. It was concluded that measurable dose anisotropy existed around Au-198 seeds. However, it was too small to be of any clinical significance.

Brachytherapy↗

PubMedDate not supplied

Tissue compensators with use of vinyl lead sheets for head and neck portals on 4-MV x rays.

Tissue compensators made with a high atomic number material are compact and retain the skin-sparing effect of megavoltage beams. In this note, we have described the use of a commercially available vinyl lead as a material for tissue compensators for use with 4-MV beams on a Clinac-4/80. The dosimetric data obtained included the sheet's lead equivalence, thickness ratios, surface dose and buildup region, and transmission factors. The presence of a compensator did not alter the beam's skin-sparing effects. It was concluded that the vinyl lead sheets allowed an easy and rapid fabrication of a tissue compensator for head and neck portals of up to 15 X 15 cm.

Filtration↗

PubMedDate not supplied

Depth dose characteristics of 24-MV x-ray beams at extended SSD.

Depth dose characteristics of 24-MV beams from a medical linear accelerator at various SSDs have been measured. The dose buildup, percent depth dose (PDD), and output were measured using ionization chambers and a diode detector under full scatter. Surface dose ranged from 8% to 51% depending upon the collimator setting at 100-cm SSD. It decreased by 10%-15% as distance was increased to 200 cm, but remained unchanged beyond this distance. The dmax migrated rapidly toward the surface with increasing field size at 100-cm SSD. At extended SSD, the dmax occurred at greater depths and shifted a little toward the shallow depths with increasing field size. Large field PDDs measured at extended SSD agreed well with those predicted from 100-cm data. The output followed the 1/(distance)2 relationship quite well. The extended SSD beam data are clinically useful when treating hemibody or performing total body irradiation (TBI) procedures.

Humans↗