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Biomedical subjects

S C Prasad

Publications and source records attributed to S C Prasad.

44 records · Page 3Linked to original sources
PubMedDate not supplied

Buildup factors and dose around a 137Cs source in the presence of inhomogeneities.

The effect of inhomogeneities on dose near a gamma-ray source has been investigated. Experimental measurements were made with a 137Cs source in a polystyrene phantom at a distance of 5 cm from the source. Inhomogeneities consisted of 2-cm-thick slabs interposed between the source and the plane of measurement. Dose correction factors (DCF) for 2-cm-thick aluminum, bone, lung, and air have been determined. Our results show that for 137Cs source the dose correction factors are of the order of 3% to 8%. A theoretical model for calculation of dose correction factor in brachytherapy in the presence of inhomogeneities has been developed. The model calculates DCF using buildup factors for the water-equivalent path between the source and the point of calculation. Good agreement was found between calculations and experimental measurements.

Brachytherapy↗
PubMedDate not supplied

Calculation of dose in off-central axis planes using off-axis ratios.

Accuracy of dose calculation for regular fields in off-central axis planes was investigated on a RAD-8 treatment planning computer for 4- and 10-MV x-ray beams produced by Varian Clinac -4 and Clinac -18 linear accelerators. These calculations, which are based on central axis depth dose and off-axis ratios in the principal planes, can be in error by as much as 25%-30% at locations well within the irradiated volume for the 4-MV x-ray beam. These large errors for the Clinac -4 beam result from the falloff in dose beyond the peak dose along a diagonal of a large field at distances greater than 14 cm from the central axis due to the lead flattening filter. The profile data stored in the computer along the principal planes cannot be used to calculate the dose accurately in such a situation. Computed doses for the 10-MV x-ray beam agreed with the measured doses within 4%-6% at all locations.

Computers↗
PubMedDate not supplied

Lung density effect on 125I dose distribution.

Perturbations in 125I implant dose distribution due to lung tissue variations have been investigated. Dose correction factors for a point source, a planar, and a volume implant have been calculated using a model which accounts for the changes in primary photon attenuation and in buildup factor when the medium is lung rather than water. Results of our calculation show that the change in dose is about 9% and 20% in the core and the periphery, respectively, of a representative implanted volume whose density is 0.25 relative to water.

Brachytherapy↗
PubMedDate not supplied

Dose distribution for 125I implants due to anisotropic radiation emission and unknown seed orientation.

Variations in dose distribution due to anisotropic radiation emission around 125I seeds and a lack of knowledge about the orientation of the implanted seeds have been investigated. Upper and lower bounds for dose distributions have been calculated for planar implants using the experimentally determined angular dose distribution around a typical 125I seed. Results of our study suggest that significant dose variations in the center and the periphery of the implanted area are possible.

Brachytherapy↗
PubMedDate not supplied

Comparison of absorbed dose in bone substitute material and water using ionization measurements.

Absorbed dose in a liquid substitute for bone has been compared with absorbed dose in water for 9-, 12-, and 15-MeV electron beams using ionization chamber measurements. The ionization readings were converted to dose using collisional mass stopping power ratios. The collisional mass stopping powers for the liquid substitute of bone were calculated using the Monte Carlo Code PEGS4. The results of our study show that there is an increase in dose in the liquid bone substitute compared to water at shallow depths. The maximum increase in dose was 5%, 4%, and 2% at depths of 1.2, 1.5, and 2.0 cm, respectively, for 9-, 12-, and 15-MeV electrons. The density of the liquid bone substitute was 1360 kg/m3 and the effective atomic number was 11.

Bone and Bones↗
PubMedDate not supplied

Corner transmission in several linear accelerator photon beams.

Maximum x-ray field sizes on many linear accelerators are obtained only with truncated corners. Transmissions through the corners of such fields has been measured utilizing film and ion chamber dosimetry for a number of accelerators. Transmissions are found to be significantly larger than for the movable jaws.

Humans↗
PubMedDate not supplied

Determination of dose enhancement in cortical bone substitute material for electron beams.

Dose measurements in Witt liquid, which simulates cortical bone, have been compared with dose in water for 6-, 9-, 12-, and 15-MeV electron beams. Measurements were made using a Farmer ionization chamber. The results of the study show dose enhancement in Witt liquid of 5%, 7%, 4%, and 0.4% for 6-, 9-, 12-, and 15 MeV electrons at shallow depths. The dose to a small mass of soft tissue in bone has also been estimated using ionization measurements. The results show a significantly higher dose in bone.

Bone and Bones↗