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Biomedical subjects

B R Paliwal

Publications and source records attributed to B R Paliwal.

At least 55 records · Page 3Linked to original sources
PubMed1980

Comparison of Burlin cavity theory with LiF TLD measurements for cobalt-60 gamma rays.

Cobalt-60 gamma-ray absorbed dose measurements were carried out using LiF thermoluminescent (TL) dosemeters to obtain additional insight into cavity theory as applied to dosemeters of a size intermediate relative to the range of secondary electrons. Stacks of dosemeters (0.4 mm x 3 mm x 3 mm, hot-pressed LiF TLD-100) were surrounded by homogeneous LiF, polystyrene, aluminium, copper or lead. The absorbed dose in the stack (0.1 to 0.7 g cm-2 thick) was derived from TL readings of the individual dosemeters, and was compared with calculations based on the Burlin cavity theory. A relatively strong asymmetry in the experimental absorbed-dose distribution was found between the front, rear and middle dosemeters of the stack, especially when lead was the surrounding medium. Cavity theory shows good agreement with the experimental results obtained in polystyrene, while theory tends to overestimate the dose in the LiF dosemeters enclosed in Al, Cu, and Pb. The general trend predicted by the Burlin theory is somewhat steeper relative to cavity size than that observed experimentally. The theory was modified to make it more applicable to the dose at the centre of the stack. For lead this clearly agreed more closely with experimental results than did the conventional theory.

Cobalt Radioisotopes↗
PubMed1980

Heating patterns produced by 434 MHz erbotherm UHF 69.

Thermal distributions of 434 MHz Erbotherm H 69 hyperthermia/diathermy generators in tissue-equivalent medium have been investigated. Thermocouples were used to measure temperatures at a set of grid points. Cylindrical and abdominal phantoms were heated and sectioned after heating to measure the temperatures at depth. Results indicate that there is potential for heating deep-seated tumors. The distributions were influenced by the number, shape and geometry of the applicators employed.

Diathermy↗
PubMed1980

High Z elements in human sarcomata: assessment by multienergy CT and neutron activation analysis.

"Tumor equivalent" phantoms containing inorganic salts (KH2PO4, CH3COOK, NaCl and Kl) were scanned on an EMI 5005 body scanner at 140 kVp, 28 mA; 120 kVp, 33 mA; and 81 kVp, 42 mA. Significant signal gain for the detection of higher atomic number elements by multiple energy scanning was noted. Certain sarcomas are known to accumulate high Z elements. Accordingly, excised specimens of various histologies of human sarcomata (chondrosarcoma, liposarcoma, and malignant fibrous histiocytoma) were scanned at 140 kVp and 81 kVp. Using selected areas of interest in the computed tomographic (CT) image to direct the in vitro biopsy of various regions of excised tumors, interesting correlations between the CT number variation and the respective, high Z elemental composition variation, as determined by thermal neutron activation analysis were observed. Further investigation with phantoms and excised sarcomata at 62 kVp and 42 mA suggested that dual energy CT scanning (at 140 kVp and 62 kVp) may be a method of monitoring "effective Z" and heavy element compositional changes. The authors are also attempting to develop these same low kilovoltage techniques as a method for the noninvasive clinical monitoring of an antisarcoma chemotherapeutic agent, cis-diammineaichloroplatinum (11).

Activation Analysis↗
PubMed1979

Magnetic modification of electron beam dose distributions.

A 10 kG magnetic field has been used to modify the conventional electron beam isodose distributions. Isodose curves of modified distributions for single and multiple beams measured in a polystyrene phantom are presented. The results suggest a potential use of this technique to achieve dose distributions adjusted to the various tumor contours and corrected for heterogeneity effects.

Electromagnetic Fields↗
PubMed1977

A tissue compensation source-to-skin measurement system.

An easy to use, low-cost system for measuring the source-to-skin distance used in determining tissue compensation requirements is described. These measurements can be performed with accuracy and precision for any treatment field size and at any treatment beam angle. The system can be used in conjuction with several different tissue compensation methods.

Humans↗
PubMed1975

Extended field treatment flatness filter for 4 MV linear accelerators.

An auxiliary flatness filter for large field treatment on 4 MV linear accelerator provides an isodose distribution which does not vary more than 6% over the central 80% of the axis of a 32 x 32-cm field at 80 cm in air, and at a 2.5, 6, and 12-cm depth in water. This lead disc filter does not affect the central axis depth dose values, and results in only a 16% reduction in dose rate on the central field axis.

Radiotherapy, High-Energy↗
PubMedDate not supplied

Characteristics of Clinac-18 wedged fields for 10-MV x rays.

The characteristics of wedged fields which affect their clinical use have been examined for the 10-MV x-ray beam from the Clinac-18. The methods used for obtaining and analyzing the wedged-field data are discussed. These characteristics have been examined in terms of (a) the wedge angle, (b) the variation of the wedge angle with field size, (c) the variation in the angle through which isodose curves between the approximate depths of 5 and 15 cm are turned relative to the central axis as a function of depth, and (d) the variation of wedge central-axis transmission factor with field size. Analysis of the data pertinent to these categories is presented for the 15-, 30-, 45-, and 60-deg wedges. In addition, the effect of the 60-deg wedge upon the position of maximum dose on the central axis was determined.

Particle Accelerators↗
PubMedDate not supplied

Magnetic field modification of electron-beam dose distributions in inhomogeneous media.

Modern curative radiotherapy requires higher doses to the tumor volume and, necessarily, minimal doses to the surrounding normal tissues. Attempts to use heavy charged particles to achieve such optimization are currently under investigation in many centers. Our data indicate that a static, superimposed magnetic field on a clinical electron-therapy beam also offers the capability of some "tailoring" of isodose distributions. Furthermore, a variable, superimposed magnetic field minimizes those tissue-generated dose heterogeneities which are inherent with all charged-particle beams. We suggest that magnetically modified, clinically available electron beams also offer a practical and less expensive means of achieving tailored, heterogeneity-corrected isodose distributions.

Electrons↗
PubMedDate not supplied

Characterization of Clinac-18 electron-beam energy using a magnetic analysis method.

Clinac-18 electron-beam energies and spectral energy distributions were measured by a magnetic analysis method to verify results obtained by the standard range method. A comparison of these measurements with nominal energies quoted by the manufacturer indicates that electron-beam measured by both methods are in agreement with the manufacturer's specifications to +/- 0.6 MeV. Spectral energy distribution of the electron beam was measured to be approximately 1 MeV (FWHM) at all energies.

Electrons↗
PubMedDate not supplied

Perturbation of electron beam doses as a function of SSD due to the use of shielding blocks on the Clinac-18a.

Many radiotherapy linear accelerators use electron beam applicators which extended close to the patient's surface when treating at the regular distance. The relatively large size of these applicators often necessitates the use of a larger SSD than that designed by the manufacturer. In such cases, the addition of shielding blocks to the applicator can significantly alter the factors which should be used to calculate the dose rate at the new SSD as compared with the unshielded beam condition. In some typical clinical situations, errors of greater than 60% may result from failure to account for this perturbation. This paper presents the proper correction of the dose vs SSD function for the presence of such shielding blocks for the Clinac-18 linear accelerator.

Electrons↗