Search PubMed⌕ Search

Biomedical subjects

B R Paliwal

Publications and source records attributed to B R Paliwal.

70 records · Page 4Linked to original sources
PubMedDate not supplied

A solid water phantom material for radiotherapy x-ray and gamma-ray beam calibrations.

The formulation, manufacture and testing of an epoxy resin-based solid substitute for water is presented. This "solid water" has radiation characteristics very close volumetrically to those of water. When it is used as a dosimetry phantom for x- and gamma-ray beams in the radiotherapy range, phantom-to-water corrections and density corrections are eliminated. Relative transmission measurements have shown that the transmission through 10 cm of solid water is within 0.2% of that through an equal thickness of water for x and gamma rays. The use of this material for calibration phantoms can help achieve the goal of radiotherapy beam calibrations within +/- 1.0% of the true dose rate, easier to achieve.

Calibration↗
PubMedDate not supplied

Electron contamination in 60Co gamma-ray beams.

All radiotherapy photon beams are accompanied to some extent by secondary electrons which originate in interactions with source hardware, collimator, shadow tray, and/or the air through which the beam passes. Skin sparing, the shape of the dose buildup curve, and the depth of the dose maximum are all influenced by this electron "contamination." The present study of a 60Co source employs a flat ion chamber to measure dose buildup curves in polystyrene at source distances of 72 to 200 cm, with an open beam or a filter of Lucite, Cu, Pb-loaded acrylic, or Ba- or Pb-loaded nonbrowning glass placed 57 cm from the source, using 5 X 5, 20 X 20, and 35 X 35-cm2 beams as defined at 80 cm SSD. The effect of electron generation in the air was studied by placing a He-gas-filled plastic bag in the beam. A value of about 12% is estimated for the lowest relative dose obtainable with a polystyrene phantom in a "clear" 60Co gamma-ray beam of 1-cm diameter.

Cobalt Radioisotopes↗
PubMedDate not supplied

Calculational methods for estimating skin dose from electrons in Co-60 gamma-ray beams.

Several methods have been employed to calculate the relative contribution to skin dose due to scattered electrons in Co-60 gamma-ray beams. Either the Klein-Nishina differential scattering probability is employed to determine the number and initial energy of electrons scattered into the direction of a detector, or a Gaussian approximation is used to specify the surface distribution of initial pencil electron beams created by parallel or diverging photon fields. Results of these calculations are compared with experimental data. In addition, that fraction of relative surface dose resulting from photon interactions in air alone is estimated and compared with data extrapolated from measurements at large source-surface distance (SSD). The contribution to surface dose from electrons generated in air is 50% or more of the total skin dose for SSDs greater than 80 cm.

Cobalt Radioisotopes↗
PubMedDate not supplied

Some phantom designs for radiation dosimetry and CT applications.

Solid phantoms designed with nesting, rotatable cylinders, allow increased flexibility in the positioning of dosimeters or test objects, permitting continuous movement as in a water phantom, but with increased ease of handling. Using such phantoms, measurements can be made at almost any point in the cross section of a phantom, an ability not achievable with currently existing solid or water phantoms. A separate phantom design allows easy measurement at points along circles or radii in a phantom.

Equipment Design↗
PubMedDate not supplied

Stopping-power and mass energy-absorption coefficient ratios for Solid Water.

The AAPM Task Group 21 protocol provides tables of ratios of average restricted stopping powers and ratios of mean energy-absorption coefficients for different materials. These values were based on the work of Cunningham and Schulz. We have calculated these quantities for Solid Water (manufactured by RMI), using the same x-ray spectra and method as that used by Cunningham and Schulz. These values should be useful to people who are using Solid Water for high-energy photon calibration.

Absorption↗
PubMedDate not supplied

Charge storage in electron-irradiated phantom materials.

A recent article by Galbraith et al. [Med. Phys. 11, 197 (1984)] revealed the existence of dose errors due to charge storage in electron-irradiated plastic phantoms. We have subsequently studied the same effect using similar materials, plus some others including "solid water," which is an epoxy-based phantom material manufactured by Radiation Measurement, Inc. Our work shows that there is minimal charge storage in solid water, as compared to polymethylmethacrylate (PMMA) and polystyrene. Since existing dosimetry protocols allow PMMA and polystyrene to be used for calibration phantoms, users should beware of the possible dosimetry errors resulting from charge storage in those plastics, and consider choosing other water-substitute media, such as solid water, that do not display this effect.

Electrons↗
PubMedDate not supplied

Thermal and scatter effects on the radiation sensitivity of well chambers used for high dose rate Ir-192 calibrations.

High dose rate (HDR) iridium sources must be calibrated regularly because of the short half-life of Ir-192. High dose rate sources can now be calibrated using a new well-type chamber that allows easy, reproducible source calibrations. The chamber includes a styrofoam insulator that surrounds the source in the well. A study of the radiation sensitivity of the well chamber exposed to an HDR Ir-192 source at two different activities (300 and 230 GBq) revealed that the sensitivity of the chamber varies by as much as 1.1% as the chamber is moved toward a scattering surface. Second, with the styrofoam insulator removed, the air temperature within the ion collecting volume increased during exposure, causing a gradual decrease in chamber sensitivity of 0.15% in 30 min. This temperature increase was caused by heat transfer from radiation emitted by the Ir-192 source, and diminished as the source decayed. However, with the styrofoam insulator around the central aluminum tube in the well, the source cannot heat the collecting volume and thus thermal equilibrium between the ion collecting volume and its environment is maintained throughout an exposure. The radiation sensitivity of the commercial well chamber was found to be constant for exposure times of 30 min.

Aluminum↗
PubMedDate not supplied

On the cause of the variation in tissue-maximum ratio values with source-to-detector distance.

While tissue-maximum ratios (TMR) for 60cobalt treatment units have been shown to be independent of source-to-axis distance (SAD), high-energy photon beams demonstrate variations in their TMR as a function of SAD. Some authors have asserted that the distance dependence of the TMR stems from electron contamination in the beams, while others have suggested low-energy, scattered photons as the cause. Using a magnet to sweep contaminant electrons out of the photon treatment beam eliminates any variation in TMR with distance. Thus, electron contamination accounts for all of the distance dependence, and any low-energy, scattered photons behave indistinguishably like the high-energy photons.

Cobalt Radioisotopes↗
PubMedDate not supplied

Dosimetry of large wedged high-energy photon beams.

The dependence of the wedge factor and central axis depth dose on field size was evaluated for 6-, 10-, and 24-MV wedged photon beams for field sizes up to 40 x 40 cm2. The wedge factor for 60 degrees, 45 degrees, 30 degrees, and 15 degrees wedges in a 24-MV beam was found to vary by as much as 25%, 12%, 9%, and 5%, respectively, over a field size range of 5 x 5 to 40 x 40 cm2. For 10 and 6 MV wedged beams, the wedge factors varied by up to 17% and 15%, respectively, over the same field size range. The depth dose curves for the wedged beams differed significantly from the open beam profiles. At 6 MV, the wedges caused beam hardening while at 24 MV, with the exception of the 15 degrees wedge, all wedged beams were softer than the open beams, for all field sizes. At 10 MV, wedged fields of size less than 20 x 20 cm2 were hardened relative to the open beam, whereas larger wedged fields had depth dose values within +/- 1% of the 10-MV open-beam depth dose data. Accurate treatment planning for large wedged fields and high-energy photon beams thus requires the use of wedged beam depth dose curves and field size specific wedge factors. It was established that an equivalent square field for a rectangular wedged field can be determined using the standard open beam formulation. The largest difference between the wedge factor for a rectangular beam and its equivalent square beam was 2.5% and occurred for 24-MV elongated fields.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMedDate not supplied

Hyperthermia quality assurance results.

The Hyperthermia Physics Center (HPC), under contract with the National Cancer Institute (NCI), has conducted review-type quality assurance (QA) measurements at the five Hyperthermia Equipment Evaluation Centers involved in evaluating the clinical efficacy of a variety of devices for delivering heat treatments to deep-seated human tumours. A summary of the QA protocol, results, testing procedures, standards, criteria, conclusions and recommendations are presented in this paper. The QA review measurements indicate (a) that 81.5 per cent of temperatures surveyed were within the 0.2 degrees C HPC criterion (91 per cent were within 0.4 degrees C), (b) that only 66 per cent of power indications were within the 10 per cent criterion, (c) that the heat patterns in a phantom produced by the BSD Annular Phased Array (AA) had significant variability, (d) that each treatment facility had at least a few potentially occupiable locations where the maximum permissible American National Standards Institute standards of electromagnetic leakage were exceeded, and (e) that these levels of accuracy and safety were achieved only after stringent inhouse QA efforts. From the combined data, it is concluded that the temperature accuracy in this cooperative trial was sufficient to justify a common analysis of clinical data as presented in this series. Also, stringent quality control of every parameter must continue to be stressed in all future hyperthermia trials.

Electromagnetic Phenomena↗
PubMedDate not supplied

Temperature distributions, microangiographic and histopathologic correlations in normal tissue heated by ferromagnetic needles.

Rabbit hind limb musculature implanted with nine ferromagnetic nickel-copper alloy needles was inductively heated in a 120 kHz oscillating magnetic field. Rabbits were heated every third day for a total of three 30 min heating periods. Ferromagnetic needles with Curie points of 42 degrees C, 45 degrees C, and 48 degrees C were used. Fourteen days following the third heating the animals were sacrificed and the tissues processed for microangiography and histopathology. Implanted muscle heated rapidly and achieved a stable temperature within 10 min. Two weeks after heating for three 30 min heat cycles at temperatures above 47.1 degrees C, both vasculature destruction and muscle necrosis were noted. However, at temperatures below 45.5 degrees C, histopathologic and microangiographic findings were indistinguishable from unheated, implanted controls. Myocentric granuloma formation surrounding the thermoseed tracks with vascular preservation characterized tissue at a steady-state temperature between 45.5 degrees C and 47.1 degrees C. Nickel-copper alloy thermoseeds were effective at producing reproducible, localized interstitial hyperthermia. Chronic vascular and histopathologic alterations correlated closely with previous steady-state temperatures.

Angiography↗
PubMedDate not supplied

3D rendering of SAR distributions from Thermotron RF-8 using a ray casting technique.

A comprehensive 3D visualization package developed for CT-based 3D radiation treatment planning has been modified to volume-render SAR data. The program accepts data from sequential thermographic thermometry measurements as well as calculated data from thermal models. In this presentation sample data obtained from a capacitive heating system 'Thermotron-RF8' is presented. This capability allows the generation of accurate standardized volumetric images of SAR and provides a valuable tool to better preplan hyperthermia treatments.

Evaluation Studies as Topic↗
PubMedDate not supplied

Effect of implant variables on temperatures achieved during ferromagnetic hyperthermia.

Effects of ferromagnetic implant variables on steady-state temperature were studied in both in vitro (phantom) and in vivo (rabbit hind limb musculature) models. Thermoseed implant variables included: (1) the presence and number of thermoseed sleeves; (2) variations in thermoseed alignment within the oscillating electromagnetic field; (3) generator power levels of 300 W, 600 W, and 1200 W; and (4) separation of thermoseed tracks by 0.8 cm versus 1 cm. When the thermoseeds were aligned parallel to the electromagnetic field, temperature distributions in the in vivo model using bare thermoseeds and thermoseeds encased in a single sleeve (0.1 mm wall thickness) of polyethylene tubing were statistically higher than in tests performed with thermoseeds encased in a double sleeve (0.25 mm over 0.1 mm wall thickness) of tubing (p = 0.006). Nonetheless, average steady-state temperatures above a therapeutic minimum (greater than or equal to 42 degrees C) were achieved at all generator power levels using thermoseeds encased in a double sleeve of tubing and aligned parallel to the electromagnetic field. Gross misalignment of thermoseeds with the electromagnetic field was partly compensated for by utilizing higher generator power levels. Thermoseed tracks separated by 0.8 cm and aligned parallel to the electromagnetic field yielded average steady-state temperatures that were 0.4-2.2 degrees C higher than those obtained with a thermoseed track separation of 1 cm.

Animals↗
PubMedDate not supplied

Concurrent ferromagnetic hyperthermia and 125I brachytherapy in a rabbit choroidal melanoma model.

Ferromagnetic (FM) thermoseeds and radioactive (125I) seeds were combined in an episcleral plaque to give concurrent hyperthermia and irradiation for enhanced tumour destruction. A Greene melanoma cell line was utilized to study the interaction between these treatment modalities. We attached five FM thermoseeds (with an operating temperature of 48 degrees C) in parallel with alternating rows of 125I seeds onto the inner surface of each 14 mm Silastic plaque. Plaques were centred over a 3-6 mm (diameter) intraocular melanoma in each rabbit. Some rabbits were then placed within a heating coil, and their eye tumours were warmed rapidly to therapeutic temperatures (43.6 degrees C across the tumour base) while the temperature of normal conjunctiva across the globe did not exceed 38.5 degrees C. Analysis of 49 treated eye melanomas showed 50% local tumour control at 41.7 Gy for 125I alone, whereas only 9.5 Gy were needed to give the same local control rate after 125I with concurrent FM hyperthermia. Thus, a thermal enhancement ratio of 4.4 was obtained. Hyperthermia alone gave a 20% tumour response rate, but responses were only temporary. We conclude that FM thermoseeds can be used to deliver biologically effective hyperthermia concurrently with radiation, thereby reducing the dose of radiation needed for tumour control.

Animals↗
PubMedDate not supplied

Temperature-dependent versus constant-rate blood perfusion modelling in ferromagnetic thermoseed hyperthermia: results with a model of the human prostate.

Finite-element solutions to the Pennes bioheat equation are obtained with a model of a tumour-containing, human prostate and surrounding normal tissues. Simulations of ferromagnetic hyperthermia treatments are conducted on the tissue model in which the prostate is implanted with an irregularly spaced array of thermoseeds. Several combinations of thermoseed temperatures with different Curie points are investigated. Non-uniform, constant-rate blood perfusion models are studied and compared with temperature-dependent descriptions of blood perfusion. Blood perfusions in the temperature-dependent models initially increase with tissue temperature and then decrease at higher temperatures. Simulations with temperature-dependent versus constant-rate blood perfusion models reveal significant differences in temperature distributions in and surrounding the tumour-containing prostate. Results from the simulations include differences (between temperature-dependent and constant-rate models) in (1) the percentage of normal tissue volume and tumour volume at temperatures > 42 degrees C, and (2) temperature descriptors in the tumour (subscript t) and normal (subscript n) tissues including Tmax.t, Tmin.t and Tmax.n. Isotherms and grey-scale contours in the tumour and surrounding normal tissues are presented for four simulations that model a combination of high-temperature thermoseeds. Several simulations show that Tmin.t is between 1.7 and 2.6 degrees C higher and Tmax.n is between 2.1 and 3.3 degrees C higher with a temperature-dependent versus a comparable constant-rate blood perfusion model. The same simulations reveal that the percentages of tumour volume at temperatures > 42 degrees C are between 0 and 68% higher with the temperature-dependent versus the constant-rate perfusion model over all seed combinations studied. In summary, a numerical method is presented which makes it possible to investigate temperature-dependent, continuous functions of blood perfusion in simulations of hyperthermia treatments. Simulations with this numerical method reveal that the use of constant-rate instead of temperature-dependent blood perfusion models can be a conservative approach in treatment planning of ferromagnetic hyperthermia.

Body Temperature Regulation↗