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R J Schulz

Publications and source records attributed to R J Schulz.

80 records · Page 5Linked to original sources
PubMedDate not supplied

Calorimetric determination of the cavity-gas calibration factor Ngas.

In the recently published AAPM protocol for the dosimetry of high-energy photons and electrons, the response of an ionization chamber is defined as the dose to the gas in the chamber per unit electrometer reading, Ngas. Using a graphite calorimeter, Ngas has been determined for a Farmer-type ionization chamber using 4- and 25-MV x rays. The procedure was to measure the dose to graphite using the calorimeter, and then obtain the response of the chamber at the same depth in a graphite phantom. Equation (9) of the AAPM protocol was then used to calculate Ngas. The values of Ngas determined with the calorimeter are within 1% of Ngas calculated according to the AAPM protocol, using the 60Co exposure-calibration factor.

Calorimetry↗
PubMedDate not supplied

Selenium-75: a potential source for use in high-activity brachytherapy irradiators.

A study was made to find a high-specific-activity radioisotope having a longer half-life and lower gamma-ray energies than 192Ir in order to replace 192Ir in high-activity afterloading brachytherapy devices. Based upon a number of considerations, 75Se was selected. The present paper describes the physical properties of 75Se, its method of production, and the properties of an encapsulated source that was fabricated to prove the accuracy of our initial calculations.

Biophysical Phenomena↗
PubMedDate not supplied

The direct determination of dose-to-water using a water calorimeter.

A flexible, temperature-regulated, water calorimeter has been constructed which consists of three nested cylinders. The innermost "core" is a 10 X 10 cm right cylinder made of glass, the contents of which are isolated from the environment. It has two Teflon-washered glass valves for filling, and two thermistors are supported at the center by glass capillary tubes. Surrounding the core is a "jacket" that provides approximately 2 cm of air insulation between the core and the "shield." The shield surrounds the jacket with a 2.5-cm layer of temperature-regulated water flowing at 51/min. The core is filled with highly purified water the gas content of which is established prior to filling. Convection currents, which may be induced by dose gradients or thermistor power dissipation, are eliminated by operating the calorimeter at 4 degrees C. Depending upon the power level of the thermistors, 15-200 microW, and the insulation provided by the glass capillary tubing, the temperature of the thermistors is higher than that of the surrounding water. To minimize potential errors caused by differences between calibration curves obtained at finite power levels, the zero-power-level calibration curve obtained by extrapolation is employed. Also the calorimeter response is corrected for the change in power level, and therefore thermistor temperature, that follows the resistance change caused by irradiation. The response of the calorimeter to 4-MV x rays has been compared to that of an ionization chamber irradiated in an identical geometry.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetry↗
PubMedDate not supplied

A comparison of ionization-chamber and water-calorimeter dosimetry for high-energy x rays.

The temperature-regulated, flexible, water calorimeter developed in the authors' laboratory was shown previously to yield a dose-to-water from 4-MV x rays that is in very close agreement with ionization measurements made in accordance with the AAPM dosimetry protocol. The range of beam energies for this type of comparison has been increased to include 60Co, and 4-, 6- and 25-MV x rays. The grand mean of the ratios of doses obtained from the calorimeter and ionization chamber, the Cal/Ion ratio, for the four beam energies studied is 1.001 +/- 0.001. As no significant trend with beam energy was detected, it is concluded that the calorimeter and ionization chamber yield equally accurate results. Because the calibration of the calorimeter depends solely upon the accuracy with which water temperatures in the range 2-10 degrees C can be measured, and dose is given by the product of the specific heat of water and the temperature change produced by irradiation, the water calorimeter has the potential to place radiation dosimetry on a much firmer foundation than presently exists.

Calorimetry↗
PubMedDate not supplied

Assessment of the accuracy of stereotactic radiosurgery using Fricke-infused gels and MRI.

The treatment plans for stereotactic radiosurgery employ small, circular, noncoplanar fields applied in a series of arcs, or with synchronous rotation of the accelerator gantry and patient support assembly. Primary or metastatic brain tumors and arterial-venous malformations are localized in relation to a stereotactic head frame using CT, MRI, and angiography. As x-ray doses in the range of 20-40 Gy are delivered in a single treatment, it is critical that the dose distribution produced by the accelerator accurately reflect the one developed by the treatment planning computer. Until the advent of Fricke-infused gels, whose NMR characteristics are changed by irradiation, there was no practical method for assessing the accuracy of x-ray beam positioning on a target that was localized by both CT and MRI. A stereotactic head frame was attached to a hollow glass head filled with a Fricke-infused gel. A 2-mm target point at approximately the center of this manikin was localized by CT and MRI. The head frame was then mounted to the patient support assembly of a linear accelerator, and given a dose of 40 Gy to the isocenter from 6-MV x rays using a modified version of the dynamic stereotactic radiosurgery plan developed in Montreal. Subsequent MRI showed the target point at the center of the dose distribution, thus confirming the accuracy of the stereotactic radiosurgery procedure. This demonstrated the unique characteristics of the Fricke-infused gel for the simultaneous localization of x-ray beams in three dimensions.

Gels↗