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Biomedical subjects

R J Schulz

Publications and source records attributed to R J Schulz.

At least 73 records · Page 4Linked to original sources
PubMedDate not supplied

The combined effects of oxygen tension, X radiation and 5-thio-D-glucose on the survival of mammalian cells.

The glucose analogue 5-thio-D-glucose (5-TG) is toxic to hypoxic cells but does not affect aerated cells. Toxicity is markedly dependent upon oxygen tension: exposure of Chinese hamster cells to 5 mM 5-TG and 50 ppm O2 (in the overlying atmosphere) results in a survival of 0.01 in six hours, whereas increasing the oxygen to 100 ppm causes the survival to increase to about 0.9. The combined effect of X rays and 5-TG is to reduce the survival of hypoxic cells to a level far below that attainable with either agent alone. Cells made hypoxic with 5 ppm O2 have 0.1 survival after three hours exposure to 5 mM 5-TG. The same cells have a 0.5 survival when given a dose of 1000 rad. When the x-irradiation is given at the end of a three-hour exposure to 5-TG, the survival is reduced to 0.0055.

Animals↗
PubMedDate not supplied

Computer applications to radioactive-seed: brain-tumor implants.

Malignant brain tumors, in general, and anaplastic astrocytoma and glioblastoma multiforme in particular, have been highly refractory to conventional treatments including surgery, chemotherapy and external-beam irradiation. Although better local control can be achieved with high-dose, external beam irradiation, necrosis of normal brain tissue reduces the quality of life and survival. In order to localize the radiation dose given to brain tumors, the temporary implantation of 125I and 192Ir seeds is undergoing clinical trials at several medical centers. Computers play a key role in this treatment modality: in addition to being essential for image reconstruction of CT scans, a computer is used to reconstruct a tumor volume from outlined regions on individual cuts; a programable calculator is used in conjunction with a stereotaxic head holder to obtain the coordinates of the radioactive seeds; a radiation-therapy, treatment-planning computer is used to optimize the radioactive-seed positions and strengths, and to generate the corresponding dose distribution.

Astrocytoma↗
PubMedDate not supplied

Determination of high-energy x-ray spectra by photoactivation.

The determination of high-energy x-ray spectra has required scintillation spectrometers with massive shielding, neutron time-of-flight spectrometers, or the tedious counting of electron tracks in nuclear emulsions. A new approach has been developed which takes advantage of the energy dependence of photoactivation cross sections. Radioactivity is produced in a small packet of C, Cu, Co, Y, Zr, and Au foils by approximately 5000 rad (tissue). Since the amount of radioactivity produced in each foil is given by the integral of the product of photonuclear cross section and differential photon fluence, a numerical method for unfolding the spectrum is required, and the orthonormal expansion has been employed for this purpose. The photoactivation method has been used to determine the x-ray spectra produced by 30-MeV electrons incident upon thin and thick tungsten targets, and filtered by equivalent amounts of lead and aluminum. These spectra have been compared to calculated thin-target spectra as well as to those determined by a neutron time-of-flight spectrometer. The central-axis and off-axis x-ray spectra produced by a 33-MeV betatron have also been determined.

Radiometry↗
PubMedDate not supplied

Computer generation of high-energy x-ray dose distributions.

A computer program for the generation of high-energy x-ray dose distributions has been developed. The program takes account of the fact that the x-ray beam profile, as affected by penumbra, wedges, beam blocks, or flattening filters, is one of the prime determinants of the dose distribution. Doses from primary and scattered radiation are calculated separately by using published tissue-air ratio (TAR) and scatter-air ratio (SAR) data. The dose from scattered radiation is determined by dividing the x-ray field into a series of scattered strips and summing the contribution from each. Wedges or beam blocks are taken into account simply by earning their actual coordinates and linear attenuation coefficients. The program may be used to design wedges, beam blocks, or flattening filters prior to fabrication. Complete libraries of dose distribution for 60Co, 2-, 4-, and 6-MV x rays have been generated.

Computers↗
PubMedDate not supplied

Photoactivation ratios for specification of high-energy x-ray quality: part I, 15-40-MV x rays.

A sensitive and relatively simple method for obtaining an index of the spectral quality of high-energy x rays is presented. The method is based upon the use of photoactivation threshold detectors. Photonuclear reactions produce measurable amounts of radioactivity for radiation exposures of several thousand roentgens. Since the threshold energies for photonuclear reactions vary for different nuclei, the ratio of radioactivity induced in two appropriately selected foils is very sensitive to the x-ray spectrum. Photoactivation ratios (PAR) have been measured for 20-35-MV x rays using Co, Cu, Y, Zr, and Au activation foils. It is shown that the PAR method offers a sensitive and practical means for quality control of x-ray spectra, comparison of high-energy accelerators, and the measurement of variations of spectral quality control at different point in an irradiated volume.

Energy Transfer↗
PubMedDate not supplied

Photoactivation ratios for specification of high-energy x-ray quality: part II, 4-15-MV x rays.

A sensitive and relatively simple method for obtaining an index of the spectral quality of x rays in the 4-15-MV range is presented. The method is based upon the simultaneous production of 115mIn by inelastic scattering of x rays by an indium foil and the production of 116mIn by neutron capture, where the neutrons are obtained from the photodisintegration of deuterium. Since the cross sections for the production of 115mIn and 116mIn have markedly different energy response functions, the ratio of radioactivities is very sensitive to the incident x-ray spectrum. Using a detector consisting of an indium foil sandwiched between two plastic bottles of D2O, photoactivation ratios (PAR) have been measured for 4-, 6-, and 10-MV x rays. It is shown that the PAR method offers a sensitive and practical means for quality control of x-ray spectra and comparison of high-energy accelerators.

Deuterium↗
PubMedDate not supplied

On the choice of material for half-value-layer measurements for megavoltage x rays.

The relative sensitivity of various materials for the measurement of half-value-layer thickness has been calculated for bremsstrahlung x ray spectra in the energy range 5-40 MeV. It is concluded that low-atomic-number materials such as water are more sensitive to changes in spectral quality of megavoltage x rays than high-atomic-number materials such as lead.

Aluminum↗
PubMedDate not supplied

Modification of electron-beam dose distributions by transverse magnetic fields.

By applying a transverse magnetic field to a dosimetry phantom, an incident high-energy electron beam is made to follow a spiral path in the course of slowing down. Certain levels, determined by the electron energy and the magnetic field strength, will be traversed several times by the same electrons. The net result of this process is an enhancement of the depth dose in relation to the entrance dose, and a more sharply defined depth of penetration. Experiments with 50- and 55-MeV electrons traversing a 20.5-kG field are shown to support the predictions of a detailed Monte Carlo calculation.

Electrons↗
PubMedDate not supplied

Monte Carlo calculation of the wall correction factors for ionization chambers and Aeq for 60Co gamma rays.

The application of cavity-ionization chambers to the standadization of 60Co gamma-ray beams, in terms of exposure, requires that the specific ionization of air Jg, be corrected for the attenuation and scatter of the incident rays by the wall, central electrode, and supporting stem of the chamber. A Monte Carlo photon-electron transport code has been developed for the purpose of calculating this correction for spherical and cylindrical chambers. The code has been applied to a spherical graphite chamber having dimensions typical of the chambers used by the NBS, the calculated wall-correction factor is in close agreement with the average of the NBS factors which were determined experimentally. The code was also used to calculate Aeq, which is central to the determination of tissue-air ratios. The calculated value, 0.989 +/- 0.003, is very close to the generally accepted value, 0.985.

Cobalt Radioisotopes↗
PubMedDate not supplied

Argon/propane ionization-chamber dosimetry for mixed x-ray/neutron fields.

The photoneutrons produced by high-energy x-ray machines can diffuse through the mazes usually employed at the treatment-room entrance and readily penetrate the lead-lined doors used for x-ray shielding. The measurement of these neutrons in the presence of x-rays and the determination of dose equivalent poses a problem for which there is currently no standard method of solution. In order to separate x-ray dose from neutron dose, the author employed an ionization chamber alternately filled with argon or propane. The response characteristics of this chamber to x-ray and neutrons are described. Quality factors were determined from a calculated neutron spectrum. As a result of these measurements, a 10-in. polyethylene door was added to the entranceway of a 25-MV linear accelerator.

Argon↗
PubMedDate not supplied

On the constancy in composition of polystyrene and polymethylmethacrylate plastics.

Variations in the atomic composition, and mass and electron densities of polystyrene and polymethylmethacrylate (PMM) plastics were assessed from experimentally determined mass attenuation coefficients for 125I and 137Cs gamma rays. The means and standard deviations in the mass densities of 16 samples of PMM and 10 samples of polystyrene were found to be 1.174 +/- 1.4% and 1.042 +/- 0.6% g/cm3, respectively. Based upon transmission measurements on various solutions of ethyl alcohol in water, the standard deviations in the effective atomic numbers of PMM and polystyrene were determined to be 0.77% and 1.3%, respectively. Based upon experimentally determined mass attenuation coefficients for 137Cs, the standard deviations in electron density for PMM and polystyrene were 0.5% and 1.2% respectively. Similar measurements on tap water and two grades of distilled water failed to detect any differences in atomic composition.

Methylmethacrylates↗
PubMedDate not supplied

Determination of the exposure rate constant for 125I using a scintillation detector.

A scintillation spectrometer was employed to determine the exposure rate constant for an 125I seed of the type used in therapeutic radiology. The method consisted of converting counting rate to photon fluence incident upon the detector, and then calculating the exposure rate from the photon fluence. Correlations are required for absorption of the 125I photons in the air path, in the aluminum window of the NaI crystal, and in the aluminum-oxide reflector. Corrections are also required for the escape of iodine K-characteristic radiation, and the intrinsic peak efficiency of the crystal. The experimentally determined exposure rate constant in a direction perpendicular to the long axis of the seed is 1.309 Rcm2mCi-1 h-1 +/- 5.8%, and this compares favorably with a calculated value of 1.284 R cm2mCi-1 h-1. The photon fluence as a function of angle of rotation about an axis perpendicular to the seed's long axis was also measured. These data were used to estimate an average exposure rate constant for 125I seeds of 1.089 Rcm2mC-1 h-1.

Brachytherapy↗
PubMedDate not supplied

Yale University School of Medicine, New Haven, Connecticut.

The response and wall correction factors for various ionization chambers in a cobalt-60 gamma-ray field have been calculated using a Monte Carlo photon-electron transport code. Among the chamber parameters studied are chamber wall material and its thickness, central electrode material and its dimensions, and the shape and size of the sensitive volume. The calculations show that the response and wall correction factors are sensitive to the shape and volume of the ionization chamber, but relatively independent of the choice of material for the chamber wall and electrode when these are compared on the basis of electron density. Data are presented for cylindrical, plane-parallel, and spherical ionization chambers constructed from carbon, magnesium, aluminum, water, Lucite, polystyrene, and ICRU muscle, as well as for a number of commercially available ionization chambers.

Gamma Rays↗
PubMedDate not supplied

Fraction of ionization from electrons arising in the wall of an ionization chamber.

The accuracy of high-energy x-ray dosimetry can be improved by taking account of differences between the compositions of the chamber wall and the buildup cap or dosimetry phantom. The fraction of the ionization due to secondary electrons arising in the chamber wall has been determined as a function of wall thickness for 60Co gamma rays and x rays in the range of 2-25 MV for Farmer-type chambers. Secondary electrons arising in the accelerator head were removed from the x-ray beams by a magnetic field placed just in front of the ionization chamber. For 60Co gamma rays, the fraction increases from 40% to 100% as the wall thickness increases from 0.05 to 0.55 g cm-2. For a 0.05 g cm-2 wall, fraction decreases from 60% to 10% as the x-ray energy is increased from 2 to 25 MV. Limited data obtained with different chambers suggest that the fraction is independent of chamber wall composition when the thickness is expressed in g cm-2.

Cobalt Radioisotopes↗
PubMedDate not supplied

On the selection of stopping-power and mass energy-absorption coefficient ratios for high-energy x-ray dosimetry.

A method for the selection of average stopping-power (L/rho)medair and energy-absorption coefficient (mu en/rho)medair ratios has been developed. The quality of the x-ray beam is characterized by the ratio of ionization chamber readings at depths of 20 and 10 cm in water (TMR)2010. For convenience, a relationship is established between experimental (TMR)2010 and the nominal accelerating potential (MV) of the accelerator. Experimental (TMR)2010 are related to (L/rho)medair and (mu en/rho)medair in a three-step process. First, using experimental and theoretical spectra in the range 60Co to 45 MV, (TMR)2010 were calculated for primary and first-scatter photons, and a graph of experimental versus calculated (TMR)2010 for these same spectra was constructed. Second, (L/rho)medair and (mu en/rho)medair were calculated for a large number of primary spectra [for most of which experimental (TMR)2010 were not available] and a graph constructed that related these quantities and (TMR)2010 calculated as above for this group of spectra. Third, using the graphs from the preceding steps, graphs relating the calculated (L/rho)medair and (mu en/rho)medair with experimental (TMR)2010 were constructed. Data are presented for water, polystyrene, acrylic, graphite, A-150, C-552, Bakelite, and nylon for beams with nominal accelerating potentials in the range 2-45 MV.

Humans↗
PubMedDate not supplied

Enhancement of electron beam dose distributions by longitudinal magnetic fields: Monte Carlo simulations and magnet system optimization.

A Monte Carlo electron-photon transport code was developed in order to determine the effects of static, longitudinal, magnetic fields on dose distributions produced by high-energy electron beams, and to optimize the design of a superconducting magnet system. As a result of these simulations, a 20-cm-i.d., 30-cm-o.d., 15-cm-tall, single-coil, magnet system was designed that could be incorporated into a mobile treatment table for use with a standard radiation therapy accelerator. Operating at a current density of 18 kA/cm2, the magnet would produce field strengths of 1-4 T in the phantom and 0.01 T at the accelerator exit window. Magnetically enhanced dose distributions, calculated for 20- and 30-MeV electron beams, show a pronounced Bragg peak, steeper gradients to the sides and rear, and a roughly fourfold increase in the peak dose to entrance dose ratios relative to those similarly calculated without a magnetic field. These magnetically enhanced dose distributions have the potential for sparing intervening tissue when high-energy electrons are used for the treatment of deep-seated tumors.

Electrons↗