An international neutron dosimetry intercomparison.
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Biophysical Phenomena↗
Biomedical subjects
J L Horton
Publications and source records attributed to J L Horton.
A look at home health agencies. Non-certified agency.
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Certification↗
Physical characteristics of the M.D. Anderson Hospital clinical neutron beam.
The physical characteristics of the M.D. Anderson Hospital (MDAH) clinical neutron beam are presented. The central-axis percent depth-dose values are intermediate between a 4 and 6 MV X-ray beam. The build-up curves reach a depth of maximum dose at 1.2 cm and have surface dose values of approximately 30%. Teflon flattening filters are employed to flatten the beam at the depth of the 75% dose level. Two wedges are available for shaping the beam; they are made of Teflon and produce wedge angles of 31 degrees and 45 degrees as defined by the ICRU. Output factors ranged from 0.88 for a 4 x 4 cm field to 1.12 for a 20 x 20 cm field. Tungsten blocks reduced the dose received at Dmax to 25% of the unblocked value but only 52% of the unblocked value at a depth of 22.8 cm.
Filtration↗
Arachidonic acid metabolism by neonatal lungs perfused with Krebs bicarbonate buffer.
We characterized and quantified products of exogenous arachidonic acid metabolism by neonatal lamb lungs perfused with Krebs solution and determined effects of alveolar hypoxia upon this metabolism. The predominant metabolite was PGI2 (62.4 +/- 5.8 (SEM)% of the total prostanoids (PGs) produced). An additional 23.1 +/- 6.7% was metabolized PGI2. PGE2 (1.7 +/- 3.0%) was produced also. Little thromboxane B2 (TXB2) (3.3 +/- 1.7%) was synthesized by these platelet-free lungs. Although the pulmonary vasoconstrictor response to arachidonic acid infusion reached a plateau in 71 +/- 19 seconds, the PG efflux was greater 8-15 minutes after beginning arachidonic acid infusion (3066 +/- 1353 ng/min) than 1-8 minutes into the infusion (899 +/- 343 ng/min). Reduction of FIO2 from 20% to 3% O2 which produces vasoconstriction in the absence of exogenous arachidonic acid, did not alter the PG profile nor quantity produced. With respect to the neonatal lamb pulmonary vasculature perfused with Krebs solution, we conclude: 1) exogenous arachidonic acid ultimately is metabolized predominantly to PGI2, 2) arachidonic acid-induced pulmonary vasoconstriction is not caused by TXA2, 3) short term hypoxia does not alter the conversion of excess arachidonic acid to PGs, and 4) a lag between intrapulmonary PG synthesis and pulmonary venous efflux may occur.
Animals↗
The use of MUMPS in a departmental computerized clinical data system.
The development of clinical data systems is an important task for family medicine academic units and residency programs. A computer system offers family medicine a unique opportunity to achieve its educational, clinical service, and research potential. MUMPS is recommended as a language and operating system. It was designed for medical information systems and is easily adapted to meet multiple needs.
Computers↗
Dosimetry of the Siemens Mevatron 67 linear accelerator.
Measurements have been made on a Siemens Mevatron 67 linear accelerator. The change of beam quality has been measured as a function of position off-axis and compared to another Siemens 6 MV linear accelerator. Similarly, beam profiles are compared to a Siemens Mevatron VI. Additional measurements include entrance dose, inverse square applicability, central axis percent depth dose, tissue-maximum ratios, output factors, wedge factors and block transmission factors. Comparison is made with an Atomic Energy of Canada Limited Therac 6 and Varian Clinac 6-100.
Particle Accelerators↗
Status report of the Cleveland Clinic Foundation National Aeronautics and Space Administration Neutron Therapy Center.
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Cancer Care Facilities↗
Calculation of the 3-D dose distribution surrounding a 103Pd stent.
PURPOSE: This study was designed to assess the suitability of a 103Pd-implanted stent for use in intravascular brachytherapy. MATERIALS AND METHODS: A stent was modeled as a superposition of 201 identical struts and the EGS4/DOSRZ Monte Carlo code was used to calculate the dose distribution for each strut. To verify the simulation parameters, doses along the transverse axis of a Model 200 103Pd interstitial seed were calculated and compared to those calculated by the TG43 method. RESULTS: Dose profiles within 1 mm of the stent's outer surface were heterogeneous and reflected the stent's structure. For a 2-mm outer-diameter 103Pd-implanted stent, approximately 2.68 x 10(7) Bq were required to deliver 31.5 Gy in 28 days at a distance of 0.5 mm along the perpendicular bisector from the stent's outer surface. The Monte Carlo simulation of the 103Pd seed showed relative doses within 7% of the values calculated by the TG43 method. CONCLUSION: The dosimetry about a 103Pd-implanted stent suggests that the stent is suitable for use in intravascular brachytherapy.
Brachytherapy↗
Physical factors affecting absorbed dose to the skin from cobalt-60 gamma rays and 25-MV x rays.
When there is an absence of scattering material adjacent to the skin on the exit side of a megavoltage beam the dose to the skin is less than would be calculated using depth dose tables or isodose distributions measured in "semi-infinite" phantoms. Ionization measurements using a thin-window parallel-plate chamber show that the dose at 4 mg/cm2 from the exit surface is 14% to 16% less for cobalt-60 gamma rays and about 8% less for 25-MV x rays compared to the dose with full backscatter. As the angle of incidence increases the skin dose increases due to radiation scattered toward the surface. A method for the calculation of skin doses from tangential therapy beams is described.
Cobalt Radioisotopes↗
Dose in the buildup region outside the primary beam.
The dose received in the buildup region outside the primary beam has been measured for 60Co gamma rays and 6- and 18-MV X rays. The variation of this dose with field size and depth was evaluated. The effect of blocking trays and the importance of their design was determined.
Cobalt Radioisotopes↗
Correlation of microdosimetric measurements with relative biological effectiveness from clinical experience for two neutron therapy beams.
Microdosimetric measurements were made for the neutron therapy beams at the University of Chicago and at the Cleveland Clinic with the same geometry and phantom material using the same tissue-equivalent spherical proportional counter and standard techniques. The energy deposition spectra (dose distributions in lineal energy) are compared for these beams and for their scattered components (direct beam blocked). The model of dual radiation action (DRA) of Kellerer and Rossi is employed to interpret these data in terms of biological effectiveness over this limited range of radiation qualities. The site-diameter parameter of the DRA theory is determined for the Cleveland beam by setting the biological effectiveness (relative to 60Co gamma radiation) equal to the relative biological effectiveness value deduced from radiobiology experiments and clinical experience. The resulting value of this site-diameter parameter is then used to predict the biological effectiveness of the Chicago beam. The prediction agrees with the value deduced from radiobiology and clinical experience. The biological effectiveness of the scattered components of both beams is also estimated using the model.
Humans↗
A microdosimetric characterization of a cyclotron-produced therapeutic neutron beam.
The therapeutic neutron beam of the Cyclotron Corporation's CP-42 negative-ion cyclotron is generated by protons of 42 MeV bombarding a thin beryllium target. Microdosimetric measurements were made for this neutron beam in a full-scatter water phantom at nine positions inside and outside the useful beam. The lineal energy distribution and the variations of dose-mean, frequency-mean, and saturated lineal energy are compared for these positions. The dose fraction due to gamma rays is also calculated at each of these positions, based upon previously published techniques. A theoretical relative biologic effectiveness, based upon the dual radiation action model of Kellerer and Rossi [Curr. Top. Radiat. Res. 8, 85 (1972)] is also shown for the positions of measurement.
Gamma Rays↗
Measurement of the dose equivalent of leakage radiation through an isocentric gantry used for neutron therapy.
The leakage radiation through the shielding on an isocentric gantry of a neutron therapy machine was measured with a Rossi-type proportional counter. The dose equivalent of the leakage radiation was determined at two positions: (1) in the plane of the patient and (2) in the plane of the target. The dose equivalent of the leakage radiation is approximately the same as the leakage of a high-energy x-ray linac.
Humans↗
Calibration frequency as determined by analysis of machine stability.
Simple statistical analysis is applied to the evaluation of the output measurements of equipment used in radiotherapy. The calibration frequency is calculated based on the stability of the equipment and the performance parameters required by the quality control criteria.
Calibration↗
Determination of the neutron and photon spectra of a clinical fast neutron beam.
A simple technique to determine the neutron and photon spectra of a clinical fast neutron beam is described. This technique involves making narrow beam attenuation measurements with a pair of ionization chambers and an iterative fitting program to analyze the data. A method is also described for determining the first-guess neutron spectrum for input into the iterative program. The results of the analysis yield spectra suitable for use in dose calculation algorithms and dosimetry protocols. Presented here is the first-known published photon spectrum from a clinical machine.
Humans↗
Gadolinium neutron capture therapy for brain tumors: a computer study.
A Monte Carlo computer study of the total dose distribution from neutrons and prompt gamma emissions (but excluding the contribution from conversion and Auger electrons) for gadolinium neutron capture therapy of brain tumors has been carried out in order to test the theoretic feasibility of this modality using commercially available magnetic resonance contrast media. The three-dimensional dose distribution calculations were performed in a spherical head phantom with a spherical tumor at the center. Potentially achievable gadolinium concentrations of 150 micrograms/g of tissue in tumor and 3 micrograms/g in normal tissue were assumed with enrichment to 79.9% gadolinium-157, as supplied by Oak Ridge National Laboratory. Irradiation was assumed to be with a 2-keV monoenergetic cylindrical epithermal neutron beam having a radius of 4 cm. The three-dimensional thermal neutron fluence resulting from the 2-keV beam propagation through the tissue was modeled. For a single neutron beam, the maximum dose is delivered within the tumor but the dose is very inhomogeneous across the tumor volume due to rapid decrease of thermal neutron fluence with depth. Two parallel opposed neutron beams deliver to the interface of normal and malignant tissue 70%-80% of the maximum dose received at the center of the tumor. To deliver an average tumor dose of 500 cGy in 10 min would require a 2-keV source neutrons number of 8.0 x 10(11) per s within the geometry of the beam.
Brain Neoplasms↗