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At least 559 records · Page 31Linked to original sources
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↗
PubMed1998

Evaluation of a diode detector array for use as a linear accelerator QC device.

A series of tests were conducted to evaluate the accuracy of a diode detector array in measuring flatness and symmetry of x-ray and electron energies, and the ability to determine change in beam energy. The results show the diode detector array accurately measures flatness and symmetry and can accurately measure or detect change in beam energy. The evaluation also demonstrates the utility of using this model detector system in a linear accelerator QC program.

Biophysical Phenomena↗
PubMed1998

Photon beam skin dose analyses for different clinical setups.

A comprehensive set of data on skin dose for 8 MV and 18 MV photon beams from a medical linear accelerator was measured using a parallel-plate chamber to document the effect of field size, source-to-surface distance (SSD), off-axis distance, acrylic block tray, wedge (external standard wedge), Lipowitz's metal block, multileaf collimator (MLC), and dynamic wedge. The skin dose increased as field size increased from 5 X 5 cm2 to 40 X 40 cm2 (6% to 38% for 8 MV and 5% to 44% for 18 MV beam). With the use of an acrylic block tray, the skin dose increased for all field sizes (7% to 59% for 8 MV and 5% to 62% for 18 MV beam), but the increase was minimal for small fields. The skin dose with a wedge showed a much more complex trend. It was generally lower than the dose for an open field, but higher in the case of large fields and higher degree wedges. When both wedge and block tray were used, the tray was a major contributor to the skin dose because some of the contaminant electrons from the wedge assembly were absorbed by the block tray. Field-shaping blocks increased the skin dose, but, interestingly, the block tray reduced the skin dose for small blocked fields treated with a high-energy photon beam. The effect of an MLC on skin dose was very similar to that of a Lipowitz's metal block, but its magnitude was less. The skin dose was higher for dynamic wedge fields than it was for standard wedge fields. As SSD decreased, the skin dose increased, and this effect was dominant in larger field sizes. The SSD effect was enhanced in the presence of an acrylic block tray. The skin dose off-axis was the same as at the central axis, or smaller. A similar pattern of behavior of the skin dose is expected for photon beams from other linear accelerators.

Acrylic Resins↗
PubMed1998

Application of a video-optical beam imaging system for quality assurance of medical accelerators.

Method validation techniques were developed and experiments were carried out using a beam imaging system (BIS, Wellhöfer Dosimetrie, Schwarzenbruck, Germany) for routine quality assurance of medical accelerators. The routine quality assurance tasks include x-ray beam flatness and symmetry check, light/radiation field congruence test, beam energy constancy for electrons and mechanical checks for couch and collimator rotations. Comparisons were made between the BIS application and conventional quality assurance methods that use radiographic films or detector arrays. In this work, we have demonstrated efficiency and accuracy of the BIS to perform some of the routine quality assurance tasks for medical linear accelerators.

Electrons↗
PubMed2001

Radiation protection at high energy proton accelerators.

The radiological problems associated with proton accelerators having maximum energies higher than a few GeV are discussed. Examples are given from accelerators where the authors have had practical experience for a number of years. The main focus will be on those problems which are unique to high energy proton accelerators, and which may not be necessarily associated with the proton beam operation itself.

Humans↗
PubMedDate not supplied

Advances in linear accelerator design for radiotherapy.

The microwave-powered electron linear accelerator, or linac, is becoming the dominant radiotherapy treatment unit. Several technical advances, combined with attention to how patients are most effectively set up and treated, have led to continuing improvements in linac radiotherapy. This review describes: improvements in accelerator structures, widely variable energy linacs , microtrons , beam transport systems, and treatment head design.

Humans↗
PubMedDate not supplied

Investigations on the near surface dose for three 10-MV x-ray beam accelerators with emphasis on the reduction of electron contamination.

A study of three 10-MV x-ray clinical accelerators with emphasis on the reduction of electron contamination was conducted. This study, which was performed with different types of trays and filters, suggests that, at 100-cm source-surface distance (SSD), Pb can be used as an effective filter material up to 30 X 30 cm2; however, due to its transparency, a Clear-Pb tray is useful for field sizes up to a 20 X 20 cm2. Percent depth doses for a few selected depths and field sizes at this nominal SSD were examined. No significant differences, with the exception of the location of Dmax, amongst the three accelerators were noticed.

Electrons↗
PubMed1980

[Dosimetry on the linear accelerator. Second communication: moving field therapy with ultrahard X-radiation (author's transl)].

Further to the description of the stationary-field irradiation with ultra-hard X-radiation (first communication) the planning of a moving field therapy is now discussed. The authors give simple criteria how to choose as exactly as possible the dose distribution adequate to the clinical plan among the available isodose plots. The indication of the necessary equipment parameters in facilitated by an irradiation table containing the average length of the moving beam. Test measures taken since 1974 on the linear accelerator Philips SL 75/10 show how good the scanning of a certain sector is reproducible.

Humans↗
PubMed2005

Diet as a factor in behavioral radiation protection following exposure to heavy particles.

Major risks associated with radiation exposures on deep space missions include carcinogenesis due to heavy-particle exposure of cancer-prone tissues and performance decrements due to neurological damage produced by heavy particles. Because exposure to heavy particles can cause oxidative stress, it is possible that antioxidants can be used to mitigate these risks (and possibly some health risks of microgravity). To assess the capacity of antioxidant diets to mitigate the effects of exposure to heavy particles, rats were maintained on antioxidant diets containing 2% blueberry or strawberry extract or a control diet for 8 weeks prior to exposure to 1.5 or 2.0 Gy of accelerated iron particles at Brookhaven National Laboratory. Following irradiation rats were tested on a series of behavioral tasks: amphetamine-induced taste aversion learning, operant responding and spatial learning and memory. The results indicated that the performance of the irradiated rats maintained on the antioxidant diets was, in general, significantly better than that of the control animals, although the effectiveness of the diets ameliorating the radiation-induced deterioration in performance varied as a function of both the specific diet and the specific endpoint. In addition, animals fed antioxidant diets prior to exposure showed reduced heavy particle-induced tumorigenesis one year after exposure compared to the animals fed the control diet. These results suggest that antioxidant diets have the potential to serve as part of a system designed to provide protection to astronauts against the effects of heavy particles on exploratory missions outside the magnetic field of the earth.

Aging↗
PubMed2001

Techniques for hazard analysis and their use at CERN.

CERN, The European Organisation for Nuclear Research is situated near Geneva and has its accelerators and experimental facilities astride the Swiss and French frontiers attracting physicists from all over the world to this unique laboratory. The main accelerator is situated in a 27 km underground ring and the experiments take place in huge underground caverns in order to detect the fragments resulting from the collision of subatomic particles at speeds approaching that of light. These detectors contain many hundreds of tons of flammable materials, mainly plastics in cables and structural components, flammable gases in the detectors themselves, and cryogenic fluids such as helium and argon. The experiments consume high amounts of electrical power, thus the dangers involved have necessitated the use of analytical techniques to identify the hazards and quantify the risks to personnel and the infrastructure. The techniques described in the paper have been developed in the process industries where they have been to be of great value. They have been successfully applied to CERN industrial and experimental installations and, in some cases, have been instrumental in changing the philosophy of the experimentalists and their detectors.

Equipment Failure↗
PubMed1986

A comparison of build-up and depth dose characteristics of different photon beams for the treatment of Hodgkin's disease.

Measurements have been performed of build-up and depth-dose characteristics of photon beams under Hodgkin's disease treatment conditions as applied in two hospitals (AVL, Amsterdam and IGR, Villejuif). Although different types of accelerators, photon energies, field sizes and SSD are employed, similar dose distributions along the beam axis have been obtained in both centers. In order to explain this unexpected good agreement, the influence of the geometrical conditions of irradiation on the build-up and depth-dose distribution has been studied in detail for five photon beams (8 MV-25 MV) of three types of accelerators.

Evaluation Studies as Topic↗
PubMed2001

Personal dose monitoring of employees at the institute of modern physics, China.

The radiation field at the accelerator facility consists of radiation produced immediately and of secondary radiation induced by activation etc. As the accelerator building and the experimental hall are closed and inaccessible during accelerator operation, the exposure received by the employees at the IMP (Institute of Modern Physics) comes almost totally from the induced radiation. The methods and the results of personal dose monitoring from 1986 to 1999 at the IMP are presented. During the period, the total number of monitored individuals was 1960, and the average annual effective dose was 0.10 mSv. The number recording less than 0.1 mSv of effective dose was 1471 individuals. amounting to 77% of the total. Only six individuals had received effective doses between 5.0 mSv and 10 mSv. The maximum effective dose of 10 mSv was received by workers repairing the accelerator.

Calibration↗
PubMedDate not supplied

A particle system for interactive visualization of 3D flows.

We present a particle system for interactive visualization of steady 3D flow fields on uniform grids. For the amount of particles we target, particle integration needs to be accelerated and the transfer of these sets for rendering must be avoided. To fulfill these requirements, we exploit features of recent graphics accelerators to advect particles in the graphics processing unit (GPU), saving particle positions in graphics memory, and then sending these positions through the GPU again to obtain images in the frame buffer. This approach allows for interactive streaming and rendering of millions of particles and it enables virtual exploration of high resolution fields in a way similar to real-world experiments. The ability to display the dynamics of large particle sets using visualization options like shaded points or oriented texture splats provides an effective means for visual flow analysis that is far beyond existing solutions. For each particle, flow quantities like vorticity magnitude and wavelength2 are computed and displayed. Built upon a previously published GPU implementation of a sorting network, visibility sorting of transparent particles is implemented. To provide additional visual cues, the GPU constructs and displays visualization geometry like particle lines and stream ribbons.

Algorithms↗