SEARCH · PubMed Health

Results for “Particle Accelerators”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 919 records · Page 51Linked to original sources

PubMedDate not supplied

Results of photon absorbed-dose measurements using the AAPM TG-21 protocol for accelerating potentials up to 26 MV.

The AAPM Task Group 21 protocol for the calibration of high-energy photon and electron beams was produced to accomplish essentially two goals: (1) incorporate the latest physical data available for calculating absorbed dose from ionization measurements and (2) to eliminate inconsistencies in absorbed dose measurements made with various ion chamber and phantom combinations. The ability of the protocol was assessed to consistently determine x-ray absorbed dose from measurements made with four Farmer-type chambers and one parallel-plate chamber in water, polystyrene, and acrylic phantoms. The measurements were performed using seven high-energy x-ray beams from 60Co to 26-MV nominal accelerating potential. The absorbed dose to water calculated from measurements made with the various chamber and phantom combinations were found to be consistent. The doses calculated for the two most common phantom materials, water and polystyrene, were found to be in excellent agreement. This resolved a 1.6% discrepancy in the absorbed dose determined from the two phantoms using the SCRAD protocol. The doses for acrylic phantoms were found to be approximately 1.2%, low for nominal accelerating potentials less than 8.8 MV. For accelerating potentials of 8.8 MV or greater the agreement was considerably better. The mean dose determined for the parallel-plate chamber from measurements in polystyrene was found to be within 0.7% of the mean dose determined using Farmer-type ion chambers in all phantom materials.

Humans↗

PubMed1995

X-ray source and the output factor.

When the collimator setting of a linear accelerator is made sufficiently small, the output factor in air, R, is greatly reduced because the collimators obstruct the periphery of the x-ray source. This has been utilized to examine the size of the source by varying the width y of a narrow field and determining how R(y) varies. The sources diameters in the two principal directions were clearly influenced by the design of the accelerators. The x-ray sources of two accelerators with bending magnets were found to be noncircular while that of a linear accelerator without a magnet showed circular symmetry. The position of the source relative to the axis of collimator rotation was determined by measuring R for offset narrow fields. For one of the accelerators, the source was initially moving and off the central axis by about 2 mm for the first five monitor units. The results correlated well with sharpness in portal-film images. The technique can serve to evaluate the major source characteristics in acceptance testing and quality control.

Equipment Design↗

PubMed1978

A study of dose distribution patterns from cobalt-60 and Varian Clinac-4 beams.

Dose distribution patterns of 4 MV x-rays from a Varian Clinac 4 linear accelerator are compared with those from a 60Co teletherapy machine. Field flatteners of lead and depleted uranium were used. Dose distribution maps display the off-axis high dose or "horns" with lead filters and circular dose distribution patterns. The uranium field flattener clearly "clipped the horns" and eliminated circular dose distribution patterns with larger fields.

Cobalt Radioisotopes↗

PubMed1992

Physics and dosimetry of the gamma knife.

Since 1968, the gamma knife has been one of the major radiosurgical devices. Although approximately 4300 patients worldwide had been treated with the gamma knife units through June 1990, gamma knife installments in the United States are still rather scarce compared to linear accelerators adapted for radiosurgery. This article describes the basic physical characteristics of the gamma knife, patient set-up procedures, the existing treatment-planning system, the measurements of dosimetry and physical parameters, dose delivery accuracy, and quality assurance procedures. It also includes a vision of future developments and improvements in these areas.

Brain Neoplasms↗

PubMedDate not supplied

Stereotactic radiosurgery of arteriovenous malformations.

Stereotaxis has been introduced at the Joint Center for Radiation Therapy in Boston, MA, to aid in the precise localization and treatment of arteriovenous malformations (AVMs). A Brown-Roberts-Wells stereotactic system and a 6 MV linear accelerator have been modified for these treatments. By using multiple isocentric rotational fields, radiation doses from 1500 to 2500 cGy can be safely prescribed to the AVM in efforts to occlude the blood vessels without risking damage to nearby radiosensitive structures. Sixteen patients have been treated from February 1986 to July 1988 using the technique described.

Arteriovenous Malformations↗

PubMed2005

[Cost of quality assurance in radiotherapy: human and material requirements].

In 2004, three new important laws were passed concerning radiotherapy services. The first two concerns the internal and external quality control of linear accelerators and the last concerns the role of the medical physicist, whose presence was made mandatory during the whole length of the treatments. These laws, which aim to improve the quality and the security of treatments, represent an increase in price that we have calculated, and which prevents them being implemented, as a joint study realised by the SFRO and the SFPM has shown. The cost of quality in radiotherapy requires investment in material and manpower and improvement in availability of the accelerators which entails a complete reorganisation of the services. Cost analysis is included. The difficulties in implementing these laws have also been evaluated and this evaluation already enables us to propose certain elements enabling us to go forward to globally improve the quality and security in radiotherapy.

Costs and Cost Analysis↗

PubMed1997

Errors in three-dimensional doses calculated from a two-dimensional database--case report: wedged fields at 6 MV.

This report discusses the calculation of x-ray doses in three dimensions using a treatment planning database which was measured in two dimensions only. It concerns the common assumption that wedged field profiles in the non-wedged direction are similar to open-field profiles for the same field size and depth. It shows the extent to which this assumption can lead to errors in wedge dose calculation for both solid and dynamically wedged fields on Varian linear accelerators at 6 MV. Finally it shows that this assumption tends to produce more accurate results when used to calculate doses for dynamically wedged fields and why, even in the wedged direction, some of the simpler treatment planning algorithms are more suitable for dynamic wedges than they are for solid wedges.

Algorithms↗

PubMed2003

Low-dose radiosurgery for benign intracranial lesions.

This study assesses the efficacy and neurotoxicity of radiosurgical treatment of benign intracranial tumors using a linear accelerator, with relatively low dose and homogeneous dosimetry. Between June 1998 and July 2000, 27 patients were treated for benign lesions with radiosurgery using a 6-MV linear accelerator-based X-knife system and circular collimators. The lesions included schwannoma, meningioma, papillary cyst adenoma, and hemangioblastoma. Five patients had tissue diagnosis. The mean peripheral dose to the tumor margin was 12.8 Gy. The mean dose to the isocenter was 16.3 Gy. One to five isocenters were used to treat these lesions, with a mean of 10 arcs per isocenter and mean collimator size of 1.25 cm. Follow-up information was available on all patients, with a mean follow-up duration of 33 months. Six patients (22%) had improved symptoms and 21 (78%) had stable symptoms. Eight patients (30%) had regression of tumor and 19 had stable disease (70%). No patient had tumor progression, and Radiation Therapy Oncology Group (RTOG) grade III or IV toxicity did not occur in any patients. In 3 patients (11%), RTOG grade I or grade II neurotoxicity developed. Of these, one patient had worsening of a preexisting VIIth nerve deficit that required temporary oral methylprednisolone, and in two patients a mild trigeminal deficit developed that did not require any medical intervention. Low-dose homogeneous radiosurgery using a linear accelerator is an effective treatment for benign intracranial tumors. If lower, more homogeneous radiation doses produce responses as durable as higher doses, then toxicity might be further reduced.

Adult↗

PubMed2005

Derivation of the distribution of extrafocal radiation for head scatter factor calculation.

Head scatter factors for high energy photon beams from linear accelerators can be modeled using a two-source model consisting of focal and extrafocal radiation. The focal radiation can be approximated as a point source, and the distribution of the extrafocal radiation is a two-dimensional (2D) radial symmetric function. Various methods, including analytical, Monte Carlo, and empirical trial functions, have been used to determine the radial symmetric function of extrafocal radiation distribution. This article describes a method for directly determining the extrafocal radiation distribution without assuming any empirical trial function. The extrafocal radiation distribution is determined with measured head scatter factors for rectangular fields defined by the lower jaw (X) fixed at 40 cm and the upper jaw (Y) varying from 3 to 40 cm. The derivatives of the measured head scatter factors, with respect to the Y jaw position projected in the plane of extrafocal radiation, are proportional to the one-dimensional (1D) projection (also called the line spread function) of the extrafocal radiation distribution. Two methods are used to solve the radial function of extrafocal radiation from the 1D projection. The first method uses a 2D filtered backprojection algorithm, originally developed for parallel beam computed tomography reconstruction, to directly derive the radial dependence of the extrafocal radiation distribution. The method has been applied to 6 and 18 MV photon beams from a Siemens linear accelerator and has been tested by comparing measured and calculated head scatter factors for square and rectangular fields. The second method uses a Fourier transform followed by a Fourier-Bessel transform to solve the problem. The distributions of extrafocal radiation derived from these two methods are virtually identical.

Algorithms↗

PubMedDate not supplied

Demonstration of relatively new electron dosimetry measurement techniques on the Mevatron 80.

A comprehensive set of electron dosimetry measurements at 7, 10, 12, 15, and 18 MeV was made on a Mevatron 80. Dosimetry measurements presented include percentage depth dose, dose in the buildup region, field size dependence of output, output at extended distances, lead transmission measurements, and isodose curves. These beam measurements are presented to document the electron beam characteristics of this linear accelerator. Three relatively new dosimetry techniques, which have not been standardly used in the past, are illustrated. One technique determines the depth dose of fields too small to measure. A second technique accurately converts depth dose measured in polystyrene to depth dose in water. A third technique calculates the output at extended distances.

Electrons↗

PubMed2003

Gentlemen (and ladies), choose your weapons: Gamma knife vs. linear accelerator radiosurgery.

This article compares and contrasts Gamma Knife radiosurgery with linear accelerator-based radiosurgery; where appropriate, Cyberknife technology is discussed. Topics covered are: positioning of the head (invasive versus non-invasive positioning systems); collimator construction; beam properties; beam arrangements; treatment planning; and issues regarding manpower (including a discussion of patient repositioning during treatment), machine availability, and financial considerations.

Brain Neoplasms↗

PubMed2000

Evaluation of the mechanical alignment of a linear accelerator with an electronic portal imaging device (EPID).

Mechanical misalignment of a medical linear accelerator can be caused by any combination of source position displacement relative to the collimator rotational axis, collimator jaw asymmetry, or when the rotational axes of the gantry and the collimator do not intersect. A test procedure sensitive to all of these problems has been developed using an Electronic Portal Imaging Device (EPID). Each marker is placed on top of the EPID housing and on the treatment couch, then two images are acquired for gantry positions 180 degrees apart. By comparing the positions of the markers and their distances to the beam centre, mechanical alignment of the linear accelerator can be assessed. By comparing the positions of the beam centre for another two images acquired at collimator angles 180 degrees apart, the three potential sources of misalignment can be distinguished. Results with Siemens' Beam View were presented and determination accuracy of better than 0.25 mm can be achieved.

Humans↗

PubMed1987

[Comparison between 2 methods of determining photon-induced air activity around medical accelerators].

The authors compare two different methods to determine the activity of the air induced by the bremsstrahlung of medical accelerators. The activities of the air resulting from maximum photon energies of 16 to 42 MeV are measured with a proportional counter tube ("direct measurement") and by means of activation analyses of imidazole, triazole, malonic acid, and urea specimens ("indirect measurement"). The results are compared.

Activation Analysis↗

PubMed1981

[Nuclear processes on operation of linear accelerators (author's transl)].

The primary nuclear reactions possibly occurring in accelerators operated on an energy level between 8 and 20 MeV are outlined, and their probability is estimated. Furthermore, the authors discuss which photoneutron rates par micron As are to be expected in case of different threshold energies in a tungsten sheet with a thickness of 6 mm and which secondary nuclear processes are induced by the photoneutrons.

Neutrons↗

PubMed1995

Harvesting backscatter electrons for radiation therapy.

PURPOSE: An innovative technique is used to harvest backscatter electrons for the treatment of superficial small lesions of skin, oral cavity, and rectum where a significant dose gradient and maximum surface dose is desired. METHODS AND MATERIALS: Backscatter electrons are harvested out of the primary electron beams from the linear accelerators. The design consists of a short cylindrical cone that fits snugly over a long cylindrical electron cone. The short cylindrical cone has a thick circular plate of high atomic number medium (Pb) attached to the distal end, and a lateral slit of variable length and width. The width of the slit could be closed as desired by rotating the two cones and the length can be increased by lowering the short cylindrical cone. Primary electrons strike the Pb plate perpendicularly and produce backscatter electrons that pass through the lateral slit for treatment. Using film and a parallel plate ion chamber, backscattered electron dose characteristics are studied. RESULTS: The depth dose characteristic of the backscatter electron is very similar to that of the 0.2 mm Al half-value layer x-ray beam that is commonly used for the intracavitary and superficial lesions. The backscatter electron energy is nearly constant and effectively < or = 1 MeV from the clinical megavoltage beams. The backscatter electron dose rate of 0.32-0.8 Gy/min could be achieved from modern accelerators without any modification. The beam flatness is dependent on the slit size and the depth of treatment, but is satisfactory to treat small lesions. CONCLUSIONS: The measured data for backscatter electron energy, fluence, depth dose, flatness, dose rate, and absolute dose indicates that the harvested backscattered electrons are suitable for clinical use.

Electrons↗

PubMed2005

[Computerized radiotherapy planning: retrospective analysis and current methods].

PURPOSE: To present the development of treatment planning methods at the National Institute of Oncology (NIO) from 1969 till 2005. METHODS: The methods and devices of treatment planning is described chronologically. RESULTS: First we did the treatment planning with in-house made devices: body contour drawing instrument, simplified anatomical cross sections, treatment planning table for the cross section projection, archives of isodose curves adjusted to body contours, etc. It was a significant improvement when the graphical addition of isodose curves was followed by computerized dose calculation. In 1978 the work of the Computerized National Treatment Planning Network was started. The Network was organized by IAEA, Ministry of Health Hungary and NIO. The modern treatment planning started at NIO in 1981. From this year, the treatment planning was based on CT, using the CT apparatus of the Medical Postgraduate University. In 1991 a Siemens MEVAPLAN treatment planning system was installed at NIO. The CT data were transferred to the system via floppy disk. The 3D treatment planning program (Pinnacle software of ADAC) started in 2000. The CT, the treatment planning system and newer linear accelerators are connected through the computerized radiotherapy network. Patient positioning, fixing and control devices (mask, EPID etc.) increased the efficacy of the treatment. In-house made devices help this aim too: mirrors at the linear accelerators, special skin marks for CT, block verification unit, multileaf collimator for x-ray treatment simulator. In this year the intensity-modulated radiotherapy (IMRT) will be started at NIO. CONCLUSIONS: The treatment planning at NIO developed to high degree during the investigated time, and it had a considerable effect on the efficacy of radiotherapy.

Humans↗

PubMed2002

[Quality control of equipment used for radiotherapy].

Due to the modifications of components, to unexpected breakage of elements or to electronic dysfunctions, the performance of radiotherapy machines may decrease with age. Quality Assurance procedures and maintenance program are necessary to guarantee the performances. For linear accelerators, modus operandi of control tests and their frequency are based on regulations and recommendations widely published, that are presented here. Concerning accessories, especially those recently developed (multileaf collimators, dynamic wedges, ...), recommendations remains to be defined. Simple tests are proposed. Concerning numerical imaging systems, widely used for three dimensional dosimetry, image quality and geometry controls must be performed with phantom tests. For portal imaging, a quality assurance program is proposed. A strict and complete Quality Assurance program is essential to guarantee quality and safety of the treatment. A regular control of linear accelerator is one of the important component of this program. It suppose the implementation of permanent tests procedures, periodically modified following technological progresses and treatment techniques. Measurements must be sensible to variations below the tolerance level defined during the installation process. The analysis of the variations of measurements with time are an objective criterion of quality.

Equipment Safety↗

PubMed2001

Radiation protection at high energy electron accelerators.

An overview is presented of radiation protection at high energy electron accelerator facilities. By 'high energy' is meant the energy domain beyond a few tens of MeV, where electromagnetic showers are the determining and dominant factor for beam interactions with matter. The basic components of electron accelerators are described and their potential impact on radiation safety. The paper then concentrates on sources of prompt radiation which distinguish these machines from other accelerator facilities and briefly describe other features such as shielding or safety systems as relevant to electron machines. A more comprehensive description of these aspects can be found elsewhere in these proceedings. In addition, general concepts presented in this review are complemented and illustrated by specific examples in the authors' synchrotron radiation work in these proceedings.

Electromagnetic Fields↗