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 1,387 records · Page 77Linked to original sources

PubMedDate not supplied

Reconstruction of 4-MV bremsstrahlung spectra from measured transmission data.

Transmission data for 4-MV bremsstrahlung beams have been measured with a combination of lead and aluminum attenuators. From these data, the original energy spectra have been reconstructed using an iterative least-squares technique, previously evaluated by simulation studies. The spectra on the central axis for three similar 4-MV linear accelerators indicated no significant differences. When studying the spectra at 5 degrees and 9 degrees off the central axis, that at 9 degrees showed the expected increase of low-energy photons. All these spectra showed a maximum photon energy of 4.5 +/- 0.2 MeV. When the magnetron power was reduced, the spectrum on the central axis shifted to lower energies and the maximum photon energy decreased to 3.5 +/- 0.2 MeV. The result of this experimental study confirms the conclusions from the previous stimulation, that the numerical technique for analysis of transmission data can accurately represent 4-MV bremsstrahlung spectra and detect differences in energy distribution with changes in machine tuning and position in the radiation field for a 4-MV bremsstrahlung beam.

Particle Accelerators↗

PubMed1994

Beam profiles along the nonwedged direction for large wedged fields.

Beam profiles along the nonwedged direction of a wedged field produced by a linear accelerator exhibit more "sagging" than that of an open field at the same depth. For large fields, the profiles of open and wedged fields can differ by as much as 7%. The extra "sagging" of wedged profiles is mainly due to the difference in penetration between on- and off-axis rays caused by the variation of beam quality across the field. An algorithm was developed to estimate an "effective" depth such that the profile of a wedged field can be approximated by the open-field profile at the effective depth. The algorithm was verified by measured beam profiles for 6- and 15-MV x-ray beams for 15 degree, 30 degree, 45 degree, and 60 degree wedges.

Algorithms↗

PubMed1990

[Leakage radiations in a medical electron accelerator facility--calculation of neutron doses in the facility].

Neutron doses often come dominant in mazes of electron accelerator facilities in which X-rays of energies more than 10 MV are produced. A simple analytical method to calculate neutron doses in such a facility is developed. In the calculation procedure, it is assumed that the irradiation room is spherical in shape and the maze is cylindrical. Multiple reflection of neutrons is also considered using the albedo concept in the calculation. The procedure allows to exist a hanging wall over the entrance of the irradiation room and also multiple legs in the maze. All the parameters used in the calculation are given definitely in the procedure, and any experiment is unnecessary to determine value of the parameters. Comparison of the calculated results with experimental ones will be described in the following report.

Hospital Departments↗

PubMed1997

Local temperature and pressure variations and its effects on daily ionometric measurements of radiation beam outputs from linear accelerators.

As part of the quality assurance protocol for linear accelerators at West Virginia University Hospitals, the local temperature and pressure are recorded on a daily basis by the radiation therapists. These temperature and pressure values are not used for temperature and pressure correction unless the ionometric measurements of radiation beam outputs from linear accelerators are outside the established tolerance levels. This paper presents a study on the variation of temperature and pressure over the past two years. Analysis of the data showed that the variation of pressure is within +/- 1.5% of the average value. Likewise, the variation of temperature is within +/- 1.5% of the average value. the worst possible effects of temperature and pressure on ionometric measurements would be within +/- 3%, which is at the tolerance levels recommended by the American Association of Physicists in Medicine (AAPM) Task Group No. 13. By maintaining machine output fluctuation to within +/- 2%, the maximum deviation of uncorrected ionometric measurements would be within +/- 5%. This study collaborates with the tolerance levels suggested by the AAPM Task Group No. 40.

Humans↗

PubMed1996

HPRT mutations in V79 Chinese hamster cells induced by accelerated Ni, Au and Pb ions.

Mutation induction by accelerated heavy ions to 6-TG resistance (HPRT system) in V79 Chinese hamster cells was investigated with Ni (6-630 Me V/u), Au (2.2, 8.7 Me V/u) and Pb ions (11.6-980 Me V/u) corresponding to a LET range between 180 and 12895 ke V/microns. Most experiments could only be performed once due to technical limitations using accelerator beam times. Survival curves were exponential, mutation induction curves linear with fluence. From their slopes inactivation- and mutation-induction cross-sections were derived. If they are plotted versus LET, single, ion-specific curves are obtained. It is shown that other parameters like ion energy and effective charge play an important role. In the case of Au and Pb ions the cross-sections follow a common line, since these ions have nearly the same atomic weight, so that they should have similar spatial ionization patterns in matter at the same energies. Calculated RBEs were higher for mutation induction than for killing for all LETs.

Animals↗

PubMed1999

Linear accelerator radiosurgery in brain tumor management.

This article begins with a brief introduction to the concepts and techniques of linear accelerator-based stereotactic radiosurgery. The expanding role of radiosurgery in the treatment of brain tumors is explored in depth, including detailed discussions of the five intracranial neoplasms most frequently treated with radiosurgery. These include both benign (i.e., vestibular schwannoma, meningioma, pituitary adenoma) and malignant (i.e., cerebral metastasis, malignant glioma) pathologies. For each of these, a thorough review of published radiosurgical results is presented along with a discussion of common treatment modalities. The role of radiosurgery in the treatment of brain tumors continues to be defined, but an effort is made to provide reasonable indications for and against radiosurgery based on the current state of the art.

Adenoma↗

PubMed1995

Wedge factor changes with depth and field size on a linear accelerator with a motorized wedge.

The wedge factor was experimentally determined as a function of field size and phantom depth for 6-MV X-rays; measurements were performed on a linear accelerator with a motorized wedge filter (universal wedge, nominal wedge angle = 60 degrees). As a result of our experimental work, we can state that: 1) The wedge factor increases linearly with phantom depth (0.275%/cm), almost independent of field size; this change was experimentally related to the beam "hardening" caused by the high Z value of wedge material, as shown by the increase in wedged-field PDD with respect to the corresponding open-field PDD. Neglecting this variation can induce systematic errors in delivered dose, especially when deep-seated tumors are treated with wedge fields. 2) The wedge factor changes, for rectangular fields, according to the "equivalent square law", and not to the dimension in the wedged direction.

Filtration↗

PubMedDate not supplied

Neutron leakage measurements from a medical linear accelerator.

The McCall method has been used to measure neutron leakage from the Mevatron 77, 18- and 15-MV photon beams. Gold foil activation has been used employing a beta counting technique for the 18-MV beam and a gamma counting technique for both the 18- and 15-MV beam. The two counting techniques were used to evaluate their relative merit. The measurements were made at various locations in the patient-treatment plane for different field sizes. The results show that the thermal-neutron dose equivalent contributes only about 1%-2% of the total neutron dose equivalent. At 100 cm, the neutron dose equivalent for the 18-MV beam is approximately six times that of the 15-MV beam, slightly exceeding the 0.1% of the useful beam criteria used by some of the regulatory agencies. In light of the uncertainty in fluence to dose equivalent conversion factors, the increased dose equivalent above 0.1% is insignificant.

Neutrons↗

PubMed1994

Head-scatter factors in blocked photon fields.

The behavior of the head-scatter factor in shielded 6 and 25 MV X-ray beams from a Philips SL25 linear accelerator was investigated by measuring incident fluences by direct (in-air) and indirect (in-phantom) methods. It was found that perturbations in head-scatter produced by shielding blocks arranged to define a slit-shaped field are considerably less than 1% in unwedged beams, even when 80% of a 20 x 20 cm2 field is shielded. The results are independent of beam energy and orientation of the slit with respect to the collimator jaws. When a 60 degrees wedge is inserted, the head-scatter factor decreases by up to a few percent, depending on slit direction but not on energy. The contributions to head-scatter from the block tray and the shielding blocks are negligible.

Acrylic Resins↗

PubMed2005

Electron beam irradiation of dental composites.

OBJECTIVES: Electron beam irradiation can be used to influence the mechanical properties of polymers. It was the aim of this study to investigate whether dental composites can benefit from irradiation in order to achieve increased fracture toughness, work of fracture, hardness or less wear. METHODS: Two hundred rectangular specimens of five veneering composites were electron beam irradiated with 25, 100 and 200 kGy using an electron accelerator of 10 MeV. Fracture toughness, work of fracture, Vickers hardness, color changes and three-medium wear were measured and compared with non-irradiated specimens. RESULTS: Visible color changes (DeltaE>3) were observed with all composites and with all dose rates. Fracture toughness, work of fracture, Vickers hardness and resistance against wear increased significantly with few exceptions. Composites with a simple curing process needed higher dose rates while systems with a more complex curing procedure should be irradiated with lower dose rates. SIGNIFICANCE: Electron beam irradiation can significantly change the mechanical properties of dental composites. However, color changes can limit the use of irradiation for dentistry.

Chemical Phenomena↗

PubMed2003

Effectiveness of customised neutron shielding in the maze of radiotherapy accelerators.

An investigation was performed to examine the neutron dose equivalent in a radiotherapy maze lined with a customised neutron shielding material. The accelerator investigated was a Varian Clinac 2100C/D using 18 MV photons, and the neutron shielding utilised at this centre was Premadex commercially available neutron shielding. Based on Monte Carlo simulations, properly installed customised neutron shielding may reduce the neutron dose equivalent by up to a factor of 8 outside the maze, depending upon the installation. In addition, it was determined that the neutron dose near the entrance to the maze may be reduced by approximately 40% by using customised neutron shielding in the maze, as compared with a facility not using this shielding. This would have a positive dose-saving effect in doorless maze designs.

Body Burden↗

PubMed1990

Use of electrons for radiation therapy in France.

The development of high energy radiotherapy equipments in France between 1950 and 1990 is analyzed. The use of betatrons after a growth between 1960-1970 decreased progressively from 1975 in favour of linear accelerators (133 in 1989). Cobalt-therapy, rapidly grown up between 1960 and 1980, has reached a plateau phase with a trend to being progressively substituted by low energy linac. The dosimetric aspects are considered: calibration of electron beams, quality control and treatment planning. About 22000 cancer patients were treated in 16 Centres in 1983; of these only 23% (mean value) received electron therapy, but the number varies greatly from one center to another (9-45%). The major indications for electrons in France are: neck nodes, operated breast, internal mammary chain. The general trend is to restrict their use to lesions not deeper than 4-5 cm and first of all to employ them as a boosting treatment, after an X ray irradiation.

Electrons↗

PubMed1989

[Radiation treatment of intracranial arteriovenous malformations--experiences with a semi-stereotactic technic].

Since 1982 79 patients with intracranial AVM's were irradiated at linear accelerator with 10 MV photons. Diagnosis, localization and therapy were done immobilizing the head for identical positioning. In a controlled study the therapy was done with fractionated irradiation up to 50 Gy in 25 fractions within 5 weeks or with 20 Gy in 4 fractions within 7 days. Complications, due to therapy, did not occur. Suffering from convulsion was not affected. Five patients have died, one patient got a hemorrhage 36 months later. The angiographical analysis of the first 25 patients until 30 months after radiotherapy gave complete obliterations and reduction of volume in more than 50% with minor AVM's. The result are not detrimental compared with a group after photon irradiation.

Adolescent↗

PubMed2006

Analysis of quality control data of eight modern radiotherapy linear accelerators: the short- and long-term behaviours of the outputs and the reproducibility of quality control measurements.

Quality control (QC) data of radiotherapy linear accelerators, collected by Helsinki University Central Hospital between the years 2000 and 2004, were analysed. The goal was to provide information for the evaluation and elaboration of QC of accelerator outputs and to propose a method for QC data analysis. Short- and long-term drifts in outputs were quantified by fitting empirical mathematical models to the QC measurements. Normally, long-term drifts were well (< or =1%) modelled by either a straight line or a single-exponential function. A drift of 2% occurred in 18 +/- 12 months. The shortest drift times of only 2-3 months were observed for some new accelerators just after the commissioning but they stabilized during the first 2-3 years. The short-term reproducibility and the long-term stability of local constancy checks, carried out with a sealed plane parallel ion chamber, were also estimated by fitting empirical models to the QC measurements. The reproducibility was 0.2-0.5% depending on the positioning practice of a device. Long-term instabilities of about 0.3%/month were observed for some checking devices. The reproducibility of local absorbed dose measurements was estimated to be about 0.5%. The proposed empirical model fitting of QC data facilitates the recognition of erroneous QC measurements and abnormal output behaviour, caused by malfunctions, offering a tool to improve dose control.

Calibration↗

PubMed2004

In vivo dosimetry for external photon treatments of head and neck cancers by diodes and TLDS.

In vivo dosimetry was implemented for treatments of head and neck cancers in the large fields. Diode and thermoluminescence dosemeter (TLD) measurements were carried out for the linear accelerators of 6 MV photon beams. ESTRO in vivo dosimetry protocols were followed in the determination of midline doses from measurements of entrance and exit doses. Of the fields monitored by diodes, the maximum absolute deviation of measured midline doses from planned target doses was 8%, with the mean value and the standard deviation of -1.0 and 2.7%. If planned target doses were calculated using radiological water equivalent thicknesses rather than patient geometric thicknesses, the maximum absolute deviation dropped to 4%, with the mean and the standard deviation of 0.7 and 1.8%. For in vivo dosimetry monitored by TLDs, the shift in mean dose remained small but the statistical precision became poor.

Calibration↗

PubMed2004

A procedure to determine the radiation isocenter size in a linear accelerator.

Measurement of radiation isocenter is a fundamental part of commissioning and quality assurance (QA) for a linear accelerator (linac). In this work we present an automated procedure for the analysis of the stars-shots employed in the radiation isocenter determination. Once the star-shot film has been developed and digitized, the resulting image is analyzed by scanning concentric circles centered around the intersection of the lasers that had been previously marked on the film. The center and the radius of the minimum circle intersecting the central rays are determined with an accuracy and precision better than 1% of the pixel size. The procedure is applied to the position and size determination of the radiation isocenter by means of the analysis of star-shots, placed in different planes with respect to the gantry, couch and collimator rotation axes.

Biophysical Phenomena↗

PubMedDate not supplied

Wedge-shaped dose distributions by computer-controlled collimator motion.

We have recently installed a linear accelerator, modified to allow computer control of several machine parameters during irradiation of the patient. As an initial feasibility study of computer-controlled radiation therapy, its application to produce wedge-shaped dose distributions by moving the collimator jaws has been evaluated. The required collimator motions have been calculated with an iterative technique. When these routines were used during irradiations of phantoms containing radiographic film, a good correspondence between calculated and measured dose distributions was observed. It is concluded that computer-controlled motion of the collimator jaws to shape the dose distribution is technically feasible. Additionally, this technique has the advantage that the wedge angle can be continuously adjusted and the isodose curves optimized for a particular depth and field size.

Computers↗

PubMed1989

[Treatment technic and clinical dosimetry in whole-body irradiation].

At the Istituto Nazionale Tumori, Milan, total-body irradiation (TBI) is delivered by a 15 MV linear accelerator, with two lateral opposed beams. Maximum build-up at the skin is achieved by lateral slabs of perspex 3 cm thick. Attenuation filters or bolus are used for dose compensation, or reduction, to the head and lungs. The dose delivered to clinically relevant anatomic regions is determined by "in vivo" dosimetry. For this purpose, calibrated diodes are employed, which are positioned at the entrance and at the exit of the beams. "In vivo" dosimetry data show our TBI technique to allow an homogeneous irradiation of all body areas, with maximum deviation of the mean dose value from reference point dose of -11% in the posterior abdomen, at the spinal cord shielded by arms.

Humans↗