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 1,513 records · Page 84Linked to original sources

PubMed1995

Investigation of applicability of alanine and radiochromic detectors to dosimetry of proton clinical beams.

Cancer therapy studies using proton accelerators are underway in several major medical centers in the U.S., Russia, Japan and elsewhere. To facilitate dosimetry intercomparisons between these laboratories, alanine-based detectors produced at the National Institute of Standards and Technology and commercially available radiochromic films were studied for their possible use as passive transfer dosimeters for clinical proton beams. Evaluation of characteristics of these instruments, including the LET dependence of their response of proton energy, was carried out at the Institute of Theoretical and Experimental Physics. Results of absolute dose measurements were regarded as a preliminary step of dose intercomparison between ITEP and NIST. Measurements made in a number of experiments showed average agreement between the ITEP and NIST dosimetry standards to 2.5%.

Alanine↗

PubMed2003

The use of trans-vinylene formation in quantifying the spatial distribution of electron beam penetration in polyethylene. Single-sided, double-sided and shielded irradiation.

The use of the quantification of trans-vinylene unsaturations in irradiated ultrahigh molecular weight polyethylene (UHMWPE) in determining the spatial distribution of electron beam penetration was investigated. UHMWPE was irradiated with a 10MeV linear electron beam accelerator to 100kGy at both room temperature and 125 degrees C in air. The irradiation was carried out in shielded and unshielded single-sided and unshielded double-sided irradiation modes. Dose-depth profiles were measured by quantifying the yields of trans-vinylene unsaturations as a function of depth away from the e-beam incidence surfaces using infrared spectroscopy. The extent of beam penetration increased with increasing irradiation temperature as was manifested by the increase in the iso-dose penetration with the unshielded single-sided irradiation and increased dose overlap with the unshielded double-sided irradiation. The optimum thickness with maximum uniformity in dose-depth distribution for double-sided irradiation was 85 and 90mm for 25 degrees C and 125 degrees C irradiation temperatures, respectively.

Biocompatible Materials↗

PubMed2002

Simultaneous exposure of mammalian cells to heavy ions and X-rays.

Crews of space missions are exposed to a mixed radiation field, including sparsely and densely ionizing radiation. To determine the biological effectiveness of mixed high-/low-LET radiation fields, mammalian cells were exposed in vitro simultaneously to X-rays and heavy ions, accelerated at the HIMAC accelerator. X-ray doses ranged from 1 to 11 Gy. At the same time, cells were exposed to either 40Ar (550 MeV/n, 86 keV/micrometers), 28Si (100 MeV/n, 150 keV/micrometers), or 56Fe (115 MeV/n, 442 keV/micrometers) ions. Survival was measured in hamster V79 fibroblasts. Structural aberrations in chromosome 2 were measured by chemical-induced premature chromosome condensation combined with fluorescence in situ hybridization in isolated human lymphocytes. For argon and silicon experiments, measured damage in the mixed radiation field was consistent with the value expected using an additive function for low- and high-LET separated data. A small deviation from a simple additive function is observed with very high-LET iron ions combined to X-rays.

Animals↗

PubMed1997

The irradiation of blood and blood components to prevent graft-versus-host disease: technical issues and guidelines.

In recent years, there have been several advances in blood irradiation practice. These include a better definition of the most appropriate dose level that should be used when irradiating blood components. Commercial innovation has provided the tools for a quality assurance program to assess the dose that is delivered throughout the canister in a free-standing irradiator, and, through the use of radiation-sensitive indicator labels, to confirm that the irradiation process has taken place. With the apparent increased use of linear-accelerators to irradiate blood components, appropriate quality assurance measures need to be developed. The maximum storage period for irradiated red cells should be shorter than for nonirradiated red cells if the treatment is performed early during the storage period because irradiation reduces the in vivo 24-hour red cell recovery parameter. The storage period for irradiated platelets does not need to be modified. Some questions are being raised regarding whether fresh-frozen plasma should be irradiated to inactivate a small number of immunocompetent progenitor cells that may be present. Table 4 summarizes the practices that should be followed in connection with the technical issues that have been addressed in this article. These guidelines follow the recommendations issued in July 1993 by the FDA in the United States. This article and Tables 1 and 2 contain additional guidelines.

Blood Cells↗

PubMedDate not supplied

[Comparative dosimetry study of two methods of intensity modulation performed on the same accelerator].

Intensity modulated radiation therapy (IMRT) is an advanced method of conformal radiotherapy. It permits optimal dose distribution to the target volume while preserving surrounding normal tissues. IMRT, with a multileaf collimator, can be realised in two different ways: either the segmented mode, which consists of combining small elementary static field, or the dynamic mode, which consists of moving the leaves while irradiating. The purpose of this work was to study these two methods of modulation on a Varian linear accelerator equipped with a collimator consisting of 40 pairs of one-centimetre-wide leaves. The measurements, obtained by using a diode array, showed that the quality of the irradiation in the dynamic mode does not depend on either the dose rate or the duration of the irradiation. In the segmented mode, weak magnitude segments are preferable, but increase the errors in the delivered dose. Comparisons of various profiles showed that the measured profiles are consistent with those programmed. Both modes seem to be equivalent for step-shaped profiles. In the case of profiles with constant slope, the segmentation generated by the segmented method deteriorates the profile. Even though the choice of technique is strongly dependent on the material available, the dynamic mode presents greater flexibility of use and has been chosen in our institution for IMRT.

Calibration↗

PubMed1997

Determination of aluminium-26 in biological materials by accelerator mass spectrometry.

Studies of the biological chemistry of aluminium can gain significantly from the use of the long-lived isotope 26Al as a tracer, although the cost of the isotope often precludes its determination by radiochemical counting techniques. Accelerator mass spectrometry (AMS) provides an ultra-sensitive method of determination, free from isobaric interference from atomic (26Mg) or molecular species. The source materials for AMS can be aluminium oxide or phosphate, both of which can be readily prepared at a sufficient level of purity from biological substrates. Natural aluminium (27Al, 100%) is added to the preparations as a chemical yield monitor and to provide the reference for the isotope ratio measurement. 26Al/27Al ratios can be determined over the range 10(-14)-10(-7), implying a limit of detection for 26Al of around 10(-18) g. The precision of measurement and long-term reproducibility are < 5% and < 7% (RSD), respectively. Chemical methodologies for routine measurements on blood and urine samples have been developed.

Aluminum↗

PubMed2005

A preliminary dosimetric characterization of chemical vapor deposition diamond detector prototypes in photon and electron radiotherapy beams.

Three radiation detectors based on polycrystalline diamond films with different thickness and resistivity, obtained by microwave chemical vapor deposition, were tested to assess their suitability for relative dosimetry of photon and electron beams supplied by clinical linear accelerators. All samples showed a linear response as a function of the absorbed dose. The sensitivity per unit of detector sensitive volume spanned between 7 and 43 nC Gy(-1) mm(-3) with an applied electric field of 40 kV/cm. The dose rate dependence was evaluated following the Fowler theory and delta coefficient values between 0.95 and 1.00 were found for the three samples when polarized at 40 kV/cm. Percentage depth dose curves, output factors, and normalized dose profiles were determined for 6 and 10 MV photon beams and for 6 and 15 MeV electron beams. The results obtained with the diamond detectors were in good agreement with those obtained by reference detector measurements [all the data were within the experimental uncertainty of 1% (1sigma)].

Crystallization↗

PubMedDate not supplied

Reduction of the "horns" observed on the beam profiles of a 6-MV linear accelerator.

The presence of large "horns" was found while plotting beam profiles during acceptance testing of a 6-MV linear accelerator. The in-phantom off-axis ratio (OAR), measured at 22 cm off the central axis along the diagonal of a 40 X 40 cm field at dmax was found to be 1.19, while beam uniformity was within specifications at 10-cm depth. A change in the gun injection voltage and the replacement of the magnet surrounding the magnetron with one of greater strength resulted in a reduction of the OAR to 1.085. The beam uniformity at depth was maintained within specifications. An alternative solution of adding a modifying filter in the primary beam was considered undesirable because of the 20%-25% reduction in dose rate caused by such filters. The relationship between the energy, the intensity distribution of the beam, and the magnitude of the horns is discussed, and the beam profiles, isodoses, and central axis depth doses before and after the changes are compared.

Humans↗

PubMedDate not supplied

Optimization of parameters for fitting linear accelerator photon beams using a modified CBEAM model.

Measured beam profiles and central-axis depth-dose data for 6- and 25-MV photon beams are used to generate a dose matrix which represents the full beam. A corresponding dose matrix is also calculated using the modified CBEAM model. The calculational model uses the usual set of three parameters to define the intensity at beam edges and the parameter that accounts for collimator transmission. An additional set of three parameters is used for the primary profile factor, expressed as a function of distance from the central axis. An optimization program has been adapted to automatically adjust these parameters to minimize the chi 2 between the measured and calculated data. The average values of the parameters for small (6 X 6 cm2), medium (10 X 10 cm2), and large (20 X 20 cm2) field sizes are found to represent the beam adequately for all field sizes. The calculated and the measured doses at any point agree to within 2% for any field size in the range 4 X 4 to 40 X 40 cm2.

Humans↗

PubMed1999

Lung compensator design using an electronic portal imaging device.

Using a liquid filled electronic portal imaging device (EPID) installed on a linear accelerator and a composite chest phantom, exit dose measurements were carried out to establish an empirical relationship between the pixel values of the imaging detector and the corresponding equivalent thickness of the overlying phantom material. Results for 6 and 10 MV photons show that the relationship depends on the so-called input/output characteristics of the imaging device for a particular photon energy. For a chest irradiation, an EPID image obtained under treatment geometry provides the pixel value information that is used to calculate the tissue deficit over the lung region. The compensators are made of lead whose thickness is calculated from the established empirical relationship to replace the tissue deficit over lungs. The effectiveness of the method is demonstrated with thermoluminescent dosimetry (TLD) for 6 and 10 MV beams. With compensators in place, the dose uniformity was found to be within +/- 5%.

Humans↗

PubMed2002

Optimization of accelerator target and detector for portal imaging using Monte Carlo simulation and experiment.

Megavoltage portal images suffer from poor quality compared to those produced with kilovoltage x-rays. Several authors have shown that the image quality can be improved by modifying the linear accelerator to generate more low-energy photons. This work addresses the problem of using Monte Carlo simulation and experiment to optimize the beam and detector combination to maximize image quality for a given patient thickness. A simple model of the whole imaging chain was developed for investigation of the effect of the target parameters on the quality of the image. The optimum targets (6 mm thick aluminium and 1.6 mm copper) were installed in an Elekta SL25 accelerator. The first beam will be referred to as A16 and the second as Cu1.6. A tissue-equivalent contrast phantom was imaged with the 6 MV standard photon beam and the experimental beams with standard radiotherapy and mammography film/screen systems. The arrangement with a thin Al target/mammography system improved the contrast from 1.4 cm bone in 5 cm water to 19% compared with 2% for the standard arrangement of a thick, high-Z target/radiotherapy verification system. The linac/phantom/detector system was simulated with the BEAM/EGS4 Monte Carlo code. Contrast calculated from the predicted images was in good agreement with the experiment (to within 2.5%). The use of MC techniques to predict images accurately, taking into account the whole imaging system, is a powerful new method for portal imaging system design optimization.

Bone and Bones↗

PubMed2004

Correction for dose-response variations in a scanning liquid ion chamber EPID as a function of linac gantry angle.

The response of electronic portal imaging devices (EPIDs) of the scanning liquid ionization chamber (SLIC) type is known to vary with linear accelerator gantry angle. This work considered several contributing factors, quantified the artefacts, monitored their reproducibility and investigated the effects of repeated gantry rotations. Unflatness of up to 5% was found. A correction technique was devised using nonlinear regression of a three-variable sinusoidal modulation. Comparison with two existing techniques found our method to be the most effective, providing a flatness well within 2%. This improved accuracy is expected to benefit more accurate dosimetric studies in particular. The post-acquisition correction process required no change in imaging protocols. Applicability of the new technique was demonstrated on images acquired on different days and with different beam sizes. Since the artefacts compromise both accurate dosimetry and image quality, their successful removal should benefit a broad range of SLIC EPID applications.

Calibration↗

PubMed2003

Reference photon dosimetry data: a preliminary study of in-air off-axis factor, percentage depth dose, and output factor of the Siemens Primus linear accelerator.

The dosimetric characteristics for modern computer-controlled linear accelerators with the same make, model, and nominal energy are known to be very similar, as long as the machines are unaltered from the manufacturer's original specifications. In this preliminary study, a quantitative investigation of the similarity in the basic photon dosimetry data from the Siemens Primus linear accelerators at eight different institutions is reported. The output factor, percentage depth dose (PDD), and in-air off-axis factor (OAF) for the 6 and 18 MV photon beams measured or verified by the Radiological Physics Center (RPC) were analyzed. The RPC-measured output factors varied by less than about 2% for each field size. The difference between the maximum and minimum RPC-verified PDD values at each depth was less than about 3%. The difference between the maximum and minimum RPC-measured in-air OAF was no more than 4% at all off-axis distances considered in this study. These results strongly suggest that it is feasible to establish a reference photon dosimetry data set for each make, model, and nominal energy, universally applicable to those machines unaltered from the manufacturers' original specifications, within a clinically acceptable tolerance (e.g., approximately +/-2%).

Particle Accelerators↗

PubMed1985

External stereotactic irradiation by linear accelerator.

Stereotactic radiotherapy has two advantages: (a) the possibility of giving high radiation doses to small but spatially well-defined target volumes and (b) the presence of a stepped dose gradient between the target volume and the surrounding healthy tissues. To utilize these advantages, the authors built a new stereotactic head frame by which the intracranial target is fixed to the rotational isocenter of a 4-MV linear accelerator. The collimator openings are selected according to the volume and the three-dimensional configuration of the target, and the radiation dose is based on the radiosensitivity of the lesion. After the patient is fixed to the frame, the radiation source and the patient are rotated so that the target is irradiated through infinite portals distributed over the convexity of the skull. It is thereby possible to obtain very high radiation doses centered into the target with a stepped dose gradient. The preliminary radiodosimetric tests and the operative technique are described. The advantages of this technique compared to interstitial radiotherapy and Leksell's radiosurgery are emphasized. This noninvasive procedure has been used to treat a series of intracranial tumors.

Adolescent↗

PubMed2000

The 1998 Australian external beam radiotherapy survey and IAEA/WHO TLD postal dose quality audit.

The results of an updated Australian survey of external beam radiotherapy centres are presented. Most of the centres provided most of the requested information. The relative caseloads of various linear accelerator photon and electron beams have not changed significantly since the previous survey in 1995. The mean age of Australian LINACs is 7.1 years and that of other radiotherapy machines is 14.7 years. Every Australian radiotherapy centre participated in a special run of the IAEA/WHO TLD postal dose quality audit program, which was provided for Australian centres by the IAEA and WHO in May 1998. The dose quoted by the centres was in nearly every case within 1.5% of the dose assessed by the IAEA. This is within the combined standard uncertainty of the IAEA TLD service (1.8%). The results confirm the accuracy and precision of radiotherapy dosimetry in Australia and the adequate dissemination of the Australian standards from ARL (now ARPANSA) to the centres. The Australian standards have recently been shown to agree with those of other countries to within 0.25% by comparison with the BIPM.

Australia↗

PubMedDate not supplied

An integrated radiotherapy treatment system and its clinical application.

A new total system for radiation therapy installed in our department consists of a linear accelerator unit with microcomputer-controlled multileaf collimators, along with a CT scanner installed in the same treatment room. Also included are a digitally controlled communal couch, a minicomputer, and a system of treatment planning devices. We have developed various kinds of external radiation treatment planning techniques (including dynamic ones) with this system on the basis of the concept of dose corresponding technique. A reference point setup method is needed to maintain a unified coordinate system through each step of the treatment procedure, and, as developed by us, permits a volume-oriented setup. This system has been evaluated for accuracy in a variety of clinical setups. A true three-dimensional dose calculation algorithm is required for treatment planning associated with multileaf collimator treatments in order to realize the potential accuracy of such a system.

Computer Simulation↗

PubMed2002

[Long term registration of linear accelerator parameters for quality assurance in stereotaxic radiosurgery].

Long-term observations of mechanical parameters have been performed in Freiburg for the last 7 years within the quality assurance of stereotactic irradiation at linear accelerators. The deviations between the laser indication and the position of the beam isocentre are recorded, as well as parameters for the stability of the couch rotation and the additional devices for patient adjustment and beam collimation. The deviations are used for the correction of the calculated isocentre coordinates. The long-term observation of the measured values and their standard deviations allow conclusions about the quality of the measuring procedure itself, the laser adjustment, the extension of the isocentre sphere, as well as the mechanical slackness of the treatment couch and the fixation device of the stereotactic ring. The standard deviations of the isocentre position were within 0.3 to 0.8 mm, those of the measurement quality approximately 0.2 mm. The analysis of long-term observation has contributed to improvements of the equipment and of quality assurance procedures.

Calibration↗

PubMed1998

Spectral reconstruction of clinical megavoltage photon beams and the implications of spectral determination on the dosimetry of such beams.

An analysis technique, based on simulated annealing, is described which is employed to derive megavoltage photon beam spectral information from narrow beam attenuation measurements. Megavoltage photon beam spectra have been determined using this technique for linear accelerators from different manufacturers, and different models from individual manufacturers at a range of energies from nominal 6 MV to nominal 25 MV. All of the photon beams included in the study are in routine clinical use. The subsequent effects on dosimetry of employing derived primary spectra to specify beam quality are examined. The results suggest that the quality index TPR(20)10 may be insensitive to beam quality changes for high-energy beams in the range of 15 MV to 25 MV. Although the quality index may be insensitive as a beam quality specifier at these higher qualities, the actual difference in the calculated dose delivered using derived spectra as the quality specifier rather than TPR(20)10 is likely to be small, the results obtained indicating a difference of between 0.2% and 0.7% in the calculated dose delivered.

Biophysical Phenomena↗