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At least 595 records · Page 33Linked to original sources
PubMed2002

Stability of electron-beam energy monitor for quality assurance of the electron-beam energy from radiotherapy accelerators.

Information on electron energy is important in planning radiation therapy using electrons. The Geske 3405 electron beam energy monitor (Geske monitor, PTW Nuclear Associates, Carle Place, NY, USA) is a device containing nine ionization chambers for checking the energy of the electron beams produced by radiotherapy accelerators. We wondered whether this might increase the likelihood of ionization chamber trouble. In spite of the importance of the stability of such a quality assurance (QA) device, there are no reports on the stability of values measured with a Geske monitor. The purpose of this paper was therefore to describe the stability of a Geske monitor. It was found that the largest coefficient of variation (CV) of the Geske monitor measurements was approximately 0.96% over a 21-week period. In conclusion, the stability of Geske monitor measurements of the energy of electron beams from a linear accelerator was excellent.

Electrons↗
PubMed1987

[Distribution of absorbed dose of electron beams from a linear accelerator of 8-17.5 MeV energy].

The authors presented some experimental and theoretical data on dose distribution in a tissue equivalent phantom during irradiation with electrons at 8; 10; 12.5; 15 and 17.5 MeV. The proposed mathematical apparatus based on the method of dose functions of a punctate unidirectional source, can be used for the development of programs of dosimetric planning of electron therapy using the LUEV-15MI accelerator.

Electrons↗
PubMed1983

The contribution of the physical sciences to the development of radiation therapy.

Radiation therapy is used in the treatment of about half of all cancer patients in the United States. The development of this specialty has been intimately involved with major advances in the physical sciences, from the discovery of x-rays by Roentgen and radium by the Curies, to the technological breakthroughs of World War II that led to the 60cobalt teletherapy unit and the modern linear accelerator--the principal treatment machines in use today. These historical events are described and the principal characters introduced.

Elementary Particles↗
PubMed1990

A technique for treating local breast cancer using a single set-up point and asymmetric collimation.

Using both pairs of asymmetric jaws of a linear accelerator local-regional breast cancer may be treated from a single set-up point. This point is placed at the abutment of the supraclavicular fields with the medial and lateral tangential fields. Positioning the jaws to create a half-beam superiorly permits treatment of the supraclavicular field. Positioning both jaws asymmetrically at midline to define a single beam in the inferoanterior quadrant permits treatment of the breast from medial and lateral tangents. The highest possible matching accuracy between the supraclavicular and tangential fields is inherently provided by this technique. For treatment of all fields at 100 cm source to axis distance (SAD) the lateral placement and depth of the set-up point may be determined by simulation and simple trigonometry. We elaborate on the clinical procedure. For the technologists treatment of all fields from a single set-up point is simple and efficient. Since the tissue at the superior border of the tangential fields is generally firmer than in mid-breast, greater accuracy in day-to-day set-up is permitted. This technique eliminates the need for table angles even when tangential fields only are planned. Because of half-beam collimation the limit to the tangential field length is 20 cm. Means will be suggested to overcome this limitation in the few cases where it occurs. Another modification is suggested for linear accelerators with only one independent pair of jaws.

Breast Neoplasms↗
PubMed2006

Molecular beams with a tunable velocity.

The merging of molecular beam methods with those of accelerator physics has yielded new tools to manipulate the motion of molecules. Over the last few years, decelerators, lenses, bunchers, traps, and storage rings for neutral molecules have been demonstrated. Molecular beams with a tunable velocity and with a tunable width of the velocity distribution can now be produced, and are expected to become a valuable tool in a variety of physical chemistry and chemical physics experiments. Here we present a compact molecular beam machine, capable of producing 3D spatially focused packets of state-selected accelerated or decelerated molecules.

Chemistry, Physical↗
PubMed1986

Observations on personnel dosimetry for radiotherapy personnel operating high-energy LINACs.

A series of measurements were conducted to determine the cause of a sudden increase in personnel radiation exposures. One objective of the measurements was to determine if the increases were related to changing from film dosimeters exchanged monthly to TLD-100 dosimeters exchanged quarterly. While small increases were observed in the dose equivalents of most employees, the dose equivalents of personnel operating medical electron linear accelerators with energies greater than 20 MV doubled coincidentally with the change in the personnel dosimeter program. The measurements indicated a small thermal neutron radiation component around the accelerators operated by these personnel. This component caused the doses measured with the TLD-100 dosimeters to be overstated. Therefore, the increase in these personnel dose equivalents was not due to changes in work habits or radiation environments. Either film or TLD-700 dosimeters would be suitable for personnel monitoring around high-energy linear accelerators. The final choice would depend on economics and personal preference.

Film Dosimetry↗
PubMed2002

A simple method for bremsstrahlung spectra reconstruction from transmission measurements.

A new method for evaluation of bremsstrahlung spectra from transmission measurements has been developed. In this method some very well known facts relating to thick target bremsstrahlung spectra are a priori included in the calculation procedure. Some characteristics of the method are preliminarily illustrated on a 6 MV therapy linear accelerator.

Algorithms↗
PubMedDate not supplied

Corner transmission in several linear accelerator photon beams.

Maximum x-ray field sizes on many linear accelerators are obtained only with truncated corners. Transmissions through the corners of such fields has been measured utilizing film and ion chamber dosimetry for a number of accelerators. Transmissions are found to be significantly larger than for the movable jaws.

Humans↗
PubMed2003

Shielding evaluation for IMRT implementation in an existing accelerator vault.

A formalism is developed for evaluating the shielding in an existing vault to be used for IMRT. Existing exposure rate measurements are utilized as well as a newly developed effective modulation scaling factor. Examples are given for vaults housing 6, 10 and 18 MV linear accelerators. The use of an 18 MV Siemens linear accelerator is evaluated for IMRT delivery with respect to neutron production and the effects on individual patients. A modified modulation scaling factor is developed and the risk of the incurrence of fatal secondary malignancies is estimated. The difference in neutron production between 18 MV Varian and Siemens accelerators is estimated using Monte Carlo results. The neutron production from the Siemens accelerator is found to be approximately 4 times less than that of the Varian accelerator resulting in a risk of fatal secondary malignancy occurrence of approximately 1.6% when using the SMLC delivery technique and our measured modulation scaling factors. This compares with a previously published value of 1.6% for routine 3D CRT delivery on the Varian accelerator.

Air Pollutants, Radioactive↗
PubMed1989

Stereotactic LINAC radiosurgery.

A system of stereotactic radiosurgery using a linear accelerator is described and the experience with a variety of neurosurgical lesions including gliomas, acoustic neurinomas and arteriovenous malformations is recorded. The system is simple, inexpensive and effective and can be used with any CT scanner and linear accelerator.

Adult↗
PubMed1992

Radiosurgery using a modified linear accelerator.

Stereotactic radiosurgery using modified LINACs is a powerful new technique that offers a versatile method for administering high-dose radiation in a single session. Treatment-planning techniques currently in use enable individual isocenter field shaping, three-dimensional optimization of isocenter placement (including the use of multiple isocenters), and real-time analysis of isodose distribution in three dimensions relative to specifically defined anatomic structures from MR imaging or CT data. Modified linear accelerators offer successful control of both arteriovenous malformations and benign tumors (acoustic neuromas, craniopharyngiomas, and meningiomas), with results equal to the best series reported by users of the gamma knife or heavy-particle systems. More recent work indicates that radiosurgery may play an important role in controlling recurrent pediatric malignant tumors, metastatic cerebral lesions, and even malignant astrocytomas in some patients. Improved correlation of isodose distributions with imaging data allows for sophisticated analysis of results and specific complications, ultimately enabling improved patient care using this exciting new technique.

Adolescent↗
PubMed1994

An inductive assessment of radiation risks in space.

Procedures for the assessment of risks or vulnerabilities from radiation in space are evaluated in terms of model-independent inductive approaches. The reliability of risks calculated for space applications on the basis of accelerator-based physical and biological data is examined from a microdosimetric perspective. Probability distributions for energy deposition in biologically significant sites extend over several decades in lineal energy even for monoenergetic high-energy particles of relatively high atomic number. Because the response depends on a large number of variables and because of the difficulty of incorporating all such factors into calculations, a precise correlation between a physical descriptor of the field and observed effects in space is not feasible. For the same reasons, it is equally difficult to estimate the accuracies of such risk assessments. We use recently published microdosimetric spectra for HZE particles and biological weighting functions, including those derived from biological measurements with maximum entropy techniques, to illustrate some problems associated with the evaluations of risks from radiation fields in space.

Animals↗
PubMed1994

Comparison of performance standards for radiation beam uniformity characteristics of a linear accelerator.

The recent international standards published by the International Electrotechnical Commission (IEC) on the performance of medical electron accelerators describe suggested test procedures for the performance of radiotherapy accelerators. The recommendations of the Nordic Association of Clinical Physics for testing of the radiation beam characteristics include test conditions, methods and suggested tolerances that are different from those of the IEC. In this work the two publications were compared for acceptance testing of a Philips SL25 linear accelerator. It is important to gain experience on the practical use of these standards.

Europe↗
PubMed2002

Depth dose enhancement of electron beams subject to external uniform longitudinal magnetic fields: a Monte Carlo study.

We studied the dose distributions from electron beams subjected to a longitudinal magnetic field applied to them before they reach the phantom. We found that specific combinations of the length and intensity of the magnetic field produced enhancement of the peaks of the central-axis depth-dose distributions. The EGS4 Monte Carlo system was used in this study. In the simulations, a uniform axial magnetic field parallel to the electron beam direction was applied to the air gap between the collimation and the phantom. We extensively studied the simplified case of an 18 MeV electron beam point source. Dose deposition was calculated for various magnetic field strengths, distances through which the magnetic field was applied, collimation sizes, and source to collimation distances. The magnetic field strengths varied from 0 to 3 T, the source-to-collimation distances studied were 50 and 95 cm, the collimation sizes studied were 10 x 10 and 20 x 20 cm2, and the distance through which the field was applied ranged from 10 to 20 cm. Specific combinations of these variables resulted in as much as a 70% enhancement of the peak dose relative to the surface dose. Finally, to determine how the geometry of a real accelerator affects the resulting dose distribution, we performed a full simulation of an Elekta SL20 linear accelerator and compared the results with the ideal case.

Electromagnetic Fields↗