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At least 217 records · Page 12Linked to original sources
PubMed1992

Wedge filter effects on dosimetric parameters of a linear accelerator.

The open-field and wedged-field output factors as a function of field size for two linear accelerators were measured. Wedge factors were determined by taking the ratio of the outputs with and without the wedge filter. For one linear accelerator, the difference in the output factors between the wedged field and open field can be as large as 5%. The wedge factor for this linear accelerator also varies with the field size. On the other hand, the other linear accelerator shows smaller variation of output factors between wedge field and open field. The variation of wedge factor is less than 1% for a 60 degree wedge. In addition to modifying the isodose distributions, the wedge filter also changes the percent depth dose curves, the output factor, and the wedge factor. The degree of wedge effects on these dosimetric parameters is different for different linear accelerator.

Filtration↗
PubMedDate not supplied

Measurements of backscattered radiation from Therac-20 collimator and trimmer jaws into beam monitor chamber.

The field size dependent photon output is known to be influenced by the existence of backscattered radiation (BSR) generated in the collimator or trimmer jaws of a linear accelerator. This paper describes the results of measurements made to study the existence of such backscatter by simulating the geometry of the treatment head of a Therac-20 linear accelerator. The machine's monitor chamber, flattening filter, and collimator jaws were simulated by another real monitor chamber, a 1-cm thick lead sheet and 2.5-cm thick low-melting-point alloy divergent blocks, respectively. BSR from the simulated collimator jaws (SCJ) was measured with the simulated monitor chamber (SMC) as a function of the openings of the SCJ and as a function of distance between SMC and SCJ. The present results demonstrate the presence of BSR in an 18-MV photon beam from a Therac-20 linear accelerator.

Humans↗
PubMed1983

The cost of radiotherapy treatments on a linear accelerator.

The cost of delivering a radiotherapy treatment on a linear accelerator is calculated. Items included in the cost are: cost of the accelerator itself, a simulator, a treatment planning computer, the treatment room, and the salaries of the radiographers and physicists concerned, as well as machine operating costs and interest charges. The cost of a beam direction shell is also calculated. The different costs are reduced to statements of cost per year (see Table IV for summary), and from these the cost of a course of treatment for a patient is arrived at on the basis of 800 patients treated per year. The cost comes out at pounds 137, plus pounds 70 for a beam direction shell, if one is used. Because the capital costs of radiotherapy treatment equipment are very substantial, it is often assumed that radiotherapy is a very expensive form of treatment. An analysis of treatment cost is presented, initially in terms of cost per year for equipment, special buildings, and staff at 1981 prices. Reduction of these figures to mean treatment cost per patient is based on the DHSS guideline for provision of radiotherapy services (HC(78)32), which suggests a figure of 800 patients to be treated per year as the number justifying the provision of a linear accelerator.

Costs and Cost Analysis↗
PubMed1978

Absorbed dose distributions for X-ray beams and beams of electrons from the Therac 20 Saturne linear accelerator.

After a brief description of the Therac 20 Saturne linear accelerator a complete set of absorbed-dose distribution values is given. These values define the depths on the axis as a function of the depth dose and define the penumbra (as characterized by the positions of the intersections of the isodose curves with planes parallel to the phantom surface) for beams of X-rays and for beams of electrons. Tissue-maximum ratios are given for beams of X-rays. Analytical values for the electron depth dose curve are compared with the values obtained on the Sagittaire linear accelerator.

Electrons↗
PubMed1980

[A Cerenkov detector to determine the energy in electron accelerators (author's transl)].

The authors present a Cerenkov threshold value detector consisting of elements originating from the commercial vacuum industry. This detector allows to measure electron energies between 8 MeV and 20 MeV. By the comparison of the signals of two photo-multipliers, a considerable independence of external disturbing factors is achieved. It is demonstrated that the results achieved with normal propane are not worse than those of super-purified gas and, therefore, a cheap and simple method is available in order to control the electron energy of accelerators.

Electrons↗
PubMed1999

Vacuum electron acceleration by coherent dipole radiation.

The validity of the concept of laser-driven vacuum acceleration has been questioned, based on an extrapolation of the well-known Lawson-Woodward theorem, which stipulates that plane electromagnetic waves cannot accelerate charged particles in vacuum. To formally demonstrate that electrons can indeed be accelerated in vacuum by focusing or diffracting electromagnetic waves, the interaction between a point charge and coherent dipole radiation is studied in detail. The corresponding four-potential exactly satisfies both Maxwell's equations and the Lorentz gauge condition everywhere, and is analytically tractable. It is found that in the far-field region, where the field distribution closely approximates that of a plane wave, we recover the Lawson-Woodward result, while net acceleration is obtained in the near-field region. The scaling of the energy gain with wave-front curvature and wave amplitude is studied systematically.

Journal Article↗
PubMed2000

TLD efficiency of 7LiF for doses deposited by high-LET particles.

The efficiency of 7 LiF TLDs (TLD-700) in registering dose from high-LET (> or = 10 keV/micrometers) charged particles (relative to 137Cs gamma rays) has been measured for a number of accelerated heavy ions at various particle accelerator facilities. These measured efficiency values have been compared with similar results obtained from the open literature and a dose efficiency function has been fitted to the combined data set. While it was found that the dose efficiency is not only a function of LET, but also of the charge of the incident particle, the fitted function can be used to correct the undermeasured value of dose from exposures made in mixed radiation fields where LET information is available. This LET-dependent dose efficiency function is used in our laboratory in determining total absorbed dose and dose equivalent from combined TLD and CR-39 plastic nuclear track detector measurements.

Calibration↗