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 1,063 records · Page 59Linked to original sources

PubMedDate not supplied

A water phantom controller for automated acquisition of linac beam parameters.

Three-dimensional water phantoms are routinely used when acquiring the data necessary to commission medical linear accelerators. A new water phantom controller has been developed at our institution that is unique in several aspects. The water phantom controller is based on an IBM XT compatible computer. This has been interfaced to an Artronix three-dimensional water phantom and a commercially available linac setup controller. Some of the unique features of the new controller are (i) its ability to perform three-dimensional coordinate transformations on the fly (this removes the need to level and align the water tank with the beam axes and greatly reduces setup time). (ii) Its ability to communicate with a device which can control the linac operating parameters (this allows the water phantom controller to adjust, for example, the jaw size of the accelerator as it acquires data). (iii) Its ability to output the acquired data in a number of modes, including screen display, hardcopy plot, or disk file that may be transferred to a central treatment planning computer). (iv) Its ability to digitally process the acquired data. This water phantom controller may be used to gather machine data in a highly automated manner, greatly reducing the time required to gather the desired data.

Computers↗

PubMed1992

[Application of CT simulation system to stereotactic radiosurgery--experimental study in phantoms].

Stereotactic radiosurgery with linear accelerator requires accurate localization of target and accurate spatial delivery of radiation. In phantom study, geometric accuracy of radiosurgery was assessed in combination of CT simulation system (CTSS), which had been developed in our institute, and linear accelerator with supplemental collimator. After determination of target and its isocenter with CTSS, phantom was placed on treatment table so that isocenter meet at the intersection of mechanical axes (gantry, turn table). Displacement of the isocenter from the center of the radiation field was 1 mm in average. It was concluded that this combination could be applied to radiosurgery.

Computer Simulation↗

PubMed1983

[Deviations in the control of the monitor calibration of a circular accelerator].

The operation of an accelerator requires to perform at regular intervals a control measurement of the basic calibration. This measurement showed a deviation growing from week to week. Finally, the test measurements gave occasion to suppose a defect of the used ionizing chamber, but this was not confirmed by measurements at a gamma irradiation unit. When the chamber had been repaired by the manufacturer, there were no more deviations with respect to the basic measurement.

Particle Accelerators↗

PubMed1991

Depth dose and profile analysis using Library Least-Squares method.

The Library Least-Squares method has been applied to detect changes in the depth dose and in the profiles of 6 and 20 MeV electron beams, and in 6 MeV photon beams from medical linear accelerators. The changes in the beam energy of 0.78% and 0.32% at low and high electron energies, respectively, induced by the insertion of 0.1 mm A1 absorber were clearly visible in the residual distributions. The changes in the photon beam were introduced by rotation of the gantry or the collimator. The applicability of the method to quantitatively monitor any changes in the treatment beam characteristics is presented.

Calibration↗

PubMed2004

New equations for matching a low neck field to oblique upper neck fields with collimator rotation in a 3-field monoisocentric setup for head-and-neck cancers.

With a monoisocentric 3-field technique for treating head-and-neck cancer, collimator rotation may be needed for the upper lateral fields to avoid cephalad-positioned shoulders or to avoid unnecessary arytenoid irradiation while maintaining more anterior coverage. For patients with unilateral lymphadenopathy, lateral oblique-opposed boost fields can be used to encompass the primary tumor and ipsilateral lymph nodes without junctioning through gross disease. When initial collimated lateral fields are also rotated with a gantry angle to produce oblique boost fields, however, the resulting matchline with a low anterior neck (LAN) boost field is no longer nondivergent. This can be corrected by manual adjustment of collimator and gantry angles for the LAN field using 3D treatment planning software. The goal of this study was to derive mathematical formulas to simplify this process. We used a transformation matrix to define formulas that could predict the appropriate modifications to the LAN boost field. Output from the formulas was (1) visually tested within 3D treatment planning software and (2) verified using a solid water head-and-neck phantom and radiographic film dosimetry to confirm that a nondivergent matchline was obtained in several clinical scenarios. When evaluated with 3D treatment planning software, the formulas accurately predicted the appropriate gantry and collimator angles of the LAN boost field for a variety of possible beam combinations. When evaluated with film dosimetry, the formulas were shown to accurately predict the appropriate gantry and collimator angles of the LAN boost field to within the +/- 2 mm/1 degrees tolerance specifications of the linear accelerator and acceptable for routine clinical use. The presented formulas are simple and geometrically precise. They predict the necessary manipulations of the LAN boost field to maintain a geometrically precise matchline, as verified by 3D treatment planning software, phantom dosimetry, and actual patient setups.

Algorithms↗

PubMed2002

Calibration results obtained with Liulin-4 type dosimeters.

The Mobile Radiation Exposure Control System's (Liulin-4 type) main purpose is to monitor simultaneously the doses and fluxes at 4 independent places. It can also be used for personnel dosimetry. The system consists of 4 battery-operated 256-channel dosimeters-spectrometers. We describe results obtained during the calibrations of the spectrometers at the Cyclotron facilities of the University of Louvain, Belgium and of the National Institute of Radiological Sciences-STA, Chiba, Japan with protons of energies up to 70 MeV. The angular sensitivities of the devices are studied and compared with Monte-Carlo predictions. We also present the results obtained at the HIMAC accelerator with 500 MeV/u Fe ions and at the CERN high energy radiation reference fields. Records made during airplane flights are shown and compared with the predictions of the CARI-6 model.

Aerospace Medicine↗

PubMed1998

Tin shielding thicknesses for electrons.

The transmission of 6, 9, 12, 16, and 20 MeV electrons from a linear accelerator through tin and lead is studied at 0.5 cm depth in polystyrene. These measurements are performed using a 10 x 10 cm2 cone and extend well into bremsstrahlung region. The results show certain advantages of using tin to shield sensitive organs from electron beams. Tin is non-toxic and creates less additional bremsstrahlung even though it must be thicker than the equivalent lead shield.

Electrons↗

PubMed1998

Clinical commissioning of Laitinen Stereoadapter for fractionated stereotactic radiotherapy.

The Laitinen Stereoadapter 5000 from Sandstroem Trade and Technology was acceptance tested and commissioned for clinical use in a Fractionated Stereotactic Radiotherapy Program at our facility. The frame was implemented to function as a localization device for target delineation rather than as an immobilization device. The frame is of non-invasive nature utilizing ear plugs and a nasion bridge adapter as the connecting points with the patient's head. The reproducibility of the head frame position with respect to external skull reference points was tested. CT and MRI imaging studies were performed on a patient phantom with the stereoadapter in place. The target was delineated and target coordinates were calculated for two implanted targets. The phantom was positioned according to the target coordinates on a Siemens MXE Linear Accelerator by aid of the target positioning lasers. Radiographic port film images were taken with the circular fields typically used in stereotactic radiosurgery. A complete treatment isodose plan was performed and dosimetric accuracy was tested by positioning a small volume ionization chamber at the center of the target volume in the head phantom. The results of these tests were found to be clinically acceptable.

Dose Fractionation, Radiation↗

PubMed1994

On-axis and off-axis primary dose component in high energy photon beams.

The depth dose of the primary dose component, on axis and off axis of six different x-ray beams, has been determined from transmission measurements in narrow beam geometry with and without flattening filter using a Perspex column of a cross section large enough to ensure electronic equilibrium. In order to derive the primary photon fluence, a correction for the scatter from the column has been applied according to the following method: A number of spectra taken from the literature have been used for computing a scatter coefficient Sc at different depths by convolution of dose spread arrays. Using the relationship between Sc and the single attenuation coefficient mu i to represent each entire spectrum, it has been possible to correct the experimental transmission curves iteratively, until the corresponding values of mu were stabilized and representative of the primary. The measured attenuation coefficients were found to have a linear increase as a function of the distance from the central axis for all the energies and types of linear accelerators. For the same nominal energy, this increase is different from one accelerator to another. The same phenomenon was observed for the attenuation coefficients obtained without the flattening filter in the same experimental conditions. The results are tentatively interpreted considering the angular variation of bremsstrahlung energy spectra with and without a flattening filter as calculated by a Monte Carlo method and they are consistent and useful to take accurately into account the softening of the beam as the off-axis distance increases.

Humans↗

PubMed1990

[Preliminary results with digital luminescence radiography in radiotherapy].

Port films in radiation therapy suffer from low image contrast und exposure errors. We examined the potential of computed radiography with a storage phosphor system (FCR-901/Siemens Digiscan) to overcome these drawbacks. Port films of various treatment sites were obtained for both, conventional films and digital storage phosphor screens with a DuPont cassette and steel intensifier screens under identical exposure settings. We examined port films from a linear accelerator and from a 60Co source. We found a superior contrast in the digital images and an almost total elimination of exposure errors. The differences between digital and conventional images, however, were less pronounced with films from the 60Co source. Unsharp mask filtering (medium kernel size, moderate enhancement) further improved image quality. Several sources of artefacts in the digital images could be described.

Cobalt Radioisotopes↗

PubMed1980

[Radiotherapy of basal cell epitheliomas of the face (author's transl)].

Great advances have been made in the techniques used for radiotherapy, both in safety measures to avoid accidents due to dosage errors, and by the use of new rays such as electrons from accelerators which permit more selective irradiation avoiding surrounding tissues. The authors discuss the indications for the different radiotherapy methods used in basal cell epitheliomas of the face, when compared with or used in association with other procedures such as operative therapy, in relation to the characteristics of the tumor, its proximity to delicate organs, the age of the patient. The possible esthetic and functional sequelae, and the cost to society.

Basal Cell Carcinoma↗

PubMedDate not supplied

[Lethal action of accelerated heavy ions on mammalian cells as affected by inhibitors of DNA synthesis. A theoretical analysis].

Günter-Schulz's model and the authors' own model were used to study the dependence of radiosensitivity (D0(-1)) of Chinese hamster cells on linear energy transfer (LET) upon irradiation in standard conditions and in the presence of DNA synthesis inhibitors, arabinosylcytosine and hydroxyurea. A better agreement of the experimental and theoretical results was obtained using Kozubek-Krasavin's model than the model of Günter and Schulz.

Animals↗

PubMed1988

Dosimetric characteristics of a 6 MV photon beam from a linear accelerator with asymmetric collimator jaws.

Dosimetric measurements have been made of a 6 MV photon beam from a linear accelerator equipped with asymmetric jaws. The field size factors for asymmetrically set fields are compared to those for symmetrically set fields. The change of beam quality has been measured as a function of off-axis position of the asymmetric fields to assess its effect on depth dose. Additional measurements include beam penumbra and shape of isodose curves for open and wedge fields as the field opening is moved asymmetrically from the central ray.

Particle Accelerators↗

PubMed1980

Stopping powers of a tissue-equivalent gas for 40--200 keV protons.

The stopping powers of a tissue-equivalent gas were measured for protons using the proton beam from a Cockcroft and Walton type accelerator and a differentially pumped gas cell. The stopping powers of each component of the tissue-equivalent gas were also measured and used to calculate expected values of the stopping powers of the tissue-equivalent gas. Results from direct measurement and from calculation showed good agreement. The results of the measurements on the component gases were also used to calculate the stopping power of Rossi's gas.

Carbon Dioxide↗

PubMed1996

The influence of air humidity on an unsealed ionization chamber in a linear accelerator.

The safe and accurate delivery of the prescribed absorbed dose is the central function of the dose monitoring and beam stabilization system in a medical linear accelerator. The absorbed dose delivered to the patient during radiotherapy is often monitored by a transmission ionization chamber. Therefore it is of utmost importance that the chamber behaves correctly. We have noticed that the sensitivity of an unsealed chamber in a Philips SL linear accelerator changes significantly, especially during and after the summer season. The reason for this is probably a corrosion effect of the conductive plates in the chamber due to the increased relative humidity during hot periods. We have found that the responses of the different ion chamber plates change with variations in air humidity and that they do not return to their original values when the air humidity is returned to ambient conditions.

Air↗

PubMedDate not supplied

Neutron spectral measurements in an intense photon field associated with a high-energy x-ray radiotherapy machine.

High-energy x-ray radiotherapy machines in the supermegavoltage region generate complex neutron energy spectra which make an exact evaluation of neutron shielding difficult. Fast neutrons resulting from photonuclear reactions in the x-ray target and collimators undergo successive collisions in the surrounding materials and are moderated by varying amounts. In order to examine the neutron radiation exposures quantitatively, the neutron energy spectra have been measured inside and outside the treatment room of a Sagittaire medical linear accelerator (25-MV x rays) located at Yale-New Haven Hospital. The measurements were made using a Bonner spectrometer consisting of 2-, 3-, 5-, 8-, 10- and 12-in.-diameter polyethylene spheres with 6Li and 7Li thermoluminescent dosimeter (TLD) chips at the centers, in addition to bare and cadmium-covered chips. The individual TLD chips were calibrated for neutron and photon response. The spectrometer was calibrated using a known PuBe spectrum Spectrometer measurements were made at Yale Electron Accelerator Laboratory and results compared with a neutron time-of-flight spectrometer and an activation technique. The agreement between the results from these independent methods is found to be good, except for the measurements in the direct photon beam. Quality factors have been inferred for the neutron fields inside and outside the treatment room. Values of the inferred quality factors fall primarily between 4 and 8, depending on location.

Neutrons↗

PubMedDate not supplied

A statistical analysis of ionometrically measured 6-MV x-ray beam parameters.

A statistical analysis has been made of ionometrically measured parameters related to the central-axis output, the beam symmetry, the beam flatness, and the depth dose for the 6-MV x-ray beam from the Therac-6 linear accelerator. The means and coefficients of variation of the measurements are determined on a monthly basis for each parameter. Mean coefficients of variation less than +/- 1% have been determined and are useful in establishing the optimal limits of acceptance of these parameters. The results of this study indicate that the long-term stability of the radiation parameters associated with this unit can be maintained within acceptable limits with a consistent quality-assurance program.

Humans↗

PubMedDate not supplied

Medical accelerator safety considerations: report of AAPM Radiation Therapy Committee Task Group No. 35.

Ensuring safe operation for a medical accelerator is a difficult task. Users must assume more responsibility in using contemporary equipment. Additionally, users must work closely with manufacturers in promoting the safe and effective use of such complex equipment. Complex treatment techniques and treatment modality changeover procedures merit detailed, unambiguous written procedural instruction at the control console. A thorough "hands on" training period after receiving instructions, and before assuming treatment responsibilities, is essential for all technologists. Unambiguous written instructions must also be provided to guide technologists in safe response when equipment malfunctions or exhibits unexpected behavior or after any component has been changed or readjusted. Technologists should be given a written list of the appropriate individuals to consult when unexpected machine behavior occurs. They should be assisted in identifying aberrant behavior of equipment. Many centers already provide this instruction, but others may not. Practiced response and discussion with technologists should be a part of an ongoing quality assurance program. An important aspect of a safety program is the need for continuous vigilance. Table III gives a summary of a comprehensive safety program for medical accelerators. Table IV gives a list of summary recommendations as an example of how one might mitigate the consequences of an equipment failure and improve procedures and operator response in the context of the environment described. Most of these recommendations can be implemented almost immediately at any individual treatment center.

Biophysical Phenomena↗