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Results for “Particle Accelerators”

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At least 1,153 records · Page 64Linked to original sources

PubMed1994

Laminated primary ceiling barriers for medical accelerator rooms.

In this work the performance of a number of laminated shields used as the primary ceiling barrier for high-energy accelerator rooms was investigated. The neutron and photon dose equivalent rates were measured outside each shield. Based on the measured photon level and the calculated x-ray leakage level the dose equivalent rate due to photons produced by neutron interactions with the shield materials was estimated. Shielding parameters for polyethylene were established for photons and neutrons. It was found that the barriers designed using the techniques given in an NCRP Report suffered from excessive radiation leakage.

Algorithms↗

PubMedDate not supplied

Calculative technique to correct for the change in linear accelerator beam energy at off-axis points.

The change in energy of linear accelerator x-ray beams from the central ray to off-axis points causes errors in the dose calculated by conventional techniques for large, irregularly shaped fields. A modification of conventional calculative methods to correct for the change in beam energy is presented. The results of measurements in irregular fields on a Clinac-4 are reported which verify the validity of the calculative method. A discussion of the clinical significance will point out errors of 3% to 4% in conventional dose calculations.

Particle Accelerators↗

PubMed2000

Electronic portal imaging with an avalanche-multiplication-based video camera.

The aim of this study is to investigate the degree to which the imaging quality of an existing (video-based) electronic portal imaging device (EPID) system may be improved by using an avalanche-multiplication-based video camera (called the avalanche-gain method). Due to avalanche multiplication in the target of the video camera tube, the new camera can be made up to several hundred times more sensitive than a camera using a conventional video (e.g., Saticon) tube. As a result, the camera noise which limits the performance of current video-based EPIDs should be overwhelmed and made negligible. The detective quantum efficiency (DQE) of an EPID using the avalanche-gain method has been measured with 6 MV and 18 MV beams obtained using a linear accelerator. It is shown that the camera noise is indeed much smaller than quantum noise and that the DQE of the system is significantly increased compared to conventional video-based EPIDs. Variation of DQE of the avalanche-gain video portal system with a change of demagnification was also investigated. It has been shown that the improvement of optical coupling has less effect in this system than that in a conventional video-based EPID system.

Humans↗

PubMed1998

[Treatment of Graves' ophthalmopathy with irradiation].

Orbital radiation therapy appears to be an effective and safe treatment of Graves' ophthalmopathy. Five patients with Graves' ophtalmopathy were treated with external beam radiotherapy during 1996 and 1997 in St. Ann University Hospital. Four of them patients had a past history of hyperthyreoidism, one hypothyreoidism. The radiotherapy was performed with high-energy beam of a linear accelerator. The technique of irradiation employs 2 opposed beams tilted 5 degrees posteriorly. The use of the asymetric jaws is necessary to protect surrounding normal tissues. A course of radiotherapy 20.0 Gy (2.0 Gy per fraction) was given to the patients. The therapy was well tolerated. We have not observed any side late effects of radiotherapy. All patients showed good or excellent responses. The improvement of symptoms appeared within 3-6 months after the irradiation.

Aged↗

PubMed1990

[Possibilities of using beams of the Mevatron linear accelerator].

The output beams of fast electrons and high energy fotons of a linear accelerator Mevatron KD 80/67 provide broad spectrum of therapeutic application. The properties of static fields of bremsstrahlung, modification of dose distribution by means of wedge filters, properties and advantages of asymmetric diaphragms and possibilities of shaping the irradiation fields are described. Dose distribution in rotation therapy with bremsstrahlung is presented. In electron beams the author presents cohorts of selected isodoses of static fields for all energies and define important therapeutic values. A possibility of shaping basic fields and the use of rotation therapy with electron beam is suggested.

Humans↗

PubMedDate not supplied

Intraoperative radiotherapy. Literature updating with an overview of results presented at the 6th International Symposium of Intraoperative Radiation Therapy.

Intraoperative radiotherapy is a technique that can be integrated into multidisciplinary treatment strategies in oncology. A radiation boost delivered with high energy electron beams can intensify locoregional antitumor therapy in patients undergoing cancer surgery. Intraoperative radiotherapy can increase the therapeutic index of the conventional combination of surgery and radiotherapy by improving the precision of radiation dose location, while decreasing the normal tissue damage in mobile structures and enhancing the biological effect of radiation when combined with surgical debulking. Intraoperative radiotherapy has been extensively investigated in clinical oncology in the last 15 years. Commercially available linear accelerators require minimal changes to be suitable for intraoperative radiotherapy. Its successful implementation in clinical protocols depends on the support given by the single institutions and on a clinical research-oriented mentality. Tumors where intraoperative radiotherapy as a treatment component has shown promising rates of local control include locally advanced rectal, gastric and gynecologic cancer, bone and soft tissue sarcoma. Intraoperative radiotherapy can be applied to brain tumors, head and neck cancer, NSCLC and pancreatic carcinoma.

Combined Modality Therapy↗

PubMed1996

Neutrons and carcinogenesis: a cautionary tale.

The best estimates for radiation induced cancer and leukemia are based on the Japanese survivors of Hiroshima and Nagasaki. With the earlier dosimetry systems of the 1960s, it was possible to drive an RBE (relative biological effectiveness) for neutrons from the Japanese data, because it was thought that there was a significant neutron dose at Hiroshima compared with Nagasaki. The estimated RBE of about 20 was consistent with laboratory estimates for oncogenic transformation in vitro and tumors in animals. The revised dosimetry of the 1980s [DS 86] essentially eliminated the neutron component at Hiroshima, and consequently removed the only neutron RBE estimate based on human data. However, recent neutron activation measurements indicate that there may indeed have been thermal neutrons at Hiroshima, and measurements of the ratio of inter- to intra-chromosomal aberrations in peripheral lymphocytes of survivors also tend to indicate that the biologically effective dose was dominated by neutrons. Another area in which the large biological effectiveness of neutrons assumes importance is the production of photoneutrons in high energy medical linear accelerators (Linacs). An increasing number of accelerators operating in the 18 to 20 MV range are coming into routine clinical use and at this energy, photoneutrons generated largely in the collimators result in a total body dose to the patient. The increased risk of second malignancies must be balanced against the slight improvement in percentage depth doses compared with more conventional machines operating at 6 to 10 MV, below the threshold for photoneutron production.

Cell Transformation, Neoplastic↗

PubMed1991

Technical and therapeutic aspects of dynamic stereotactic radiosurgery.

A treatment procedure that results in a uniform dose in a single fraction over the entire treatment volume while minimizing the dose to other tissues has been developed to treat intracranial lesions such as arteriovenous malformations (AVMs). This technique is called dynamic stereotactic radiosurgery. Its main characteristic is simultaneous and continuous gantry and couch motion during the treatment procedure. It employs an isocentrically mounted linear accelerator as the source of radiation. Target localization is determined by digital subtraction angiography and CT. From results obtained in other centres, this technique has shown that an AVM less than 2.5 cm in diameter has an 85% chance of being completely obliterated within two years after a single treatment of stereotactic radiosurgery. This technique is suitable for those patients with inoperable, surgically inaccessible lesions, or whose current medical profile shows them to be high-risk candidates for surgical intervention.

Angiography, Digital Subtraction↗

PubMed1987

A study of the mantle technique for Hodgkins disease using thermoluminescent dosimetry.

A study of the mantle technique for Hodgkins disease patients was embarked upon using thermoluminescent dosimetry (TLD) and a RANDO phantom. There was concern that the technique using Co-60 and turning the patient over for the posterior field may not be optimal as it was felt that the axillary areas of these patients were being under-dosed. A variety of treatment set-ups were performed on a 60-Cobalt unit at extended SSD. The phantom was planned for treatment lying supine for the anterior field and prone for the posterior field. Measurements were made in the neck, hilum and axillae using various combinations of compensators and lead filters. The combination of full field compensators and a lead filter gave the best results in the phantom but did not give satisfactory axillary readings in the patient situation. This was attributed to the difference in shape and AP separation of the patient when lying prone. RANDO, being solid, retains its shape. Treatment was then planned for a 6 MV linear accelerator using a vertical couch extender which enables the patient to remain supine throughout the treatment by increasing the table height to allow the posterior portal to be treated through the couch. Full field compensation was found to give an adequate dose to all the measured areas. This technique has since been performed on 28 patients being treated for Hodgkins disease at the Hillbrow Hospital, and the average axillary dose recorded in these patients is 88.6% of the prescribed dose.

Cobalt Radioisotopes↗

PubMed2000

Evaluation of a second set of lateral lasers for easier patient positioning in radiotherapy.

On modern linear accelerators patient set-up is often awkward due to the height of the isocentre. A second set of lateral lasers was mounted approximately 20 cm below the isocentre allowing patient positioning at an ergonomical height. A small randomised study involving 16 heavy patients treated for pelvic malignancies demonstrated that using the lower lasers for patient set-up is not only likely to reduce strain on staff but also speed up the set-up process.

Aged↗

PubMed2005

Properties of the Feynman-alpha method applied to accelerator-driven subcritical systems.

A Monte Carlo study of the Feynman-method with a simple code simulating the multiplication chain, confined to pertinent time-dependent phenomena has been done. The significance of its key parameters (detector efficiency and dead time, k-source and spallation neutrons multiplicities, required number of fissions etc.) has been discussed. It has been demonstrated that this method can be insensitive to properties of the zones surrounding the core, whereas is strongly affected by the detector dead time. In turn, the influence of harmonics in the neutron field and of the dispersion of spallation neutrons has proven much less pronounced.

Algorithms↗

PubMed1984

Heavy ion effects on yeast cells: reappearance of the oxygen effect with very high LET.

Survival of a diploid and a haploid wild type and a radiation-sensitive rad52-mutant was investigated after exposure to accelerated ions in the presence or absence of oxygen. Ar, Kr, Xe, Sm, Pb and U ions were used with specific energies between 0.3 and 12 MeV/u. The results demonstrate that the oxygen enhancement ratios (o.e.r.) do not only depend on LET or Z*2/beta 2 but even more so on ion specific energy. The o.e.r.s are always higher with greater E/m values pointing to the importance of delta-electron action.

Cell Survival↗

PubMed1999

Monte Carlo modelling of a virtual wedge.

Compared with a set of physical photon wedges, a non physical wedge (virtual or dynamic wedge), realized by a moving collimator jaw, offers an alternative that allows creation of a wedged field with any arbitrary wedge angle instead of the traditional four physical wedges (15 degrees, 30 degrees, 45 degrees and 60 degrees). It is commonly assumed that non-physical wedges do not alter the photon spectrum compared with physical wedges that introduce beam hardening and loss of dose uniformity in the unwedged direction. In this study, we investigated the influence of a virtual wedge on the photon spectra of a 6-10 MV Siemens MD2 accelerator with the Monte Carlo code EGS4/BEAM. Good agreement was obtained between calculated and measured lateral dose profiles at the depth of maximum dose and at 10 cm depth for 20 x 20 cm2 fields for 6 and 10 MV photon beams. By comparing Monte Carlo models of a physical wedge and the virtual wedge that was studied in this work, it is confirmed that the latter has an insignificant effect on the beam quality, whereas the former can introduce significant beam hardening.

Computer Simulation↗

PubMed2004

Calculation of conversion coefficients for clinical photon spectra using the MCNP code.

In this work, the MCNP4B code has been employed to calculate conversion coefficients from air kerma to the ambient dose equivalent, H*(10)/Ka, for monoenergetic photon energies from 10 keV to 50 MeV, assuming the kerma approximation. Also estimated are the H*(10)/Ka for photon beams produced by linear accelerators, such as Clinac-4 and Clinac-2500, after transmission through primary barriers of radiotherapy treatment rooms. The results for the conversion coefficients for monoenergetic photon energies, with statistical uncertainty <2%, are compared with those in ICRP publication 74 and good agreements were obtained. The conversion coefficients calculated for real clinic spectra transmitted through walls of concrete of 1, 1.5 and 2 m thick, are in the range of 1.06-1.12 Sv Gy(-1).

Computer Simulation↗

PubMed2005

Radiation safety design for the J-PARC Project.

The High-Intensity Proton Accelerator Project, named J-PARC, is in progress, with the aim of enabling studies on the latest basic science and the advancement of nuclear technology. In the project, a high-energy proton accelerator complex with the world's highest instantaneous intensity is under construction. In order to establish a reasonable shielding design, both simplified and detailed design methods were used in the shielding design of J-PARC. This paper reviews the present status of the radiation safety design study for J-PARC.

Computer Simulation↗

PubMed1982

A double-junction technique for total central nervous system irradiation with a 4-MV accelerator.

An analysis is presented of a double-junction technique for the treatment of the complete central nervous system using orthogonal brain and spine ports from a 4-MV linear accelerator with a lead flattening filter. Experimental results are compared with calculated results in the junction zone. This study shows that clinically homogeneous doses (+/- 6%) can be delivered to the total central nervous system, provided that the irradiation of the junction zone is terminated about 10% sooner than irradiation of the rest of the brain and cord. The number of reduced fractions to be delivered in this region can be determined from the results of scatter dose calculations.

Central Nervous System Diseases↗

PubMed1992

[Comparison of acceptance tests of 5 linear electron accelerators for radiotherapy].

The acceptance tests of five electron linear accelerators have been analyzed to compare their completeness, the standards specified by the manufacturers and the measured deviations. Involved tests were relative to geometrical and mechanical parameters, photon and electron beam characteristics, dose monitoring systems, treatment table and safety systems. The comparison of acceptance tests has shown a significant nonuniformity, also due to the absence of a specific legislation; anyway, the measured deviations generally appeared to be within internationally recommended values.

Particle Accelerators↗

PubMedDate not supplied

A parameterization of the electron beam output factors of a 25-MeV linear accelerator.

A new parameterization of the output factors of an electron beam has been developed. The output factors for the electron beams of an AECL Therac-25 have been determined for a variety of square and rectangular fields using ionization measurements and thermoluminescent dosimetry. The data were then least-squares fit by a semiempirical equation which treats the two field dimensions as variables. Such a parameterization allows computer-generated tables of output factors to be manufactured. The calculated values agree with the measured data in most cases to within the +/- 1% experimental uncertainty. A comparison between this method of calculating output factors and two conventional methods is also presented.

Humans↗