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At least 1,333 records · Page 74Linked to original sources

PubMed1992

Fractionated stereotactic radiotherapy for intracranial neoplasms.

Fractionated stereotactic radiotherapy is a method which attempts to combine the radiobiological advantages offered by dose fractionation with a technique for focal delivery of radiation. At McGill University, fractionated stereotactic radiotherapy is given with a linear accelerator-based dynamic stereotactic radiosurgery unit. The first treatment is given using the stereotactic frame for target localization and head immobilization. Subsequent treatments are given using skin tattoos and laser alignment for target placement within the isocenter of the linear accelerator, and a modified portable halo-ring device is used for skull immobilization. Typically, a marginal dose of 42 Gy was prescribed at the margins of the lesion, divided in 6 fractions and given over a 2-week period. We report the pathological profile and treatment results in a series of 21 patients with a variety of intracranial tumors, treated in this manner between May 1987 and April 1990. Fractionated stereotactic radiotherapy appears to be a worthwhile procedure for the treatment of well-selected patients with intracranial neoplasms.

Adult↗

PubMed1990

[Leakage radiations in a medical electron accelerator facility--measurement of neutrons and comparison with calculation].

Neutron dose equivalent rates are measured in the irradiation room and the maze of a medical electron accelerator facility. An m-counter used for measurement is associated with a gate circuit to reject piled up signals caused by pulsed X-rays, so that it is possible to measure the neutrons without obstruction by the piled up signals even in the irradiation room. Counting loss of neutrons is mathematically corrected, and the correction factor comes nearly equal to 1.5 at dose points close to the entrance to the irradiation room when 14 MV X-rays are generated. The measurement in the maze agrees with the calculation reported in the preceding report in a factor less than 1.5 except for dose points close to the entrance to the irradiation room.

Hospital Departments↗

PubMed1990

A needed modification to vendor supplied oversized 45 degree wedges.

A recent change in the source-to-blocking tray distance on at least one new linear accelerator restricts simultaneous use of the vendor supplied oversized (i.e., 20 cm) 45 degree wedge and 7.5 cm or higher custom cerrobend blocks. Truncating the "heel" end of the vendor supplied steel wedge as well as substituting a 45 degree lead wedge (designed and supplied by the vendor) allows simultaneous wedge and 7.5 cm high block usage. Central axis depth dose, transverse dose profile, wedge transmission factor, and surface dose measurements were made for each wedge for 6 through 18 MV x-rays. Dosimetric data for the truncated steel wedge is insignificantly different from that of the untruncated steel wedge, whereas the lead substitute wedge dosimetric data is substantially different and requires remeasurement.

Humans↗

PubMed1989

Measurements of radioactive gaseous releases to air from target halls at a high-energy proton accelerator.

Measurements of induced radioactivity concentration in air were made at five target halls associated with the Fermilab high-energy proton synchrotron. A gas flow-through ionization chamber was used to continuously sample air released from an exhaust stack at each location. The radioactive decay of a grab sample of air from each target hall was recorded, and a best-fit of each decay curve was made via iterative steps to determine the mixture of radionuclides present. The radionuclides 11C, 13N, 15O, 38Cl, 39Cl, and 41Ar were identified. Two distinct radionuclide mixtures or "signatures" were found and were correlated to the geometry of the proton loss point. The normalized activity release rates for the five target areas agree within a factor of two of the average value of 2.7 muBq per proton for 800 GeV protons.

Air Pollutants, Radioactive↗

PubMed1981

[Dosimetry in case of whole-body irradiations with a linear accelerator (author's transl)].

The described whole-body irradiations are effected in order to prepare patients with acute myeloic leukemia for bone marrow transplantation. The patient is irradiated with equal doses from the right and the left side. So a dose of 10 Gy in the middle of the body is achieved which is determined individually by means of phantom measurements. In order to avoid the risk of an interstitial pneumonitis, the lung region is temporarily blinded in such a way that a dose of 8 Gy is obtained there in the middle of the body. During the irradiations, the accumulated dose is measured in different body regions with thermoluminescence dosimeters, a semi-conductor diode and, eventually, a ionization chamber. The scattering width is +/- 13%. We do our best to make the patient's position as comfortable as possible.

Bone Marrow Transplantation↗

PubMed2004

Multinational assessment of some operational costs of teletherapy.

BACKGROUND AND PURPOSE: Decisions in planning radiotherapy facilities in countries with limited financial resources require information on economic factors to make provision for sustainability. This study aims at acquiring data on some of these factors involved in delivery of teletherapy in 11 countries of different economic status. PATIENTS AND METHODS: Representatives of three European, one African, three Latin American and four Asian countries, were identified from radiation oncology institutions that operated both cobalt and linac teletherapy machines. Productivity data were prospectively collected for the year 2002. A detailed log was recorded for each machine over an arbitrary two-week period. Data on quality assurance (QA), maintenance, the capital costs of each machine, and the source replacement costs for the cobalt units were also recorded. RESULTS: Both linear accelerators and cobalt machines treat more than 10,000 fractions per year per machine with 2.5 and 2.3 fields per fraction, respectively. The capital costs of the machines vary considerably, with a factor of more than 10 for linear accelerators. Cobalt sources show a huge variation in price. The median costs of QA and maintenance of a linac was US$ 41,000 compared to US$ 6000 for cobalt machines. This results for the economic factors considered in median costs per fraction of US$ 11.02 for linear accelerators and US$ 4.87 for cobalt machines. These figures do not include the costs for physicians. CONCLUSIONS: The variation of the costs per fraction is more due to the result of differences in machine usage and costs of equipment than of national economic status. A treatment fraction on a linac with functionality comparable to cobalt, costs 50% more than cobalt therapy. This project shows that it is possible to collect data on economic factors prospectively as well as retrospectively.

Capital Expenditures↗

PubMed2006

Mitotic arrest caused by an X-ray microbeam in a single cell expressing EGFP-aurora kinase B.

Although the highest radiosensitivity of cells in the M phase among the other cell phases, such as the G(1), S and G(2) phases, has been known, the exact mechanism of radiosensitivity in mitotic cells remains unclear. Recently, mitotic arrest caused by DNA-damaging reagents has been shown, and the molecular mechanism in the arrest has been discussed in detail. In this study, abnormal cell-cycle progression in the M phase was investigated when a single mitotic cell in each mitotic stage was irradiated with a 5.35 keV X-ray microbeam focused on the cell nucleus. An X-ray microbeam irradiation system installed at BL-27 in Photon Factory, High Energy Accelerator Research Organization (HEARO, Tsukuba) was used. HeLa cells, genetically modified and expressing enhanced green fluorescent protein-tagged aurora kinase B, were used as irradiated samples in order to recognise the stage of each cell in the M phase. Thus, 10 Gy irradiation concentrated at the nucleus of a single cell elongated the cell-cycle progression in the M phase by delaying the metaphase/anaphase transition. The dose dependence of the elongation of the M phase was also examined. An irregular distribution of DNA in anaphase cells was observed after irradiation.

Aurora Kinase B↗

PubMed1998

Neutron fluence and energy spectra around the Varian Clinac 2100C/2300C medical accelerator.

We have simulated the head geometry of a Varian Clinac 2100C/2300C medical accelerator in a Monte Carlo calculation to produce photoneutrons and transport them through the head shielding into a typical therapy room (modeled by a test cell at Varian Associates). The fast neutron leakage fluence and energy spectra have been calculated at 7 positions around the linac head for typical beam operation at 10, 15, 18 and 20 MV. The results of these calculations have been compared with limited measurements made using the same model accelerator operating in a Varian test cell. Calculations were also made for the fluence and energy spectra outside the head with no surrounding concrete walls, floor or ceiling to eliminate the effects of scattering from concrete. Comparisons were also made with calculations using a much simplified head geometry. The results indicate that the calculations using the complex head geometry compare, within the uncertainties, with the measurements. The simple head geometry leads to differences of a factor of 2 from the complex geometry. Results of these calculations can be used to calculate fast neutron transmission through various shielding configurations and through labyrinths.

Monte Carlo Method↗

PubMed2002

Deterministic calculations of photon spectra for clinical accelerator targets.

A method is proposed to compute photon energy spectra produced in clinical electron accelerator targets, based on the deterministic solution of the Boltzmann equation for coupled electron-photon transport in one-dimensional (1-D) slab geometry. It is shown that the deterministic method gives similar results as Monte Carlo calculations over the angular range of interest for therapy applications. Relative energy spectra computed by deterministic and 3-D Monte Carlo methods, respectively, are compared for several realistic target materials and different electron beams, and are found to give similar photon energy distributions and mean energies. The deterministic calculations typically require 1-2 mins of execution time on a Sun Workstation, compared to 2-36 h for the Monte Carlo runs.

Computers↗

PubMed1995

The race for megavoltage. X-rays versus telegamma.

Roentgen's discovery was announced in January, 1896, and x-ray therapy trials followed in 1897. Becquerel rays and radioactive minerals were identified during 1896 through 1898. Radium was used for therapy by 1901, even though a pure standard was not achieved until 1910-1912. Quantities of radium finally became available after 1919, and for 20 years telegamma therapy machines underwent progressive development. Their megavoltage beam was much preferred over the standard 200-250 KV x-ray units of that time. Nuclear physicists during the Great Depression modified electron accelerators into giant 600-900 KV medical x-ray therapy machines and achieved one MV by 1937-1939. These were huge, complex, expensive, and unique to major academic and/or metropolitan centers. During World War II nuclear reactors superseded cyclotrons as efficient factories for few new radioisotopes, including "artificial radium". Few seemed interested in the latter for use in telegamma therapy until 1949-1951, when three competing teams from Canada and the USA designed telecobalt machines. From this competition, among then unknown innovators, emerged three future giants in radiation therapy: A.E.C.L., H. Johns, and G.H. Fletcher. The clinical application of telecobalt therapy was to revolutionize cancer care in community hospitals worldwide.

Canada↗

PubMed1990

The role of stereotactic radiosurgery in the management of intracranial tumors.

With the availability of gamma knife units and the development of modified linear accelerators, there is new interest in radiosurgery, especially as it applies to intracranial tumors. Among its advantages are the ability to precisely localize tumor and through small beam sizes deliver steep dose gradients at the field edges. This sharp dose gradient allows for the prescribed dose of radiation to be given to a tumor while avoiding vital structures only millimeters away. Radiosurgery has produced good results in the treatment of inoperable arteriovenous malformations and can be used as the sole curative therapy for small, radiographically distinct, benign noninvasive tumors. It is also used as salvage therapy in recurrent benign or malignant tumors that have previously been irradiated.

Brain Neoplasms↗

PubMedDate not supplied

[Lethal action of accelerated heavy ions on mammalian cells as affected by inhibitors of DNA synthesis. Results of experimental research].

Radiosensitivity of Chinese hamster cells exposed to 137Cs-gamma-radiation and accelerated heavy ions of 4He (L = 22 and 60 keV/micron), 12C (L = 231 keV/micron), and 20Ne (L = 690 keV/micron) was studied in standard conditions and in the presence of arabinosylcytosine and hydroxyurea. These agents were shown to exert a radiosensitizing effect in the case of gamma-radiation. The effect was less pronounced with 4He ion-radiation and was absent upon irradiation with 12C and 20Ne ions. The radiosensitivity was maximum upon irradiation with 4He ions at L = 60 keV/micron. The RBE coefficients of heavy ions under study decreased in the presence of DNA synthesis inhibitors.

Animals↗

PubMed1995

A new sigmoidal function describing the small field dose profile data from a linear accelerator.

Dose profile data from small circular fields have been used in treatment dose planning for stereotactic radiosurgery. Generally, a two-dimensional interpolation of the measured beam profiles from circular collimators is used to calculate the dose at any axial depth and radial distance from the central axis. Instead, the dose profile data can be transformed into a sigmoidal form. A new three parameter sigmoidal function was developed to fit the transformed (sigmoidal) dose profile data. The values of the three estimated parameters were found to follow either linearly or exponentially as a function of axial depth. Thus, instead of linear interpolation, these formulas can be used to calculate dose at any axial depth and radial distance from the central axis for circular collimators of various diameters. This new sigmoidal function provides another formula to describe dose profile data from circular collimator of small fields.

Humans↗

PubMed1996

A simple and generally applicable method to estimate the peripheral dose in radiation teletherapy with high energy x-rays or gamma radiation.

PURPOSE: Many articles have been published on the measurement of the dose to points outside the primary beam, often called the peripheral dose (PD), for instance, to the gonads, for specific treatment machines and/or techniques. We investigated the possibilities for developing a generalized method based on the data from several publications. METHODS AND MATERIALS: The data from several publications were recalculated for a reference situation, then averaged, and the frequency distributions around the mean were determined. Published data were available for 60Co, 4, 6, 8, and 10 MV, and 18 to 25 MV for a large variety of treatment machines. Furthermore, an analysis of possible corrections for depth dependence, field elongation, irregularly shaped fields, wedges, and shielding blocks was carried out. RESULTS: The frequency distributions of all published PD values for square fields for photon energies of 4 MV to 25 MV showed a standard deviation of 33%. The PD values of 60Co are significantly different with a standard deviation of 25%. A difference in the leakage radiation between cobalt machines and linear accelerators can possibly explain this difference, especially for large distances, where leakage radiation predominates. Taking the uncertainty of the risk factors into consideration, we conclude that the use of average values is justified. Although statistically not significant, the peripheral dose appears to be dependent on photon energy with a minimum around 6 MV. CONCLUSIONS: It is possible to estimate the peripheral dose for photon energies of 4 MV to 25 MV with an accuracy of +/- 33%; for 60Co, the accuracy is even better. The variation of the PD between different treatment machines is so small that it is justified to use average PD values, irrespective of the treatment machine.

Cobalt Radioisotopes↗

PubMed1998

Evidence for a significant timer error on a linear accelerator: consequences for a special therapy application.

A significant timer error has been found on a linear accelerator used for intraoperative radiation therapy. Since typical treatment doses range up to 20 Gy, calibrations that do not account for this effect can incur a large dosimetry error. The effect is dose rate dependent, but energy independent. For a dose rate of 900 cGy min(-1), the error in dose calibration varies between 2 and 3% for an average of 2.5%. At dose rates of 600 and 300 cGy min(-1), the error was 1.7% and 0.8% respectively. The average timer error at 900 cGy min(-1) was found to be 1.2 +/- 0.1 monitor units. It is argued that even a small timer error has serious dosimetry consequences for the implementation of intensity modulated radiation therapy.

Calibration↗

PubMed1993

Shielding at the egress of a 23 mV linear accelerator.

Radiation shielding is required in radiotherapy accelerators to limit the dose equivalent rate from x-rays produced at the target in all directions except that of the useful forward directed beam. Calculations were made in terms of weekly workload of 50,000 cGy delivered to the isocenter per 5-day week, treating 25 patients per day at 400 cGy per patient. It is desirable to reduce the total exposure at the maze door below 30 Rem (0.30 Sv)/wk assuming full-time occupancy. The maze was designed to reduce the neutron dose to an acceptable level by slowing the neutrons to thermal velocities and then absorbing them in a low Z material in the egress.

Biophysical Phenomena↗

PubMed1993

Stereotactically guided radiosurgery using the linear accelerator.

Leksell initiated the concept of stereotactic radiosurgery in 1951. This last decade has seen a rapid proliferation in the development of the methodology which is certainly related in part to the simultaneous growth of high-resolution neuro-imaging techniques. By focusing the beams of 201 hemispherically arrayed cobalt 60 sources, the gamma-knife delivers a high dose of radiation to a small target. Another possibility proposed by several authors is the bragg peak cyclotron-generated irradiation with accelerated protons or helium ions. In Lille, since 1988, we have chosen to develop stereotactic radiosurgery, according to the system of Betti, by the association of Talairach's stereotactic methodology and external single-dose encephalic irradiation with high energy X-rays, delivered by means of a linear accelerator. The major indication for the use of this method is an arteriovenous malformation. Stereotactic radiosurgery may be proposed alone or in combination with surgery and embolisation. It has been shown to be a potentially effective treatment and an attractive alternative in carefully selected patients with intracranial tumours: slow-growing, well limited, deep-seated tumours, such as some gliomas, acoustic neurinomas, skull base meningiomas, pituitary adenomas. This treatment is also used to deliver a focal boost of radiation to previously administered fractionated radiotherapy in patients with small gliomas and solitary brain metastases.

Adenoma↗

PubMed1990

Radiosurgery with photon beams: physical aspects and adequacy of linear accelerators.

The question of the adequacy of isocentric linear accelerators (linacs) for use in radiosurgery is addressed. The general physical requirements for radiosurgery, mainly a high spatial and numerical accuracy of dose delivery, reasonable treatment time, and low skin and leakage dose as well as cost considerations are examined. Various linac-based procedures are analyzed in view of their ability to meet these requirements and are contrasted with the clinically proven system of the Gamma unit. It is shown that the linac-based multiple converging arcs techniques and the dynamic rotation meet the stringent physical requirements on dose delivery and are thus viable alternatives to radiosurgery with the commercially available and dedicated Gamma unit.

Costs and Cost Analysis↗