PubMed Health⌕ Search

Biomedical subjects

G K Svensson

Publications and source records attributed to G K Svensson.

18 recordsLinked to original sources
PubMed1991

Quality assurance in stereotactic radiosurgery using a standard linear accelerator.

Methods have recently been developed for using standard linear accelerators to perform stereotactic radiosurgery. The accuracy necessary to perform this procedure requires an intensive quality assurance program to encompass all aspects of dose calibration and mechanical integrity of the treatment unit, the treatment planning process, and treatment delivery. The programs developed at the Joint Center for Radiation Therapy (JCRT) include testing of the linear accelerator and the stereotactic system, cross checking of the treatment planning process, and a quality assurance check list of the treatment delivery procedure. This report outlines in detail the quality assurance program currently in use at the JCRT.

Humans↗
PubMed1988

An external beam treatment technique for retinoblastoma.

The main difficulty in the irradiation of retinoblastoma has been to deliver a high uniform dose to the entire retinal surface and spare the lens. Conventional techniques are inadequate to deliver an acceptable dose distribution especially for cases when there are both anterior and posterior lesions. We have developed a procedure to deliver a high dose anteriorly at the ora serrata for a compromise of about 30-35% of the target dose to the lens. The technique consists of 3 pairs of non-coplanar arcs using a 4 MV accelerator. This technique may offer a higher probability of tumor control and cure when gross tumor is present at the ora serrata when compared to the conventional techniques using lateral techniques.

Eye Neoplasms↗
PubMed1987

Measurements of dose distributions in small beams of 6 MV x-rays.

Dose distributions produced by small circular beams of 6 MV x-rays have been measured using ionisation chambers of small active volume. Specific quantities measured include tissue maximum ratios (TMR), total scatter correction factors (St), collimator scatter correction factors (Sc) and off-axis ratios (OAR). Field sizes ranged from 12.5 to 30 mm diameter, and were defined by machined auxiliary collimators with the movable jaws set for a 4 cm x 4 cm field size. Due to the lack of complete lateral electronic equilibrium for these small fields, the accuracy of the measurements was also investigated. This was accomplished by studying dose response as a function of detector size. Uncertainties of 2.5% were observed for the central axis dose in the 12.5 mm field when measuring with an ionisation chamber with a diameter of 3.5 mm. The total scatter correction factor exhibits a strong field size dependence for fields below 20 mm diameter, while the collimator scatter correction factor is constant and is defined by the setting of the movable jaws. Off-axis ratio measurements show larger dose gradients at the beam edges than those achieved with conventional collimator systems. Corrected profiles measured with an ionisation chamber are compared with measurements made with photographic film and LiF thermoluminescent dosemeters.

Humans↗
PubMed1985

A beam alignment device for matching fields.

When radiation treatment is delivered to adjacent or contiguous tumor volumes with multiple fields, the geometric alignment of the beams is particularly critical. A beam alignment device has been developed that enables precise matching of such fields.

Breast Neoplasms↗
PubMed1985

Progress in 3-D treatment planning for photon beam therapy.

The purpose of this report is to study the feasibility of improving dose distributions using non-coplanar photon beams from a linear accelerator. Non-coplanar beams may enter the patient in any arbitrary configuration. This type of treatment technique requires a three-dimensional (3-D) planning system. Clinical examples are used to illustrate the general problems in 3-D treatment planning, and the potential improvement over coplanar beam treatments. Features of a treatment planning system for 3-D planning are discussed.

Brain Neoplasms↗
PubMed1983

Measurements of dose from secondary radiation outside a treatment field.

Radiation dose to organs outside the radiotherapy treatment field can be significant and therefore is of clinical interest. We have made measurements of dose at distances up to 70 cm from the central axes of 5 X 5, 15 X 15 and 25 X 25 cm radiation fields of 300 kVp, 4 MV and 8 MV X rays, and 60Co gamma rays, at the surface and at depths in water of 5 and 10 cm. Contributions to the total secondary radiation dose from water scatter, machine (collimator) scatter and leakage radiation have been separated. We have found that the component of dose from water scatter can be described by a simple exponential function of distance from the central axis of the radiation field for all energies and field sizes. Machine scatter contributes 20 to 40% of the total secondary dose depending on machine, field size and distance from the field. Leakage radiation contributes very little dose, but becomes the dominant component at distances beyond 60 cm from the central axis. Estimates of the risk of second tumors in long term survivors indicate a small incremental increase above the natural incidence rate based on information from the 1980 BEIR Committee report.

Adult↗
PubMed1975

The use of wedge filters to improve dose distribution with the partial rotation technique.

Rotational arcs covering less than 360 degrees are frequently used in order to spare critical structures in radiation therapy; however, this results in a lack of dose uniformity in the target volume. Wedge filters can be used to restore dose uniformity in the treatment volume when limited arcs are employed. The authors emphasize the use of small rotational arcs (less than 90 degrees) in treating eccentrically located tumors such as those of the spinal cord and describe a simple model which can be used to calculate the wedge angle as a function of the arc angle.

Dose-Response Relationship, Radiation↗
PubMedDate not supplied

Electron scattering and collimation system for a 12-MeV linear accelerator.

The original scattering and collimation system for the Siemens Mevatron XII linear accelerator used a lead scattering foil and box-type plastic collimators. This arrangement achieves excellent field flatness by repeated electron scattering. The electrons reaching the patient are widely distributed in energy and direction. This has detrimental effects on the depth-dose curves: slower falloff and increased surface dose. We have developed an alternative system for this accelerator, designed to minimize electron scatter and improve the safety of patient setup. Primary-electron scatter occurs in the bending-magnet exit window. Field uniformity is accomplished with a flattener of thin aluminum discs of different diameters, piled concentrically. An adjustable electron collimator 25 cm from the patient limits beam size, and a final electron collimator, either a cutout from lead sheet or a custom-made collimator of Lipowitz's metal, in contact with the patient, define the area to be treated. This design results in lower surface dose, sharper dose falloff, bremsstrahlung contamination less than or equal to 1%, and a field flatness expressed by a homogeneity index greater than 0.8 for large fields. Since there is no mechanical connection between the machine and the final collimator, the safety aspects of the system are considerably improved.

Electrons↗
PubMedDate not supplied

Image quality of an analog radiation therapy simulator-based tomographic scanner.

The spatial resolution and noise level of images produced by a commercial analog tomographic scanner have been measured and compared to those of images reconstructed digitally from projections from the same detector. The full width at half maximum of the line spread function was 3.6 mm for images from the analog scanner and 1.1 mm for the digitally reconstructed images. The standard deviation of the CT numbers over a 10-cm2 circular area at the center of a large water phantom, calculated as a percentage of the linear attenuation coefficient of water, was 3.5% for the analog images, 15.4% for high-resolution digital images, and 3.2% for digital images reconstructed using a convolution filter which reduced the resolution to that of the analog images. The data contributing to each digital image were fewer than those contributing to each analog image by a factor of 10. The noise level did not depend on tube current in either the analog or the digital images. The utility of this analog device in radiation therapy planning will depend upon whether errors in contour localization resulting from transferring data from diagnostic CT scanners exceed the errors due to its poorer image quality.

Analog-Digital Conversion↗
PubMedDate not supplied

Head scatter data for several linear accelerators (4-18 MV).

Measurements were made in air on the central axis of radiation beams from linear accelerators operating in the energy range from 4 to 18 MV, to determine the magnitude and source of head scattered radiation. Machines of several manufacturers were studied. The data indicate that, except for one unique collimator design, head scatter originates primarily in the flattening filter and is relatively independent of energy and machine.

Humans↗
PubMedDate not supplied

A high-resolution digital dosimetric system for spatial characterization of radiation fields using a thermoluminescent CaF2:Dy crystal.

A high-resolution digital dosimetric system has been developed for the spatial characterization of radiation fields. The system comprises the following: 0.5-mm-thick, 25-mm-diam CaF2:Dy thermoluminescent crystal; intensified charge coupled device video camera; video cassette recorder; and a computerized image processing subsystem. The optically flat single crystal is used as a radiation imaging device and the subsequent thermally stimulated phosphorescence is viewed by the intensified camera for further processing and analysis. Parameters governing the performance characteristics of the system were measured. A spatial resolution limit of 31 +/- 2 microns (1 sigma) corresponding to 16 +/- 1 line pairs/mm measured at the 4% level of the modulation transfer function has been achieved. The full width at half maximum of the line spread function measured independently by the slit method or derived from the edge response function was found to be 69 +/- 4 microns (1 sigma). The high resolving power, speed of readout, good precision, wide dynamic range, and the large image storage capacity make the system suitable for the digital mapping of the relative distribution of absorbed doses for various small radiation fields and the edges of larger fields.

Health Physics↗