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Biomedical subjects

M R Sontag

Publications and source records attributed to M R Sontag.

30 records · Page 2Linked to original sources
PubMed1987

Rapid three-dimensional treatment planning: I. Ray-tracing approach to primary component dose calculations.

Algorithms for fully three-dimensional divergent-beam radiotherapy treatment planning have been developed to achieve very high sampling of dose in heterogeneous (inhomogeneous density) tissue throughout an arbitrarily oriented patient volume, in clinically acceptable times of calculation. Dose is calculated at points along numerous rays which sample each beam. To display the dose distribution, the calculated dose values for each beam are interpolated onto rectilinear grids of (arbitrary) parallel planes, scaled for beam weight and finally merged with the weighted dose contributions of other beams. In this paper we describe and demonstrate the algorithm for the primary component of the three-dimensional photon dose distribution delivered to a patient.

Clinical Trials as Topic↗
PubMed1985

A clinically operational method for three-dimensional dose calculations.

Three-dimensional dose calculations can now be performed for both photon and neutron beam therapy in reasonable times on a minicomputer. The method described is a scatter-air ratio-tissue-air ratio (SAR-TAR) model which, for any beam, finds the correct depth at each point in a patient even when the beam passes obliquely through more than one transverse section of the patient. Scatter dose is determined by performing a Clarkson integration over both angle and radial distance, with the correct depth of the beam calculated at both accumulation and scatter points. Doses are computed and displayed over a 0.33 cm sampling grid superimposed on each transverse CT slice used in treatment planning. Dose-volume histograms are made for the entire patient volume and for internal organs and target volumes outlined by the physician. The clinical efficacy of the method in therapy planning is demonstrated and a comparison is made between this method and single section two-dimensional methods.

Breast Neoplasms↗
PubMed1978

Clinical application of a CT based treatment planning system.

An AECL (TP-11) treatment planning system has been modified to allow multiple CT scans to be used directly for dosage calculation and display. This CT/TP-11 system has incorporated the "equivalent tissue-air ratio method" to make corrections for the effects of tissue heterogeneity and the three-dimensional nature of patient shape. In test situations, using an anatomical phantom, an accuracy of ca. 2% has been attained. An investigation has also been made to assess the importance of the reproducibility of patient position and its effect on the validity of such dosage calcualtions. When representing an advance in sophistication of existing facilities, this system has also been designed with practically in mind.

Humans↗
PubMed1978

The equivalent tissue-air ratio method for making absorbed dose calculations in a heterogeneous medium.

The CT scanner makes three-dimensional anatomical information available for treatment planning, the calculational algorithm being the limiting factor in dose calculations. A method has been developed, called the "effective tissue-air ratio method," that uses all the information but reduces it to manageable proportions. The accuracy is satisfactory for a wide range of photon beam energies and clinical applications. It has been implemented in such a way that the CT scan can be used directly without the intermediate step of manually obtaining structures from either a viewing screen or a hard copy of the CT image.

Air↗
PubMed1977

Implications of computed tomography for inhomogeneity corrections in photon beam dose calculations.

Patient inhomogeneity information is investigated for use in radiotherapy planning. Absorbed doses measured in a phantom are compared to doses calculated for photon beams using various treatment planning inhomogeneity correction methods. Delineation of inhomogeneities with a spatial resolution of 5 mm and with an electron density accuracy of 2% in usually sufficient to allow doses to be calculated with a mean accuracy of better than 2% for 60Co and 3% for 25-MV x rays if the authors' Equivalent Tissue-Air Ratio Method is used.

Cobalt Radioisotopes↗
PubMedDate not supplied

Dose measurements in the build-up region for cobalt-60 therapy units.

The dose in the build-up region for four different Cobalt-60 therapy units was measured. It was found that, for large collimator openings and relatively short SSDs, a new dose peak occurs at a depth very much smaller than 0.5 cm. The dose at this peak is a function of collimator openings and SSDs and, in some extreme case, could be 15% or more higher than the dose at the conventional peak dose of 0.5 cm. The new dose peak is probably due to electrons produced in the source capsule and the part of the collimator close to the source; it can be almost eliminated by a filter placed just below the collimator.

Cobalt Radioisotopes↗
PubMedDate not supplied

Corrections to absorbed dose calculations for tissue inhomogeneities.

Traditional methods for correcting for the presence of tissue inhomogeneities may produce errors as great as 10% at points within or closese to the inhomogeneities. Measurements were made in phantoms containing aluminum or cork inhomogeneities. Agreement between measured and predicted results was usually within 2%-3%.

Methods↗
PubMedDate not supplied

Displacement corrections used in absorbed dose determination.

When an (exposure) calibrated ionization chamber is used for the determination of absorbed dose from a photon beam, the reading of the instrument must be multiplied by a number of factors, one of which is an attenuation correction for phantom material displaced by the chamber. the magnitude of this correction must depend on the size and shape of the ionization chamber as well as the energy of the radiation beam. For cobalt-60 radiation, a single number, 0.985, has generally been used. Recent measurements, however, and "first scatter" calculations, of kerma suggest that a more appropriate value for a Farmer-type chamber used in a water phantom would be 0.975. Such a change is small but would be important when dose calculations based on "in phantom" measurements are compared to calculations that are based on in air measurements. Values for the attenuation factor for other beam energies have not been generally available. We have carried out "first scatter' calculations for a rather wide range of energies and spectra. Measurements in 60Co beams and in a high-energy (25 MV) photon beam support the calculations. A set of proposed displacement correction factors is presented.

Cobalt Radioisotopes↗
PubMedDate not supplied

The use of lead-lined cones to improve the beam characteristics of a 50-kVp contact therapy x-ray beam.

Measurements were made to calibrate a Philips RT-50 contact therapy unit using an EG&G extrapolation chamber. The measured half-value layer of 0.27 mm in aluminum agrees closely with published data. Exposure rates in air for 4-cm source-to-surface distance (SSD) cones fall off faster than can be explained by the inverse square law alone. Lining the cone with lead results in exposure rates that follow the inverse square law at all distances from the cone end. Depth dose measurements made for 2- and 4-cm SSD cones produced the anomalous result of greater depth dose for the shorter SSD. This is due to a low-energy component arising from the end of the 4-cm SSD cone, producing an artificially high surface dose. Lining the cone with lead reduces the component and significantly increases the depth dose. The low-energy component is believed to be 6-8 keV characteristic x rays that arise from the stainless-steel cone. Lining the cone with lead either absorbs these x rays or prevents their production.

Brachytherapy↗
PubMedDate not supplied

An algorithm for three-dimensional visualization of radiation therapy beams.

A computer algorithm to display radiation beams superimposed on three-dimensional (3-D) views of patient anatomy has been developed. It may be implemented as a postprocessing step to existing software for 3-D presentation and display. The algorithm takes as input a shaded 3-D view (reconstructed, for example, from computed tomography scans), together with the associated depth map, and generates as output an enhanced 3-D view highlighting in color the visible points which lie within the projected beam outlines. The algorithm is independent of the method used to generate the 3-D view (surface or volume rendering techniques may be used) and is independent of beam shape (beams may be modified with shielding blocks). It is not restricted to external surfaces and will correctly show radiation beams projected onto cut-away views of internal organs. The method is illustrated by application to a tangential pair for breast malignancy, using 3-D views generated with volume rendering software.

Algorithms↗
PubMedDate not supplied

3-D dose-volume compensation using nonlinear least-squares regression technique.

A method for external beam dose-volume optimization is presented. The Gauss-Marquardt nonlinear least-squares regression technique is applied to compensator design and determination. The dose distribution (uniform or otherwise) desired throughout a volume is specified. Compensators optimized to produce the necessary variation of beam intensity across the surface of each beam are simultaneously determined for all the beams. Solutions for homogeneous dose, homogeneous target dose, and restricted dose to exterior target volume structures, and inhomogeneous target dose cases are presented. Dependence of the results on the number of parameters as well as the role of degree of desirability weighting is explained and illustrated via examples. Discussion of the significance and limitations of this optimization method is also presented.

Humans↗