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Biomedical subjects

G Starkschall

Publications and source records attributed to G Starkschall.

At least 37 records · Page 2Linked to original sources
PubMed1995

Water bolus for electron irradiation of the ear canal.

PURPOSE: To demonstrate that water bolus in the external ear can decrease the dose inhomogeneity caused by auricular surface irregularities when the ear is in an electron-beam field. METHODS AND MATERIALS: Three-dimensional (3D) dose distributions with and without water bolus in the external ear were calculated for a representative patient. The electron dose calculations were made using the Hogstrom pencil beam algorithm as implemented in 3D by Starkschall. To demonstrate the use of water bolus in the ear clinically, the case of a patient with squamous carcinoma of the concha who was treated with electrons is presented. RESULTS: Water bolus markedly lessens the dose heterogeneity caused by the surface irregularities of the ear and the air in the external auditory canal. In the test case, the maximum dose was reduced by 25% using this technique. CONCLUSION: When the ear is in an electron beam field, warm water should be placed in the external auditory canal and concha. This maneuver may reduce the incidence of auricular complications that occur after electron-beam therapy.

Aged↗
PubMed1993

Dosimetric evaluation of total scalp irradiation using a lateral electron-photon technique.

PURPOSE: To evaluate the radiation dosimetry of a new technique for total scalp irradiation. METHODS AND MATERIALS: A treatment technique described by Akazawa (1989) has been studied. During each fraction, two electron and two photon fields are treated. While most of the lateral scalp is treated with the electron fields, a rind of scalp close to the midsagittal plane is irradiated by parallel-opposed lateral photon fields. A wax bolus is used to build up skin dose and to protect the brain from electron dose. The dose distribution and dose-volume histograms were evaluated for different field arrangements using a 3-dimensional treatment planning system. After modifying the technique, in-vivo thermoluminescent dosimetry were used to evaluate the dose distributions for the first two patients. RESULTS: To compensate for the lack of dose from the opposed photon field at the junction, the technique was modified using overlapped fields instead of abutting fields. A field overlap of 3 to 4 mm between the electron and photon fields was found optimal. When used with the field junction shift of 1 cm midway through the treatment, this scheme resulted in a dose uniformity of -5% to +15% of the prescribed dose in the region of abutment. Results of the 3-dimensional dose calculation were supported by in-vivo thermoluminescent dosimetry on two patients. CONCLUSION: On the basis of computer dose calculations and in-vivo dosimetry. Akazawa's technique for scalp irradiation can be improved by using a 3 to 4 mm overlap of electron and photon fields. This modified technique is practical and produces clinically acceptable dosimetry.

Clinical Protocols↗
PubMed1991

Effect of dimensionality of heterogeneity corrections on the implementation of a three-dimensional electron pencil-beam algorithm.

Electron beam dose distributions were calculated on a three-dimensional grid using three pencil-beam algorithms, each taking into account irregularities in field shape. The algorithms differ in that patient anatomy in either one, two, or three dimensions is used in the calculation of dose to a point. Algorithms were optimized for speed by such techniques as precalculation and storage of several quantities, reordering of pencil-beam and grid-point loops, selection of cut-off values for some calculated quantities, and invoking error function symmetries. Execution times for optimized versions of each of the algorithms as implemented on a three-dimensional treatment planning system were comparable for both the one- and two-dimensional heterogeneity correction requires an additional calculational loop over fan lines. Execution times for the three-dimensional heterogeneity correction were approximately a factor of four longer than those for the two-dimensional correction. For certain geometries, three-dimensional heterogeneity corrections were necessary to calculate dose distributions accurately, in spite of the additional cost in calculation times.

Algorithms↗
PubMed1990

An interactive system for point dose optimization.

An interactive system has been developed to aid in determining optimal photon and electron beams and beam weights for radiotherapy treatment planning. Dose constraints at various points are selected and an algorithm searches for a set of beams and weighting factors that satisfy these constraints. In the event that no combination of beam weights satisfies the choice of treatment modalities and dose constraints, the treatment modalities and dose constraints can be modified interactively. The goal of this procedure is different from that of more conventional optimization schemes in which optimal dose values are specified and the optimization algorithm determines the set of beam weights that yields a dose distribution closest to optimal.

Algorithms↗
PubMed1989

A two-dimensional pencil-beam algorithm for calculation of arc electron dose distributions.

A two-dimensional pencil-beam algorithm is presented for the calculation of arc electron dose distributions in any plane that is perpendicular to the axis of rotation. The dose distributions are calculated by modelling the arced beam as a single broad beam defined by the irradiated surface of the patient. The algorithm is two-dimensional in that the anatomical cross section of the patient and the skin collimators are assumed identical in parallel planes outside the plane of calculation. The broad beam is modelled as a collection of strip beams, each strip beam being characterised by its planar fluence, mean projected angular direction and a root-mean-square spread about the mean direction. Using these parameters, the dose distribution is calculated using pencil-beam theory. Examples of strip-beam parameters and resulting dose distributions for patient geometries are presented. Features of the algorithm, which include (1) incorporation of pencil-beam theory for the calculation of dose in heterogeneous tissue, (2) run times of only about twice that of comparable-sized fixed electron fields and (3) the input requirement of only a single depth dose and four off-axis dose profiles of measured data, make the algorithm practical for clinical use.

Algorithms↗
PubMed1986

A computer-aided instructional module for radiological physics.

A computer program and instructional module have been written to assist the physicist in radiation therapy physics instruction. The program, written in Pascal, presents the student with a screen of instructional material, followed by a multiple choice question pertaining to that material. A correct response allows the student to proceed to the next screen. If the answer is incorrect, another screen is presented which explains to the student why the answer is incorrect. Then the original screen and question are repeated. The program is general and can be used with any instructional module. It also is transportable, with a minimum of machine-dependent instructions.

Computer-Assisted Instruction↗
PubMedDate not supplied

Surface dose for megavoltage photon beams outside the treatment field.

Measurements made on photon beams from four different radiotherapy machines have demonstrated that skin dose several centimeters outside the boundary of a treatment field may be as much as 20% of the central axis maximum dose. This surface dose has been measured for an AECL Theratron 80, Siemens Mevatron VI, Varian Clinac 20, and CGR Sagittaire for distances up to 12 cm outside the field boundary and for depths up to the depth of maximum central axis dose. This dose has also been measured as a function of field size and of source-to-skin distance. For the lower energy photon beams, this radiation is significantly attenuated in the first 2-3 mm of tissue, while for higher energy beams, a buildup phenomenon with a dmax of 2-3 mm is observed. The magnitude of this radiation is approximately linearly dependent upon field dimension for all energies.

Cobalt Radioisotopes↗
PubMedDate not supplied

A constrained least-squares optimization method for external beam radiation therapy treatment planning.

A method has been developed to optimize the selection of beam weights for external beam radiation therapy treatment planning. This method uses a matrix transformation method to achieve a least-squares fit to doses at preselected points in a target volume. This method is more general than the conventional least-squares method in that it allows one to incorporate as constraints maximum allowable doses delivered to critical organs, as well as the requirement that all beam weights be non-negative. A computer algorithm DOSEOPT has been written which takes a matrix of doses to various points from beams whose field sizes and orientations have been predetermined, and performs the constrained least-squares fit to desired doses in the target volume and maximum doses to critical organs. Implementation of this algorithm and its application to several cases are discussed.

Esophageal Neoplasms↗
PubMedDate not supplied

Analytic evaluation of depths of dose calculation points for external beam radiation therapy treatment planning.

A method has been developed to calculate the depth of a point in a patient contour analytically. In this method, the patient contour is approximated by a Fourier series comprising 5-10 terms. The depth of a calculation point is the distance between the calculation point and the intersection of the ray line with the contour line. Depth calculations using this method are compared with calculations using a more conventional method in which the intersection is found by representing the contour as a set of straight-line segments between pairs of contour points. This method gives depth values comparable in accuracy to the more conventional method and computed approximately twice as fast.

Biophysical Phenomena↗
PubMedDate not supplied

Extension of Fourier methods to the calculation of effective depths in heterogeneous media of arbitrary contour.

The description of patient contours and internal structures by means of truncated Fourier series can be extended to continuous contours of arbitrary shape and location by expressing the x and z Cartesian coordinates of the contour as independent Fourier series in a parameter t. An analytic equation for the intersection of the contour and a ray line is then written as an equation in the parameter t. The equation can be solved using numerical methods yielding the Cartesian coordinates of the intersection point directly.

Fourier Analysis↗
PubMedDate not supplied

Analytic evaluation of heterogeneity locations for external beam treatment planning.

A method has been developed to calculate depths of calculation points analytically in patient contours containing internal heterogeneities. In this method, each contour is approximated by a Fourier series comprising five to ten terms. Using this approximation, the intersections of a ray line with each contour are found as the solution of a transcendental equation. Once these intersections have been located, effective depths used in heterogeneity calculations can readily be calculated.

Algorithms↗
PubMedDate not supplied

A convolution method for constructing primary beam profiles in the presence of beam modifiers.

Empirical functions that describe primary beam profiles for radiotherapy treatment planning systems generally account for finite source size only on beams unmodified by blocks, wedges, or compensating filters. To incorporate the effects of extended sources on such modified beams and to treat the effect of an extended source consistent with the manner in which the unmodified beam is treated, the unmodified beam profile can be written as a convolution of an unknown source function with a collimator transmission profile. Using Fourier transforms, one can then solve for the source function. This source function is then convolved with a beam transmission function that has been modified by blocks, wedges, or compensating filters to obtain a primary beam profile. A number of examples are presented that demonstrate the calculations of the effects of beam modifiers on primary beam profiles.

Algorithms↗
PubMedDate not supplied

Electron bolus design for radiotherapy treatment planning: bolus design algorithms.

Computer algorithms to design bolus for electron beam radiotherapy treatment planning were investigated. Because of the significant electron multiple scatter, there is no unique solution to the problem of bolus design. However, using a sequence of operators, a bolus can be designed that attempts to meet three important criteria: adequate dose delivery to the target volume, avoidance of critical structures, and dose homogeneity within the target volume. Initial calculation of bolus shape was based upon creation operators forcing either the physical or the effective depths of the distal surface of the target volume to a specified value. Modification operators were then applied to the bolus to alter the shape to better meet the design criteria. Because the operators each address a single dosimetric issue, they can often adversely affect some other attribute of the dose distribution. In addition, an extension operator is used to design the bolus thickness outside the target volume. Application of these operators is therefore carried out in certain sequences and each may be used more than once in the design of a particular bolus. The effects of these operators on both the bolus and the resulting dose distribution are investigated for test geometries and patient geometries in the nose, parotid, and paraspinal region.

Algorithms↗
PubMedDate not supplied

An interactive beam-weight optimization tool for three-dimensional radiotherapy treatment planning.

A computer software tool has been developed to aid the treatment planner in selecting beam weights for three-dimensional radiotherapy treatment planning. The program consists of a feasibility search algorithm embedded in an interactive, user-friendly driving program. The feasibility search algorithm is based on the iterative relaxation algorithm of Cimmino [La Ricerca Scientifica, Vol. I, pp. 326-333 (1938)] as applied to the radiotherapy inverse problem by Altschuler et al. [Med. Phys. 13, 590 (1986)]. Relative importances of structures based upon clinical considerations can be incorporated into the algorithm. In order to speed convergence, the relaxation parameter is made to vary, with its value based upon a measure of deviation from feasibility. The interactive driving program is designed so that the treatment planner can make reasonable judgments regarding the acceptability of a plan in the event that the dose constraints yield no feasible solution. An example of the use of this program applied to a problem in three-dimensional radiotherapy treatment planning is illustrated.

Algorithms↗