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R L Siddon

Publications and source records attributed to R L Siddon.

35 records · Page 2Linked to original sources
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↗
PubMed1985

Prism representation: a 3D ray-tracing algorithm for radiotherapy applications.

Computing the intersection of a line with a volume, or 'ray tracing' as it is commonly known, is an integral component of three-dimensional radiotherapy dose calculation algorithms. Examples of ray tracing include the intersection of a ray from the radiation source with the patient, with heterogeneous volumes within the patient and with beam-modifying devices, such as a wedge or compensator. Ray tracing is also of primary importance in the field of three-dimensional computer graphics. Through the process of ray tracing, various display features, such as hidden surfaces, shadows, reflection and refraction, can be rendered simply and with startling realism. Advances have recently been made in three-dimensional ray-tracing methods in computer graphics. Due to the similarity of ray tracing in three-dimensional computer graphics and radiotherapy, these new techniques have an immediate application to the ray-tracing problems in three-dimensional radiotherapy dose calculations. The purpose of this paper is to present these advances and illustrate their use in radiation therapy.

Humans↗
PubMed1985

Effective wedge angles with a universal wedge.

Some recently designed x-ray-producing accelerators are equipped with a single built-in wedge, and different 'effective' wedge angles are obtained by combining an open (unwedged) and a wedged field in the appropriate proportions. This paper describes a technique for determining these proportions from measured isodose distributions of the two component fields. Our data for the Philips SL/75 6 MV accelerator are compared with two existing theoretical models. One model, in which the beams are weighted by the ratio of the tangents of the effective and nominal wedge angles, agrees with the data to within 3 degrees over the range of effective wedge angles and square field sizes examined. The second and simpler model, in which the beams are weighted by the ratio of the wedge angles directly, results in errors of as much as 11 degrees. It is shown that both of these models are approximations to an exact theoretical solution which may be formulated in terms of one free parameter. This parameter may be interpreted physically as the ratio of the slopes of the central-axis depth-dose curves for the open and wedged fields.

Humans↗
PubMed1983

Three-field technique for breast irradiation using tangential field corner blocks.

A further modification of the three-field technique for the radiotherapy of the breast has been developed. Two isocentric opposing tangential fields encompass the breast, chest wall, and may include the internal mammary lymph nodes. The third, an anterior field, encompasses the axilla and supraclavicular areas. As with our previously reported techniques, the objectives of the present modification is to make the posterior edges of the tangential fields coplanar and to match the cephalad geometric edges of the tangential fields to the caudad geometric edge of the supraclavicular field. A half-beam block is used to shield the caudad half of the anterior field, thus producing a vertical transverse plane to which the tangential fields are matched. Small corner blocks are used on the cephalad edges of the tangential fields to produce the vertical edge necessary for matching to the anterior field. It is essential that the match between the tangential fields and the anterior field be geometrically correct to ensure both local control of disease and good cosmetic results. Two advantages of the present technique are the ease with which it can be carried out and the precision of the match plane without the use of cumbersome mechanical accessories.

Breast Neoplasms↗
PubMed1982

Perspective display of patient external and internal contours.

The two-dimensional perspective display of three-dimensional patient contour data is useful in radiotherapy treatment planning as it provides a comprehensive view of the relative positions of patient internal (organ and target) and external contours. The advent of minicomputers and video display systems has made possible the integration of such display techniques into the treatment planning routine. This paper describes and gives an example of the methods we have found useful in producing perspective displays of patient contour data. Included is a solution to the problem of removing hidden line segments from the displayed image.

Anatomy↗
PubMed1982

Utilization of parasternal lymphoscintigraphy in radiation therapy of breast carcinoma.

In radiation therapy of patients with breast carcinoma, the ipsilateral internal mammary lymph nodes are either irradiated by a separate anterior field or included by isocentric opposing tangential fields, which also treat the breast and chest wall. To determine the acceptability of a particular treatment setup, the positions of the nodes must be determined with respect to the treatment fields. For the anterior field technique the problem is two-dimensional and is solved by simply superimposing the treatment field onto an anterior lymphoscintigram. For treatment by opposing tangential fields the problem is three-dimensional and more complex. The solution described in this note is to project the three-dimensional lymph node positions, obtained by a stereo-lymphoscintigraphic procedure, onto the tangential field radiographs. A mathematical expression is given to perform the required projection of the node positions onto the radiographs.

Breast Neoplasms↗
PubMedDate not supplied

Solution to treatment planning problems using coordinate transformations.

The majority of radiation treatment planning problems are relatively straightforward, involving only specified gantry angles in a treatment plane which is perpendicular to the patient longitudinal axis. In addition, there are a number of more complex three-dimensional problems which require combined rotation of the gantry, collimator, and turntable for their solutions. These include, for example, the use of non-coplanar fields and oblique treatment planes, the matching of field edges in three dimensions, the treatment of the breast with opposing tangential fields, and the treatment of inclined elongated lesions. Unfortunately, there is no general systematic approach to the solution of these more complex problems. One may attempt an analytic solution, but this approach is often too cumbersome and tedious. On the other hand, one may resort to a "trial and error" session with the simulator. This paper, therefore, presents a mathematical method which is easily applied and applicable to a wide variety of complex three-dimensional treatment planning problems. The method considers the gantry, collimator, and turntable as coordinate systems. These coordinate systems are derivable from each other by specified coordinate transformations, which contain the rotation angles of the gantry, collimator, and turntable. Within this mathematical framework, the treatment planning problems are found to reduce to two general types, of which various clinical examples are then given.

Breast Neoplasms↗
PubMedDate not supplied

Scatter transmission through an ideal grid.

An analytic expression is derived for the transmission of isotropic scattered radiation incident upon an ideal parallel grid. It is found that the expression for the transmission usually found in the literature overestimates the transmission by approximately a factor of two.

Humans↗
PubMedDate not supplied

A note on equivalent circles, squares, and rectangles.

The scatter dose in the center of a circular beam of photons can be represented by a serial expansion, the first term of which is proportional to the radius. If this term only is used in a scatter integration, the expression becomes proportional to the average radius r of the field contour, whatever its shape. For a square, the ratio between its side and the average diameter d = 2r is s/d = 0.891, the relation between the equivalent circular and square fields for small fields. For rectangles, r is given by the area-to-perimeter ratio multiplied by a function that increases slowly with the elongation ratio. The error term resulting from the use of only the linear term to describe scatter remains small under a variety of conditions.

Elementary Particles↗
PubMedDate not supplied

Scatter integration with right triangular fields.

The concept of the equivalent field is used extensively in radiotherapy dose calculation algorithms. The rationale for using equivalent fields is to allow dose calculations for a wide variety of field shapes, while maintaining dose calculational data for only a few, very regularly shaped fields. A common example is the table of equivalent squares of rectangular fields presented by Day in the British Journal of Radiology. Recently, in searching for fast dose calculation algorithms for irregular fields, we introduced the concept of the equivalent square of a right triangular field. It is shown that an arbitrary irregular field of N vertices may be decomposed into 2N right triangular fields, each with a precalculated equivalent square. The scatter at the point of calculation due to the irregular field is then obtained as a sum of the scatter contributions from the equivalent squares. The scatter integration with right triangles is compared with scatter integration using program IRREG.

Humans↗
PubMedDate not supplied

Fast calculation of the exact radiological path for a three-dimensional CT array.

Ready availability has prompted the use of computed tomography (CT) data in various applications in radiation therapy. For example, some radiation treatment planning systems now utilize CT data in heterogeneous dose calculations algorithms. In radiotherapy imaging applications, CT data are projected onto specified planes, thus producing "radiographs," which are compared with simulator radiographs to assist in proper patient positioning and delineation of target volumes. All these applications share the common geometric problem of evaluating the radiological path through the CT array. Due to the complexity of the three-dimensional geometry and the enormous amount of CT data, the exact evaluation of the radiological path has proven to be a time consuming and difficult problem. This paper identifies the inefficient aspect of the traditional exact evaluation of the radiological path as that of treating the CT data as individual voxels. Rather than individual voxels, a new exact algorithm is presented that considers the CT data as consisting of the intersection volumes of three orthogonal sets of equally spaced, parallel planes. For a three-dimensional CT array of N3 voxels, the new exact algorithm scales with 3N, the number of planes, rather than N3, the number of voxels. Coded in FORTRAN-77 on a VAX 11/780 with a floating point option, the algorithm requires approximately 5 ms to calculate an average radiological path in a 100(3) voxel array.

Humans↗
PubMedDate not supplied

Two-film brachytherapy reconstruction algorithm.

We have developed a new isocentric two-film reconstruction algorithm for brachytherapy seed and needle implants. The algorithm has no requirements that the two films be orthogonal, symmetric, or even be taken in a transverse plane. In addition, there is no requirement that the two films even have the same number of images. We have found removal of these usual constraints useful for head and neck implants where images are often obscured by patient anatomy. The inherent image matching ambiguities associated with traditional two-film techniques are minimized by considering the image end points, rather than just the image centroids. For two films, the new algorithm, which considers all image combinations at one time, matches all the end-point images on one film with those on the other, and then reconstructs the end-point positions of the seeds. The algorithm minimizes the difference between the actual images and the projected images from the reconstructed seeds. The new two-film image matching problem is shown to be equivalent to the well-known assignment problem. For an implant of N seeds, this equivalence allows the two-film problem to be solved by an algorithm (ACM algorithm 548) that scales with a polynomial power of N, rather than N! as is usually assumed. An implant of N seeds can be matched and reconstructed in approximately (N/20)2s on a VAX 11/780.

Brachytherapy↗
PubMedDate not supplied

Calculation of the radiological depth.

The concept of the radiological depth is central to all algorithms which calculate radiation dose in a heterogeneous medium. For a discrete heterogeneous medium, consisting of regions of inhomogeneity, the radiological depth is usually presented as the sum over segments of the product of the segment length and the inhomogeneity density of the region corresponding to the segment. This paper illustrates that the usual formulation is inefficient because it requires the solution of the topological problem of which region corresponds to each segment. For simple heterogeneity problems involving just three regions of inhomogeneity, it is found that the topological problem constitutes at least 85% of the time required to calculate the radiological depth. It is shown in this paper that formulating the radiological depth as a sum over regions rather than as a sum over segments allows one to avoid this topological problem entirely.

Humans↗