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Biomedical subjects

L M Chin

Publications and source records attributed to L M Chin.

35 records · Page 2Linked to original sources
PubMed1987

A coordinate transfer of anatomical information from CT to treatment simulation.

Use of CT in treatment planning requires that the anatomical information obtained from the CT scans can be transferred accurately to the treatment geometry. We have developed a technique which transfers the CT data to the simulation data and vice versa. The method uses skin marks and digital radiographs reconstructed from CT scans to reproduce the patient's position. The technique has been studied with a special phantom. Based on 10 phantom set-ups, CT coordinates can be transferred to simulation coordinates with a standard deviation of 0.25 cm. This method has been applied on a number of clinical cases that involve different disease sites. For these cases, comparisons of set-up films taken at the simulator and the digital radiographs reconstructed from CT data indicate that patient position can be reproduced to within 0.5 cm.

Computer Simulation↗
PubMed1987

A computer-aided treatment planning technique for universal wedges.

Some modern accelerators provide only one wedge built into the head of the treatment machine. A continuous range of wedge angles can be generated using wedged and unwedged beams in different proportions. However, this may pose difficulty in treatment planning since the planners will have to work with twice as many beams to achieve a desirable dose distribution. We have developed a simple algorithm to solve the treatment planning problem involving wedged and open beam combinations. The algorithm is based on the assumption that the user has some knowledge of the dose distribution to be optimized. Some properties of the distributions are then used to set up the system of equations for optimization. Unlike other optimization techniques, it does not require specification of dose value to every interest point selected, and it allows each specified dose value to vary within a clinically acceptable range. Examples are given to illustrate the application of the algorithm for some standard treatment plans.

Algorithms↗
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

Dose optimization with computer-controlled gantry rotation, collimator motion and dose-rate variation.

The applications of a computer-controlled radiation therapy system to optimize dose distributions in two dimensions are explored. This study is limited to a target volume with constant cross-section along an axis parallel to the long axis of the patient. The machine components that are continuously varied during treatment are the dose rate, the gantry angle, and the four independent collimator jaws, two of which can cross the beam centerline. Basic control strategies, treatment planning and delivery techniques are illustrated with clinical examples. We conclude that the computer-controlled radiation therapy system can easily and reliably deliver dose distributions which are significantly better than those produced by conventional multiple-field techniques.

Computers↗
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↗
PubMed1982

Optimization of radiation therapy: integral-response of a model biological system.

Several radiotherapy treatment planning criteria have been proposed for dose distribution optimization. Here we present a simple mathematical model of an idealized biological system. From it we have derived an objective function designed to achieve an extremum for that particular plan which minimizes the probabilities of occurrence of unacceptable complications in healthy tissue and of recurrence or spread of disease. The model assumes that an organism is separable into physiologically discrete compartments or organs, each consisting of a set of microscopic functional units with their own dose-response characteristics. In analogy to the integral-dose, we define an integral-response parameter v as a measure of radiation-induced damage; the value of this v may be calculated for any given spatial distribution of dose in a compartment or organ. A Probability of Serious Complications function, PSC(v), then provides an estimate of the likelihood of occurrence of unacceptable complications. Special problems arising with paired organs (kidneys), "series" organs (spinal cord), and the recurrence and spread of disease are addressed. The PSC for the various organs and neoplasia can be combined to form a compound Complication Factor (CF) objective function; the lower the value of the CF, the better the overall plan. Prospects for making the model explicitly time/fractionation dependent, and for incorporating utility theoretic ideas, are discussed.

Humans↗
PubMed1978

Computer-controlled radiation therapy.

Radiation therapy is often hampered in important body regions by the need to transit sensitive normal tissues which act as dose-limiting barriers. Computer-controlled radiation therapy permits the simultaneous variation of multiple treatment parameters during irradiation of the patient, producing improved dose distributions with the potential for improved local control. Equipment used for this purpose includes a Mevatron XII linear accelerator, redesigned for automatic control, and a PDP 11/45 minicomputer. Dose distributions are shown and potential clinical gains discussed.

Computers↗
PubMedDate not supplied

Wedge-shaped dose distributions by computer-controlled collimator motion.

We have recently installed a linear accelerator, modified to allow computer control of several machine parameters during irradiation of the patient. As an initial feasibility study of computer-controlled radiation therapy, its application to produce wedge-shaped dose distributions by moving the collimator jaws has been evaluated. The required collimator motions have been calculated with an iterative technique. When these routines were used during irradiations of phantoms containing radiographic film, a good correspondence between calculated and measured dose distributions was observed. It is concluded that computer-controlled motion of the collimator jaws to shape the dose distribution is technically feasible. Additionally, this technique has the advantage that the wedge angle can be continuously adjusted and the isodose curves optimized for a particular depth and field size.

Computers↗
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

Scattered photons produced by beam-modifying filters.

When a beam-modifying filter such as a wedge or a compensator is placed in an x-ray beam, scattered photons are generated in the filter material. The magnitude of the dose contribution from these photons for a 4-MV x-ray beam was measured. At a distance of 30 cm from the filter, a copper sheet of 1-cm thickness produced a dose contribution on the centerline of about 6% of the transmitted primary dose in a 20 X 20 cm2 field. At the edges of the beam, this contribution was only about one-half that on the centerline. The presence of these scattered photons leads to only minor dose errors in clinical applications if simple procedures are followed to account for their contribution.

Humans↗
PubMedDate not supplied

Heterogeneity model for photon beams incorporating electron transport.

A method of calculating photon doses in heterogeneous media incorporating electron transport is studied. The dose is represented as the convolution of kerma with an exponential longitudinal electron spread function which describes the penetration of electrons from one medium to another. At large distances from an interface, the dose approaches an asymptotic value equal to the kerma multiplied by a constant describing the degree of longitudinal and lateral electron equilibrium. For the simple situations studied, this asymptotic dose is adequately described by O'Connor's scaling theorem. The method is compared with both Monte Carlo calculations and measurements for a 15-MV photon beam for various geometries and field sizes. It predicts the dose in regions of electron disequilibrium to within 2% in most cases. In situations of extreme electron disequilibrium, such as within low-density regions at high energies and small field sizes, this represents a significant improvement over many existing techniques.

Electrons↗
PubMedDate not supplied

The influence of ionization chamber and phantom design on the measurement of lung dose in photon beams.

Lung dose correction factors, commonly defined as the ratios of ionization chamber readings in the heterogeneous and homogeneous phantoms, have been compared with those based on accurately determined doses. An analysis of stopping power values, Pwall values, and measurements in lunglike and waterlike materials showed that the wall material and thickness are not very critical in the determination of lung dose correction factors under conditions of electronic equilibrium. When lateral electronic equilibrium is not established due to the extended range of scattered electrons in the low density material, Prepl differs significantly from unity for ionization chambers with thick walls which do not match the lung material in density. An attempt has been made to characterize this effect as a function of photon energy, lung density, field size, and wall thickness.

Humans↗
PubMedDate not supplied

The effect of differences in data base on the determination of absorbed dose in high-energy photon beams using the American Association of Physicists in Medicine protocol.

Exposure rates were adjusted at the National Institute of Standards and Technology (NIST) on January 1, 1986 to take into account more recent values for some physical parameters, mainly in electron stopping power ratios. Exposure calibration factors for 60Co gamma rays Nx will therefore be lowered by 1.1%. Consequently, absorbed dose determinations in high-energy photon beams will be reduced by the same amount if the values for these physical parameters remain unchanged in the American Association of Physicists in Medicine (AAPM) protocol. If the same data base as used at NIST is applied in the AAPM protocol, then Ngas/Nx values, water-air stopping power ratios, and Pwall values will be different. The overall change in absorbed dose determinations using a consistent set of data will be a reduction of 0.8% for 60Co gamma rays and 1.5% for a 20-MV x-ray beam compared to the values before January 1, 1986. Since the net effect is small when different sets of data are applied, the new NIST exposure calibration factors may be used in combination with the AAPM protocol without significant error.

Cobalt Radioisotopes↗
PubMedDate not supplied

Automated data collection and analysis system for MOSFET radiation detectors.

Metal oxide semiconductor field effect transistors (MOSFET) have been used as radiation dosimeters. Because of their small detector size, minimal power requirements, and signal integration characteristics, they offer unique possibilities as real-time dose monitors in radiotherapy. An automated data collection and analysis system for use with MOSFET radiation dosimeters has been designed and built. The objective was to design a system which can acquire and process the MOSFET signals in real time, in any radiation field encountered in radiotherapy. In particular, major problems have been solved arising from the intrinsic drifts of the MOSFET signal during low dose rate measurements. These signal drifts are significant when the MOSFET detector is used in applications such as on-line monitoring of radiation dose delivery in brachytherapy or radioimmunotherapy. The data collection and analysis system includes a portable IBM-compatible personal computer fitted with digital-to-analog and analog-to-digital converter boards. A single-chip programmable current supply is used to power the MOSFET dosimeters. Intrinsic and extrinsic drifts in signal due to ion diffusion and electron tunneling are corrected by deconvolution of the collected data in real time or after data collection. The data acquisition system and signal-processing methodologies are described.

Data Collection↗
PubMedDate not supplied

Sampling techniques for the evaluation of treatment plans.

Sampling techniques using randomly distributed points and regular Cartesian grids were compared for the evaluation of volume, dose-volume histogram, tumor control, and normal tissue complication probabilities in radiation treatments. Particularly, the uncertainties associated with each sampling technique in estimating the dose-volume histograms for several dose distributions are analyzed in detail. It is found that the estimation of these parameters using sampling points on a regular Cartesian grid is, in general, significantly more efficient than using random points. This finding is different from other published results. The choice of grid size for sampling was analyzed according to the AAPM recommended uncertainty on the dose delivered to the patient. It was concluded that when grid sampling is used, a grid size of 0.5 cm is adequate for most plans to meet the guidelines.

Biometry↗
PubMedDate not supplied

Dosimetry of 125I sources in a low-density material using scaling.

One may apply O'Connor's scaling theorem to dose measurements with brachytherapy sources in order to overcome the difficulties associated with the need for high spatial accuracy. This possibility has been evaluated by measuring the dose distribution around 125I sources in a low-density styrofoam phantom and comparing it with the dose distributions in water and solid water. Some generalization of the scaling theorem is proposed to allow for the minor differences in atomic composition between styrofoam and water, and the distances are scaled according to the ratio of the linear attenuation coefficients, instead of the physical densities, of the two media. The validity of this application of the scaling theorem has also been tested using Monte Carlo calculations. The results indicate that the scaling of the styrofoam measurements to water is a useful approximation in brachytherapy dosimetry.

Brachytherapy↗