Biomedical subjects

A L Boyer

Publications and source records attributed to A L Boyer.

At least 55 records · Page 3Linked to original sources

PubMed1996

Use of a multileaf collimator as a dynamic missing-tissue compensator.

A dynamic multileaf collimator (D-MLC) was used to investigate the feasibility of producing missing-tissue compensators. The modulation of the x-ray field in two dimensions produced by conventional physical compensators was mimicked by delivering a sequence of D-MLC-shaped subfields. A method is introduced to calculate monitor units (MU) for dynamically compensated fields that is analogous to and expands upon methods used for conventional compensating filter MU calculations. In this investigation, the tissue deficit at the surface of an anatomical phantom was measured using a Moiré camera. The tissue deficit data were used to generate a series of D-MLC subfields that, delivered in sequence, provided the compensated treatment. Film was used to integrate the dose delivered to a specified depth of compensation. Isodose distributions were measured for uncompensated fields, fields compensated with a conventional lead or plastic filter, and fields compensated with the D-MLC. A comparison of the dose distributions shows the compensation achieved with the dynamic compensating filter is comparable to that achieved using conventional physical compensating filters.

Biophysical Phenomena↗

PubMed1995

An image correlation procedure for digitally reconstructed radiographs and electronic portal images.

PURPOSE: To study a procedure that uses megavoltage digitally reconstructed radiographs (DRRs) calculated from patient's three-dimensional (3D) computed tomography (CT) data as a reference image for correlation with on-line electronic portal images (EPIs) to detect patient setup errors. METHODS AND MATERIALS: Megavoltage DRRs were generated by ray tracing through a modified volumetric CT data set in which CT numbers were converted into linear attenuation coefficients for the therapeutic beam energy. The DRR transmission image was transformed to the grayscale window of the EPI by a histogram-matching technique. An alternative approach was to calibrate the transmission DRR using a measured response curve of the electronic portal imaging device (EPID). This forces the calculated transmission fluence values to be distributed in the same range as that of the EPID image. A cross-correlation technique was used to determine the degree of alignment of the patient anatomy found in the EPID image relative to the reference DRR. RESULTS: Phantom studies demonstrated that the correlation procedure had a standard deviation of 0.5 mm and 0.5 degrees in aligning translational shifts and in-plane rotations. Systematic errors were found between a reference DRR and a reference EPID image. The automated grayscale image-correlation process was completed within 3 s on a workstation computer or 12 s on a PC. CONCLUSION: The alignment procedure allows the direct comparison of a patient's treatment portal designed with a 3D planning computer with a patient's on-line portal image acquired at the treatment unit. The image registration process is automated to the extent that it requires minimal user intervention, and it is fast and accurate enough for on-line clinical applications.

Dose-Response Relationship, Radiation↗

PubMed1995

Validation of a dose-point kernel convolution technique for internal dosimetry.

The objective of this study was to validate a dose-point kernel convolution technique that provides a three-dimensional (3D) distribution of absorbed dose from a 3D distribution of the radionuclide 131I. A dose-point kernel for the penetrating radiations was calculated by a Monte Carlo simulation and cast in a 3D rectangular matrix. This matrix was convolved with the 3D activity map furnished by quantitative single-photon-emission computed tomography (SPECT) to provide a 3D distribution of absorbed dose. The convolution calculation was performed using a 3D fast Fourier transform (FFT) technique, which takes less than 40 s for a 128 x 128 x 16 matrix on an Intel 486 DX2 (66 MHz) personal computer. The calculated photon absorbed dose was compared with values measured by thermoluminescent dosimeters (TLDS) inserted along the diameter of a 22 cm diameter annular source of 131I. The mean and standard deviation of the percentage difference between the measurements and the calculations were equal to -1% and 3.6%, respectively. This convolution method was also used to calculate the 3D dose distribution in an Alderson abdominal phantom containing a liver, a spleen, and a spherical tumour volume loaded with various concentrations of 131I. By averaging the dose calculated throughout the liver, spleen, and tumour the dose-point kernel approach was compared with values derived using the MIRD formalism, and found to agree to better than 15%.

Animals↗

PubMed1995

Radiation dose distribution within the bone marrow of patients receiving holmium-166-labeled-phosphonate for marrow ablation.

The primary objective of this work was to estimate the absorbed dose distribution to the bone marrow of six multiple myeloma patients who received holmium-166 (166Ho) DOTMP (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylene-phosphonic acid) for the purpose of bone marrow ablation. A methodology based on gamma camera images was developed to estimate the regional absorbed dose distributions delivered to the bone marrow, and this was compared with values calculated from the MIRD technique and bone marrow biopsies. The activity concentration in various skeletal regions was calculated from the activity in the region of interest (ROI) drawn on whole body gamma camera images, and the mass of bone in each ROI was derived from a dual x-ray absorptiometry image. The radiation absorbed dose to the bone marrow was calculated from this activity concentration using an adaptation of Bragg-Gray cavity theory. The radiation absorbed dose delivered to the bone marrow in the six patients calculated from the MIRD "S" factors ranged from 15.0 to 46.3 Gy. The gamma camera measured activity concentration in skeletal regions predominantly composed of trabecular bone was approximately five to six times higher than that in cortical regions. The skeletal activity concentration in each patient ranged from highs in such regions as the ribs to lows in such regions as the shin and foot by a factor of nearly 20, producing a heterogeneous distribution of absorbed dose within the marrow. Dose volume histograms calculated for these patients indicated that 15%-20% of the marrow received an absorbed dose significantly larger than the average value, while 5%-10% of the marrow received a substantially lower dose. Weighted mean dose estimates from the regional technique were typically 30% greater than the average dose estimates calculated with the MIRD "S" factors. Finally, absorbed dose estimates for the marrow calculated from the regional technique correlated more closely with the clinical response of blood cells and abnormal proteins measured in bone marrow aspirates and peripheral blood samples than estimates from the MIRD "S" factors.

Absorptiometry, Photon↗

PubMed1994

Realization and verification of three-dimensional conformal radiotherapy with modulated fields.

PURPOSE: We describe the experimental demonstration of the delivery of a three-dimensional conformal radiotherapy dose distribution using in-field modulation of nine fixed-gantry fields. METHODS AND MATERIALS: Two-dimensional in-field modulation profiles, varying from field to field, were realized by quasi-dynamic multileaf collimation using the prototype of a commercially available multileaf collimator installed on a medical linear accelerator. The profiles were calculated to deliver an optimal dose distribution for a patient with a prostate carcinoma. The target volume surface was invaginated and bifurcated. The calculated dose distribution was delivered to a homogeneous polystyrene phantom consisting of 1 cm thick slices that were cut to match the patient's outer contour. Seven therapy verification films were placed between the phantom slices. RESULTS: Analysis of the films revealed a degree of conformation of the high-dose region to the target shape that would not be possible with unmodulated conformal therapy. However, small observed spatial displacements of the dose distribution confirm the need for very accurate positioning. CONCLUSIONS: It is feasible to deliver clinically relevant, three-dimensional dose distributions that conform to invaginated and bifurcated target volumes using fields modulated by multileaf collimators.

Feasibility Studies↗

PubMed1994

X-ray field compensation with multileaf collimators.

PURPOSE: It has been proposed that conformal therapy can be carried out with static ports that are each individually compensated to deliver an optimal total dose distribution. If this proposal is to be implemented, one must have a means of compensating or modulating the fluence distributions within the boundaries of individual treatment fields. A theory was developed and implemented to achieve this goal. METHODS AND MATERIALS: The theory allowed creation of a leaf-setting sequence for a desired level of field-modulation precision. This method of beam modulation was experimentally verified using radiographic film to integrate the dose delivered by the sequence of discrete static multileaf collimator-defined subfields. RESULTS: Beam profiles were generated that matched the planned beam profiles to within the specified degree of precision. CONCLUSION: This methodology is a candidate for implementation of inverse planning for conformal therapy.

Humans↗

PubMed1994

The potential and limitations of the inverse radiotherapy technique.

The objective of the work presented in this paper is to explore the scope of the applicability of the inverse radiotherapy technique for designing optimized intensity distributions to achieve a desired dose distribution. A specified desired uniform dose to the target volume is inverted, subject to constraints on the surrounding normal tissue dose, to produce optimum intensity distributions in a set of beams arranged around the target volume. We employed the inverse technique and software developed by Bortfeld and evaluated results both qualitatively and quantitatively using dose distribution displays, dose-volume histograms and biological indices including tumor control probability and normal tissue complication probabilities. So far we have applied this methodology to prostate and lung treatment plans. For prostate the inverse technique produces satisfactory approximations of the desired dose distributions. However, for lung its performance is considerably inferior. Our investigations point to a number of factors for this difference, the primary ones being differences in the tolerance doses of neighboring normal tissues, magnitudes of volume effect, tissue architectures, and the achievability of the specified desired dose distributions. We conclude that, for certain clinical situations, it is not sufficient to specify the objectives of optimization purely in terms of the desired pattern of the dose. The objectives must also include dose-volume effects and biological indices. Furthermore, the mathematics of optimization must be able to incorporate these factors into the process. We find that the inverse technique is not suitable for situations where dose-volume considerations and biological indices are important and that other methods of optimization of intensity distributions should be explored.

Adenocarcinoma↗

PubMed1994

An analytic calculation of the energy fluence spectrum of a linear accelerator.

Calculation of photon dose by convolution methods requires a knowledge of the fluence spectrum of photons produced by the linear accelerator treatment head. But this spectrum is very difficult to measure accurately, and is often derived by Monte Carlo calculations modeling the elements of the treatment head. In this paper an analytic calculation technique and the corresponding computer tool for calculating the photon energy fluence spectral distribution at any point in a bremsstrahlung field are presented. Primary bremsstrahlung photon distributions are computed by modeling electron dispersion in layers in a thick target and using the thin target bremsstrahlung cross section formulas of Schiff. The first Compton scatter from all materials in a linear accelerator treatment head is computed analytically. Higher-order Compton scatter events and pair production annihilation photons are ignored, but the attenuation of both primary and first scattered photon fluence is computed. Predictions of the computer implementation of the model are compared to measurements of bremsstrahlung production in a thick target and to Monte Carlo calculations of the energy fluence emerging from a linear accelerator. Finally, the computer tool is used to investigate the source of collimator-dependent fluence fluctuations in air. In agreement with other measurements, the principal contribution to fluence outside the geometric field is found to be from scatter in the flattening filter.

Biophysical Phenomena↗

PubMed1994

Dosimetry of effective wedge fields produced by an internal wedge.

A procedure is described to calculate the monitor unit ratios required to produce effective wedge fields having a desired wedge angle by combining an internal 60 degrees wedge with an open field. Complementary procedures are derived and demonstrated for calculating the effective wedge dose distributions with wedge angles of 15 degrees, 30 degrees, and 45 degrees using the central axis depth dose data and off-axis ratios of the open field and the 60 degrees wedged field. Measurements at five points on and off the central axis within each field and measurements of the effective wedge factor demonstrated that the calculated wedge distributions were correctly delivered to within 2% in all cases.

Biophysical Phenomena↗

PubMed1994

Comprehensive analysis of electron beam central axis dose for a radiotherapy linear accelerator.

This work evaluates the application of AAPM task group 25 (TG25) methodology for determination of central axis depth dose for a radiotherapy linear accelerator, whose dual scattering foil system and applicators were recently modified. The percent depth dose (%DD) and the dose output factor have been measured for square and rectangular fields at 100- and 110-cm source-to-surface distance (SSDs). At 100-cm SSD, results showed that %DD for a specific energy and field size can vary with applicator, the largest variation being for the 20-MeV, 10 x 10-cm field where a spread of +/- 2.5% or +/- 3 mm about the mean %DD is observed. The square-root method determines rectangular field %DD within 1%. Output factors for rectangular fields are calculated from square field values more accurately using a square-root method than the equivalent-square method recommended by TG25. At 110-cm SSD, the %DD calculated from that at 100-cm SSD using an inverse square factor does not agree with measured values for all fields. The maximum difference observed for the 20-MeV, 6 x 6-cm field was 5.5% or 10 mm. Output data at the 110-cm SSD show that the square-root method is suitable for determination of the air-gap correction factors of rectangular fields. In summary, the recommendations of TG25 work reasonably well for central axis electron beam dosimetry for this version of a radiotherapy linear accelerator, except in limited cases where applicator-scattered electrons apparently cause minor but clinically significant discrepancies.

Biophysical Phenomena↗

PubMed1994

An objective method for evaluating electronic portal imaging devices.

A statistical technique is described for analyzing the image of a contrast-detail phantom acquired by a radiotherapy electronic portal imaging device. By using Student's t test, the image analysis algorithm computes which holes in the contrast-detail phantom can be objectively resolved from the background signal. The specific pattern of the resolved holes and the total number of the resolved holes characterize the performance of the electronic portal imaging device. Image processing techniques are used to align the holes automatically. Because the use of this algorithm is completely automated, the procedure removes operator subjectivity from the evaluation.

Algorithms↗

PubMed1992

Dose distributions of x-ray fields as shaped with multileaf collimators.

Multileaf collimators (MLC) with various blade widths were simulated using standard cerrobend blocks, and three-dimensional dose computations were carried out to study the resultant radiation field edges. Film measurements made with 6 and 18 MV x-ray beams were compared with calculations that employed a three-dimensional Fourier convolution. A spatial accuracy of better than 3 mm was found in the 50% isodose line of the penumbral region with a calculation voxel size of 5 mm x 5 mm x 5 mm. The computer simulation was used to study the deviation of the calculated 50% isodose line from the desired geometric field edge using various MLC blade positions. The study suggests that multileaf collimation to the outside of the desired field edge will lead to overdose outside the field, whereas multileaf collimation to the inside of the desired field edge will lead to underdose inside the field. When the direction of travel of the leaves with respect to the field edge is near 45 degrees, the 50% isodose of a multileaf-collimated beam will fall close to the desired edge with no underdose when the leaf corners are allowed to insert into the desired field edge by 1.2 mm for 6 MV x-rays and 1.4 mm for 18 MV x-rays using a 1 cm wide leaf. These blade offsets account for the scattering of photons and electrons in the medium within the penumbral region.

Computer Simulation↗

PubMed1991

Dosimetry characteristics of metallic cones for intraoperative radiotherapy.

Dosimetry data were obtained on the first dedicated linear accelerator of its type designed for electron intraoperative radiotherapy (IORT) within an operating room. The linear accelerator uses a high dose rate, 9 Gy.min-1, to reduce the treatment time. Its chrome-plated brass treatment cones, designed with straight ends and 22.5 degrees beveled ends, are not mechanically attached to the collimator head, but are aligned using a laser projection system. Dosimetry measurements were made for each combination of energy (6, 9, 12, 15, and 16 MeV), cone size (diameters range from 5 to 12 cm), and cone type (22.5 degrees beveled or straight). From these data, depth-dose curves, cone output, and air-gap correction factors were generated that allow the calculation of the monitor setting for delivering a prescribed dose at any depth for any irradiation condition (energy, cone, air gap). Isodose data were measured for every cone using film in a solid water phantom. Scatter off the inside wall of the cone resulted in peripheral dose horns near the surface that were energy and cone dependent, being as large as 120%.

Electrons↗

PubMed1991

Dose-response characteristics of a ferrous-sulphate-doped gelatin system for determining radiation absorbed dose distributions by magnetic resonance imaging (Fe MRI).

The nuclear magnetic resonance (NMR) longitudinal relaxation rate R1 dose-response characteristics of a ferrous-sulphate-doped chemical dosimeter system (Fe MRI) immobilized in a gelatin matrix were explored. Samples containing various concentrations of the FeSO4 dosimeter were irradiated to absorbed doses of 0-150 Gy. R1 relaxation rates were determined by imaging the samples at a field strength of 1.5T(1H Lamor frequency of 63.8 MHz). The response of the system was found to be approximately linear up to doses of 50 Gy for all FeSO4 concentrations studied (0.1-2.0 mM). Changing concentrations in the range of 0.1-0.5 mM affected both the slope and intercept of the dose-response curve. For concentrations of 0.5-2.0 mM, the slope of the dose-response curves remained constant at approximately 0.0423 s-1 Gy-1 in the dose range of 0-50 Gy. However, the intercept of the curve continued to increase in that region, as expected, because of the additional paramagnetic ions. The reproducibility of the absorbed dose estimates for measurements made over a 22 cm field of view was found to be 5% in the range of 20-50 Gy (an uncertainty of 0.81 Gy on average), decreasing to approximately 10% in the dose range of 5-10 Gy.

Dose-Response Relationship, Radiation↗

PubMed1990

Shielding considerations for an operating room based intraoperative electron radiotherapy unit.

The leakage radiation characteristics of a dedicated intraoperative radiotherapy linear accelerator have been measured on a machine designed to minimize the shielding required to allow it to be placed in an operating room suite. The scattering foil design was optimized to produce a flat beam for the field sizes employed while generating minimal bremsstrahlung contamination over the available energy range. More lead shielding was used in the treatment head than is used in conventional accelerators. A small amount of borated polyethylene shielding was also employed since neutron production was present at measurable levels. The room shielding installed in the operating room was demonstrated to be adequate to treat at least 20 patients each month to an average dose of 20 Gy. The worst case exposure was found to be 73% maximum permissible exposure. Administrative control was required for adjoining areas when calibrations and maintenance were performed.

Intraoperative Period↗

PubMed1990

Design of metallic electron beam cones for an intraoperative therapy linear accelerator.

A set of circular collimators and treatment cones from 5 to 12 cm diameter has been designed for an intraoperative accelerator (6-18 MeV) that has an optical docking system. Electron beam scattering theory has been used to minimize their weight while minimizing leakage radiation. Both acrylic and brass were evaluated as possible materials; however, because of substantial electron leakage through the lateral cone wall for acrylic, we have concluded that 2 mm thick brass walls are more desirable than acrylic walls. At 18 MeV, isodose measurements beneath the cones showed hot spots as great as 120% for both materials. The placement and dimension of an internal trimmer ring inside the brass cone was studied as a method for reducing the hot spots, and it was found this could only be accomplished at the expense of decreasing coverage of the 90% isodose surface. The effects of 1 degree cone misalignment on the dose distribution has been studied and found to generate changes of less than 5% in the dose and 3 mm in position of the 90% isodose surface. In a study of the contribution of the cone and its matching collimator assembly to x-ray room leakage, it was noted that although the treatment cone had a negligible contribution, the upper annuli of the upper collimator assembly contributed as much as 80% of the leakage at 16 MeV for the 5-cm cone.

Humans↗

PubMed1988

Effects of dosimetric and clinical uncertainty on complication-free local tumor control.

A mathematical description is developed to demonstrate the effects of uncertainty on complication-free local tumor control. Responses of tumors and normal tissues are modeled by conventional radiobiological models. Uncertainties are considered in the delivery of absorbed dose to the target volume and the normal tissue at risk for complications. The degree by which uncertainties compromise complication-free local control for target volumes with various tumor cell burdens is calculated.

Computer Simulation↗

PubMed1988

Relationship between attenuation coefficients and dose-spread kernels.

Dose-spread kernels can be used to calculate the dose distribution in a photon beam by convolving the kernel with the primary fluence distribution. The theoretical relationships between various types and components of dose-spread kernels relative to photon attenuation coefficients are explored. These relations can be valuable as checks on the conservation of energy by dose-spread kernels calculated by analytic or Monte Carlo methods.

Radiometry↗