Biomedical subjects

H B Giap

Publications and source records attributed to H B Giap.

5 recordsLinked to original sources

PubMed2001

Source displacement during the cardiac cycle in coronary endovascular brachytherapy.

PURPOSE: Preliminary clinical trials employing catheter-based endovascular brachytherapy show promising results in reducing restenosis after coronary intervention. Failure analysis of these studies showed a significant number of failures at the treatment margin. It is hypothesized that one of the possible causes for marginal failure is the longitudinal seed movement during the brachytherapy procedure. In this study a quantitative analysis was performed to determine the magnitude of the source displacement during the cardiac cycle. METHODS AND MATERIALS: Cine-angiograms of the Iridium-192 (Ir-192) active source seeds or dummy source seeds in place were reviewed frame by frame for 30 patients enrolled from various clinical trials using the Cordis catheter delivery system with a Ir-192 seed ribbon. The proximal and distal source points were measured in reference to branching vessels closest to the respective seed during the contrast phase of the cine-angiogram. The two frames showing the maximum source displacement were captured. After appropriate demagnification, longitudinal source displacement was measured. The data were tabulated for proximal vs. distal ends and for different coronary vessels. RESULTS: The longitudinal source displacement is significant with overall mean and standard deviation of 1.1 and 0.8 mm, respectively. The range is from 0.0 to 5.4 mm. CONCLUSION: The contribution of source movement should be included into the treatment length to avoid "geographic miss" and the subsequent marginal failure.

Angioplasty, Balloon, Coronary↗

PubMed1999

Derivation of isoeffect dose rate for low-dose-rate brachytherapy and external beam irradiation.

PURPOSE: The combination of external beam irradiation and brachytherapy has been used effectively in the management of many malignancies. Brachytherapy dose is typically prescribed to an isodose rate line, from which the implant duration is derived. In this study, the linear-quadratic model is used to derive the brachytherapy dose rate at which biological effectiveness is equivalent to that of external beam irradiation. METHODS AND MATERIALS: Relative effectiveness per unit dose (RE) for brachytherapy was based on Dale's formalism. Isoeffect dose rate, defined as the brachytherapy dose rate at which the biological effectiveness is equivalent to that of external beam irradiation, was derived. RESULTS: The functional dependencies of brachytherapy RE on dose rate, alpha/beta ratio, and implant duration were investigated. The isoeffect dose rate depends only on the dose per fraction, sublethal damage repair (SLDR) constant, and the implant duration. The isoeffect dose rate does not depend on alpha/beta ratio. For sufficiently long implant duration >10-15 hours, the value for isoeffect dose rate approaches a constant value around 40 to 50 cGy/hr. CONCLUSION: The isoeffect dose rate may be useful in treatment planning and optimization for low-dose-rate (LDR) brachytherapy, especially when brachytherapy is used in combination with external beam irradiation.

Brachytherapy↗

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

Improved conjugate view quantitation of I-131 by subtraction of scatter and septal penetration events with a triple energy window method.

The majority of radiation absorbed dose estimates for radioimmunotherapy (RIT) with I-131 labeled antibodies have been calculated based on in vivo quantitation of activity using the conjugate view approach with planar Anger camera images. Scatter and septal penetration events contributed by a small fraction of high-energy photons emitted by I-131 with an energy exceeding 600 KeV lead to a significant degradation of I-131 images acquired with an Anger camera, which blurs the images of uptake sites and complicates the definition of background regions. The objective of this study was to evaluate a triple energy window (TEW) subtraction method that has been used to remove these interfering events from I-131 images. In the method, a primary photopeak image for I-131 is obtained after sequential subtraction of septal penetration and scatter events by using scatter multipliers derived from a photopeak window and two adjacent scatter window images. Qualitative improvement in image contrast was demonstrated with this technique, together with more accurate and reproducible quantitation for I-131 in the organs of an abdominal phantom. This TEW scatter subtraction method can be used to provide more precise dosimetry estimates for radionuclide therapy and RIT with I-131.

Abdomen↗

PubMed1995

Development of a SPECT-based three-dimensional treatment planning system for radioimmunotherapy.

UNLABELLED: Two major obstacles in the development of improved methods for more accurate dose estimates for radioimmunotherapy have been the difficulty in obtaining an accurate patient-specific three-dimensional activity map in vivo and calculating the resulting absorbed dose. We propose a method for three-dimensional internal dosimetry that integrates the three-dimensional activity map from SPECT with a dose-point kernel convolution technique to provide the three-dimensional distribution of absorbed dose. METHODS: Accurate activity quantitation was achieved with appropriate methods. The count density map from SPECT images was converted into an activity concentration map with a calibration phantom approach. This map was then convolved with an 131I dose-point kernel and three-dimensional fast Fourier transform to yield three-dimensional distribution of absorbed dose, which was then processed to provide the absorbed dose distribution in regions of interest. RESULTS: The accuracy of quantitative SPECT was validated to be within 16%. The calculated penetrating radiation absorbed dose was verified with thermoluminescent dosimeter measurements to be within 8%. With standard organs and configuration, the method calculated absorbed dose in good agreement with the MIRD formalism (less than 14%). CONCLUSION: This method overcomes the limitations of planar imaging techniques and the current routine implementation of the MIRD formalism. The results can be processed to provide the absorbed dose distribution in regions of interest and parameters for treatment optimization. Absorbed dose distribution from any plane can be graphically displayed in various ways.

Calibration↗