Biomedical subjects
Publications and source records attributed to J W Scrimger.
Quality Assurance (QA) on computerized treatment planning systems is currently an area that requires considerable attention. To meet this objective a simple yet effective quality assurance program has been developed and implemented at the Cross Cancer Institute.
Large anterior electron fields are sometimes used to irradiate the neck when treating head & neck tumors. To offer a degree of spinal cord shielding, wax bolus, approximately the width of the vertebral bodies, is placed on the immobilization shell. The thickness of the bolus is adjusted so that the radiological depth of the anterior edge of the vertebral bodies is equal to the R80 depth for the energy used. This approach ignores electron scattering. Using a CT study of a thyroid cancer patient, neck contours were generated at 0.5 cm intervals and entered into the Alberta Treatment Planning system. Internal contours for the trachea and vertebral bodies were added and CT information was used for treatment planning purposes. The bolus outline was added as described above, and the dose calculated using a 3D implementation of the M.D. Anderson (Hogstrom) algorithm. The calculation shows that the simple bolus technique described above is inappropriate. The spinal cord is adequately shielded, but the target volume is not covered by the 80% isodose line. Qualitatively, the results can be explained by the lateral scatter non-equilibrium introduced by the bolus. By iteratively adjusting the shape and thickness of the wax bolus and recalculating the dose distribution, we were able to better fulfill the dose prescription. Comparison with measured data shows reasonable, but not perfect agreement. In conclusion, electron beam treatments must be examined closely to ensure that the treatment goals are met. In some cases, treatment integrity may be compromised by incorrect assumptions regarding the nature of the electron transport and dose deposition.
Beam collimators on newer linear accelerators may be collimated asymmetric to the central axis. The asymmetric beam has a non-flat profile adjusted to yield fields whose half widths are not symmetric about the central axis. While some treatment planning systems modify their programs to mimic the nonuniformity, ideally it is preferred to have a flat profile under the open beam. We have developed a universal wedge that can be used to flatten the field for a variety of jaw sizes and positions and energies for the Varian 2100C. The wedge flattens the field to +/- 3% over 80% of the field.
There exist numerous occasions in which it is desirable to approximate the polyenergetic beams employed in radiation therapy by a beam of photons of a single energy. In some instances, commonly used rules of thumb for the selection of an appropriate energy may be valid. A more accurate approximate energy, however, may be determined by an analysis which takes into account both the spectral qualities of the beam and the material through which it passes. The theoretical basis of this method of analysis is presented in this paper. Experimental agreement with theory for a range of materials and beam qualities is also presented and demonstrates the validity of the theoretical approach taken.
The introduction of an attenuating medium into a photon beam serves both to reduce the intensity of the primary beam and to create secondary radiation due to scatter. When retracted missing tissue compensators are employed to compensate for irregular surface contour or internal inhomogeneities, they are often fabricated without regard to the scattered radiation that they introduce into the system. The study of a mathematically describable conical geometry has clearly demonstrated the need for improved compensator design. Experimental results obtained with this geometry can be reproduced with good agreement, using theoretical calculations based on primary and first order scattered radiation. This method of analysis may be extended to predict the shape of a compensator which will produce an optimized dose distribution at a given depth in a phantom, equivalent to that which would be obtained when an irregular surface is filled with unit density material (bolus), producing a flat surface. An optimized compensator was constructed based on these theoretical considerations and excellent agreement was observed between theory and experiment. Dramatic improvement in the restoration of bolus dose is obtained with this optimized compensator. Finally, an anthropomorphic phantom of the neck region has been constructed and the performance of a compensator designed according to current clinical methods for this geometry has been evaluated. The performance of an optimized compensator specific to this geometry is presented and good agreement between theoretical predictions and experimental results is observed. Dramatic improvement in bolus dose restoration over that obtained with the clinically designed compensator is realized.
The introduction of an attenuating medium into a photon beam serves both to reduce the intensity of the primary beam and to create secondary radiation due to scatter. When retracted missing tissue compensators are employed to compensate for irregular surface geometry or internal inhomogeneities they are almost always fabricated without regard to the scattered radiation that they introduce to the system. Analysis of such a retracted compensator designed for use with an anthropomorphic phantom reveals an inability to provide true compensation. Subsequent analysis of a mathematically describable conical geometry demonstrates the need for improved compensator design.
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The preliminary assessment of many radiopharmaceuticals is often carried out with the help of "normal volunteers". These volunteers are drawn from the general public, are fully informed of the procedure to be performed and its attendant risks, and in many cases are compensated financially for their trouble. The cooperation of such people is of vital importance to the full understanding of the normal kinetics and metabolism of many new radiopharmaceuticals. The restrictions on the choice of normal volunteers, and the radiation dose limits which must be observed are not explicitly defined in any of the current guidelines, and in this paper we propose a rationale, based upon available information, which sets acceptable limits for volunteers, and provides a framework within which scientists and physicians can work.
Brachytherapy in the treatment of prostate cancer is an accepted modality. A considerable experience has been accrued using Iridium interstitial therapy. The major problem associated with this choice is that of radioprotection. We describe the first clinical use of the MicroSelectron, a remote afterloading device, in prostate brachytherapy. Twenty-six patients have now been treated using this system. Approximately 55 treatment interruptions with an average 5 hr increase in overall treatment time occurs. The initial problems associated with instituting remote afterloading to prostate brachytherapy and their solutions are discussed.
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Dose distributions were measured and computed within inhomogeneous phantoms irradiated with beams of electrons having initial energies of 10 and 18 MeV. The measurements were made with a small p-type silicon diode and the calculations were performed using the pencil beam algorithm developed originally at the M D Anderson Hospital (MDAH). This algorithm, which is available commercially on many radiotherapy planning computers, is based on the Fermi-Eyges theory of electron transport. The phantoms used in this work were composed of water into which two- and three-dimensional inhomogeneities of aluminum and air (embedded in wax) were introduced. This was done in order to simulate the small bones and the air cavities encountered clinically in radiation therapy of the chest wall or neck. Our intent was to test the adequacy of the two-dimensional implementation of the pencil beam approach. The agreement between measured and computed doses is very good for inhomogeneities which are essentially two-dimensional but discrepancies as large as 40% were observed for more complex three-dimensional inhomogeneities. We can only trace the discrepancies to the complex interplay of numerous approximations in the Fermi-Eyges theory of multiple scattering and its adaptation for practical computer-aided radiotherapy planning.
An iterative algorithm has been developed to reduce blur artefacts in images obtained from the Pho/Con multiplane tomographic scanner. In this technique estimates of the activity distribution are convolved with the detector response function and compared with the original blurred image planes. Successive estimates of the activity distribution are modified so as to minimise the square of the difference in pixel count between the two sets of blurred images. The technique has been tested with images containing non-correlated noise provided by computer simulation and images with correlated noise acquired in experimental phantom studies.
A technique has been developed to remove out-of-focal-plane blurring from coronal and axial images made using a multiplane tomographic scanner. The technique uses a combined smoothing and differential operator that is applied to the axial images. It has been tested using computer-simulated images, with favorable results. The usefulness of the technique in a real system has yet to be determined.
High atomic number materials in high-energy photon beams produce an increased radiation level on the side of the material towards the radiation source. This added radiation has been termed backscattered radiation, although it must also contain characteristic radiation from the scattering material. The magnitude and distribution of this radiation were investigated principally for the photon beam from an 8 MeV medical linear accelerator, and also for the beam from a 60Co teletherapy unit.
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Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.