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J W Scrimger

Publications and source records attributed to J W Scrimger.

At least 19 recordsLinked to original sources
PubMed2000

Shielding considerations for tomotherapy.

Tomotherapy presents an evolutionary modality that holds forth the promise of better dose conformation to tumor volumes with a concomitant reduction in radiation-induced damage to surrounding normal structures. This delivery technique also presents a new set of radiation protection challenges that impact upon the design of the shielding vault required to house such a unit. A formalism is presented to determine the requisite amounts of shielding for both the primary beam and leakage radiation associated with a generic tomotherapy unit. A comparison is made with the shielding requirements for a conventional linear accelerator operated in a standard manner. Substantial differences in the amount of both primary and secondary shielding are indicated. A tomotherapy primary beam shield is both reduced in width by a factor of almost 10 and increased in thickness by more than a tenth value layer in comparison to a conventional accelerator. Furthermore, the secondary shielding requirements are enhanced by more than two tenth value layers with respect to conventional shielding demands.

Humans↗
PubMed1997

Experimental validation of dose distributions predicted by a treatment planning system for complex models.

The trend towards dose escalation in radiation therapy creates the need for non-coplanar beam arrangements to help minimize healthy tissue morbidity in the proximity of treatment volumes. Treatment planning software packages are able to generate external beam non-coplanar isodose distributions and this must be validated before use in a clinical setting. Kodak XV film was used to measure the relative dose within various tissue equivalent phantoms for comparison with the distributions predicted by the G.E. Target ("Target") treatment planning software. The results confirm that Target is able to predict the isodose distribution for coplanar and non-coplanar beams incident on patient equivalent phantoms containing relatively large, semi-infinite inhomogeneities well enough to warrant its implementation into routine clinical use. However, we have found that Target may not be able to adequately predict dose distributions around smaller inhomogeneous inclusions. Further work will be required to investigate this potential problem.

Film Dosimetry↗
PubMed1995

Optimization of pencil beam widths for electron-beam dose calculations.

The pencil beam method of calculating dose distributions for electron-beam radiotherapy has been very useful, however, several limitations in the approach have been recognized. One such limitation is the lack of a mechanism to model range straggling of electrons. For stationary electron-beam calculations, range straggling is incorporated incompletely in the planar-fluence-to-dose conversion factor, which uses measured percentage depth dose curves to force the calculated percentage depth dose to reproduce the measurement. When calculating the dose distribution for an arced beam using a pencil beam algorithm, insufficient modeling of the pencil beams leads to larger errors than when using a stationary beam algorithm. The calculated depth of maximum dose is systematically over-estimated by the pencil beam calculations. We will show that the lack of a way to account for range straggling in the arc-electron pencil beam calculation is primarily responsible for this discrepancy. Methods of incorporating range straggling into the electron pencil beam dose calculation have been presented before, but no data have been shown to support their use for heterogeneous phantoms (patients). This paper presents a similar range-straggling modification, as well as data to show that this model can predict pencil beam width to within 20% for heterogeneous slab phantoms. For stationary electron-beam calculations, the calculated isodose lines follows the measured isodose lines to within 1 mm down to the 10% dose level.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗
PubMed1992

Optimization of a cord shielding technique for electrons.

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.

Humans↗
PubMed1991

The use of an universal wedge for asymmetric fields.

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.

Humans↗
PubMed1991

Monoenergetic approximately of a polyenergetic beam: a theoretical approach.

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.

Cobalt Radioisotopes↗
PubMed1990

An analytic approach to optimized retracted missing tissue compensators.

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.

Humans↗
PubMed1989

Limitations of retracted missing tissue compensators: an experimental analysis.

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.

Humans↗
PubMed1989

Irradiation of volunteers in nuclear medicine.

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.

Adult↗
PubMed1988

Remote interstitial afterloading in cancer of the prostate: preliminary experience with the MicroSelectron.

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.

Brachytherapy↗
PubMed1987

Electron dose distributions in experimental phantoms: a comparison with 2D pencil beam calculations.

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.

Humans↗
PubMed1985

Iterative deblurring algorithm for a multiplane tomographic scanner.

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.

Humans↗
PubMed1983

An assessment of the physical properties of a multiplane tomographic imager.

The Pho/Con is a dual Anger camera multiplane tomographic scanner, capable of producing longitudinal tomograms through the subject. The physical characteristics of this device have been investigated. The resolution capabilities in three dimensions have been determined, and sensitivity, linearity, and uniformity of each detector have been studied with a view to establishing the influence of each of the subcomponents of the detector on overall performance. Experimental measurements have been made using both analogue and computerised reconstruction techniques.

Tomography, Emission-Computed↗
PubMed1982

Contamination of a 15-MV photon beam by electrons and scattered photons.

The 15-MV photon beam of a linear accelerator (Siemens Mevatron 20) was studied for electron and scattered photon contamination. The surface dose, attributable almost entirely to contamination electrons, has a Gaussian lateral distribution, a linear dependence on field width for square fields, and an inverse square dependence on distance from the bottom of the fixed head assembly. This geometrical dependence is consistent with the proposal that the field flattening filter is the main source of electron contamination when accessories are absent. A tissue-maximum-ratio curve in the build-up region for the electron and photon contamination was produced utilizing the linearity of dose with respect to field width. The derived contamination curve inside was similar to the measured build-up curve outside the field. The primary photon component, obtained by subtracting the contaminant contribution, showed no dependence on field size, source-to-probe distance, or presence of accessories.

Electrons↗
PubMed1979

Correction for out-of-focal-plane blurring in a simulated multiplane tomographic scanner.

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.

Computers↗