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A G Meek

Publications and source records attributed to A G Meek.

18 recordsLinked to original sources
PubMed1992

Assessment of geometric treatment accuracy using time-lapse display of electronic portal images.

During the past two years, several electronic portal imaging systems have been introduced to the market by therapy accelerator manufacturers and other vendors. While these systems differ substantially in their detection technology, they are all capable of displaying portal images on a video screen in near real-time, and of creating multiple static (or "movie") images during each treatment. Major questions confront the users of such systems as to the best utilization of this wealth of information, and to its value in comparison to traditional weekly portal film methods. Using an "in-house" video based system, a new technique was established to aid in the assessment of on-line images so that immediate "go/no-go" decisions can be made by the therapy technologist. A video "movie-loop" is displayed which consists of the static image of the initial (approved) set-up, and the current treatment image. Multiple images of successive treatments can also be viewed in this "time-lapse" display mode to provide a quick visual means for review of an entire course of therapy. The on-line imaging system hardware is composed of a combination copper-plate/fluorescent-screen detector, a front surface mirror angled at 45 degrees to remove the camera from the direct radiation beam, and a high sensitivity SIT video camera. This assembly is attached to a rigid base and mounted directly to the isocentric gantry. The geometry is fixed to within +/- 1 mm and assures the precise day-to-day reproducibility which is necessary for the success of the time-lapse display technique. Experience with this technique shows it to enhance the user's ability to notice small changes in patient's position with respect to the radiation field. Radiation treatment sites reviewed using this procedure were Hodgkin's (mantle), Lung, Brain and extremities. Shifts in patient position on the order of several millimeters were readily detectable, as will be demonstrated in this paper. Somewhat surprisingly, grosser movements (greater than 1 cm) were also noted despite overall technical excellence as assessed by weekly portal filming. The eye senses day-to-day movement with greater ease when the fields are seen in time-lapse display than when compared as discrete portal images. Ultimately, persistent movement appreciated on the time-lapse display can suggest the need for a change in patient set-up or immobilization technique.

Humans↗
PubMed1991

Depth dose and profile analysis using Library Least-Squares method.

The Library Least-Squares method has been applied to detect changes in the depth dose and in the profiles of 6 and 20 MeV electron beams, and in 6 MeV photon beams from medical linear accelerators. The changes in the beam energy of 0.78% and 0.32% at low and high electron energies, respectively, induced by the insertion of 0.1 mm A1 absorber were clearly visible in the residual distributions. The changes in the photon beam were introduced by rotation of the gantry or the collimator. The applicability of the method to quantitatively monitor any changes in the treatment beam characteristics is presented.

Calibration↗
PubMed1990

Clinical use of a wing field with transmission block for the treatment of the pelvis including the inguinal node.

Treatment planning of photon and electron beams to include the pelvis and the groin poses a technical difficulty of positioning beams, and a dosimetric problem of abutting fields at the groin. We have analyzed a simpler AP/PA method using a central transmission block. The posterior portal is smaller and opposes only the pelvic portion of the anterior portal under the transmission block, while the anterior extended portion (hence the wing) is unattenuated to treat the inguinal region. By calculating the attenuation thickness according to the patient's separation and the beam quality, the dose distribution is tailored to yield the proper dose to the pelvic mid-plane and the inguinal nodes while minimizing the dose to the femora. Measured dose distribution (6MV) using film dosimetry in a tissue-equivalent phantom indicates that a 30% hot spot is created by the posterior portal diverging into the wings of the anterior field. Therefore, the pelvic attenuator is tapered at its lateral edges, thereby significantly reduced the dose inhomogeneity (5%) at the groin. Clinical methods are outlined for the verification of the patient portal films against possible mismatch in beam divergence.

Groin↗
PubMed1990

A prospective study of prostate specific antigen levels in patients receiving radiotherapy for localized carcinoma of the prostate.

Prostate Specific Antigen (PSA) is becoming the preferred tumor marker in the management of prostate cancer. Prostate Specific Antigen levels fall exponentially after radical prostatectomy with a half-life of between 2 and 3 days. Persistently elevated Prostate Specific Antigen levels beyond 7 half-lives suggest occult residual disease and may serve as an indication for post operative adjunctive therapy. The change in Prostate Specific Antigen levels during a course of radical external beam radiotherapy for prostate cancer has not been described. In this study of 81 patients receiving radiotherapy for primary prostate cancer, 47 had elevated Prostate Specific Antigen levels prior to therapy and 35 had serial measurement of Prostate Specific Antigen during their course of treatment. Working on an assumption that in patients with radioresponsive localized prostate cancer Prostate Specific Antigen levels will fall exponentially during the radiotherapy, a half-life of 43 +/- 11 days was derived. Prostate Specific Antigen half-life appears independent of stage, grade, or pretreatment Prostate Specific Antigen level and may be an independent prognostic indicator. A prolonged Prostate Specific Antigen half-life may suggest untreated or resistant disease and serve as an indication for adjuvant hormonal treatment in patients receiving radiotherapy for primary prostate cancer.

Aged↗
PubMed1990

An assessment of a film enhancement system for use in a radiation therapy department.

The clinical uses of a radiotherapy film enhancement system are explored. The primary functions of the system are to improve the quality of poorly exposed simulator and portal films, and to perform comparisons between the two films to determine whether patient or block positioning errors are present. Other features include: the production of inexpensive, high quality hardcopy images of simulation films and initial portal films for chart documentation, the capacity to overlay lateral simulation films with sagittal MRI films to aid in field design, and a mode to zoom in on individual CT or MRI images and enlarge them for video display during chart rounds or instructional sessions. This commercially available system is comprised of a microcomputer, frame grabber, CCD camera with zoom lens, and a high-resolution thermal printer. The user-friendly software is menu driven and utilizes both keyboard and track ball to perform its functions. At the heart of the software is a very fast Adaptive Histogram Equalization (AHE) routine, which enhances and improves the readability of most portal films. The system has been evaluated for several disease sites, and its advantages and limitations will be presented.

Humans↗
PubMed1990

Clarification.

Explore the source record for details and available documents.

Humans↗
PubMed1987

Preoperative preparation of high-risk, specifically hyperimmunized canine renal allograft recipients with total-lymphoid irradiation and cyclosporine.

Hyperimmunized subjects are a particularly high-risk and rapidly growing group in the patient population awaiting renal transplantation. In a search for methods designed to ameliorate the prognosis in such cases, dogs of defined DLA genotype were sensitized with DLA incompatible skin allografts and injections of buffy coat. Each recipient was challenged with a renal allograft bearing the same DLA incompatibilities. Five dogs received kidney transplants, without any other treatment, and rejected their transplants at 2.5, 4, 5, 6, and 6.5 days, respectively. Another four dogs were given a 9-11-week course (1760 +/- 35 cGy) of total-lymphoid irradiation (TLI), followed by rabbit antithymocyte globulin (ATG); these animals rejected their renal allografts at 7, 8, 14, and 17 days, respectively. Five other dogs were treated with TLI and received cyclosporine (CsA) and methylprednisolone (MPd) daily until graft rejection. Their renal allografts survived for 7.5, 8.5, 20, 62, and 227 days, respectively. Renal allografts placed in normal recipients under the same conditions of donor-recipient DLA incompatibility had a mean survival time of 12.4 days (range: 10-18 days). At the time of transplantation, the specific anti-DLA antibody titers in the recipients were 81 to 243 in the untreated dogs; 27 to 81 in the TLI-ATG-treated group, and 3 to 243 in the TLI-CsA/MPd-treated group. The titers fell within 24-48 hr after renal transplantation, to 3 to 81 in the untreated sensitized dogs; they were 3 to 9 in the TLI-ATG-treated group, and were 9 to 243 in the TLI-CsA/MPd treated group. The cytotoxic antibody titers reached postoperative peaks of 6500 to 200,000 in the untreated dogs; 729 to 6500 in the TLI-ATG-treated dogs, and 243 to 6500 in the TLI-CsA/MPd-treated recipients. The combined use of TLI and CsA/MPd can significantly inhibit the capacity of immunized recipients to muster a secondary humoral response to the DLA antigen(s) used in the sensitization process; such treatment also abrogates the ability of the recipients to reject renal allografts bearing the same DLA specificities in accelerated fashion. This effect of TLI and cyclosporine may be of relevance to current severe problems in high-risk hyperimmunized human renal transplant candidates.

Animals↗
PubMed1979

Digoxin -- quinidine interaction.

A patient with chronic paroxysmal atrial fibrillation, receiving maintenance doses of digoxin, was admitted for addition of quinidine therapy. With stable serum digoxin levels, the institution of oral quinidine sulfate resulted in a rise in the serum digoxin level in less than 24 hours. The serum digoxin concentration increased more than threefold before digoxin was discontinued. The rise and fall of the digoxin serum concentration appeared to correlate directly with an increase and decrease in the PR interval. With the reinstitution of digoxin at lower doses, apparently stable therapeutic levels of both digoxin and quinidine were achieved. However, discontinuation of quinidine alone was followed by a prompt fall in the serum digoxin level. This study demonstrates that quinidine produces a significant increase in the serum digoxin level. The increased digoxin concentration appears to correlate with enhanced electrophysiologic effects of digoxin and emphasizes the caution required when these two drugs are used simultaneously.

Atrial Fibrillation↗
PubMedDate not supplied

Continuous three-dimensional radiation dosimetry in tissue-equivalent phantoms using electron paramagnetic resonance in L-alpha-alanine.

A new tissue-equivalent phantom material has been developed which also acts as a dosimeter. The new phantom material has a similar elemental composition to that of soft tissue and has a density 1.1 g/cm3. The phantom has an agar-gel base, and contains crystallized L-alpha-alanine which traps radiation-induced free radicals. Samples from the phantom were analyzed by an electron paramagnetic resonance (EPR) spectrometer and the intensity of the EPR signal was related to the absorbed dose. When calibrated, the phantom material acts as a dosimeter, with applications in radiation therapy.

Alanine↗
PubMedDate not supplied

Tests of an electron monitor for routine quality control measurements of electron energies.

The depth dose for electrons is sensitive to energy and the AAPM Task Group 24 has recommended that tests be performed at monthly intervals to assure electron beam energy constancy by verifying the depth for the 80% dose to within +/- 3 mm. Typically, this is accomplished by using a two-depth dose ratio technique. Recently, a new device, the Geske monitor, has been introduced that is designed for verifying energy constancy in a single reading. The monitor consists of nine parallel plate detectors that alternate with 5-mm-thick absorbers made of an aluminum alloy. An evaluation of the clinical usefulness of this monitor for the electron beams available on a Varian Clinac 20 has been undertaken with respect to energy discrimination. Beam energy changes of 3 mm of the 80% dose give rise to measurable output changes ranging from 1.7% for 20-MeV electron beams to 15% for 6-MeV electron beams.

Electrons↗