Search PubMed⌕ Search

Biomedical subjects

A Wu

Publications and source records attributed to A Wu.

At least 19 recordsLinked to original sources
PubMed1995

Quality assurance for gamma knife stereotactic radiosurgery.

PURPOSE: This quality assurance program is designed for stereotactic radiosurgical units, gamma knife, to check and maintain the unit to preclude accidents and comply with current regulations. MATERIALS AND METHODS: Over 58 stereotactic radiosurgical units using 201 focused 60Co beams have been installed in the last 7 years and are in use at hospitals throughout the world, with at least 11 additional units being prepared to come on-line in the next year. This system has been in use at the University of Pittsburgh Medical Center (UPMC) for 7 years. A comprehensive quality assurance program has been developed. It includes the physics and dosimetry parameters and safety checks required by regulatory agencies. The program, based on over 7 years of experience in measurements, and used during the treatment of over 1500 patients, is separated into three aspects, namely physics, dosimetry, and safety. The UPMC program hopefully will indicate out-of-tolerance problems. Some quality assurance items are checked on a daily basis prior to patient treatment, while other aspects are checked on a weekly, monthly, and/or annual basis. A complete list of items with their respective time tables and tolerances is provided. RESULTS: Although experience shows very small margins of error, larger values were chosen to account for variations in equipment and techniques. CONCLUSIONS: Items included in this quality assurance program should indicate and/or preclude problems encountered in the use of this unit.

Cobalt Radioisotopes↗
PubMed1995

A simple technique for fabrication of transmission block for concomitant treatment of bilateral anterior inguinal lymphatics and pelvic/perineal structures.

The efficacy of transmission block technique with contiguous photon irradiation of inguinal as well as pelvic and perineal structures has been reported in literature. In this paper, a simple and accurate method for the proper fabrication of transmission block has been described and also demonstrated to be effective and useful. The procedures for a precise geometrical alignment and dose verification are also detailed.

Anus Neoplasms↗
PubMed1995

[Magnetic resonance imaging in the diagnosis and staging of endometrial carcinoma].

The value of magnetic resonance imaging (MRI) in the diagnosis and staging of endometrial carcinoma was studied in 43 cases of clinically suspected endometrial carcinoma and 7 normal women. All of the 43 cases showed abnormal endometria measured by MRI, among which were pathologically proven 40 cases of endometrial carcinoma, 2 cases of endometrial polyps and 1 case of adenomyosis. This suggested that MRI showed a high susceptibility in the diagnosis of endometrial carcinoma, and a lack of specificity. MRI could predict myometrial invasion, its accuracy being verified by surgico-pathological findings in 11 of 13 cases. MRI staging was correct in 10 of the 13 primarily operated cases, and only half of the clinical staging was in conformity with surgical staging. The results suggest that MRI is useful in the staging of endometrial carcinoma and therefore is of value in the choice of treatment planning, although it is not yet an ideal diagnostic aid to detect small metastatic pelvic lesions.

Adult↗
PubMed1994

Studies of the mechanism underlying increased Na+/Ca2+ exchange activity in Alzheimer's disease brain.

The Na+/Ca2+ exchanger was characterized in plasma membrane vesicles derived from frozen human postmortem tissues. The frontal cortex, temporal cortex and cerebellum of control and Alzheimer's disease (AD) tissues were compared. Na+/Ca2+ exchange activity was defined as the change in vesicular Ca2+ content seen after Na+ loaded vesicles were diluted into choline buffer. The time course of changes in Ca2+ content after dilution was similar in all three regions of control brain. In AD brain, both frontal and temporal cortex vesicles showed elevated Ca2+ content, most evident as an increased peak Ca2+ content at 2 min. The AD cerebellar cortex time course was similar to control and did not show an elevated peak at 2 min. No differences were seen in the passive permeability to Ca2+ when comparing plasma membrane vesicles prepared from control and AD brain. Vesicles from the frontal and temporal cortex of AD brain showed increases in the Vmax of the initial velocity of Ca2+ uptake when compared to control brain, whereas, the cerebellum did not. There were no significant effects of AD on the Km for Ca2+ activation of the initial velocity. Ca2+ influx measured during the rise in vesicular Ca2+ content was elevated in vesicles from AD temporal cortex when compared to control. Two known inhibitors (exchange inhibitory peptide and dichlorobenzamil) of the cardiac Na+/Ca2+ exchanger inhibited the human brain exchanger equally well in control and AD vesicles. Increased Na+/Ca2+ exchange activity was not due to astrocytic gliosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Alzheimer Disease↗
PubMed1994

Validity of universal wedge equation over the range of 60Cobalt to 25 MV photon beam energies.

PURPOSE: Evaluating the validity of the universal wedge equation over the range of 60Cobalt (60CO) to 25 MV photon beam energies. METHODS AND MATERIALS: The universal wedge equation relates the beam weight of the wedged field to the effective wedge angle produced by combining the wedged field to the nonwedged field and is expressed as tan (theta) = B tan (theta w), where B is the normalized weight imposed on the wedged field, theta w is the maximum wedge angle of the wedge filter, and theta is the effective wedge angle. The isodose distributions from 60Co to 25 MV photon beam energies were used. For each photon energy, the isodose distributions of wedged and nonwedged fields were combined in different proportions. The effective wedge angle was determined from each resultant isodose distribution. RESULTS: The relationship between the weight of wedged field and the effective wedge angle was found to be nonlinear for wedge filters with maximum wedge angle greater than 30 degrees. The universal wedge equation predicts the effective wedge angle to within 2 degrees compared to the measured value.

Cobalt Radioisotopes↗
PubMed1994

Dosimetric considerations of commercially fabricated esophageal applicators.

Two commercially fabricated esophageal applicators for low dose rate intraluminal brachytherapy were compared using the dose non-uniformity ratio (DNR). One of the applicators has a single afterloading catheter while the other applicator has six afterloading catheters to accommodate low dose rate 192Ir ribbon or wire sources. Five sets of source configurations, each with a different ribbon length in the range of 5-17 cm were analysed using the DNR. The DNR shows the delivery of relatively higher total doses at higher dose rates to tissues closer to the applicators. The difference in the inhomogeneity of dose rates and the treatment volume exposed to higher dose rates than the prescribed dose rate between the two applicators are minimal. The dose homogeneity is better for the single-source applicator while the worst dose homogeneity for the six-source applicator is caused by the placement of each source closer to the surface of the applicator. The implications of higher dose rates were discussed using the alpha/beta model.

Brachytherapy↗
PubMed1994

Effects of extended SSD on electron-beam depth-dose curves.

The effects of extended source-to-surface distance (SSD) on the electron-beam depth dose curves were examined for five electron beam energies ranging from 6 MeV to 20 MeV. The depth dose curves were measured with the water phantom surface set at 100 cm, 110 cm, and 120 cm from the source. Except for the dose in the build-up region, negligible depth dose curve changes due to extended SSD were observed for electron energies less than or equal to 12 MeV. For electron beams greater than 12 MeV, the percent surface dose decreases and, additionally, the depth of maximum dose shifts deeper into the phantom as the SSD increases. Changes in the rapid falloff region of the depth dose curves were apparent only for the higher energy electron beams, in particular 20 MeV. Extended SSD has no observable effect on the tail portion of the depth dose curves. As such, the energies of the electron beams are not changed significantly by increasing the SSD.

Electrons↗
PubMed1994

Characterization of exchange inhibitory peptide effects on Na+/Ca2+ exchange in rat and human brain plasma membrane vesicles.

The inhibitory effects of Na+/Ca2+ exchange inhibitory peptide (XIP), which corresponds to residues 219-238 of the Na+/Ca2+ exchange protein from canine heart, were studied in both rat and human brain plasma membrane vesicles. XIP had very high potency with respect to the inhibition of the initial velocity of intravesicular Na(+)-dependent Ca2+ uptake in both rat brain [IC50 = 3.05 +/- 0.69 microM (mean +/- SE)] and human brain (IC50 = 3.58 +/- 0.58 microM). The maximal inhibition seen in rat brain vesicles was approximately 80%, whereas human brain vesicles were inhibited 100%. XIP also inhibited extravesicular Na(+)-dependent Ca2+ release, and the inhibitory effect was enhanced by increasing the extravesicular Na+ concentration. In contrast, the inhibitory effect of bepridil was competitive with respect to extravesicular Na+. When XIP was added at steady state (5 min after the initiation of intravesicular Na(+)-dependent Ca2+ uptake), it was found that the intravesicular Ca2+ content declined with time. Analysis of unidirectional fluxes for Ca2+ at steady state showed that 50 microM XIP inhibited Ca2+ influx and efflux approximately 85 and 70%, respectively. This result suggested that XIP inhibited both Na+/Ca2+ exchange and Ca2+/Ca2+ exchange but had no effect on the passive release pathway for Ca2+. The results suggest structural homology among cardiac, rat, and human brain exchangers in the XIP binding domain and that the binding of Na+ or other monovalent cations, e.g., K+, is required for XIP to have its inhibitory effect on Ca2+ transport.

Amino Acid Sequence↗
PubMed1994

Nonvalvular atrial fibrillation and primary stroke prevention from a geriatric point of view.

Cerebrovascular disease constitutes one of the major causes of death. Patients with atrial fibrillation are known to be at high risk for stroke. The efficacy of oral anticoagulant therapy in the primary prevention of stroke in individuals with nonvalvular atrial fibrillation has been demonstrated in several large randomized trials. However, many geriatricians remain reluctant to prescribe anticoagulants. This article focuses on the risks and benefits of stroke prophylaxis with anticoagulant therapy in elderly patients with nonvalvular atrial fibrillation.

Aged↗
PubMed1994

Prognosis-based futility guidelines: does anyone win? SUPPORT Investigators. Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatment.

OBJECTIVE: Advocates for health care reform and others claim that significant savings could be achieved if "futile" care were eliminated. Our objective was to provide an initial estimate of the effects of a public policy that would preclude futile life-sustaining treatments, defined as those employed despite < or = 1% chance of surviving for 2 months. DESIGN: Simulation using data from an observational cohort study. SETTING: Five academic medical centers. PATIENTS: Seriously ill hospitalized adults enrolled in the Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatment (SUPPORT). METHODS: We examined the impact of prognosis-based futility guidelines on survival and hospital length of stay on a cohort of seriously ill adults. We calculated the number of days of hospitalization that would not be used if, on the third study day, life-sustaining treatment had been stopped or not initiated for subjects with estimated 2-month survival probability of < or = 1%. RESULTS: Of the 4301 patients, 115 (2.7%) had an estimated chance of 2-month survival of < or = 1%. All but one of these 115 subjects died within 6 months. Almost 86% died within 5 days of prognosis. At the time of death, 92 subjects (80.0%) had had no attempt at resuscitation; 35 (30.4%) had had a life-sustaining mechanical ventilator withdrawn. A Do-Not-Resuscitate order was written either before (n = 61) or within 5 days (n = 18) of reaching this prognosis for 68.6% of the patients. These 115 subjects had total hospital charges of $8.8 million. By forgoing or withdrawing life-sustaining treatment in accord with a strict 1% futility guideline, 199 of 1,688 hospital days (10.8%) would be forgone, with estimated savings of $1.2 million in hospital charges. Nearly 75% of the savings in hospital days would have resulted from stopping treatment for 12 patients, six of whom were under 51 years old, and one of whom lived 10 months. CONCLUSIONS: Patients at a high risk of dying can be identified prospectively. Implementation of a strict, prognosis-based futility guideline on the third day of a serious illness would result in modest savings.

Adolescent↗
PubMed1994

The use of a radiochromic detector for the determination of stereotactic radiosurgery dose characteristics.

The measurement of absorbed dose as well as dose distributions (profiles and isodose curves) for small radiation fields (as encountered in stereotactic surgery) has been difficult due to the usual large detector size or densitometer aperture (> 1 mm) relative to the radiation field (as small as 4 mm). The radiochromic direct-imaging film, when read with a scanning laser microdensitometer (laser beam diameter 0.1 mm), overcomes this difficulty and has advantages over conventional film in providing improved precision, better tissue equivalence, greater dynamic range, higher spatial resolution, and room light handling. As a demonstration of suitability, the calibrated radiochromic film has been used to measure the dose characteristics for the 18-, 14-, 8-, and 4-mm fields from the gamma-ray stereotactic surgery units at Mayo Clinic and the University of Pittsburgh. Intercomparisons of radiochromic film with conventional methods of dosimetry and vendor-supplied computational dose planning system values indicate agreement to within +/- 2%. The dose, dose profiles, and isodose curves obtained with radiochromic film can provide high-spatial-resolution information of value for acceptance testing and quality control of dose measurement and/or calculation.

Film Dosimetry↗
PubMed1992

More RIpH.

Explore the source record for details and available documents.

Chemical Phenomena↗
PubMed1992

Radiosurgery and brain tolerance: an analysis of neurodiagnostic imaging changes after gamma knife radiosurgery for arteriovenous malformations.

In order to analyze complications and the factors responsible for the development of serial imaging changes after stereotactic radiosurgery for intracranial arteriovenous malformations, we reviewed serial post-treatment magnetic resonance imaging scans in 72 patients. Median follow-up was 23 months (range 12 to 35 months). Twenty patients developed post-radiosurgical imaging changes consisting of new regions of increased T2 signal on magnetic resonance imaging in brain surrounding the arteriovenous malformation (two year actuarial incidence of 31%). Imaging changes were associated with headache or new neurological deficits in nine of these 20 (45%) and remained asymptomatic in 11 (55%). Symptoms developed in three of 13 patients with imaging changes in the cerebral cortex or cerebellum, in contrast to six of seven patients who had symptoms with imaging changes in the brainstem (p = .028). The onset of imaging changes varied from five to 18 months after radiosurgery (median, 12 months). Serial follow-up scans four to 25 months after the onset of imaging changes were available for review in 16 patients. Post-radiosurgical imaging changes completely resolved within 4 to 19 months in ten patients and have not yet completely resolved after 6 to 25 months in six patients. The projected actuarial rate for resolution of imaging changes was 88%, 19 months after onset; the median time for resolution was 14 months. Univariate analysis revealed that the development of imaging changes was significantly associated with treatment volume (p = .025), the risk predicted from the integrated logistic formula (p = .042), and the number of isocenters treated (p = .042). In multivariate analysis, volume was the only factor significantly associated with the development of imaging changes.

Adolescent↗
PubMed1992

High energy photon beam percent depth dose curves.

The percent depth dose of 8 MV and 18 MV photon beams were measured for various field sizes. The percent depth dose curves exhibited crossed over at selective depth in phantom. Hence, the percent depth dose increases at shallower depth, is independent at crossed over depth, and decreases at deeper depth with decreasing field size. This peculiar phenomenon, which cannot be explained using the primary and scattered radiation concept, should be noted.

Humans↗
PubMed1992

Determination of basal dose rates in Paris system of interstitial implants.

The isodose distributions are related to the source configurations in the Paris system of interstitial implants. In this system, the basal dose rates are determined at those points identified relative to the implant. Once these points have been located, the basal dose rates are merely the summation of the dose rate contributions from all the sources. By applying the symmetry properties of the implant, the number of basal dose rate computations and the number of dose rate summations are reduced. The reduction in both the number of computations and the number of summations has facilitated the determination of the average basal dose rate. After the average basal dose rate has been determined, the reference dose rate that is used clinically is calculated. Examples of determining the basal dose rates for a few interstitial implants are presented.

Brachytherapy↗
PubMed1992

Various wedge isodose angles for treatment planning.

Various wedge isodose angles or simply wedge angles smaller than the nominal wedge angle were created by combining the isodose distributions generated from a single physical wedge with the isodose distributions of the open field for the 8-MV photon beam. The particular wedge angle generated depends on the weights imposed on these isodose distributions. The relationship between these weights and the wedge angle were examined and found to be nonlinear. The difference between the wedge angles defined at 10 cm depth and those defined using the 50% isodose curve is less 6 degrees. The present data was fitted using two proposed empirical equations.

Humans↗