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W C Lam

Publications and source records attributed to W C Lam.

At least 37 records · Page 2Linked to original sources
PubMed1992

Phosphorescence and optically detected magnetic resonance measurements of the 2'AMP and 2'GMP complexes of a mutant ribonuclease T1 (Y45W) in solution: correlation with X-ray crystal structures.

Phosphorescence and ODMR measurements have been made on ribonuclease T1 (RNase T1), the mutated enzyme RNase T1 (Y45W), and their complexes with 2'GMP and 2'AMP. It is not possible to observe the phosphorescence of Trp45 in RNase T1 (Y45W). Only that of the naturally occurring Trp59 is seen. The binding of 2'GMP to wild-type RNase T1 produces only a minor red shift in the phosphorescence and no change in the ODMR spectrum of Trp59. However, a new tryptophan 0,0-band is found 8.2 nm to the red of the Trp59 0,0-band in the 2'GMP complex of the mutated RNase T1 (Y45W). Wavelength-selected ODMR measurements reveal that the red-shifted emission induced by 2'GMP binding, assigned to Trp45, occurs from a residue with significantly different zero-field splittings than those of Trp59, a buried residue subject to local polar interactions. The phosphorescence red shift and the zero-field splitting parameters demonstrate that Trp45 is located in a polarizable environment in the 2'GMP complex. In contrast with 2'GMP, binding of 2'AMP to RNase T1 (Y45W) induces no observable phosphorescence emission from Trp45, but leads only to a minor red shift in the phosphorescence origin of Trp59 in both the mutated and wild-type enzyme. The lack of resolved phosphorescence emission from Trp45 in RNase T1 (Y45W) implies that the emission of this residue is quenched in the uncomplexed enzyme. We conclude that local conformational changes that occur upon binding 2'GMP remove quenching residues from the vicinity of Trp45, restoring its luminescence.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Monophosphate↗
PubMed1991

On-line portal imaging: computer-assisted error measurement.

A microcomputer-based system was implemented in an on-line portal imaging system to determine portal misalignment in radiation therapy. Reference markers derived from a patient simulation film are superposed on the daily on-line portal images for visual evaluation of placement accuracy. The ability to check the alignment after administration of 3-6 cGy of radiation and the minimal operator interaction required make this system useful in radiation therapy delivery.

Mathematics↗
PubMed1988

External beam profiles measured with film or ionization chamber.

Radiation therapy beam profiles measured with an ionization chamber in a scanning water phantom are compared with profiles measured with a standard film dosimetry technique. An 18 cm field was filmed with 4 MV photons in a polystyrene phantom at depths of 1, 7, and 13 cm. The film was scanned with a scanning densitometer and the resulting profiles were converted to dose profiles with a sensitometric curve. To determine the effect of backscatter on the film a special phantom was constructed to eliminate the backscatter from the film measurement. This provided a comparison with the solid film phantom where full backscattering was present. The beam profiles measured with the film differed by less than 3% from the ionization chamber profiles and there was less than 1% difference between the film techniques.

Film Dosimetry↗
PubMed1987

Electron arc therapy: beam data requirements and treatment planning.

In electron arc therapy a long narrow beam defined by a secondary collimator is used to sweep across the treatment area of the patient. Central axis percentage depth doses and off-axis ratios measured with such a collimator show variations with the source-to-surface distance (SSD). Errors may be introduced into isodose distributions calculated with the effective SSD method when beam data measured at a fixed SSD are used. A treatment planning algorithm using beam data measured at two SSDs to allow an interpolation and extrapolation procedure to account for SSD variations has been implemented. For calculations in off-axis planes, off-axis ratios normal to the plane of rotation are also required. Results of the calculations were checked with film dosimetry measurements and found to be in good agreement.

Electrons↗
PubMed1987

On-line measurement of field placement errors in external beam radiotherapy.

An on-line electronic portal imaging system was used in a quantitative measurement of field placement errors in external beam radiotherapy. Data from 30 patients, giving a total of 150 portals with 66% in the thoracic region and 34% in the abdominal region, were analysed. Displacements from the prescription delineated by physicians were measured. The displacements included errors to block malposition, field malposition, field malrotation and patient malposition, and these were classified in two categories of displacements, those associated with the portal boundary and those with anatomic structures. Except for one rotational error, only translational errors were found. Displacements greater than 1 cm amounted to 17% with respect to the portal boundary and between 10% and 46% with respect to anatomic landmarks within the portal. From the results of the study, a margin of 1 cm around the tumour is barely sufficient if a 5% accuracy in dose delivery is desired.

Electromagnetic Fields↗
PubMed1986

A new design of microwave interstitial applicators for hyperthermia with improved treatment volume.

New 915 MHz microwave interstitial applicators with improved treatment volume have been developed for clinical hyperthermia. The applicators are made from semi-rigid miniature coaxial cables by removing sections of the outer copper conductor to create multiple nodes while preserving the integrity of the teflon dielectric insulators. The homogeneity of the temperature distribution along the longitudinal axis is optimized by empirically adjusting the spacing of the gaps between sections of the outer conductor along the length of the applicator. In vitro and in vivo testing of the two-node and three-node microwave applicators show that the treatment volume can be improved by 100% over that of a one-node microwave applicator.

Animals↗
PubMed1986

An on-line electronic portal imaging system for external beam radiotherapy.

A digital imaging system has been constructed to obtain the treatment portal image of a patient during external beam radiotherapy. A DEC LSI-11/23 microcomputer controls a stepper motor, which moves a linear array of 256 silicon diodes to scan the radiation transmitted through the patient. The computer also processes the collected data to generate an image for displaying on a video monitor. The quality of the digital image is comparable to that of the conventional verification radiographs. Major advantages of the system include the speed and simplicity in data storage and retrieval and in its capability of direct comparison with simulation radiographs.

Humans↗
PubMed1983

Carcinoma of the colon: irradiation by delayed split whole-abdominal technique.

Locally advanced and recurrent colon cancer was treated by irradiating first the pelvis and two hours later the upper abdomen. Curative treatment consisted of 4,000-5,000 rad (40-50 Gy) in 200-rad (2-Gy) fractions to the pelvis and 3,000 rad (30 Gy) in 150-rad (1.5-Gy) fractions to the upper abdomen. Palliative treatment consisted of 2,100 rad (21 Gy) in 300-rad (3-Gy) fractions to the liver, 3,000 rad (30 Gy) in 150-rad (1.5-Gy) fractions to the upper abdomen, and 4,000-5,000 rad (40-50 Gy) in 200-rad (2-Gy) fractions to the pelvis. Treatment was tolerated well, and acute toxicity was limited. Seven of the 11 patients treated curatively remain free of abdominal disease after 10-35 months; median survival among 9 patients treated palliatively was 9 months.

Abdomen↗
PubMed1983

Tissue compensator fabrication using a simple photographic technique.

A simple method of fabricating photon beam tissue compensators is described. A plaster cast of the patient's surface contours is partially immersed in a mixture of India ink and water to obtain a topographic map relative to a reference plane which is perpendicular to the photon beam. The contours are photographed, and successive contours corresponding to different missing tissue thicknesses are used to fabricate a compensator from lead sheets cut to the same shape with the proper magnification.

Humans↗
PubMed1983

Beam data requirements of the Clinac 4 for three-dimensional treatment planning with wedged fields.

To calculate dose distributions in off-axis planes for wedged fields, some commercial radiotherapy treatment planning systems use the Milan and Bentley method and simply modify the transverse principal-plane profile in the wedge direction with the value of its orthogonal open-field profile at the off-axis distance. For the Clinac 4 this simple modification may introduce serious distortions in off-axis dose distributions. The error can be considerably reduced if the basic beam data are supplemented with longitudinal principal-plane profiles and depth doses measured for the largest wedged field (14 X 32 cm2). These added data enable the determination of position-dependent effective attenuation coefficients of the wedges and hence, with the model proposed here, the calculation of transverse off-axis and longitudinal profiles as required. Even for very large field sizes and off-axis distances, the maximum deviation of generated profiles from measured ones varies from about 1% to the worst case of slightly above 4%. This procedure requires little extra computer time and data storage. The computed profiles are of sufficient accuracy for 3-dimensional treatment planning.

Filtration↗
PubMed1983

An on-line acquisition system for radiation therapy: external photon and electron beams.

An on-line data acquisition system for external beam treatment planning has been developed. The hardware consists of a Tektronix 4054 Computer, an SHM Nuclear 3-D scanning water phantom, interfaces for 16 channels of ADC and 32 channels of digital I/O, and two detector probes for measuring dose distribution and monitoring fluctuations in machine output. The program to control the movement of the scanning probe and to collect data is written in BASIC. Results of the measurements are plotted in real time on the Tektronix screen. The beam data format consists of a set of percentage depth doses along the central axis and five sets of off-axis ratios at five different depths. In the electron mode, when ionization chambers are used, data points are averages of two measurements, one with positive bias and the other with negative bias. The beam data are stored on disk which can be assessed by the Capintec treatment planning system.

Computers↗
PubMed1983

An on-line data acquisition system for radiation therapy: external photon and electron beams.

An on-line data acquisition system for external beam treatment planning has been developed. The hardware consists of a Tektronix 4054 Computer, an SHM Nuclear 3-D scanning water phantom, interfaces for 16 channels of ADC and 32 channels of digital I/O, and two detector probes for measuring dose distribution and monitoring fluctuations in machine output. The program to control the movement of the scanning probe and to collect data is written in BASIC. Results of the measurements are plotted in real time on the Tektronix screen. The beam data format consists of a set of percentage depth doses along the central axis and five sets of off-axis ratios at five different depths. In the electron mode, when ionization chambers are used, data points are averages of two measurements, one with positive bias and the other with negative bias. The beam data are stored on disk which can be accessed by the Capintec treatment planning system.

Computers↗
PubMed1982

Risk factors in interstitial pneumonitis following allogenic bone marrow transplantation.

Total body irradiation is part of the preparatory regimen for allogeneic bone marrow transplantation because of its cytotoxic and immunosuppressive properties. A major toxicity of bone marrow transplantation has been interstitial pneumonitis, which may be, in part, related to the lung irradiation. One hundred and sixty-one consecutive patients receiving allogeneic bone marrow transplantation for leukemia and aplastic anemia at Johns Hopkins Hospital (1968-1979) were retrospectively studied. The present study demonstrated that lung shielding to 600 rad maximum in single dose total body irradiation, fractionation of total body irradiation in comparison to single dose total body irradiation, and absence of graft versus host disease in the leukemia patients, each reduced the risk of interstitial pneumonitis. Total body irradiation significantly reduced the leukemia recurrence rate and/or the failure of remission induction.

Analysis of Variance↗
PubMed1981

Localization of intravenously injected tumor cells in the rat mesentery after intraperitoneal administration of chemotactic stimuli.

The formation of metastases can be induced at sites injected with chemotactic factors in animals with circulating, chemotactically responsive tumor cells. This study was done to examine the possibilities that this phenomenon is due to enhanced tumor localization or stimulation of tumor growth. Walker carcinosarcoma cells labelled with 125I-iododeoxyuridine were injected into the tail veins of rats subsequently given intraperitoneal injections of chemotactic factor. 24 h later there was a 2.7- to 3.5-fold increase in the number of labelled cells in the mesenteries of animals given chemotactic factors compared to controls (p less than 0.001). The distribution of cells in other viscera was not affected by the injection. No effects were observed on the kinetics of tumor growth in vitro when cultures were supplemented with these chemotactic factors or with peritoneal lavage fluids from animals previously given intraperitoneal injections of chemotactic factor. Thus, in this model, the formation of metastases can be related to the arrest of circulating tumor cells in response to local chemotactic stimuli but the promotion of tumor growth has not been demonstrated.

Animals↗
PubMed1981

The chemotactic response of tumor cells. A model for cancer metastasis.

Injection of a C5-derived chemotactic factor for tumor cells into the peritoneal cavities of Sprague-Dawley rats induced diffuse mesenteric metastasis following the intravenous injection of Walker carcinosarcoma cells. Intraperitoneal injections of culture medium, histamine, or of trypsin-treated albumin resulted in many fewer metastases. Intraperitoneal injections of the chemotactic factor, unlike histamine, did not alter mesenteric vasopermeability as measured by the exudation of Evans blue into the mesentery. In vitro, tumor cells responded to the chemotactic factor by demonstrating directed migration in the Boyden chamber, by volume changes, measurable in the Coulter counter, and by demonstrating an increased adherence to nylon fibers. These phenomena are similar to the behavior of neutrophils in the presence of their chemotactic factors. All the responses in vitro were markedly depressed by the addition of 2-deoxyglucose, while the cell swelling response was slightly enhanced by cytochalasin B (again similar to the responses of leukocytes). The data suggest that movement of tumor cells from the circulation may be under chemotactic influence in the manner similar to the responsiveness of neutrophils to leukotactic stimuli in vivo.

Animals↗