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This paper provides an overview of Canadian research on snow, frozensoils and permafrost hydrology during the years 1995-98. There were significant advances in the understanding of processes and the development of models of snow accumulation and melt, including the relocation of snow by wind, snow interception in forest canopies, sublimationand energy balance during snowmelt. A major aspect was the development of physically based predictive techniques that account for the effects of heterogeneous topography, vegetation and snow properties, andcomplex boundary-layer development on snow accumulation, evaporation, melt and runoff. Another advancement is in the linkage of physical snow processes with chemical models to better describe ion accumulation and elution from snow. Snow ecology has shown the interactions in nutrient cycles that involve snow. Frozen ground research has resulted in significantly improved models of frozen soil infiltration, basedon both field observations and thermodynamic principles. Research inpermafrost regions includes the exfiltration of groundwater in the seasonally thawed zone and the occurrence of perennial springs discharged from below the permafrost. Groundwater discharge is important to features such as icings and the occurrence of wetlands in a polar desert. Processes governing runoff generation on hillslopes have been examined, both in continuous and discontinuous permafrost zones. In terms of future research directions, consideration should be given to continued intensive field studies of cold region hydrological processesand the incorporation of these processes into aquatic chemistry and hydrological models and land surface schemes used in atmospheric models. A better understanding of the role of hydrological boundaries in affecting the rates of processes is needed. The question of scaling processes up from the small scale at which they are relatively well understood, to the larger scales necessary to address global environmental co [ABSTRACT FROM AUTHOR]

Arctic snow cover usually attains maximum values at the end of winter and such information is important for hydrological investigations because most floods are associated with spring snowmelt. Snow data from weather stations or collected at some local sites are often extrapolated to large areas, but without verifying that the upscaling procedure yields correct results. This study compares maximum snow cover data gathered over two large target areas (170 to 300 km2) with weather station snow course measurements to determine the representativeness of local-scale data for areas typically occupied by largegrid cells of macro-hydrological models. The field snow survey results confirmed the controlling role of terrain on snow distribution in the High Arctic. The variability of areal mean snow water equivalencefor a grid cell (with dimensions of 1x 1 km2 to 13x 13 km2) increases with terrain complexity but decreases with grid size. Although point data do not represent the snow cover over an area, an attempt was made to upscale the weather station data to the target areas using an index method. Test results show that this indexapproach works well in the area with a shallow snow cover, but the error increases for an area with relatively deep snow. More effort is needed to refine this method, perhaps in conjunction with remote sensing, so that point data can be upscaled to yield snow information suitable for large-scale hydrological models or land surface schemes. [ABSTRACT FROM AUTHOR]

This study determines how representative the snowmelt values computed using arctic weather station data are of the melt in its surrounding area. Simultaneous measurements of meteorological variables were made at several sites to permit comparisons of their calculated snowmelt with the weather station at Resolute, Northwest Territories, Canada. Like most other stations, the Resolute site is located near the coast, at an airport and close to human settlement, making it warmer andits snow albedo lower than its adjacent sites. Snowmelt rates at Resolute are higher than that of a flat site away from the airport. Thislatter site has snowmelt conditions more typical of the rolling terrain nearby, but its melt rates are higher than those for an inland site where the snow remains longer than at the coastal zone. Through these simultaneous observations and systematic comparisons, this study indicates that the point data from coastal, arctic stations are unlikely to be representative of their surrounding areas. Thus, caution should be exercised when applying such information directly to the computation of snowmelt for entire grid cells of macro-hydrologic models. [ABSTRACT FROM AUTHOR]

A hand-held detector was developed which allows for the unambiguous and consistent determination of the edge of the light (optical) field on treatment machines conveniently and practically. The design incorporates an electronic circuit using three miniature photodiodes. The complete module is housed conveniently in a hand-held box measuring 6 cm x 10 cm x 1.5 cm. The device eliminates any observer subjectivity on the position of the edge of the light field and hence facilitates greatly any commissioning and quality assurance procedures requiring light-radiation field coincidence. Other applications are possible, including alignment of the light field on patients.

The spectrin-based membrane skeleton plays an important role in determining the distributions and densities of receptors, ion channels, and pumps, thus influencing cell shape and deformability, cell polarity, and adhesion. In the paradigmatic human erythrocyte, short tropomodulin-capped actin filaments are cross-linked by spectrin into a hexagonal network, yet the extent to which this type of actin filament organization is utilized in the membrane skeletons of nonerythroid cells is not known. Here, we show that associations of tropomodulin and spectrin with actin in bovine lens fiber cells are distinct from that of the erythrocyte and imply a very different molecular organization. Mechanical disruption of the lens fiber cell membrane skeleton releases tropomodulin and actin-containing oligomeric complexes that can be isolated by gel filtration column chromatography, sucrose gradient centrifugation and immunoadsorption. These tropomodulin-actin complexes do not contain spectrin. Instead, spectrin is associated with actin in different complexes that do not contain tropomodulin. Immunofluorescence staining of isolated fiber cells further demonstrates that tropomodulin does not precisely colocalize with spectrin along the lateral membranes of lens fiber cells. Taken together, our data suggest that tropomodulin-capped actin filaments and spectrin-cross-linked actin filaments are assembled in distinct structures in the lens fiber cell membrane skeleton, indicating that it is organized quite differently from that of the erythrocyte membrane skeleton., (Copyright 2000 Wiley-Liss, Inc.)

A single remote afterloading system can sometimes be used for the radiation treatment of two or more patients in separate rooms simultaneously. This configuration poses certain radiation protection problems, especially in a busy clinic where some of the treatment rooms have to be used for other non-radiation related patients even though not all radiation treatments have been completed. In this report we describe a door interlock system that has been designed to allow for radiation protection purposes during radiation treatment but is disabled when the radiation treatment is completed--with enough safeguard built in to prevent accidental bypass of the interlock. In addition, the quality control procedures of the radiation monitor devices for these treatment rooms are described. These radiation protection procedures could be generalized to other remote afterloading systems.

A model is developed to enable dose determination for thermoluminescent dosimeters immersed in a radioactive solution such as used in radioimmunotherapy. For low energy beta emitters used in such therapy the size of the dosimeter results in a much lower light output than when irradiated with an external Cobalt-60 (60Co) beam to the same dose as delivered to the medium. The model takes the size of the dosimeter into account and hence allows calculation of the dose in the actual medium. The application to different dosimeter sizes as well as different radionuclide energies is also illustrated. Finally, the model can be extended to dose calculation in a mixed gamma and beta irradiation geometry.

The large cytoplasmic domain form of the neural cell adhesion molecule N-CAM has been reported to interact specifically with fodrin, a submembranous cytoskeletal protein. We tested the abilities of fodrins from bovine brain and embryonic chicken brain to bind to N-CAM that had been isolated from differentiated or undifferentiated mouse N2A neuroblastoma cells or from the brains of embryonic day 11 or day 14 chickens. Labeled fodrin samples bound with immobilized fodrin at a minimum soluble fodrin concentration of 2.5 x 10(-8) M, but the labeled fodrin did not bind to the immobilized N-CAM when incubated at 20-fold higher fodrin concentrations.

Purpose: A system was developed which uses the light field on the simulator to allow easy visualization of the treatment field in the sagittal plane of a phantom for complex treatment geometries., Methods and Materials: The phantom consists of a plastic shell with a vertical metallic plate in the sagittal plane. Magnetic strips attach to the plate to mark the lateral field while the anterior field is marked by a special pointer., Conclusion: This method is useful for teaching and technique development by showing how the field is distorted after gantry, collimator, and couch rotations, as well as any overlap or gap in field matching for certain treatment techniques. It could be modified for actual patient setup.

We examined the localization of the 140- and 180-kDa transmembrane isoforms of chicken N-CAM following transfection into mouse N2A neuroblastoma cells. Both isoforms were expressed at the cell surface and became partially or completely localized at areas of cell-cell contact after several days of culture or of in vitro differentiation. These results indicate that the presence of the large cytoplasmic domain of the 180-kDa N-CAM isoform is not necessary to bring about the localization of N-CAM to points of cell-cell contact.

To measure the adhesion of cells expressing the neural cell adhesion molecule N-CAM, mouse Lmtk- fibroblast cells were transfected by a calcium phosphate precipitation technique with eucaryotic expression vectors encoding N-CAM polypeptides. We obtained cell lines expressing the 140-kDa transmembrane isoform of N-CAM at high levels by several rounds of selection by fluorescence-activated cell sorting and compared the adhesion of these cells to that of untransfected cells using a centrifugal removal assay that measures the centrifugal force required to remove radiolabeled probe cells from a cell monolayer. The adhesion of cells prepared from embryonic chicken neural retinas also was examined. Retinal probe cells remained associated with a retinal cell monolayer with an adhesive force of approximately 5 x 10(-6) dyn/cell, and this force was not reduced by treatment with specific anti-N-CAM antibody fragments. Transfected and untransfected mouse L cells each were dislodged from transfected cell monolayers with a removal force of 5 x 10(-5) dyn/cell and thus did not differ in their adhesion. These results support the hypothesis that N-CAM-mediated homophilic adhesion in retinal cells and transfected fibroblasts is relatively weak and that the major adhesive interaction involved in N-CAM-mediated cell-cell adhesion is heterophilic.

Pencil beam algorithms for the calculation of electron beam dose distributions have come into widespread use. These algorithms, however, have generally exhibited difficulties in reproducing dose distributions for small field dimensions or, more specifically, for those conditions in which lateral scatter equilibrium does not exist. The work described here has determined that this difficulty can arise from the manner in which the width of the pencil beam is calculated. A unique approach for determining the pencil beam widths required to accurately reproduce small field dose distributions in a homogeneous phantom is described and compared with measurements and the results of other calculations. This method has also been extended to calculate electron beam dose distributions in heterogeneous media and the results of this work are presented. Suggestions for further improvements are discussed.

In a homogeneous medium of soft tissue the radiation dose distribution due to a nonuniformly distributed beta source can be calculated by convolution of the beta dose point kernel of the nuclide with the source distribution. A possible extension of the technique to the calculation of the dose distribution in heterogeneous media involving relatively simple geometric interfaces requires the knowledge of the resulting perturbation to the beta point kernels in individual media. We simulated a soft-tissue-bone planar interface by a polystyrene (PST)-aluminum junction and measured the change in beta dose from the dose value in homogeneous PST due to a point source of 32P using 7LiF thermoluminescent dosimeters. With the point source at the interface, the dose rates at 0-31, 125-156, and 283-314 mg/cm2 separations from the interface were increased by (12 +/- 3)%, (8 +/- 2)%, and (3 +/- 2)%, respectively, compared with homogeneous PST. With the point source at a PST-air planar interface to simulate a soft-tissue-air junction, the dose rates at 0-31, 139-170, and 283-314 mg/cm2 from the interface were decreased by (25 +/- 4)%, (11 +/- 7)%, and (5 +/- 2)%, respectively. The changes in dose rates for these two interfaces have also been measured with degraded spectra of 32P. Comparison of the experimental data with Monte Carlo calculation for a point source and the two-group method of calculation for a plane source is also presented.