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M Fix

Publications and source records attributed to M Fix.

5 recordsLinked to original sources
PubMed1998

Calibration of a portal imaging device for high-precision dosimetry: a Monte Carlo study.

Today electronic portal imaging devices (EPID's) are used primarily to verify patient positioning. They have, however, also the potential as 2D-dosimeters and could be used as such for transit dosimetry or dose reconstruction. It has been proven that such devices, especially liquid filled ionization chambers, have a stable dose response relationship which can be described in terms of the physical properties of the EPID and the pulsed linac radiation. For absolute dosimetry however, an accurate method of calibration to an absolute dose is needed. In this work, we concentrate on calibration against dose in a homogeneous water phantom. Using a Monte Carlo model of the detector we calculated dose spread kernels in units of absolute dose per incident energy fluence and compared them to calculated dose spread kernels in water at different depths. The energy of the incident pencil beams varied between 0.5 and 18 MeV. At the depth of dose maximum in water for a 6 MV beam (1.5 cm) and for a 18 MV beam (3.0 cm) we observed large absolute differences between water and detector dose above an incident energy of 4 MeV but only small relative differences in the most frequent energy range of the beam energy spectra. It is shown that for a 6 MV beam the absolute reference dose measured at 1.5 cm water depth differs from the absolute detector dose by 3.8%. At depth 1.2 cm in water, however, the relative dose differences are almost constant between 2 and 6 MeV. The effects of changes in the energy spectrum of the beam on the dose responses in water and in the detector are also investigated. We show that differences larger than 2% can occur for different beam qualities of the incident photon beam behind water slabs of different thicknesses. It is therefore concluded that for high-precision dosimetry such effects have to be taken into account. Nevertheless, the precise information about the dose response of the detector provided in this Monte Carlo study forms the basis of extracting directly the basic radiometric quantities photon fluence and photon energy fluence from the detector's signal using a deconvolution algorithm. The results are therefore promising for future application in absolute transit dosimetry and absolute dose reconstruction.

Biophysical Phenomena↗
PubMed1997

Analysis of endotoxins by capillary electrophoresis.

Endotoxins are part of the outer membrane of gram-negative bacteria such as E. coli. Upon entering the blood stream, they cause a violent, sometimes life-threatening, response of the immune system. Endotoxins are lipopolysaccharides (LPS), lacking optically active groups, and their detection in the underivatized state can be difficult. In this paper the potential of capillary electrophoresis (CE) for LPS analysis is investigated. By using a standard phosphate buffer method, concentrations down to 100 micrograms/mL can be detected within 6 min. The detection limit can be lowered by one order of magnitude by using a sodium dodecyl sulfate (SDS)/borate buffer, pH 9.2. In this buffer, the SDS serves to homogenize the size of the LPS aggregates, while the borate forms complexes with the diol groups of the molecule, thereby enhancing its optical activity. The formation of LPS-affinity complexes with the UV-active polymyxin B or labeling of the LPS with a fluorophore (fluorescein isothiocyanate) was unsuccessful. Best results, in terms of detection limit and speed, were obtained with an indirect UV-detection CE method. By using a strongly UV-active electrophoresis buffer, endotoxins could be detected as "negative" peaks. In this case, a detection limit of 3 micrograms/mL (35 pM) was determined. Proteins and other UV-active substances did not disturb the assay, since they generated no detectable signals. The indirect UV detection was used to quantify the residual LPS content of a DNA preparation from E. coli.

Carbohydrate Sequence↗
PubMed1994

Impairment of growth and immune function of avocet chicks from sites with elevated selenium, arsenic, and boron.

Avocets (Recurvirostra americana) hatched from eggs collected from the south Central Valley of California (USA) were studied to determine the impact of elevated concentrations of selenium, arsenic, and boron on the immune system and growth to maturity. Corcoran ponds were the reference site with low selenium (1.2 ppb) and arsenic (29 ppb) (boron not measured). Westfarmers Pond had elevated concentrations of selenium (319 ppb), arsenic (127 ppb), and boron (109 ppm). Pryse ponds also had elevated selenium, arsenic, and boron concentrations (13.9 ppb, 1,100 ppb, and 29.4 ppm, respectively). Size at hatch was significantly reduced (P < or = 0.05) in birds from Westfarmers and Pryse ponds. The growth rate was faster, but mean adult size was reduced in birds from Pryse ponds. Avocet chicks from Pryse and Westfarmers ponds exposed solely through in ovo transfer of these elements had significantly increased heterophil:lymphocyte ratios. The phagocytic activity of macrophages also was significantly reduced in these birds, and Pryse Pond birds had an increased proliferative ability of lymphocytes in the presence of concanavalin A, a T-cell mitogen. Avocet chicks (< or = 5 wk old) were captured from the various ponds and the same morphometric and immune function measurements made. The birds that were most severely impacted by exposure to these compounds were those that were collected from Pryse ponds.

Animals↗
PubMedDate not supplied

Glucocorticoid effects on natural and humoral immunity in mallards.

Two studies were conducted to determine the effects of dexamethasone (DEX) on immune function in mallard ducks. Each day ducks were injected intramuscularly with DEX at doses ranging from 0.2-4.0 mg/kg for 28-30 days. Physiologic effects consistent with high dose glucocorticoid (GC) treatment were observed at the 4 mg/kg dose, and included significant body weight loss, lowered hematocrit, and elevated alanine aminotransferase (ALT) activity. At all doses, effects of DEX on the immune system were observed. When DEX was given at 0.2 mg/kg/day, significant suppression of primary IgG antibody titers to sheep erythrocytes (SRBC) was observed. At 1 mg/kg/day, primary IgM and secondary IgM and IgG titers were suppressed as well. These doses of DEX also produced significant elevation in natural killer cell (NKC) activity of peripheral blood mononuclear cells (PBMNC). Removal of adherent cells from the PBMNC prior to NKC assay eliminated the enhancement in NKC activity. Based on these results, it was postulated that the elevation in NKC activity may be due to suppression by DEX of monocyte production of prostaglandin-E2 (PGE-2) resulting in the release of NKC activity from the inhibitory effects of PGE-2. This hypothesis was supported by a measured decrease in PGE-2 production during the NKC assay by cells from DEX-treated birds. Furthermore, an enhanced NKC activity could be reproduced in vitro with the addition of indomethacin or DEX to NKC cultures containing adherent cells from PBMNC. Direct effects of DEX on nonadherent cell NKC activity and lymphocyte viability were only observed at high concentrations (10(-4) M) of DEX, while the phagocytic activity of adhered blood monocytes was inhibited at 10(-6) M DEX. The suppressed phagocytic activity may contribute to the suppressed antibody responses observed in DEX-treated birds. Together, these results support an indirect immunomodulatory effect of DEX on NKC activity and perhaps antibody responses in vivo via altered monocyte function in mallard ducks.

Animals↗