Biomedical subjects

T Cheung

Publications and source records attributed to T Cheung.

At least 19 recordsLinked to original sources

PubMed2006

Specific epitopes of the structural and hypothetical proteins elicit variable humoral responses in SARS patients.

BACKGROUND: Severe acute respiratory syndrome (SARS) is an infectious disease which was caused by a novel coronavirus (SARS-CoV). SARS has caused an outbreak in the world during 2003 and 2004, with 8098 individuals being infected and a death toll of 774 in 28 regions around the world. Specific humoral responses to viral infection remain unclear. OBJECTIVE: To analyse the antigenicity of the SARS-CoV genome and identify potential antigenic epitopes in the structural proteins. METHODS: Potential antigenic epitopes were identified in the structural proteins (nucleocapsid, membrane, spike, and small envelope proteins) and hypothetical proteins (SARS3a, 3b, 6, 7a, and 9b) that are specific for SARS-CoV. A peptide chip platform was created and the profiles of antibodies to these epitopes were investigated in 59 different SARS patients' sera obtained 6-103 days after the onset of the illness. Serial sera from five additional patients were also studied. RESULTS: Epitopes at the N-terminus of the membrane protein and the C-terminus of nucleocapsid protein elicited strong antibody responses. Epitopes on the spike protein were only moderately immunogenic but the effects were persistent. Antibodies were also detected for some putative proteins, noticeably the C-termini of SARS3a and SARS6. CONCLUSIONS: Important epitopes of the SARS-CoV genome that may serve as potential markers for the viral infection are identified. These specific antigenic sites may also be important for vaccine development against this new fatal infectious disease.

Antibodies, Viral↗

PubMed2006

Scanning orientation effects on Gafchromic EBT film dosimetry.

Gafchromic EBT film, a new high sensitivity radiochromic film has been tested for variations in optical properties due to scanning orientation. Gafchromic EBT film has been shown to produce a scanning orientation effect whereby variations in measured relative optical density are found due to the films orientation relative to the scanner direction. This relative optical density change was found to be relatively consistent for different films exposed to varying dose levels ranging from 0 Gy to 3 Gy. A maximum variation of 0.0157 +/- 0.0035 in optical density (OD) was found. This relates to an approximate 15% variation in net OD for a 50 cGy irradiated film and 4% variation for a 3 Gy irradiated film. No noticeable effects or variations were seen with changing scanning resolution or with the film placed "up or down" during scanning. Other Gafchromic film types were tested and compared to EBT for unirradiated film to assess the magnitude of this orientation effect on the scanner used and results showed that EBT produced a significantly higher effect that MD-55-2, HS, XR type T and XR type R film by up to 3 times. As such, providing the same orientation of EBT film when scanning for dosimetric analysis becomes an essential part of EBT film dosimetry.

Anisotropy↗

PubMed2006

Does mechanical pressure on radiochromic film affect optical absorption and dosimetry?

EBT Gafchromic film, a new high sensitivity radiochromic film has been tested to evaluate if external pressure on the film can affect absorption spectra analysis and thus radiation dosimetry. This question arises from the fact that Gafchromic film is often cut into smaller pieces or to certain shapes for dosimetric analysis using scissors which can apply significant pressure to the sides of the film and small film pieces are placed within a solid phantom at depth which can produce significant pressure on the film if appropriate weight distribution procedures are not performed. As expected, results have shown that films cut by scissors can produce a large increase in OD near the film edge up to 5-10 mm away due to physical damage to the EBT film layers. Films placed within a solid phantom receiving up to 39.5 kg/cm2 pressure showed negligible differences in measured absorption spectra compared with control films subject to no external pressure. This equates to negligible external pressure effects for as much as 44 cm of 30 cm x 30 cm solid water placed on a 1 cm2 area film piece. As such, we recommend based on results herein, that film analysis should be performed with a boundary around every film edge, which can be defined visually based in the film. Also film dosimetry in a phantom can be performed with weights up to 39.5 kg/cm2 (or 44 cm of 30 cm x 30 cm solid water or equivalent) placed on the film without effecting the absorption spectra and thus dosimetry of radiation beams.

Absorption↗

PubMed2005

Absorption spectra variations of EBT radiochromic film from radiation exposure.

Gafchromic EBT radiochromic film is one of the newest radiation-induced auto-developing x-ray analysis films available for therapeutic radiation dosimetry in radiotherapy applications. The spectral absorption properties in the visible wavelengths have been investigated and results show two main peaks in absorption located at 636 nm and 585 nm. These absorption peaks are different to many other radiochromic film products such as Gafchromic MD-55 and HS film where two peaks were located at 676 nm and 617 nm respectively. The general shape of the absorption spectra is similar to older designs. A much higher sensitivity is found at high-energy x-rays with an average 0.6 OD per Gy variation in OD seen within the first Gy measured at 636 nm using 6 MV x-rays. This is compared to approximately 0.09 OD units for the first Gy at the 676 nm absorption peak for HS film at 6 MV x-ray energy. The film's blue colour is visually different from older varieties of Gafchromic film with a higher intensity of mid-range blue within the film. The film provides adequate relative absorbed dose measurement for clinical radiotherapy x-ray assessment in the 1-2 Gy dose range which with further investigation may be useful for fractionated radiotherapy dose assessment.

Dose-Response Relationship, Radiation↗

PubMed2004

Prostate dosimetry in an anthropomorphic phantom.

Four field prostate treatments are a standard treatment procedure in radiotherapy. Dose in the prostate and rectum region were calculated for 6MV and 18MV photon beams on an anthropomorphic phantom with a collapsed cone convolution method using a 3-D planning system. Validation has been performed with radiographic film and thermoluminescent dosimeters. Results have shown that the pinnacle planning system has accurately modelled doses delivered to a heterogeneous phantom with calculations and measurements agreeing within +/-3% over most areas. When treating clinically, considerations such as the volume of bowel gas should be taken into account when planning. A sample of patient CT scans showed that in the absence of a heterogeneity correction, the error in estimated dose through the rectum could be as high as 8% in the presence of large volumes of rectal gas. Considerations, such as whether the patient undergoes another CT scan, the bowel gas volume ignored or assigned a specific density needs to be taken into account and brought to the attention of the radiation oncologists for accurate treatment.

Humans↗

PubMed2003

Safety and outcome after fludarabine-thiotepa-TBI conditioning for allogeneic transplantation: a prospective study of 30 patients with hematologic malignancies.

Fludarabine, thiotepa and total body irradiation (TBI) has been used as conditioning in haplo-identical transplantation. We studied this conditioning regimen in adults undergoing matched sibling transplantation and alternative donor transplantation. A total of 30 consecutive patients underwent matched related, haplo-identical related or matched unrelated donor transplantation with fludarabine, thiotepa and TBI conditioning. All but four had advanced hematologic malignancies. For haplo-identical transplant, ATG was added to the regimen. All patients received peripheral blood stem cells; these were T-cell depleted for 2-antigen or 3-antigen mismatched related transplantation. Additional graft-versus-host disease prophylaxis consisted of tacrolimus and mini-methotrexate. One recipient of haplo-identical transplant failed to engraft; all other evaluable patients had prompt engraftment. Four patients died of regimen-related toxicity. In all, 14 additional patients died of regimen-related causes including four from failure to thrive with persistent thrombocytopenia and four from delayed pulmonary toxicity. Six patients relapsed. Progression-free survival at 12 months was 47% (90% CI: 25-69%) for recipients of HLA-identical sibling transplants and 30% (90% CI: 14-46%) for all patients. Five of six long-term survivors have extensive chronic GVHD. As a result of the delayed complications and a relatively high recurrence rate, we abandoned this regimen.

Adolescent↗

PubMed2003

Variations in skin dose using 6MV or 18MV x-ray beams.

This research aimed to quantitatively evaluate the differences in percentage dose of maximum for 6MV and 18MV x-ray beams within the first 1 cm of interactions. Thus provide quantitative information regarding the basal, dermal and subcutaneous dose differences achievable with these two types of high-energy x-ray beams. Percentage dose of maximum build up curves are measured for most clinical field sizes using 6MV and 18MV x-ray beams. Calculations are performed to produce quantitative results highlighting the percentage dose of maximum differences delivered to various depths within the skin and subcutaneous tissue region by these two beams. Results have shown that basal cell layer doses are not significantly different for 6MV and 18MV x-ray beams. At depths beyond the surface and basal cell layer there is a measurable and significant difference in delivered dose. This variation increases to 20% of maximum and 22% of maximum at 1 mm and 1 cm depths respectively. The percentage variations are larger for smaller field sizes where the photon in phantom component of the delivered dose is the most significant contributor to dose. By producing graphs or tables of % dose differences in the build up region we can provide quantitative information to the oncologist for consideration (if skin and subcutaneous tissue doses are of importance) during the beam energy selection process for treatment.

Dose-Response Relationship, Radiation↗

PubMed2003

MOSFET dosimetry in-vivo at superficial and orthovoltage x-ray energies.

This note investigates in-vivo dosimetry using a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) for radiotherapy treatment at superficial and orthovoltage x-ray energies. This was performed within one fraction of the patients treatment. Standard measurements along with energy response of the detector are given. Results showed that the MOSFET measurements in-vivo agreed with calculated results on average within +/- 5.6% over all superficial and orthovoltage energies. These variations were slightly larger than TLD results with variations between measured and calculated results being +/- 5.0% for the same patient measurements. The MOSFET device provides adequate in-vivo dosimetry for superficial and orthovoltage energy treatments with the accuracy of the measurements seeming to be relatively on par with TLD in our case. The MOSFET does have the advantage of returning a relatively immediate dosimetric result after irradiation.

Equipment Design↗

PubMed2003

Polarity effect on surface dose measurement for an attix parallel plate ionisation chamber.

The effects of chamber polarity have been investigated for the measurement of 6MV and 18MV x-ray surface dose using a parallel plate ionization chamber. Results have shown that a significant difference in measured ionization is recorded between polarities at 6MV and 18MV at the phantom surface. A polarity ratio ranging from 1.062 to 1.005 is seen for 6MV x-rays at the phantom surface for field sizes 5 cm x 5 cm to 40 cm x 40 cm when comparing positive to negative polarity. These ratio's range from 1.024 to 1.004 for 18MV x-rays with the same field sizes. When these charge reading are compared to the Dmax readings of the same polarity it is found that these polarity effects are minimal for the calculation of percentage dose results with variations being less than 1% of maximum.

Equipment Failure Analysis↗

PubMed2003

Variations in 6MV x-ray radiotherapy build-up dose with treatment distance.

Dose in the build up region for high energy x-rays produced by a medical linear accelerator is affected by the x-ray source to patient surface distance (SSD). The use of isocentric treatments whereby the tumour is positions 100 cm from the source means that depending of the depth of the tumour and the size of the patient, the SSD can vary from distances of 80 cm to 100 cm. To achieve larger field sizes, the SSD can also be extended out to 120 cm at times. Results have shown that open fields are not significantly affected by SSD changes with deviations in percentage dose being less than 4% of maximum dose for SSD's from 80 cm to 120 cm SSD. With the introduction of beam modifying devices such as Perspex blocking trays, the effects are significant with a deviation of up to 22% measured at 6MV energy with a 6 mm Perspex tray for SSD's from 80 cm to 120 cm. These variations are largest at the skin surface and reduce with depth. The use of a multi leaf collimator for blocking removes extra skin dose caused by the Perspex block trays with decreasing SSD.

Humans↗

PubMed2001

Constitutive expression of LIGHT on T cells leads to lymphocyte activation, inflammation, and tissue destruction.

LIGHT, a member of the TNF family of cytokines (homologous to lymphotoxin, exhibits inducible expression and competes with HSV glycoprotein D for herpesvirus entry mediator, a receptor expressed on T cells), is induced on activated T cells and mediates costimulatory and antitumor activity in vitro. Relatively little information is available on the in vivo effects of LIGHT expression, particularly within the T cell compartment. In this work, we describe transgenic mice that express human LIGHT under the control of the CD2 promoter, resulting in constitutive transgene expression in cells of the T lymphocyte lineage. LIGHT-transgenic animals exhibit abnormalities in both lymphoid tissue architecture and the distribution of lymphocyte subsets. They also show signs of inflammation that are most severe in the intestine, along with tissue destruction of the reproductive organs. These LIGHT-mediated effects were recapitulated when immune-deficient mice were reconstituted with bone marrow from LIGHT-transgenic donor mice. T cells in the LIGHT-transgenic mice have an activated phenotype and mucosal T cells exhibit enhanced Th1 cytokine activity. The results indicate that LIGHT may function as an important regulator of T cell activation, and implicate LIGHT signaling pathways in inflammation focused on mucosal tissues.

Animals↗

PubMed2001

The incidence of cytoplasmic fragmentation in mouse embryos in vitro is not affected by inhibition of caspase activity.

OBJECTIVE: To investigate the relationship between cytoplasmic fragmentation and caspase activity in the mouse embryo. DESIGN: Experimental laboratory study. SETTING: University gynacology unit. ANIMAL(S): One-cell zygote of mouse (MF1 x BALB/c). INTERVENTION(S): Mouse embryos were treated with caspase inhibitors: benzyloxycarbonyl-Val-Ala-Asp fluoromethylketone (z-VAD-fmk) and benzyloxycarbonyl-Asp-glu-Val-Asp-fluoromethyl ketone (Z-DEVD-fmk). MAIN OUTCOME MEASURE(S): Morphological development of the embryo, proportion of fragmented embryos, caspase-3-like activity, DNA breakage, and phosphatidylserine exposure in blastomeres. RESULT(S): The proportion of embryo reaching two-cell, three- to four-cell, and morula stage at 48, 72, and 96 hours after hCG administration, respectively, were comparable between the control embryos and those treated with either z-VAD-fmk or z-DEVD-fmk, at three concentrations (10 microM, 50 microM, and 200 microM). Although the inhibitors suppressed the caspase-3-like activity in the embryo fragment before compaction and decreased DNA breakages, there was no statistically significant difference in the percentage of fragmented embryo between the control and those treated with caspase inhibitors. The inhibitors did not affect the incidence of phosphatidylserine exposure in the blastomere of the treated embryos. CONCLUSION(S): Cytoplasmic fragmentation in precompaction mouse embryos is not a consequence of caspase-related apoptosis.

Amino Acid Chloromethyl Ketones↗

PubMed2001

Spatial resolution of a stacked radiochromic film dosimeter.

PURPOSE: The spatial resolution of stacked radiochromic film dosimeters, which have increased sensitivity from a single layer radiochromic film detector, has been studied. METHODS: A 5-layer film which can easily be constructed provided a 4.3-times increase in sensitivity over a single layer film at 670 nm readout wavelength which meant that doses as low as 0.6 Gy could be measured with an accuracy of +/-4% with the stacked dosimeter. The spatial resolution was tested by comparison of the 80%/20% penumbral widths of a 5 x 5 cm 6 MV X-ray field. RESULTS: The MD-55-2 film measured the penumbral width as 3.0 mm whereas the 5-layer stack dosimeter measured the same penumbra as 3.2 mm. CONCLUSION: The stack dosimeter can provide useful in vivo information such as the position of a diverging beam edge for treatments around critical structures such as eyes during the first fraction of treatment.

Film Dosimetry↗

PubMed2001

Radiochromic film dosimetry in water phantoms.

Radiochromic film is investigated for use in dosimetry in water phantoms as opposed to solid phantoms. Investigations are performed to measure the penetration rates of water into radiochromic film and to assess the effects on optical density that this penetration causes. The effects of film orientation during irradiation in water are also tested. Results show that only a small penetration rate is seen from water into the film which only affects the outer areas of the film, with penetration being less than 0.5 mm per hour. The optical density measurements of the film at 660 nm remain unchanged in the unaffected regions of the radiochromic film. Minimal effects are seen due to beam orientation in a water phantom as opposed to solid water phantoms in which an overestimation in dose is normally seen for parallel irradiation. Radiochromic film seems to be an adequate detector for dosimetry in a water phantom where high spatial resolution is needed and angle of beam incidence at the point of interest is important.

Dose-Response Relationship, Radiation↗

PubMed2001

Use of multiple layers of Gafchromic film to increase sensitivity.

This note investigates the increase of the sensitivity of radiochromic film by layering the film together. Investigations with up to five layers of radiochromic film placed together as a single dosimeter have allowed accurate two-dimensional dosimetry at low radiotherapy doses. The sensitivity of the response of the layered film increases with the number of layers, with the five-layer dosimeter increasing the change in optical density by 4.3 +/- 0.2 times that for a single layer film at 670 nm readout wavelength. Measurements in the penumbral region of a 6 MV x-ray beam show that the spatial resolution is slightly reduced with the layered film. The distance across the region from 20 to 80% of the maximum dose at the beam edge (20/80% penumbra) measured at 1.5 cm depth was measured as 3.1 +/- 0.3 and 3.5 +/- 0.3 mm for one and five layers, respectively. Thus no major reduction in spatial resolution was found by layering the film to five layers but the sensitivity was increased by 430% at 670 nm readout wavelength.

Particle Accelerators↗

PubMed2001

Validation of blood product irradiation doses.

Dosimetry of blood irradiation using x-ray beams on a medical linear accelerator has been studied to evaluate the accuracy of a diode detector and the delivery achievable. Variations in applied doses for a standard dual field 6 MV x-ray are measured with a commercial diode detector. Results show that the diode detector measured applied in vitro doses to within 5.4% (2 standard deviations (2 SD)) of those calculated with a collapsed-cone convolution treatment-planning computer for a sample of 100 blood irradiations. Experiments involving the packing procedure of the blood products in the blood box were performed. It was found that a large proportion of the variation in the predicted and measured dose was due to the compacting of the scatter material at the base of the blood box (over a 6 month period) producing a higher density below the blood than originally scanned; hence an overall reduction of delivered dose was observed. The diode measurements (which provide an immediate printout) are recommended in conjunction with a film dosimeter such as radiochromic film, which still provides a back-up dose monitor and a visual reminder that the blood has been irradiated. It is also recommended that the blood box be completely evacuated of all scatter material every month and the base be carefully repacked to provide uniform scatter material.

Blood↗

PubMed2001

Study on examinee's dose delivered in computed tomography.

Dose profiles are presented resulting from computed tomography (CT). The profiles are positioned at the central axis, 1 cm away from the outer surface of the phantom, for single and multiple scans. A Hitachi W-1000 scanner is used with a thermoluminescent dosimeter (TLD), and standard dosimetry head and trunk phantoms. Regression equations are found linking the dose resulting from scattered radiation associated with a single scan to the distance from the scanning centre. The impact on the CT dose index value (CTDI) for varying integrating lengths is analysed. Some problems associated with CT dose measurement are noted, which may assist in the practical application of IBSS (International Basic Standard of Radiation Protection and Safety of Radiation Sources) guide levels.

Abdomen↗

PubMed2001

Visible dye light absorption properties of processed radiographic film.

The visible absorption spectra of Kodak X-Omat V film, which had been exposed to various doses of radiation, have been investigated to analyse the dosimetry characteristics of the film with various densitometers. Common densitometers can use fluorescent light (broad band visible), helium-neon (632 nm) or other spectra of specific bandwidth. The visible absorption spectra show a slight peak in absorption at approximately 580 nm and another at 630 nm caused by the base material of the film. The optical density of the film is shown to increase almost equally at all wavelengths within the visible region with increases in applied dose. By evaluating the results for the broad band spectra and specific wavelength optical density it is shown that a relatively uniform response is expected for all densitometers that work within the visible region as well as in selected infrared wavelengths. Thus similar optical density to dose response curves for X-Omat V radiographic film should be produced for all types of densitometers, no matter what type of light source is used for illumination. Thus it is most efficient to have a densitometer with a light source suitable for radiochromic film, which can also be used with radiographic film.

Absorption↗