Biomedical subjects

C M Ma

Publications and source records attributed to C M Ma.

At least 73 records · Page 4Linked to original sources

PubMed1997

Measurement of percentage depth dose and lateral beam profile for kilovoltage x-ray therapy beams.

In this work, nine commonly used dosimetry detectors have been investigated to determine suitable relative dosimeters for kilovoltage x-ray beams. By comparison with the Monte Carlo calculated data, it was determined that for the detectors studied the PTW N23342, Markus and NACP parallel-plate chambers are more suitable for the measurement of percentage depth dose (PDD) data for this beam quality range with an uncertainty of about 3%. A diode detector may be used to measure the PDD for the 100 kVp beam, but it is not suitable for higher energies (300 kVp). The Capintec parallel-plate chamber may be adequate for medium-energy photons, but it has a slightly higher uncertainty for low-energy x-rays (100 kVp). For the measurement of beam profiles, diode and film yield incorrect profile tails, which can be corrected using the RK ionization chamber.

Humans↗

PubMed1997

Characteristics of orthovoltage x-ray therapy beams at extended SSD for applicators with end plates.

The effects of extended SSD (ESSD) on orthovoltage x-ray therapy dosimetry have been studied for a Therapax DXT300 treatment unit with several open- and close-ended cones. For the open-ended cones, the beam output and percentage depth dose (PDD) at ESSD can be accurately estimated from the measured data at the nominal SSD (NSSD) by an inverse square law (ISL) and a divergence factor for gaps (distance from cone end to point of interest) up to 15 cm. In the case of close-ended applicators (with an end plate of 3.2 mm thick PMMA), extending SSD results in significant changes in output and in PDDs, but has little effect on beam penumbra and flatness for gaps up to 15 cm. The output at ESSD calculated by applying the ISL with the NSSD is found to be as much as 9% different from the measurement for the close-ended cone. The output at ESSD can, however, be calculated accurately by applying the ISL with an effective SSD or using a measured gap factor. The change in the PDDs with SSD can be calculated from a geometric divergence using a pseudo SSD which is approximately equal to 60% of the effective SSD determined in air. The calculated PDD curves at ESSD using this divergence factor agree well with measurements (within +/- 1% for beams of 100-300 kVp with cone size ranging from 10 x 10 to 20 x 20 cm2 and gap distance up to 15 cm).

Humans↗

PubMed1997

Accurate characterization of Monte Carlo calculated electron beams for radiotherapy.

Monte Carlo studies of dose distributions in patients treated with radiotherapy electron beams would benefit from generalized models of clinical beams if such models introduce little error into the dose calculations. Methodology is presented for the design of beam models, including their evaluation in terms of how well they preserve the character of the clinical beam, and the effect of the beam models on the accuracy of dose distributions calculated with Monte Carlo. This methodology has been used to design beam models for electron beams from two linear accelerators, with either a scanned beam or a scattered beam. Monte Carlo simulations of the accelerator heads are done in which a record is kept of the particle phase-space, including the charge, energy, direction, and position of every particle that emerges from the treatment head, along with a tag regarding the details of the particle history. The character of the simulated beams are studied in detail and used to design various beam models from a simple point source to a sophisticated multiple-source model which treats particles from different parts of a linear accelerator as from different sub-sources. Dose distributions calculated using both the phase-space data and the multiple-source model agree within 2%, demonstrating that the model is adequate for the purpose of Monte Carlo treatment planning for the beams studied. Benefits of the beam models over phase-space data for dose calculation are shown to include shorter computation time in the treatment head simulation and a smaller disk space requirement, both of which impact on the clinical utility of Monte Carlo treatment planning.

Biophysical Phenomena↗

PubMed1997

Correction factors for water-proofing sleeves in kilovoltage x-ray beams.

This paper investigates the effect of the waterproofing sleeve on the calibration of kilovoltage photon beams (50-300 kV). The sleeve effect correction factor, ps has been calculated using the Monte Carlo method as the ratios of the air kerma in an air cavity of a cylindrical chamber without the waterproofing sleeve to that with a sleeve. Three sleeve materials have been studied, PMMA, nylon and polystyrene. The calculations were carried out using the EGS4 (Electron Gamma Shower version 4) code system with the application of a correlated-sampling variance-reduction technique. The results show that the sleeve correction factor for 1-mm thick nylon and polystyrene sleeves, ps varies from 0.992 to 1.000 and from 0.981 to 1.000, respectively, for the same beam quality range. The ps factor varies with sleeve thickness, beam quality and phantom depth. No significant dependence of the ps factor on field size and source-surface distance has been found. Measurements for PMMA, nylon and polystyrene sleeves of various thicknesses have also been carried out and show excellent agreement with Monte Carlo calculations.

Biophysical Phenomena↗

PubMed1995

Calculations of ion chamber displacement effect corrections for medium-energy x-ray dosimetry.

This paper investigates the displacement effect corrections for ionization chambers calibrated in air in terms of air kerma and used at a depth in a phantom irradiated by medium-energy x-rat beams for tube potentials between 100 kV and 300 kV. The EGS4 (electron gamma shower version 4) Monte Carlo code system has been used to simulate the coupled transport of photons and electrons. The chamber displacement correction factor, pdis, has been calculated both using a simple photon attenuation and scattering method and by direct Monte Carlo calculation of the ration of the water kerma at a depth in a phantom to the water kerma in a low-density (the same density as that of air) water cavity with its centre placed at the same depth in the phantom irradiated by medium-energy x-ray beams. Good agreement has been achieved between the pdis factors obtained using the simple method and those obtained by the direct Monte Carlo simulation. In contrast to the values recommended by the IAEA code of practice, which exceed unity by up to 10%, our pdis values for a stemless NE2571 chamber vary between 0.994 and 1.001 for the same energy range and for a stemless NE2561 chamber between 0.974 and 0.993. The beam quality correction factor and stem correction factor have been investigated in separate works. The ratios of the overall correction factors calculated in this work for an NE2571 chamber to that for an NE2561 chamber are consistent with the recent experimental results.

Gamma Rays↗

PubMed1995

Monte Carlo calculated stem effect correction for NE2561 and NE2571 chambers in medium-energy x-ray beams.

This paper presents the stem effect correction factors for the NE2561 and NE2571 ionization chambers calibrated in air and used at a depth in a phantom irradiated by medium-energy x-ray beams. The in air and in phantom stem effect correction factors were calculated as the ratios of the absorbed dose in the air cavity of an ionization chamber without and with a chamber stem. The 'global' stem correction factor was then calculated as the ratio of the in phantom correction factor to the in air correction factor. The calculations were carried out using the EGS4 (electron gamma shower version 4) Monte Carlo code system together with the use of a correlated sampling variance reduction technique. The results show that in general the chamber stem increases the chamber response in air but decreases it in phantom. For an NE2571 chamber the 'global' stem effect correction factor varies from 1.014 +/- 0.001 at 70 kV (2.9 mm A1, 0.1 mm Cu) to 1.005 +/- 0.001 at 300 kV (21.5 mm A1, 5.1 mm Cu) while for an NE2561 chamber it varies from 1.039 +/- 0.002 to 1.010 +/- 0.002 for the same kilovoltage and HVL values. In a 60Co beam the global stem correction is 1.001 +/- 0.001 for an NE2571 chamber and 1.003 +/- 0.001 for an NE2561 chamber. This is consistent with recent experimental results.

Humans↗

PubMed1995

BEAM: a Monte Carlo code to simulate radiotherapy treatment units.

This paper describes BEAM, a general purpose Monte Carlo code to simulate the radiation beams from radiotherapy units including high-energy electron and photon beams, 60Co beams and orthovoltage units. The code handles a variety of elementary geometric entities which the user puts together as needed (jaws, applicators, stacked cones, mirrors, etc.), thus allowing simulation of a wide variety of accelerators. The code is not restricted to cylindrical symmetry. It incorporates a variety of powerful variance reduction techniques such as range rejection, bremsstrahlung splitting and forcing photon interactions. The code allows direct calculation of charge in the monitor ion chamber. It has the capability of keeping track of each particle's history and using this information to score separate dose components (e.g., to determine the dose from electrons scattering off the applicator). The paper presents a variety of calculated results to demonstrate the code's capabilities. The calculated dose distributions in a water phantom irradiated by electron beams from the NRC 35 MeV research accelerator, a Varian Clinac 2100C, a Philips SL75-20, an AECL Therac 20 and a Scanditronix MM50 are all shown to be in good agreement with measurements at the 2 to 3% level. Eighteen electron spectra from four different commercial accelerators are presented and various aspects of the electron beams from a Clinac 2100C are discussed. Timing requirements and selection of parameters for the Monte Carlo calculations are discussed.

Electrons↗

PubMed1994

An investigation of the response of a simple design of plane-parallel chamber.

This paper reports the experimental investigation of a simple design of plane-parallel electron chamber, which has very thin layers of copper (0.018 or 0.035 mm) as conducting material. Measurements comparing the prototype chambers with other ionization chambers (PTW/Markus, NACP) have been carried out, both in a 60Co gamma-ray beam and in high-energy electron beams. The results show that the Ce factors (proportional to the product of water/air stopping-power ratio and perturbation factor) for converting the in-phantom air-kerma-calibrated chamber reading to the absorbed dose to water are nearly constant for incident electron energies between 4 and 11 MeV for prototype chambers with 0.018 mm thick copper layers and between 4 and 15 MeV for chambers with 0.035 mm thick copper layers. Other aspects concerning these prototype chambers, such as polarity effect, cable effect, collecting efficiency and angular response, have also been studied and the results are presented in this paper.

Cobalt Radioisotopes↗

PubMed1993

Dose conversion and wall correction factors for Fricke dosimetry in high-energy photon beams: analytical model and Monte Carlo calculations.

This paper presents the dose conversion and wall correction factors for Fricke dosimetry in high-energy photon beams calculated using both an analytical general cavity model and Monte Carlo techniques. The conversion factor is calculated as the ratio of the absorbed dose in water to that in the Fricke dosimeter solution with a water-walled vessel. The wall correction factor accounts for the change in the absorbed dose to the dosimeter solution caused by the inhomogeneous dosimeter wall material. A usercode based on the EGS4 Monte Carlo system, with the application of a correlated sampling variance reduction technique, has been employed in the calculations of these factors and the parameters used in the cavity model. Good agreement has been achieved between the predictions of the model and that obtained by direct Monte Carlo simulation and also with other workers' experiments. It is shown that Fricke dosimeters in common use cannot be considered to be 'large' detectors and therefore 'general cavity theory' should be applied in converting the dose to water. It is confirmed that plastic dosimeter vessels have a negligible wall effect. The wall correction factor for a 1 mm thick Pyrex-walled vessel varies with incident photon energy from 1.001 +/- 0.001 for a 60Co beam to 0.983 +/- 0.001 for a 24 MV (TPR(10)20 = 0.80) photon beam. This implies that previous Fricke measurements with glass-walled vessels should be re-evaluated.

Ferrous Compounds↗

PubMed1993

Hypouricemia, abnormal renal tubular urate transport, and plasma natriuretic factor(s) in patients with Alzheimer's disease.

OBJECTIVE: To study tubular urate transport in Alzheimer's disease (AD) and measure sodium and lithium transport rates in rats exposed to AD plasma. DESIGN: Cross-sectional study in three comparison groups. SETTING: Referral private institution involving outpatient and hospitalized patients. PATIENTS: AD, multi-infarct dementia (MID) and non-demented controls (C) were selected and evaluated by a geriatrician and a psychiatrist according to availability and willingness to participate in the study. Demented patients had brain imaging, categorized according to NINCDS-DSM III criteria, and had Mini-mental status examination (MMSE) scores determined. INTERVENTIONS: Injection of 0.5 mL of plasma I.P. followed 120 minutes later by an IV plasma injection of 0.2 mL priming dose and infusion of 1.8 mL of plasma at 0.01 mL/min in Sprague Dawley rats. MEASUREMENTS: Renal clearance studies were performed in subjects and in rats exposed to the plasma of study subjects. We measured serum urate concentration and fractional excretion (FE) of urate in subjects and FE sodium and FE lithium in rats. RESULTS: Serum urate was lower and FE urate higher in 18 AD patients compared with six patients with MID, P < 0.05 and P < 0.005, and 11 C, P < 0.02 and P < 0.005, respectively. Higher FE sodium and FE lithium were noted in rats given plasma from 19 AD patients compared with 12 with MID, P < 0.005 and P < 0.0025, and 14 C, P < 0.0025 and P < 0.0005, respectively. FE sodium and FE lithium decreased progressively after serial dilutions of three AD plasmas and FE lithium was negatively correlated with MMSE scores only in AD, r = -0.71 and P < 0.0005. CONCLUSIONS: In AD there is defective tubular urate transport and a plasma natriuretic factor(s). FE sodium and/or FE lithium in rats exposed to plasma of demented patients may differentiate AD from MID and estimate the severity of AD.

Aged↗

PubMed1993

[Triterpenes and steroidal compounds from cynomorium songaricum].

From the whole parasitic plant of Cynomorium songaricum Rupr. three ursane type triterpenes, three steroidal compounds, palmitic acid and sucrose were isolated. The triterpenes were identified as acetyl ursolic acid (2), ursolic acid (3) and a new compound, ursa-12-ene-28-oic acid, 3 beta-propanedioic acid monoester (1). The steroidal compounds were identified as beta-sitosterol palmitate (4), beta-sitosterol (5) and beta-sitosterol glucoside (daucosterol, 6). Compounds 2 and 5 were found in this genus for the first time.

Drugs, Chinese Herbal↗

PubMed1992

Overexpression of glutathione S-transferase and elevation of thiol pools in a multidrug-resistant human colon cancer cell line.

A human colon cancer cell line with acquired multidrug resistance (MDR) was assayed for the intracellular GSH level and the activity of GSH-S-transferase (GST), which catalyzes the conjugation reaction of electrophilic drugs with GSH. The GSH level and GST activity (as measured with 1-chloro-2,4-dinitrobenzene) were elevated in the resistant cells by 1.7-fold and 2-fold, respectively. This elevated catalytic activity of the resistant cells was reflected in a 2-fold increase in GST-pi mRNA, which was not the result of gene amplification. In addition, buthionine sulfoximine, a specific inhibitor of GSH synthesis, significantly increased Adriamycin sensitivity in both the MDR and the parental cells, affecting the former more than the latter. The effects seen with buthionine sulfoximine were not seen with puromycin and actinomycin D. A dramatic overexpression of mdr1, a P-glycoprotein gene responsible for the MDR phenotype, was also observed in the MDR cells. In contrast, none of these products (i.e., mdr P-glycoprotein, GSH level, total GST activity, GST-pi gene copy, and GST-pi mRNA level) was elevated in HeLa cells resistant to cisplatin and some alkylating agents, supporting the notion that the acquisition of cisplatin resistance differs from the mechanism of MDR. These results indicate that the intrinsic GSH level and GST-pi activity affect anthracycline resistance per se and not MDR in the human colon cancer cells.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1992

[Glycosides of phenolic acid and flavonoids from the leaves of Glycyrrhiza uralensis Ficsh].

Seven glycosidic constituents have been isolated from the dried leaves of Glycyrrhiza uralensis Ficsh. (Leguminosae). The Structures of these compounds have been identified as 1-O-protocatechuyl-beta-D-xylose (I), apigenin-6,8-di-C- beta-D-glucopyranoside (vicenin-2, II), isorhamnetin-3-O-rutinoside (narcissin, III) kaempferol-3-O-rutinoside (nicotiflorin, IV), kaempferol-3-O-beta-D- glucopyranoside (astragalin, V), quercetin-3-O-rutinoside (rutin, VI), and quercetin-3-O-beta- D-glucopyranoside (isoquercitrin, VII) by chemical method and spectroscopic analysis. Among these compounds, I is a new compound, named uralenneoside; III was reported for the first time in the genus; III to VII were isolated for the first time from the species. II to VII are known biologically active constituents.

Glycosides↗

PubMed1991

Co-amplification and over-expression of two mdr genes in a multidrug-resistant human colon carcinoma cell line.

The human P-glycoprotein gene family contains the mdr1 and the mdr3 gene. The mdr1 P-glycoprotein is over-expressed in multidrug resistant (MDR) tumor cells and is believed to play a role in the elimination of certain cytotoxic drugs used in the chemotherapy of cancer. The mdr3 gene has not been found to be amplified or over-expressed in MDR cells. In this study, gene-specific mdr gene probes were developed for the detection of the gene and the total mRNA level. Southern and Northern hybridization analyses showed that the mdr genes and the mRNA levels were increased 30--40-fold in a MDR human colon cancer cell line. In addition, this MDR cell line had an altered growth rate and morphology and detectable double minute chromosomes.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1991

Bragg-Gray theory and ion chamber dosimetry for photon beams.

The validity of the Bragg-Gray cavity theory in photon radiation dosimetry for photon energies from 10 keV to 10 MeV has been investigated quantitatively in this paper. A crucial assumption for Bragg-Gray cavity theory to be valid is that the dose from photon interactions in the detector material is negligible. The ratio, Fair, of the absorbed dose resulting from photon interactions in an air cavity, DPA(air), to that in air under the condition of charged-particle equilibrium, DCPE(air), has been used as a parameter to determine if the air cavity can be classified as a Bragg-Gray cavity. Monte Carlo calculated results show that, for monoenergetic photon beams of energies above about 220 keV, the dose ratio, Fair, is smaller than 0.05 for an air cavity of 6 mm thickness and 6 mm diameter in vacuum. Furthermore, it is shown that the Burlin general cavity theory seriously overestimates the departure from Bragg-Gray behaviour. For clinical photon beams the dose ratio, Fair, is 0.29 for a 150 kVp beam and 0.27 for a 240 kVp beam compared to 0.006 for a 60 Co beam if the cavity is placed at a depth of 5 cm in water. This study confirms that typical air-filled ionization chambers cannot be considered to be Bragg-Gray cavities for low- and medium-energy photon radiation.

Computer Simulation↗

PubMed1990

Increased mdr gene expression and decreased drug accumulation in a human colonic cancer cell line resistant to hydrophobic drug.

An adriamycin-resistant human colonic cancer cell line was characterized. This clone exhibits the classical multidrug resistance (MDR) phenotype, being cross-resistant to hydrophobic drugs such as colchicine, and vinblastine. In contrast, this clone shows a normal response to DNA-damaging agents. The appearance of MDR in these cells was linked to a decreased accumulation of the drug [3H]colchicine as compared to the drug-sensitive cells. This MDR line expressed 80-100 fold increased levels of the specific 4.5-kb mdr mRNA, and a gene amplification. Our results indicate that MDR in human colonic cancer cells can result from increased expression of at least one member of the mdr gene family.

Antineoplastic Agents↗

PubMed1990

Small cell carcinoma of the urinary tract.

Small cell carcinomas of the urinary tract are rare, but lethal. We report 3 cases of primary small cell carcinoma of the kidney, urinary bladder and prostate with light microscopic, immunohistochemical and electron microscopic findings. One patient with small cell carcinoma of the prostate died of disseminated disease 2 years after diagnosis and another patient with small cell carcinoma of the urinary bladder was free of tumour after 6 months. A partial remission was induced in the third patient with distant metastases of small cell carcinoma of the kidney by using chemotherapy protocols similar to the drug regimens for small cell carcinomas of the lung; the patient survived for 5 months. Immunohistochemical studies revealed the absence of argyrophilic immunostaining of tumour cells in all 3 cases, positive staining for keratin in 2 and staining for neuron-specific enolase in all 3. In the third patient, reactivity for prostate-specific antigen was negative. Dense-core, membrane-bound granules were identified in the cytoplasm of 2 patients. The paraneoplastic syndrome was not found, indicating that in considering the occurrence of ectopic hormones, specific cytoplasmic granules of origin need not be implicated. Recognition of this distinct entity requires full consideration of morphological, immunohistological, ultrastructural and biological features. In order to define the origin of this tumour more clearly and to evaluate the effectiveness of chemotherapy, larger series of patients are needed.

Adult↗

PubMed1990

[Studies on flavonoid constituents isolated from the leaves of Glycyrrhiza uralensis Fisch].

Four flavonoids were isolated from the alcoholic extract of the leaves of Glycyrrhiza uralensis Fisch. (Leguminosae). On the basis of physicochemical properties and spectroscopic analysis their structures were elucidated as 3,5,7,3',4'-pentahydroxy-5'-isoprenylflavone(I), 3,6,7,3',4'-pentahydroxy-2'-isoprenylflavone(II) and 5,7,3',4'-tetrahydroxy-5'-isoprenyl-flavonone(III) and quercetin-3,3'-dimethylether(IV). Compounds I, II and III are new compounds and named uralenol, neouralenol and uralenin respectively. Compound IV was found in this genus for the first time. Compound III was shown to be the major constituent in the leaves of Glycyrrhiza uralensis Fishch.

Flavanones↗