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Biomedical subjects

R M Harrison

Publications and source records attributed to R M Harrison.

At least 55 records · Page 3Linked to original sources
PubMed1998

Tumour dose estimation using automated TLD techniques.

Lithium fluoride (TLD-700) dosimeters were used to measure exit surface absorbed doses in external beam radiotherapy using an automated TLD reader. Delivered tumour absorbed doses were derived from these measurements for head and neck, pelvis and breast treatments. For the head and neck treatments (first fraction only), the mean percentage difference between prescribed and delivered tumour absorbed doses was -0.15 +/- 3.0% (+/- 1 SD), for the pelvic treatments -0.83 +/- 2.8% and for the breast treatments +0.26 +/- 2.9%. The spread of results is approximately +/- 3% (+/- 1 SD). This is comparable with the estimated uncertainty in a single TLD absorbed dose measurement in phantom (+/- 2%; +/- 1 SD). Thus, ICRU recommended tolerances for absorbed dose delivery of +/- 5% may not be unequivocally detectable using this method. An action level of +/- 10% is suggested, allowing investigation of possible gross errors in treatment delivery at an early stage, before the course of treatment has progressed to a point at which absorbed dose compensation is impossible.

Breast Neoplasms↗
PubMed1998

Evaluation of personal exposure to monoaromatic hydrocarbons.

OBJECTIVES: To evaluate the personal exposure of members of the general public to atmospheric benzene, toluene, and the xylenes, excluding exposure from active smoking. METHOD: 50 volunteers were equipped with active air samplers for direct measurement of personal exposure to monoaromatic hydrocarbons (MAH) and an activity diary was completed during each sampling period. Exposures were also estimated indirectly by combining activity data with independent measurements of hydrocarbon concentrations in several microenvironments. RESULTS: Personal exposure were generally well in excess of those which would be inferred from outdoor measurements from an urban background monitoring station. A wide range of sources contribute to exposure, with indoor and in car concentrations generally exceeding those measured at background outdoor locations. Environments contaminated with tobacco smoke were among those exhibiting the highest concentrations. Personal exposures determined indirectly from activity diaries/microenvironment measurements were well correlated with those determined directly with personal samplers. Personal 12 hour daytime exposures to benzene ranged from 0.23-88.6 ppb (mean 3.81 ppb), with 12 hour night time exposures of 0.61-5.67 ppb (mean 1.94 ppb) compared with an annual average concentration of 1.18 ppb at the nearest suburban fixed site monitoring station. The excess of personal exposure over fixed site concentrations was greater for benzene and toluene than for the xylenes. CONCLUSION: A wide range of sources contribute to personal exposures to monoaromatic hydrocarbons with exposure duration being as important a determinant of total exposure as concentrations. Exposures generally exceed those estimated from concentrations measured by background fixed point monitors. Microenvironment sampling combined with activity diary information can provide satisfactory estimates of personal exposure to these compounds.

Adolescent↗
PubMed1997

Automated TLD system for tumor dose estimation from exit dose measurements in external beam radiotherapy.

PURPOSE: An automated TLD facility has been commissioned and calibrated, and techniques have been developed for the measurement of exit doses in external beam radiotherapy, to enable the routine estimation of delivered tumor doses. METHODS AND MATERIALS: An automated TLD system, originally intended for use in diagnostic radiology and radiation protection, has been evaluated and configured for the measurement of exit doses in radiotherapy. Linearity, optimum heating cycles and calibration procedures have been determined. At the photon energies used, encapsulated lithium fluoride chips provide insufficient buildup to ensure electronic equilibrium, necessitating calibration to allow for oblique exit surfaces. Expressions are derived to allow the calculation of delivered tumor doses. RESULTS: Under the calibration conditions described, the uncertainty in a single TLD measurement is approximately +/-2% (+/-1 standard deviation). Over the dose range 0.4-1.5 Gy, TL response is linear. The total heating cycle time, including annealing, is 75 s. Measurements of R(exit) (the ratio of exit dose with and without full backscatter), used in the estimation of tumor doses, decreases with field size for small fields and varies only slightly for field sizes greater than 7 x 7 cm. Lack of electronic equilibrium leads to a decrease in R(exit) with increasing exit surface obliquity for all energies considered. Application of the technique to a simulated treatment showed good agreement between estimated and applied tumor doses, when surface obliquity was taken into account. CONCLUSION: This work describes the commissioning and calibration of an automated TLD facility and demonstrates that exit surface measurements using TLD chips used under conditions where electronic equilibrium was not established, have the potential for identifying discrepancies in delivered tumor doses.

Calibration↗
PubMed1997

Is there a role for telemedicine in an urban environment?

Semi-structured interviews were carried out to determine the expectations of potential participants in a feasibility study of teleconsulting. General practitioners produced high positive scores in all areas. Hospital consultants produced high scores but were more critical of technical performance and administrative arrangements. Patient satisfaction with teleconsulting was generally good. The results suggested that teleconsulting was both feasible and acceptable to the users.

Family Practice↗
PubMed1997

Low energy X-ray depth dose data for use in radiotherapy--comments on the review of BJR Supplement 17.

Percentage depth dose data acquired during the revision of BJR Supplement 17: Central Axis Depth Dose Data for Use in Radiotherapy have been reviewed. Coverage of the half value layer (HVL) and field diameter rang was broad, although gaps were apparent at HVLs of 3-5 and 6-7 mm A1 and field diameters > 14 cm. Comparisons of submitted data with that in Supplement 17 indicate a generally even spread of data about the existing PDD values. Most data agreed to within +/- 5% at 2 cm and +/- 15% at 10 cm depth. Systematic discrepancies may be due to the use of different phantom materials. There is evidence that the spread of data is at least partially due to the problems of beam quality specification. The inadequacy of the first HVL as a specifier of beam quality is a well known and a more realistic specifier might be defined in terms of a broad beam dose ratio at defined depths, for a given beam geometry.

Humans↗
PubMed1996

Uncertainties in dosemeter intercomparison techniques.

The results of 180 intercomparisons of field dosemeters with an NPL secondary-standard exposure meter (type 2560, with a type 2561 chamber) have been analyzed in order to study variations in readings when ionization chambers are interchanged during the comparison process. It is suggested that the percentage standard error of the mean of the combined series of chamber intercomparisons be used to set a target uncertainty (0.3%), action level (0.5%) and limit of acceptability (1.0%). The target uncertainty is consistent with IAEA estimates of uncertainty for this part of the calibration chain. It is shown that if the calibration procedure is repeated, the percentage difference between the geometric means calculated from each calibration (i.e. a single interchange of chambers) may be as high as +/- 1%. This reinforces the need for at least one repetition of the intercomparison procedure.

Biophysical Phenomena↗
PubMed1995

Genotypic selection of simian immunodeficiency virus in macaque infants infected transplacentally.

To understand viral and host factors that contribute to transplacental transmission of human immunodeficiency virus, we developed an animal model using pregnant female macaques infected with simian immunodeficiency virus (SIV). Pregnant females were inoculated intravenously during midgestation with either a well-characterized primary isolate of SIV (SIV/DeltaB670) or a combination of SIV/DeltaB670 and the macrophage-tropic molecular clone SIV/17E-Fr. The viral genetic diversity in five infected female macaques and their in utero-infected infants was analyzed. All of the mothers harbored a genetically diverse virus population at parturition, whereas a single genotype from the maternal quasispecies was identified in the infants at birth. Only one of two variants was found in the infants: SIV/17E-Fr (two cases) or a genotype contained within the SIV/DeltaB670 quasispecies (three cases). The macrophage-tropic properties of both transmitted genotypes were suggested by productive replication in primary rhesus macrophage cultures in vitro and the clonal presence in central nervous system tissue of infected monkeys with encephalitis. These observations provide compelling evidence for both genotypic and phenotypic selection in transplacental transmission of SIV and suggest a critical role for macrophages in fetal infection in utero.

Acquired Immunodeficiency Syndrome↗
PubMed1994

A method and contrast-detail phantom for the quantitative assessment of radiotherapy portal imaging systems.

Several designs of electronic portal imaging systems for radiotherapy have been reported or are now commercially available, as well as the familiar radiographic screen-film combinations. In order to evaluate imaging performance of these, and future, systems a prototype contrast-detail test object has been developed in conjunction with a method for determining contrast from a model of the X-ray spectra produced by linear accelerators. Several existing test objects rely on qualitative or semi-quantitative estimates of contrast. In this technique, quantitative estimates of contrast may be determined for each detail size from realistic estimates of the X-ray spectrum by using a spectral model either in conjunction with narrow-beam attenuation measurements on the linear accelerator used, or by using the nominal value of maximum photon energy. This technique should facilitate the comparison of imaging systems used with linear accelerators of different energies as well as providing a quantitative quality control tool for regular measurements of imaging performance. Examples of the use of the test object in the evaluation of several commercial screen-film combinations are given.

Evaluation Studies as Topic↗
PubMed1993

A micro-computer based CT and digital fluorography system for radiotherapy simulators.

A low cost system designed to produce low resolution CT images from a radiotherapy treatment simulator is described. The system is based upon a standard micro-computer and operates using the simulator's image intensifier as the CT detector. The resulting images are intended to supply cross-sectional anatomical information appropriate for the application of inhomogeneity corrections to treatment plans for megavoltage radiotherapy of the thorax. Options for digital fluorography and hold-last-frame fluoroscopy are also described. Examples of clinical images are presented.

Computer Simulation↗
PubMed1993

Absence of long-term gonadotoxicity in primates receiving extracorporeal shock wave application.

Extracorporeal shock wave lithotripsy (SWL) is becoming popular in the management of lower ureteral calculi; however, the quantity of data on its potential gonadotoxic effects are limited. Eight nonhuman primates had baseline measurements of testicular volume, semen analysis, serum testosterone and FSH assays, and DNA flow cytometric analysis of testicular aspirates. The anesthetized primates were lowered into a Dornier XL-1 lithotripter and at a 20 kV power setting had various numbers of shocks (0, 500, 1500) directed at each testis. Sequential studies were done at 6-week intervals for 9 months. The control group showed no significant change in any measure. Semen analysis, testosterone, and FSH results were variable and without statistically significant differences. There were changes in the DNA histograms, with the tetraploid (4N) group of cells being the most sensitive. All changes caused by SWL application returned to baseline by 9 months. Data from this primate study document no apparent long-term male gonadotoxicity from SWL, but the authors suggest measures to protect the testis during clinical use.

Animals↗
PubMed1993

A perspective on lead pollution and health 1972-1992.

The health effects of exposure to lead from environmental sources are briefly reviewed and the likely benefits of removal of lead from petrol are evaluated. At current environmental levels of exposure, adverse effects of lead upon intellectual development of children are difficult to detect with certainty, but if present are extremely small. When viewed in the context of steadily increasing population IQ scores this effect appears almost negligible. The historical path of decision making over lead in petrol is traced and it is concluded that in the UK the present outcome is not necessarily the best practicable environmental option. The question is raised as to whether the considerable financial costs of removal of lead from petrol might have been better spent in other ways.

Adult↗
PubMed1993

External beam treatment planning. Can we deliver what we plan?

Many sources of uncertainty exist in the delivery of the prescribed dose to the specified target volume in external beam radiotherapy. Their magnitudes are compared with recommendations for the overall accuracy of dose delivery (+/- 5%). Five dose intercomparisons are reviewed, including a recent UK survey undertaken by the Institute of Physical Sciences in Medicine (IPSM). For phantom doses, standard deviations in the ratio of stated to delivered doses range from 1-3.5%, with larger uncertainties when irradiation of volumes within anthropomorphic phantoms is considered. In addition, uncertainties arise from geometrical discrepancies, and the role of portal films in their assessment is also considered. Imaging during radiotherapy will be increasingly important in the development of complex conformal treatments, where the shape of the target volume may be irregular.

Dose-Response Relationship, Drug↗
PubMed1990

The impact of local emissions on the formation of secondary pollutants in urban plumes.

The results of modelling studies are reported together with a further analysis of an existing field measurement dataset. Emphasis is on the maximum ozone concentrations occurring in the urban plume from London, UK, on days of photochemical activity. Nitrogen oxide and volatile organic compound emissions from automotive exhausts are expected to be a large proportion of the total emission of these species, and the effect of their distinct diurnal variation is apparent in the field data. Factorial sensitivity studies on an expanding box model of the urban plume also show the dominance of urban emissions in the determination of plume maxima 100km downwind of the upwind urban boundary. These effects are not additive and depend strongly on the level of other variables.

Air Pollution↗