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Biomedical subjects

P B Dunscombe

Publications and source records attributed to P B Dunscombe.

At least 19 recordsLinked to original sources
PubMed1993

The investigation and rectification of field placement errors in the delivery of complex head and neck fields.

PURPOSE: To evaluate a novel technique for resolving field placement errors into their components and to quantify the improvement in accuracy potentially achievable by translation and rotation of the radiation beam. METHODS AND MATERIALS: One hundred and eighty-five films (both simulator and portal) from seventeen patients receiving radiotherapy to the head and neck region were analyzed in pairs. The computer based comparisons of complex fields with curved edges employed the intersections of perpendiculars from two reference points with the field periphery to define field match points. Field placement errors were resolved into those due to patient motion within the immobilization shell and those due to incorrect beam position, orientation, or shape. RESULTS: The median and the 95 percentile of the distribution of differences between prescribed (simulator) fields and treated (portal) fields referenced to the patients anatomy were 4.4 mm and 8.9 mm, respectively. The analysis suggests that with appropriate translation and rotation of the beam with respect to the immobilization shell these figures could be reduced to 3.1 mm and 8.2 mm, respectively, confirming the large contribution of patient motion within the shell to field placement accuracy. Comparisons between treated fields indicated smaller variability during treatment than between simulation and treatment. CONCLUSION: The perpendicular intersection method described here was found appropriate for the identification of field match points. The distributions of field placement errors were similar to those in a published study of straight edged fields. Translation and rotation of the applied field with respect to the immobilization shell would generally result in only a small improvement in field placement accuracy.

Head and Neck Neoplasms↗
PubMed1991

A proposal for the classification of field placement errors in radiotherapy.

There is an increasing awareness of the high frequency of field placement errors occurring in radiotherapy. If such errors are to be rectified systematically to provide a sustainable improvement in field placement accuracy over a course of treatment, the origins of the errors require unambiguous identification. From an examination of the procedures taking place between the exposure of the prescription and treatment films, we propose resolving field placement errors into four groups: patient motion, entrance field location, field shape or size, and beam direction. We also suggest means by which the components of clinically observed field placement errors may be resolved in practice.

Classification↗
PubMed1991

The sensitivity of the thoracolumbar spinal cord of the mouse to hyperthermia.

Twelve millimeters of the thoracolumbar spinal cord of mice has been treated with a radiofrequency heating system which has been shown previously to produce localized and controllable elevation of temperature. The severity of neurological damage was assessed by measuring the reduction in the reflex leg extension of the hind legs of the mice from video-recorded images and by scoring the performance of the mice by a negative geotaxis test. The response to treatment was rapid with maximum paralysis occurring within a few days after treatment. Only minor symptoms were observed in those animals which had not developed paralysis within 2 weeks. A 40% reduction in the reflex leg extension was chosen as an end point, and the percentage of mice having reached the end point for different thermal doses was determined in groups of nine mice. The ED50 for heating for 1 h was 43.1 degrees C and for heating at 45 degrees C was 10.8 min. An increase in temperature by 1 degree C required a decrease in time by a factor of 2.25 to produce the same effect. Thermotolerance was observed 24 h after preheating at 45 degrees C for 1.9 min with a thermotolerance ratio of 1.7. The rapid response and high sensitivity of the spinal cord will have to be taken into consideration in the clinical application of hyperthermia.

Acclimatization↗
PubMed1990

Physical aspects of external beam radiotherapy for the treatment of malignant pleural mesothelioma.

The optimization of radiotherapy for the treatment of malignant mesothelioma highlights many of the currently outstanding problems in clinical radiation physics. The experimental investigation of an intuitively attractive irradiation technique with combined photon and electron beams using a specially constructed phantom has established that, due to the penetration in low density material of both primary electrons and those secondary to photon irradiation, the normal lung tissue is not spared to any significant degree by such a technique. Furthermore, great care needs to be exercised in the treatment planning calculations for this approach if absolute dosimetry errors as large as 50% are to be avoided.

Electrons↗
PubMed1990

Quantitative computed tomography of the brain with xenon enhancement: a phantom study with the GE9800 scanner.

A recently proposed application of quantitative computed tomography is in the study of cerebral blood flow and partition coefficient using stable xenon as a freely diffusible, radio-opaque tracer. Central to the method is the calibration factor describing the relationship between CT number and xenon concentration in the brain. In this paper we examine the influence of temporal fluctuations, kVp, radial position and beam hardening on this calibration factor through the analysis of a series of phantom measurements. We conclude that under clinically realistic conditions and with correlations for temporal fluctuations, the error associated with the calibration factor is less than 2%. Furthermore, errors of this magnitude translate into errors of less than 3% in derived blood flow and partition coefficient values obtained using xenon-enhanced computed tomography.

Brain↗
PubMed1990

The role of noise in the measurement of cerebral blood flow and partition coefficient using xenon-enhanced computed tomography.

Monte Carlo simulations have been used to study the accuracy which can be expected in the quantification of blood flow and the partition coefficient using xenon-enhanced computed tomography in the presence of noise. We have demonstrated that the markedly asymmetric frequency distribution of estimates increases in size rapidly with an increase in the standard error of the input CT data. On the basis of our results, we recommend that controllable sources of noise (eg. CT number drift) be corrected and that estimates be obtained by averaging CT data and then fitting, rather than averaging blood flow and partition coefficients derived from individual pixels, as the latter procedure results in the introduction of considerable bias.

Administration, Inhalation↗
PubMed1989

Precision of determining compliance with prescribed fields from conventional portal films.

Three hundred and seventy measurements of field placement errors (FPEs) have been made by a total of 16 observers on 20 prescription-treatment film pairs taken during routine radiotherapy for cancer of the prostate. Analysis of the distributions of the measured FPEs has yielded the precision of the measurement under a variety of conditions. We report here the influence on the precision of determining FPEs of the following factors: the clinical duties of the observers, the quality of the treatment film, the relative magnification of prescription and treatment films, and whether double-exposure techniques were employed.

Humans↗
PubMed1983

Differential thermometry as a monitor of blood flow in skin flaps.

An instrument for monitoring the blood flow in skin flaps by temperature measurement is described. This compares the temperature of the flap with adjacent normal skin. An experimental assessment of the technique is presented. Although many of the problems associated with absolute temperature measurement were eliminated, the reaction to circulatory change within a flap was slow. However, with further refinement, the principle may have a place in clinical practice, especially in replantation surgery.

Animals↗
PubMed1983

A search for nonthermal effects of 434 MHz microwave radiation on whole human blood.

Whole human blood was subjected to a microwave environment at 434 MHz for 6 hr with external electric fields corresponding to free space power densities up to 598 mW cm-2 and the levels of hemoglobin, sodium, and potassium in the plasma were monitored. Under geometrical conditions in which the field strength within the samples was unknown, measurements indicated increased red cell membrane fragility following irradiation. It was not possible to exclude localized heating as an explanation of this effect. However, with a known and reasonably uniform electric field distribution within spherical specimens, increased membrane fragility was not observed. We are therefore unable to confirm previously reported results which indicate a nonthermal deleterious effect of microwave radiation on erythrocytes.

Blood↗
PubMed1982

Panoramic sialography: an alternative technique.

Pantomographic sialography, referred to in this paper as panoramic sialography, was performed on 30 out of 51 patients. In 21 patients, it proved to be a good alternative technique to conventional sialography whilst in the other nine patients it was the principal method used to study the salivary glands. The techniques employed in this study for both conventional and panoramic methods are described. The radiation dose delivered to the patients was similar for both techniques. Water soluble contrast medium, Conray (325), was used in all cases.

Aged↗
PubMed1981

Thermocouple thermometry in microwave fields.

The causes of some of the artefacts and hazards resulting from the use of thermocouples in a 434 MHz microwave field have been investigated. The factors influencing their magnitudes are discussed.

Humans↗
PubMed1980

Radioactivity studies on 2 synovial specimens after radiation synovectomy with yttrium-90 silicate.

Synovial membranes surgically removed from the knees of 2 patients who had received radioactive yttrium-90 silicate have been examined. Autoradiographs showed that in both cases the activity was unevenly distributed over the synovium. One of the specimens, removed 7 weeks after administration of the isotope, was found to contain a slight degree of radioactivity, attributable in part to a long-lived radioactive contaminant.

Arthritis, Rheumatoid↗