Not available

Many research initiatives are currently underway to improve the performance of thermoelectric coolers (TECs). The primary drawback of TECs is their inefficiency. Most of the ongoing research concentrates on various materials enhancements. One neglected area of improving TECs is the method in which they are powered and controlled. Typical TEC thermocouples are powered in series although they remove heat in parallel. In other words, if one of the numerous thermocouples is powered, all the thermocouples receive the same power whether or not they are needed. By sensing and controlling the cooling of the individual devices within the system a significant increase in performance can be achieved for the entire system. This thesis presents the evidence that applying distributed control to TEC operation can realize appreciable improvement in performance. The average improvement in performance over the operating range investigated here is 80%. Additionally, this type of control can be used in conjunction with many of the existing research initiatives to further compound the benefits.

Modulated electron beam therapy (MERT) using photon multi-leaf collimators (pMLC) is an emerging technique. In this study characterization of electron beams collimted by an Elekta Precise machine pMLC for MERT applications was done. The objectives of the study were: a) to model the Elekta Precise machine using the EGSnrc MC codes to generate electron energy spectra that replicate the machine. b) study the physical characteristics of electron beams collimated by the pMLC at different source skin distances (SSDs) and field sizes and to c) apply the findings in the treatment of scalp with electron beam. For objective (a) the EGSnrc/BEAMnrc MC codes were used to extract the electron energy spectra from simulated PDD data. These spectra were used as input sources in the MC code. Benchmarking against measurements in water in terms of percentage depth dose and beam profiles were performed for 4â15 MeV over different applicators sizes. Physical characteristics of the electron beams collimated by the pMLC using the generated energy spectra were calculated. The SSDs ranged from 60â100 cm, field sizes ranged from 1 Ã 1 up to 20 Ã 20 cm2. An anthropomorphic phantom for total scalp treatment scanned using CT. Eight fields were incident on the scalp from different angles for a 4 MeV electron beam. The results showed that the generated electron energy spectra can be assumed to be a realistic approximation of the linac. Penumbra data indicated that 60 cm SSD was the most favorable treatment distance for MERT. At this distance the field size is conserved. The output factors were much higher compared to applicator collimated electron beams. This will accelerate the MERT based dose delivery. No detectable leakage through the pMLC was observed. The penumbra for single large fields can be improved by using weighted abutted smaller fields. The virtual source position was found to be at 5 cm below the exit window. The scalp treatment showed excellent dose distribution and optimal dose coverage of the target while sparing distal organs at risk as quantified by a dose volume historgam. This study domenstrated that the MERT technique can be implemented on an Elekta Precise. MC techniques showed to be valuable tools for doing forward optimization and MERT driven dose calculations.

Not available

The differing susceptibility of individuals to the toxic effects of chronic lead exposure has never been fully understood. As the major intake of lead in the human is from food and beverages, any variation between individuals of the quantity of lead absorbed from the gut, and of the distribution and excretion of this lead, may account for the differences in individual susceptibility. The food and beverages themselves may have an influence, and to investigate their effects on absorption, distribution and excretion of lead, experiments were performed on normal subjects using a short-lived radionuclide of lead, ²⁰³Pb, and instruments generally available in Nuclear Medicine. Lead absorption between different individuals showed a wide variation when ²⁰³Pb was taken as a single dose between meals. The effect of fasting was to increase absorption and reduce the variation. This suggested that food with the lead in the gut was mainly responsible for the variation in lead absorption between individuals. This was confirmed when ²⁰³Pb was mixed into a control meal and eaten by fasted subjects. Lead absorption was again low and variable. In contrast, ²⁰³Pb taken in distilled water was avidly absorbed with little variation between subjects. The absorption of lead in water could be appreciably more than lead in food amongst the general population. Minerals were found to be mainly responsible for affecting absorption when one subject ingested ²⁰³Pb in control meals from which one dietary constituent at a time was omitted. The effect of minerals in reducing absorption of lead was greatest when they were ingested in distilled water. Lead in water with a low mineral content, such as 'soft' water, could be a hazard to population groups living in 'soft' water areas. There were also indications from these experiments that dietary constituents may affect the distribution and excretion of lead. Calcium and phosphorous were found to reduce the absorption of ²⁰³Pb to approximately the same level as that produced by the total minerals. Calcium reduced absorption more than phosphorous when these minerals were ingested separately with ²⁰³Pb. Paired kinetic experiments showed that calcium and phosphorous ingested at the same time as ²⁰³Pb affected its distribution in the body but not its rate of excretion. Using the data from the kinetic experiments, a compartmental model was developed which adequately described the kinetics of orally ingested ²⁰³Pb. The model suggested that calcium is mainly responsible for reducing absorption of lead from the gut, but that the effect of phosphorous is to increase soft tissue levels of lead at the expense of red cell lead. As susceptibility is related to soft tissue levels of lead, this suggests that the 'protective' effect of calcium in the gut is reduced. This could not be confirmed, however, as the kinetic data were insufficient for the model to distinguish the tissues particularly vulnerable to lead toxicity in the soft tissue compartment. It was concluded that the calcium and phosphorous in the diet could influence susceptibility to lead toxicity through changes in the absorption of food and water lead and in the distribution of lead in the body. The results suggest that the prophylactic effect of calcium on lead absorption should be recognised and applied in this time of increased environmental levels of lead.

Includes bibliographical references / The objective was to implement a Monte Carlo-based Intra-operative radiation therapy programme at the institution of the author.

Summary in English. / Bibliography: pages 125-129. / This thesis describes the construction and use of the facemask at the National Accelerator Centre (NAC) as used to both immobilise and position patients for precision proton radiotherapy. The precision achieved using the stereophotogrammetric (SPG) positioning system is measured, and the shortcomings and errors in using the facemask by the SPG system are measured and analysed. The implementation of improvements made to the SPG system is reported upon, and alternative means of both supporting the fiducial markers and immobilising the patient are investigated and evaluated. The accuracy of positioning a facemask using the SPG system is 1.4 mm and of positioning a newly designed frame is 1.6 mm. These measurements were made without using a patient. It is estimated that the total uncertainty of positioning a patient's tumour at the isocentre is 1.6 (1SD) mm using the facemask and it is estimated that the precision using the frame will be less than this value. The largest component of this error (1.39 mm) is due to the error in obtaining the CT scanner co-ordinates. These results are comparable to those obtained by other investigators. The movement of patient bony landmarks within the facemask was measured to be 1.0 ± 0.8 mm. Three main recommendations are that the CT scanner co-ordinating procedure be improved, the SPG computer program be rewritten in parts to achieve greater speed and accuracy, and that the new frame be used. The frame is easier to manufacture than the facemask and allows real time monitoring of the position of the patient's head by the SPG system thus allowing faster throughput of patients and better positioning quality control.

Neutron beams are always accompanied by gamma rays. Because neutrons and gamma rays have different biological effects, it is essential that the neutron and gamma dose components be determined when neutron beams are used for radiotherapy and radiobiology. Since gamma ray dosimeters almost always also respond to neutrons, it is generally not possible to use two different devices to measure the separate dose components. The separate doses are usually determined by making measurements with two instruments which have different sensitivities to the two types of radiation and deducing the separate components from these measurements. One instrument usually has approximately equal sensitivity to both neutrons and gamma rays while the other has a low sensitivity to neutrons. The latter instrument is often a Geiger-Müller (G-M) counter. Although its neutron sensitivity (kᵤ) is low, it must be measured for accurate dose determinations. Previous u measurements of the neutron sensitivities of such counters have not always been in good agreement. In the present work the neutron sensitivities of two types of G-M counters, viz. ZP1320 and ZP1300 have been measured at neutron energies between 0.72 and 7.42 MeV. An absolute measurement of the neutron sensitivity of the ZP1320 counter was made at 3 MeV using the associated particle technique while the neutron sensitivities of both counters were measured at several energies using the pulsed beam time-of-flight technique. For the latter measurements a NE213 scintillation counter was used to measure the neutron flux. The detection efficiencies of this counter were calculated and the validity of the calculations were confirmed by measurements using the associated particle technique. The neutron sensitivities obtained in the present work are in reasonable agreement with most previous measurements. All available data for similar types of counters were fitted with a function of the form kᵤ(E) = a Eᵇ. The values of the fitting parameters were ZP1320 (0.72 to 7.42 MeV) : a = 0.10 ± 0.01, b = 1.32 ± 0.05 and ZP1300 (0.57 to 15.65 MeV) a = 0.25 ± 0.01, b = 1.32 -+ 0.05 b = 0.78 ± 0.03.

Bibliography: leaves 72-77. / Thermoluminescent detectors (TLD-700) have been investigated for absorbed dose measurements in a p(66)/Be neutron therapy beam at the National Accelerator Centre. Chips were selected based on their reproducibility and chip individual neutron calibration factors were derived. The dose non-linearity was determined in peak 5 and peak 6 and dose non-linearity corrections were performed. The sensitivity of TLD-700 chips with depth and off-axis distance was determined. In-vivo dose measurements were performed on seven patients (9 fields). In the entrance in-vivo dose measurements, a maximal deviation of 3.2 % was detected and a systematic difference of 1.7 % was observed. On the exit side, a maximal deviation of -7.3 % was detected and a systematic difference of -5.1 % was observed. The glow curve peak 6/5 ratio was investigated and found to correlate with the qualitative variations of the average LET in the neutron beam.

The results of a study conducted by Maree (1995) indicated that the genetically-significant dose (GSD) for the white, female population in South Africa was considerably higher than the GSD for females in Great Britain, France and the United States of America. Further to this finding, Maree's study demonstrated that the barium enema x-ray examination was the major contributor to the GSD for this population group. A study of barium enema examinations was embarked on in order to explain the findings of Maree. The study was designed to include dose-area product measurements on patients having the barium enema procedure. In addition patient data and technique factors were recorded. The x-ray equipment used for the investigation was one digital and two non-digital fluoroscopic systems in the Western Cape. The digital unit utilised an overhead tube as did one of the conventional units. The other unit had an undercouch fluoroscopic tube and an overhead tube used for the standard radiography views. Comparison of the dose-area product measurements demonstrated that the unit having an undercouch tube had a mean dose-area product of 99.69 Gy cm² which culminates in a higher dose to the patient than the equipment utilising an overhead tube. The mean dose-area product of the two units with an overhead tube was 56.57 Gy cm² and 51.94 Gy cm² respectively. Free Air Exposure tables based on "RADCOMP Entrance Skin Exposure Software Program" (Nuclear Associates and Zamenhof, 1990) were used together with average technique factors to calculate skin entrance doses. These skin entrance doses were used to calculate gonad doses with the aid of a computer program from the Food and Drug Administration in the USA (Peterson and Rosenstein, 1989). The results were compared with the results of the barium enema component of the research conducted by Maree. The comparison indicated an average gonad dose for males of 242 μGy x 10⁻¹ (present study) compared to 485 μGy x 10⁻¹ (Maree) and an average gonad dose for females of 11185 μGy x 10⁻¹ (present study) compared to 16111 μGy x 10⁻¹ (Maree). Air-kerma at skin entrance was calculated using dose-area product measurements, recorded during the present study, for individual exposures and screening. These values were used to calculate the gonad dose. A discrepancy was demonstrated between the calculation of gonad dose from calculated as opposed measured skin entrance dose. The average gonad dose calculated by Maree is 16111 μGy x 10⁻¹ and the average gonad dose calculated for the present study using the measured skin entrance dose is 4236 μGy x 10⁻¹. This seems to explain the larger GSD estimated by Maree for the white female patients. A national protocol for measuring patient doses from x-ray examinations is proposed for South Africa.