Biomedical subjects
Publications and source records attributed to D J Dawson.
A 63-year-old man with a history of increasing cough with sputum, weight loss, and lethargy was found to have pulmonary disease caused by Mycobacterium xenopi, a species rarely seen in Australia. Chemotherapy was successful. Apart from war service in Papua New Guinea, the patient had not travelled outside Australia.
Prostaglandin synthetase activity in rectal biopsy specimens from patients with ulcerative colitis has been shown to fall on treatment with sulphasalazine, local steroids, and codeine phosphate. In vitro studies have shown that sulphasalazine is an inhibitor of prostaglandin synthetase, although less potent than indomethacin, whereas prednisolone and codeine phosphate were inactive. It is suggested that the therapeutic action of sulphasalazine may be related in part to its action in inhibiting prostaglandin biosynthesis.
Five cases of infection due to an unusual mycobacterium are recorded, and the growth requirements and histological findings are discussed. On initial culture, these mycobacteria could be grown only on Lowenstein-Jensen medium containing ferric ammonium citrate, and the histology of the skin lesions was not always typical of mycobacterial disease.
During the period 1971--7, the Tuberculosis Reference Laboratory in Queensland dealt with 52 isolates of atypical mycobacteria made from non-pulmonary sites under circumstances suggesting complicity in disease. Twenty-four isolates belonging to the MAIS complex were associated with lymph node infections in children. Twelve isolates belonged to the M. fortuitum-chelonei complex; most were from superficial abscesses. Five cases of M. marinum infection and 8 of M. ulcerans disease were detected.
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Bacteriological investigation over an 11-year period has implicated Mycobacterium avium ser I as the most probable cause of chronic pulmonary disease in a resident of south-eastern Queensland. Although the patient had contact with domestic fowls and a pet budgerigar, the source of the organism is uncertain.
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Percentage depth doses for 34 MV betatron X-rays were measured ionometrically for a number of square fields of side 5-20 cm. Significant differences in the depth doses for different field sizes were found in the build-up region and in the depth of maximum dose. A systematic change with depth was also indicated in the exponential region of these curves. These differences are discussed in terms of the changes in the contributions of primary radiation and of scattered radiation from both the collimator and the phantom.
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The tissue-maximum ratios for high-energy betatron x rays, measured at target-axis distances of 90, 110, and 130 cm, exhibit a strong dependence on the target-axis distance in the buildup region as well as a significant dependence in the exponential region. This dependence and the field-size dependence of the tissue-maximum ratios at a fixed target-axis distance are studied with the use of phantom and collimator scatter factors. The depth and field-size dependence of these scatter factors are determined experimentally. The measurements indicate that the collimator scatter factor is dependent on both the collimator opening and the targetaxis distance. These components are quantitated separately and are associated primarily with high-energy electron scatter from the collimating system. An equation involving these components of the collimator scatter factor is derived for the conversion of tissue-maximum ratios from one target-axis distance to another. With the use of this equation, the tissue-maximum ratios measured for one target-axis distance can be used to calculate within 1% those at other target-axis distances.
The effects of field elongation on the central-axis output at dmax and on the depth-dose curves for the 6-MV x rays from the Therac 6 linear accelerator are considered. These parameters are independent of the collimator angle for both square and elongated fields. The exchange of collimator pairs results in significant output differences but has a negligible effect on the depth-dose curves. Both the central-axis output at dmax and the depth-dose curves agree satisfactorily with the predictions of the equivalent-square technique. An alternative method of greater accuracy is also indicated for the prediction of central-axis outputs at dmax for elongated fields.