Date of Award


Degree Type


Degree Name

Doctor of Philosophy


It has been recognized that improved methods of verifying radiation field placement in external beam radiotherapy are required in order to make frequent checks of field placement feasible. As a result, a large number of electronic portal imaging systems have been developed as possible replacements for film. These developments have produced digital systems with faster acquisition and display (few seconds), but the quality of the images acquired with such systems is still disappointing. The reasons for this are not well known, possibly because relatively few studies of the fundamental physics of imaging at megavoltage energies have been made.;This thesis examines many of the fundamental factors which limit the quality of radiographs obtained with a megavoltage radiotherapy beam. The size and shape of the radiation sources in ten radiotherapy machines were measured using a CT reconstruction technique. A novel technique was also developed to measure the extra-focal source in medical linear accelerators. It was found that the focal spots of modern medical linear accelerators should not influence significantly image quality if reasonable radiographic magnifications (1.2-1.4) are used. The extra-focal source measurements demonstrate that the x-ray source of a medical linear accelerator is composed of two components: a small focal spot surrounded by a large extra-focal component. While the extra-focal source does not influence significantly image quality, it does have important implications for radiotherapy dosimetry. In addition, a detailed Monte Carlo simulation of x-ray interactions within the patient determined that a significant fraction of the x-ray scatter generated in the patient is due to bremsstrahlung and positron annihilation and that the scatter fractions found in megavoltage imaging are lower (0.3-0.5) than those found in diagnostic imaging (0.8-0.9). The impact of the scatter fluence on the contrast and the signal-to-noise characteristics of a therapy image is presented for a variety of therapy imaging detectors. Finally, the interaction of x-rays within typical fluoroscopic imaging detectors (metal plate/phosphor screen) is examined using Monte Carlo techniques. This study shows that failure to include electron transport in determining the quantum absorption efficiency of metal plate/phosphor detectors can lead to gross (factor of 2) inaccuracies. In addition, our study demonstrates that x-ray absorption noise at megavoltage energies can reduce the detective quantum efficiency of these detectors by a factor of 2.



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