Date of Award

Fall 2011

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Medical Physics


Douglas R. Boreham




The need to understand and accurately assess the health risks of low dose ionizing radiation is more important now than ever before. The global applications of ionizing radiation in medicine, mining, manufacturing, and the nuclear industry have increased exponentially in recent years. Parallel to this increase are the health concerns regarding occupational and medical exposures to radiation. The research presented here investigates the biological and health effects of ionizing radiation, specifically from medical diagnostic exposures.

Medical diagnostic procedures such as x-rays and computed tomography (CT) scans account for a notable portion of the public's exposure to ionizing radiation. The health risk to humans associated with these low dose exposures is unknown. Often times they are correlated with risk estimates derived from much higher radiation doses. There is no doubt that very high dose ionizing radiation can be harmful; however, the same notion does not exist regarding exposures to low dose ionizing radiation such as that from medical diagnostic CT exposures.

The objective of this research is to address the effects and risks associated with diagnostic CT scans. This research focuses on the biological outcome of cancer which remains a primary concern in health care and the development of radiation risk policies. The investigation utilized various mouse models that have differing sensitivities to radiation and susceptibilities to developing radiation-induced cancer.

Results from this research found that low-dose diagnostic CT scans do not increase risk and can, in fact, induce protective effects. The hypothesis that harmful effects increase linearly with radiation dose is not supported by this research. With low doses of CT scans, protective biological effects such as reduced chromosomal aberrations, decreased radiation-induced oxidative DNA damage, and enhanced clearance of damaged cells have been observed. In cancer-prone mice, CT scans can increase longevity and reduce cancer risk by delaying the latency of specific cancers.

This research advances the understanding of the biological effects and health risk associated with low-dose medical diagnostic procedures. This research is timely and important to allow medical practitioners, policy makers, and regulators to make informed decisions about using ionizing radiation in the clinic. Such knowledge is valuable as better, more complex, and perhaps more damaging modalities are being used to image and manage disease.

McMaster University Library

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