UCSF

Breast density and obesity are two of the most common risk factors for breast cancer among women in the United States. However, the importance of these risk factors and their individual and joint effects on breast cancer risk in individuals and on a population level is relatively unknown.

The first chapter of my dissertation provides a population perspective on the impact of breast density and obesity on breast cancer incidence in U.S. women. This study used the population attributable risk proportion (PARP) to estimate of the proportion of premenopausal and postmenopausal breast cancer cases that can be attributed to breast density, obesity and their combined effects. Results from this study suggest that breast density alone accounts for 39% of premenopausal and 26% of postmenopausal cancers, and combined with obesity, accounts for 43% of postmenopausal cancers.

Volumetric breast density software measures the three-dimensional volume of breast tissue and is increasingly used in clinical and research settings. Dense tissue volume may mediate or moderate the effect of obesity on breast cancer risk; thus, identifying the joint effects of obesity and breast density on breast cancer can improve risk stratification and provide insights into pathways driving breast cancer incidence. The second chapter evaluates if the effects of volumetric breast density on breast cancer risk are greater in obese compared with non-obese women. This study finds that the effect of volumetric density on breast cancer risk is dramatically higher in obese compared with non-obese women, with the most pronounced effects in postmenopausal women. The third chapter evaluates how obesity and other risk factors affect longitudinal change in dense breast volume over the menopausal transition in healthy women. This study found no effect of obesity, but a strong effect of baseline dense volume on greater decline in volumetric breast density across the menopause transition.

As a body of work, my dissertation provides insights into the public health impact and clinical relevance of two of the most common breast cancer risk factors in U.S. women. It provides new evidence to improve clinical risk stratification and offers novel insights into the complex causal relationship between obesity, volumetric breast density and breast cancer risk.