It is feasible, in theory, that aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) could lower the total risk of breast cancer. They block an enzyme called cyclooxygenase, an activity that could disrupt breast cancer development in a number of ways -- for example, by reducing the amount of estrogen produced in the body.
Overall the results were not astonishing, but it appeared that aspirin offered some protection for estrogen eeceptor positive breast cancer.