Interactions between benign breast disease and other risk factors for breast cancer

A population-based incident case-control study of breast cancer in Northern Alberta, Canada, provided an opportunity to evaluate interactions between the occurrence of benign breast disease (BBD) and other potential risk factors. Overall there was a 2-fold excess risk of breast cancer among women with BBD. This excess persisted regardless of the number of years from first biopsy for BBD to diagnosis of breast cancer, suggesting a permanent alternation in risk with BBD. Significant variations of the BBD--breast cancer association were noted with several factors. The excess risk of breast cancer among women with BBD was enhanced by having a later age at menarche, a later age at first birth, or lower parity. The usual protective effect of late age at menarche was seen only among women without BBD, and was supplanted by a 4-fold elevated risk among women with BBD. The deleterious effects of later age at first birth and nulliparity were greater among women with BBD. These patterns generally support the notion that hormonal factors mediate the relation between BBD and the development of breast cancer.