INTRODUCTION

In the US, breast cancer is the most common female cancer, the second most common cause of cancer death in women, and the main cause of death in women ages 40 to 59. About one-half of cases can be explained by known risk factors, such as age at menarche, first live birth, menopause, and proliferative breast disease. An additional 10 percent are associated with a positive family history.

Understanding the risk factors for breast cancer permits us to identify women at increased risk and intervene to modify risk, both individually and societally.

EPIDEMIOLOGY

Incidence — Annually, approximately 182,460 American women are diagnosed with breast cancer, and 40,480 die from the disease [1]. The lifetime probability of developing breast cancer is one in six overall (one in eight for invasive disease) [2].

In the early 1980s, breast cancer rates rose steeply by 3.7 percent per year over the baseline incidence. This was most likely the result of increasing use of screening mammography, since the incidence of ductal carcinoma in situ (DCIS) and stage I carcinomas increased, while that of higher stages either decreased or remained stable.

Data collected by the SEER program between 1992 and 1998 suggested that the incidence of estrogen (ER) and/or progesterone (PR) receptor-negative breast cancer was declining while that of ER/PR-positive disease was increasing [3]. At least some of this increase might be attributable to the use of hormone therapy (HT) in the 1980s and 1990s, since HT may preferentially increase the risk of ER/PR-positive tumors [4].