Breast cancer risk and hormone replacement therapy: a review of the epidemiology.

Hormone-related cancers account for more than 20% of all newly diagnosed malignancies and more than 40% of all female cancers in the United States. The life-time odds of getting breast cancer in North America are now one in eight. Mortality rates have held almost steady over the past 20 years, even though the number of new cases has grown. The impact of exogenous hormones on risk of reproductive cancer, especially breast cancer, has been the focus of considerable interest in recent years as the use of exogenous hormones (both pre-menopausally and post-menopausally) has increased. Evidence concerning the effect of hormone replacement therapy (HRT) on breast cancer risk is presented. The epidemiology of other risk factors for breast cancer is summarized as background; this includes reproductive factors (such as age at first birth, and age at menarche and menopause), family history, and use of exogenous hormones other than HRT.