Background

In the United States, breast cancer is the most common cancer for women, except skin cancer. It is the second most common cause of death from cancer for women, after lung cancer.

The breast is mainly made up of fatty tissue. Within this tissue is a network of lobes that contain tiny, tube-like structures called lobules that contain milk glands. Tiny ducts connect the glands, lobules, and lobes, carrying the milk from the lobes to the nipple. About 90% of all breast cancer cases start in the ducts or lobes. If the disease has spread outside of the duct or lobe and into the surrounding tissue or other parts of the body, it is called invasive or infiltrating.

Breast cancer cells may have estrogen receptors (ER) on the cell surface. Breast cancer cells with these receptors depend on the hormone estrogen to grow; these types of tumors are called ER-positive. If a breast cancer is ER-negative, it means that the tumor does not need estrogen to grow. Currently, the drugs used to reduce breast cancer risk are most effective for lowering the risk of ER-positive cancer.

Several drugs that have been used or studied for other reasons have also been studied as a way to reduce breast cancer risk.

Tamoxifen is a type of drug called a selective estrogen receptor modulator (SERM). It is a hormone treatment for breast cancer for women who already have the disease. Tamoxifen blocks the effects of estrogen on tumor growth. It has been shown to lower the risk of breast cancer recurrence (cancer that comes back after treatment) and improve survival for women with early-stage breast cancer. It has been studied for 30 years and the benefits and side effects are well known. In the previous ASCO guideline, an analysis of multiple research studies showed that women who took tamoxifen lowered their risk of ER-positive breast cancer by 48%.

Raloxifene (Evista) is also a SERM. It is now used to prevent osteoporosis (thinning of the bones) for women who have gone through menopause. Recent research shows that raloxifene lowers the risk of breast cancer for women who have been through menopause.

Aromatase inhibitors (AIs) are a type of hormone treatment that reduce the amount of estrogen in a woman's body by stopping tissues and organs other than the ovaries from producing estrogen. Research to find out if AIs can reduce the risk of breast cancer is ongoing.

Retinoids are drugs made from vitamin A. One large study suggests that one of these, fenretinide, may help prevent a second breast cancer for women who had breast cancer or ductal carcinoma in situ (DCIS). DCIS is breast cancer that has not spread outside of the duct where it started.

ASCO developed this guideline to provide recommendations for the use of drugs to help lower a woman's risk of breast cancer. The guideline defines women at risk for breast cancer as those with lobular carcinoma in situ (LCIS) and those considered to have a higher risk. LCIS is a condition in which abnormal cells are found in the lobules or glands of the breast. A woman's risk of breast cancer can be calculated using the Breast Cancer Risk Assessment Tool from the National Cancer Institute (NCI). This tool uses a woman's age, race, and medical history, including whether she has had a breast biopsy, when she first started her menstrual period, if or when she gave birth, and whether any of her first-degree relatives (mother, sisters, daughters) have had breast cancer to calculate a woman's risk.