Breast cancer is second-—behind lung cancer—-as the leading cause of cancer death in women. The chance of developing invasive breast cancer at some time in a woman's life is about 1 in 8.

The female breast is composed primarily of milk-producing glands (lobules), ducts that connect the glands to the nipple, and soft tissue. Breast cancer is a malignant tumor that has grown from breast cells.

Nearly all breast cancers start in the ducts or lobules of the breast. The cancer can spread (metastasize) to other parts of the body, but it will continue to be defined as breast cancer.

There are many forms of breast cancer. Infiltrating ductal carcinoma (IDC) is the most common form. It starts in a duct, then breaks through the duct wall and invades the tissue of the breast. At this point, it can metastasize through the lymphatic vessels and the bloodstream. About 80% of invasive breast cancers are infiltrating ductal carcinomas.

Lymph plays a major role in breast cancer. It is a fluid that carries immune-system cells through lymphatic vessels. Lymph nodes are small collections of these cells in the vessels. Almost all lymphatic vessels in the breast connect to lymph nodes under the arm.

Cancer cells that enter lymphatic vessels can spread and begin to grow in lymph nodes. This is why doctors check the lymph nodes to see if  breast cancer has spread.

Ductal carcinoma in situ (DCIS) is the most common type of noninvasive breast cancer. The term “in situ” means the cancer is confined to its original site. DCIS denotes that the cancer cells are inside the ducts but have not spread through the walls of the ducts into the surrounding breast tissue. About 20% of new breast cancer cases will be DCIS. Nearly all women diagnosed at this early stage of breast cancer can be cured.

There are many risk factors for breast cancer.

The risk rises with age. About 77% of women with breast cancer are older than 50 when they are diagnosed.

Breast cancer risk is higher among women whose close relatives have the disease.

A woman with cancer in one breast is at high risk of developing a new cancer in either of her breasts.

Women who started menstruating before age 12 or who went through menopause after age 55 have a slightly higher risk of breast cancer.

Having multiple pregnancies and becoming pregnant at an early age reduces breast cancer risk.

Long-term use of hormone replacement therapy (HRT) after menopause increases your risk of breast cancer.

Drinking alcohol is linked to an increased risk of developing breast cancer.

Obesity is a breast cancer risk, especially for women after menopause.

Evidence is growing that exercise reduces breast cancer risk.