The following is provided by the National Cancer Institute

The most common type of breast cancer begins in the lining of the ducts (thin tubes that connect the nipples to the other parts of the breast) and is called ductal carcinoma. Another type, called lobular carcinoma, arises in the lobules (small sections of the breast, which contain the bulbs that produce milk).

When breast cancer spreads outside the breast, cancer cells are often found in the lymph nodes under the arm (axillary lymph nodes). If the cancer has reached these nodes, it may mean that cancer cells have spread to other parts of the body – other lymph nodes and other organs, such as the bones, liver, or lungs - via the lymphatic system or the bloodstream.

Cancer that spreads is the same disease and has the same name as the original (primary) cancer. When breast cancer spreads, it is called metastatic breast cancer, even though the secondary tumor is in another organ. Doctors sometimes call this “distant” disease.

The National Cancer Institute (NCI) states that one out of every eight women in the U.S. will have breast cancer during her lifetime, with the highest percentage of women being diagnosed between ages 40 and 50. Research has shown that the following conditions place a woman at increased risk for breast cancer:

Personal history of breast cancer: Women who have had breast cancer face an increased risk of getting breast cancer again.

Genetic alterations: Changes in certain genes (BRCA1, BRCA2, and others) make women more susceptible to breast cancer. In families in which many women have had the disease, gene testing can show whether a woman has specific genetic changes known to increase her susceptibility to breast cancer. Doctors may suggest ways to try to delay or prevent breast cancer, or improve the detection of breast cancer in women who have the genetic alterations.

Family history: A woman's risk for developing breast cancer increases if her mother, sister, daughter, or two or more other close relatives, such as cousins, have a history of breast cancer, especially at a young age.

Certain breast changes: Having a diagnosis of atypical hyperplasia or lobular carcinoma in situ (LCIS) or having had two or more breast biopsies for other benign conditions may increase a woman's risk for developing cancer.

Other factors associated with an increased risk for breast cancer include:

Breast density: Women age 45 and older, whose mammograms show at least seventy-five percent dense tissue, are at increased risk. Dense breasts contain many glands and ligaments, which make breast tumors difficult to "see," and the dense tissue itself is associated with an increased chance of developing breast cancer.

Radiation therapy: Women whose breasts were exposed to radiation during their childhood, especially those who were treated with radiation for Hodgkin's disease, are at an increased risk for developing breast cancer throughout their lives. Studies show that the younger a woman was when she received her treatment, the higher her risk for developing breast cancer later in life.

Late childbearing: Women who had their first child after the age of 30 have a greater chance of developing breast cancer than women who had their children at a younger age.

Also at a somewhat increased risk for developing breast cancer are women who started menstruating at an early age (before age 12), experienced menopause late (after age 55), never had children, or took hormone replacement therapy or birth control pills for long periods of time. Each of these factors increases the amount of time a woman's body is exposed to estrogen. The longer this exposure, the more likely she is to develop breast cancer.

In most cases, doctors cannot explain why a woman develops breast cancer. Studies show that most women who develop breast cancer have none of the risk factors listed above, other than the risk that comes with growing older. Also, most women with known risk factors do not get breast cancer. Scientists are conducting research into the causes of breast cancer to learn more about risk factors and ways of preventing this disease.

It’s critical to know, however, that survival rates increase dramatically when breast cancer is detected and treated early. Regular self breast exams every month, and mammograms every one to two years after age 40 will help with early detection.