Breast cancer is a malignant tumor that develops from breast cells. Most start in glandular tissues. Breast cancer commonly begins as "ductal carcinoma in situ', or DCIS. That's when cancer cells have formed inside the breast ducts (milk passages connecting milk-producing glands to the nipples) but haven't spread to the surrounding fatty tissue.

Once the cancer has broken through the duct wall and into the surrounding fatty tissue, the disease can "metastasize", or spread, to other parts of the body via the lymphatic system or the bloodstream.

Lymphatic vessels are like veins, but instead of flowing with blood, they carry tissue waste products and immune system cells away from the infected areas of the body. In the breasts, these lymphatic vessels lead to lymph nodes in the underarm area, where lumps can form.

Advice From the American Cancer Society

Mammography. When it comes to breast cancer, the American Cancer Society says its "most important recommendation" is that women age 40 and over get a mammogram annually.

"Regular mammograms save lives," a spokesperson for the society says. "Mammography is safe and effective. It's the best way to find breast cancers early, when they're most treatable."

Clinical Breast Examination. Get a clinical breast examination by a doctor or nurse every year if you are 40 or over. For the best results, schedule a clinical breast examination close to the time of your mammogram. If your are between 20 and 39, get a clinical breast examination every three years.

Breast Self-Examination. All women 20 and over should perform breast self-examination every month. If you have questions about the correct way to perform a self-exam, ask your doctor or nurse or contact the American Cancer Society.

By examining your breasts regularly, you and your doctor can be aware of changes in your breasts that are not normal. Most lumps are not cancer, but all lumps should be checked out by a doctor. If you notice any change in your breast, call your doctor immediately. Even after you begin having regular mammograms, clinical breast examinations and self examinations are important because mammography may miss a small number of cancers.

Breast Cancer Resources

The American Cancer Society runs a 24-hour hotline staffed by oncology nurses. Call (800) ACS-2345.

The American Cancer Society also offers two key books that answer questions about breast cancer: "Women and Cancer" by Carolyn D. Runowicz, M.D., Jeanne A. Petrek, M.D., and Ted S. Gansler, M.D., and "Informed Decisions: The Complete Book of Cancer Diagnosis, Treatment and Recovery" by Gerald P. Murphy, M.D., Lois B. Morris and Dianne Lange. The books can be obtained by calling (888) ACS-5552.

The American Cancer Society's Web site is: cancer.org.

The National Cancer Institute offers referrals, pamphlets and other forms of information. Call (800) 422-6237.