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From MayoClinic.com

Causes

Each of your breasts contains 15 to 20 lobes of glandular tissue, arranged like the petals of a daisy. The lobes are further divided into smaller lobules that produce milk during pregnancy and breast-feeding. Small ducts conduct the milk to a reservoir that lies just beneath your nipple. Supporting this network is a deeper layer of connective tissue called stroma.

The spaces between the lobes and ducts are filled with fat, which makes up about 80 to 85 percent of your breast during your reproductive years. Your breasts also contain vessels that transport lymph - a colorless fluid that carries waste products and cells of the immune system - to lymph nodes located primarily under your arm (axillary nodes) but also above your collarbone and in your chest. These nodes are collections of immune system cells that filter harmful bacteria and play a key role in fighting infection.

Cancer affects your cells, the basic units of life. Normally, cells grow and divide in an orderly way. But sometimes this growth gets out of control - cells continue dividing even when new cells aren't needed. These extra cells may form a mass of tissue called a tumor.

Tumors may be either noncancerous (benign) or cancerous (malignant). Cells from malignant tumors can invade and damage nearby tissues and organs. They may also travel through your bloodstream or lymph system to other parts of your body.

In most cases, it isn't clear what triggers abnormal cell growth in breast tissue. It is known that between 5 percent and 10 percent of breast cancers are inherited. Defects in one of two genes, breast cancer gene 1 (BRCA1) or breast cancer gene 2 (BRCA2), put you at greater risk of developing the disease. In fact, women who have mutations in these genes have a much higher chance of developing breast cancer and a higher chance of developing ovarian cancer. Both men and women can inherit these genes from either parent.

Although the discovery of these genes is important, it's only the first step. Breast cancer is a complex disease that eventually may prove to have a number of causes.

• Ovarian cancer

The American Cancer Society estimates that 75 percent of breast cancer cases occur in women with no known risk factors. At the same time, having one or even several risk factors doesn't mean you'll develop the disease. The following factors may increase your risk of breast cancer:

• Sex. Being a woman is your greatest risk factor. Although men can develop breast cancer, it's 100 times more common in women.
• Age. Your chances of developing breast cancer increase as you get older. The disease rarely affects women under 25 years of age, whereas close to 80 percent of breast cancers occur in women over age 50. At age 40, you have a one in 252 chance of developing breast cancer. By age 85, your chance is one in eight.
• A personal history of breast cancer. If you've had breast cancer in one breast, you have an increased risk of developing cancer in the other breast.
• Family history. Women who have a mother or sister with breast cancer have a greater chance of developing breast cancer themselves. In general, the more relatives you have with breast cancer who were premenopausal at the time of diagnosis, the higher your own risk. If you have one close relative with breast cancer, your risk is doubled. If you have two or more relatives, your risk increases even more.
• Genetic predisposition. Between 5 percent and 10 percent of breast cancers are inherited. Defects in one of several genes, especially BRCA1 or BRCA2, put you at greater risk of developing the disease. Usually these genes help prevent cancer by making proteins that keep cells from growing abnormally. But if they have a mutation, the genes aren't as effective at protecting you from cancer. Women who are of Ashkenazi (Eastern and Central European Jewish) ancestry are especially at risk.
• Excess weight. The relationship between excess weight and breast cancer is complex. In general, weighing more than is healthy for your age and height increases your risk if you've gained the weight as an adult and especially after menopause. The risk is even greater if you have more body fat in the upper part of your body. Although women usually have more fat in their thighs and buttocks, they tend to gain weight in their abdomens starting in their 30s, which can increase their risk.
• Exposure to estrogen. The longer you're exposed to estrogen, the greater your breast cancer risk. In general, if you have a late menopause (after age 55) or you began menstruating before age 12, you have a higher risk of developing breast cancer. The same is true for women who never had children, or whose first pregnancy occurred when they were age 35 or older.
• Race. Caucasian women are more likely to develop breast cancer than are black or Hispanic women. Black women, however, are more likely to die of the disease because they tend to be diagnosed at a later stage than are white women. But socioeconomic factors, rather than race, may account for the difference in mortality. A study of more than 5,000 Detroit-area women published in the Journal of the National Cancer Institute found that women of all races with incomes below the poverty level were more likely to be diagnosed with late-stage breast cancer and three times more likely to die of the disease than were women with higher incomes. The study's authors concluded that low-income women weren't receiving the routine medical care that would allow breast cancer to be discovered earlier.
• Hormone replacement therapy (HRT). In July 2002, a study sponsored by the National Institutes of Health (NIH) was halted as researchers reported that HRT, once considered standard treatment for menopausal symptoms, actually posed more health benefits than risks. Among those was a slightly higher risk of breast cancer for women taking the particular combination of HRT - estrogen plus progestin - used in the study. In addition, combination hormone therapy can make malignant tumors harder to detect on mammograms, leading to cancers that are diagnosed at more advanced stages, when they're harder to treat. Because combination HRT can result in serious side effects and health risks, work with your doctor to evaluate the options and decide what's best for you.
• Birth control pills. Because of the recent information on HRT, many younger women are concerned about the relationship between birth control pills and breast cancer. Unfortunately, there's no clear answer. A large study of women between the ages of 35 and 64 published in June 2002 in the New England Journal of Medicine concluded that current or former use of oral contraceptives didn't increase the risk of breast cancer. But the American Cancer Society says that women currently using the pill may have a slightly increased risk, whereas women who stopped using oral contraceptives 10 years ago probably don't have such a risk. For the latest information on the pill and breast cancer, talk to your doctor.
• Smoking. A study published in April 2001 found that smoking significantly increases the risk of breast cancer in women with a strong family history of breast and ovarian cancers.
• Exposure to certain carcinogens. Polycyclic aromatic hydrocarbons (PAHs) are chemicals found mainly in cigarette smoke and charred red meat. Studies have shown that exposure to these chemicals can significantly increase your chances of developing breast cancer. Exposure to certain pesticides also may increase your risk, but more research needs to be done to establish a clear link.
• Excessive use of alcohol. Women who consume more than one alcoholic drink a day have a 20 percent greater risk of breast cancer than do women who don't drink. The National Cancer Institute recommends limiting alcohol intake to no more than one drink daily.
• Unusual sleep patterns. You may have an increased risk of breast cancer if you work the graveyard shift or are up often during the night. The risk seems to be greatest if you don't sleep between 1 a.m. and 2 a.m., when levels of melatonin - a sleep-regulating hormone - are highest. Women who reported missing sleep during this period at least three nights a week had a 40 percent increased risk of developing breast cancer. Women who worked nights fared worse, with a 60 percent increased risk. Researchers speculate that suppression of melatonin by exposure to light may increase the release of estrogen by the ovaries.

• Alcohol addiction
• menopause
• Nicotine dependence

When to seek medical advice

Although most breast changes aren't cancerous, it's important to have them evaluated promptly. If a problem exists, you can have it identified and treated as soon as possible. See your doctor if you discover a lump or any of the other warning signs of breast cancer. And if you've been treated for breast cancer, report any new signs or symptoms immediately. These include a new lump in your breast or an ache or pain - especially in a bone - that doesn't go away after three weeks. In addition, talk to your doctor about developing a breast-screening program that's right for you.

Screening and diagnosis

Screening - looking for evidence of disease before symptoms appear - is the key to finding breast cancer in its early, treatable stages. Depending on your age and risk factors, screening may include breast self-examination, examination by your nurse or doctor (clinical breast exam), mammograms (mammography) or other tests.

For years, women have been advised to examine their breasts on a monthly basis starting around age 20. The hope was that by becoming proficient at breast self-examination and familiar with the usual appearance and feel of their breasts, women would be able to detect early signs of cancer.

But some studies have shown that teaching women to perform breast self-exams may not accomplish this goal. A large, randomized clinical study in Shanghai, China, for example, concluded that breast self-exams don't actually reduce the number of deaths from breast cancer. In addition, the study found that women who perform regular breast self-exams may be more likely to undergo unnecessary biopsies after finding breast lumps. This was one of the primary reasons that in May 2003 the American Cancer Society changed its recommendations on breast self-examination, stating that the procedure should be considered an option, rather than a requirement, for most women.

The new guidelines emphasize breast health awareness instead of a strict series of monthly self-exams. Although the guidelines don't say you shouldn't perform the exams, the importance of self-exams has been replaced by a general need to become more familiar with your breasts. If you'd like to continue performing breast self-exams, ask your doctor to review your technique.

To check for breast changes, do a self-exam once a month. Use your eyes and hands to search for lumps, thickened areas, swelling and skin changes. Move your fingertips in a circular motion, going .

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