In breast cancer, some of the cells in your breast begin growing abnormally. These cells divide more rapidly than healthy cells do and may spread through your breast, to the lymph or to other parts of your body (metastasize). The most common type of breast cancer begins in the milk-producing ducts, but cancer may also occur in the lobules or in other breast tissue.

In most cases, it isn't clear what triggers abnormal cell growth in breast tissue, but doctors do know 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 both breast and ovarian cancer. Inherited mutations in the ataxia-telangiectasia mutation gene, the cell-cycle checkpoint kinase 2 (CHEK-2) gene and the p53 tumor suppressor gene also make it more likely that you'll develop breast cancer

Yet most genetic mutations related to breast cancer aren't inherited but instead develop during your lifetime. These acquired mutations may result from radiation exposure — women treated with chest radiation therapy in childhood, for instance, have a significantly higher incidence of breast cancer than do women not exposed to radiation. Mutations may also develop as a result of exposure to cancer-causing chemicals, such as the polycyclic aromatic hydrocarbons found in tobacco and charred red meats.

A risk factor is anything that makes it more likely you'll get a particular disease. Yet all risk factors aren't created equal. Some, such as your age, sex, and family history can't be changed, whereas others, including smoking and a poor diet are personal choices over which you have some control.

But having one or even several risk factors doesn't necessarily mean you'll become sick — most women with breast cancer have no known risk factors other than simply being women. In fact, being female is the single greatest risk factor for breast cancer. Although men can develop the disease, it's 100 times more common in women.

• Age
• Personal history of breast cancer.
• Family history.
• Certain breast changes
• Genetic alterations
  

• Estrogen.
• Late childbearing.
• Breast density.
• Radiation therapy
• Alcohol

• 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.
• Breast self-examination (BSE)
  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.
• Clinical breast exam
  Unless you have a family history of cancer or other factors that place you at high risk, the American Cancer Society recommends having clinical breast exams once every three years until age 40. After that, the American Cancer Society recommends having a yearly clinical exam.
  During this exam, your doctor examines your breasts for lumps or other changes. He or she may be able to feel lumps you miss when you examine your own breasts and will also look for enlarged lymph nodes in your armpit (axilla).
• Mammogram
  A mammogram, which uses a series of X-rays to show images of your breast tissue, is currently the best imaging technique for detecting tumors before you or your doctor can feel them. For that reason, the American Cancer Society has long recommended screening mammography for all women over 40.
  Yet mammograms aren't perfect. A certain percentage of breast cancers — sometimes even lumps you can feel — don't show up on X-rays (false-negative result). The rate is higher for women in their 40s. That's because women of this age and younger tend to have denser breasts, making it more difficult to distinguish abnormal from normal tissue.
• Ultrasound
  Often, your doctor will suggest a less invasive procedure, such as ultrasound, before deciding on a biopsy. Ultrasound is a procedure that uses sound waves to create an image of your breast on a computer screen. By analyzing this image, your doctor may be able to tell whether a lump is a cyst or a solid mass. Cysts, which are sacs of fluid, usually aren't cancerous, although you may want to have a painful cyst drained with a needle.
• Biopsy -In some cases, your doctor may want to remove a small sample of tissue (biopsy) for analysis in the laboratory. Biopsies can provide important information about an unusual breast change and help determine whether surgery is needed and if so, the type of surgery required. To obtain a tissue sample, your doctor may use one of the following procedures:
•  Fine-needle aspiration biopsy.
•  Core needle biopsy.
• Stereotactic biopsy.
• Wire localization.
• Surgical biopsy.
• Estrogen and progesterone receptor tests
• Staging tests
• Genetic testing