In a healthy body, natural systems control the creation, growth and death of cells.
Cancer occurs when these systems do not work right and cells do not die at the normal rate. There is more cell growth than cell death.
This excess growth can form a tumor.
Breast cancer occurs when cells in the breast divide and grow without their normal control.
Tumors in the breast tend to grow slowly. By the time a lump is large enough to feel, it may have been growing for as long as 10 years. (Some tumors are aggressive and grow much faster.)
Between 50-75 percent of breast cancers begin in the milk ducts, about 10-15 percent begin in the lobules and a few begin in other breast tissues .
Learn more about breast anatomy.
When abnormal cells grow inside the milk ducts, but have not spread to nearby tissue or beyond, this is called ductal carcinoma in situ (DUK-tul kar-sin-O-ma in SY-too or DCIS).
The term "in situ" means "in place." With DCIS, the abnormal cells are still inside the ducts. DCIS is a non-invasive breast cancer (you may also hear the term “pre-invasive breast carcinoma”).
Although DCIS is non-invasive, without treatment, it can develop into invasive breast cancer.
Learn more about DCIS and the risk of invasive breast cancer.
Learn about treatment for DCIS.
Invasive breast cancer occurs when abnormal cells from inside the milk ducts or lobules break out into nearby breast tissue.
Cancer cells can travel from the breast to other parts of the body through the blood stream or the lymphatic system. They may travel early in the process when a tumor is small or later when a tumor is large.
The lymph nodes in the underarm area (axillary lymph nodes) are the first place breast cancer is likely to spread.
In advanced stages, breast cancer cells may spread to other parts of the body such as the liver, lungs, bones and brain (a process called metastasis). There, the breast cancer cells may again begin to divide too quickly and make new tumors. This is called metastatic breast cancer (learn more).
Breast cancer in men is rare, but it does happen.
Learn more about breast cancer in men.
Most cancers that occur in the breast are breast cancers (breast carcinomas).
In rare cases, other types of cancer such as lymphomas (cancer of the lymph system) and sarcomas (cancer of the soft tissues) can occur in the breast. And, in rare cases, cancers from other parts of the body can metastasize to the breast and mimic breast cancers.
Other types of tumors in the breast can be benign (not cancer) or cancerous. Because these cancers are not carcinomas, treatment is different than treatment for breast cancer.
For more information on other cancers that can occur in the breast, such as lymphomas and phyllodes tumors, visit the National Cancer Institute's website.
*American Cancer Society. Cancer Facts and Figures 2016. Atlanta, GA: American Cancer Society, 2016.
Facts for Life: What is Breast Cancer
Breast Cancer 101
Ductal Carcinoma in Situ
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