In a healthy body, natural systems control the creation, growth and death of cells. Cancer occurs when these systems don’t work right.
When cells don’t die at the normal rate, there’s 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 5-15 percent begin in the lobules and a few begin in other breast tissues .
Learn about breast anatomy.
Ductal carcinoma in situ (DCIS) is a non-invasive breast cancer.
With DCIS, the abnormal cells are contained in the milk ducts (canals that carry milk from the lobules to the nipple openings during breastfeeding). It’s called “in situ” (which means "in place") because the cells have not left the milk ducts to invade nearby breast tissue.
You may also hear the terms “pre-invasive” or “pre-cancerous” to describe DCIS.
Although DCIS is non-invasive, without treatment, it can develop into invasive breast cancer.
Learn 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.
If breast cancer spreads, the lymph nodes in the underarm area (axillary lymph nodes) are the first place it's likely to go.
Learn about treatment for invasive breast cancer.
Metastatic breast cancer (also called stage IV or advanced breast cancer) is invasive breast cancer that has spread beyond the breast and axillary lymph nodes to other parts of the body (most often the bones, lungs, liver or brain).
Metastatic breast cancer is not a specific type of breast cancer. It's the most advanced stage of breast cancer.
Learn about treatment for metastatic breast cancer.
Breast cancer can occur in men.
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 tumors in the breast can be benign (not cancer) or malignant (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, sarcomas and phyllodes tumors, visit the National Cancer Institute website.
*American Cancer Society. Cancer Facts and Figures 2020. Atlanta, GA: American Cancer Society, 2020.
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