“Metastasis” is the term used to describe the spread of cancer from a primary tumor site to other areas of the body. Any type of cancer can metastasize, including breast cancer. When cancer begins in the breasts and spreads to other areas of the body, this is called metastatic breast cancer, or MBC.

The vast majority of people with MBC are women. The disease is very rare among men, though it does happen.

How does breast cancer metastasize?

Cancer spreads when cells break away from the primary tumor and move to other areas of the body. With MBC, this process typically begins with cancer cells from the tumor in the breast moving through the blood vessels or the axillary lymph nodes, which are located in the armpits. Lymph nodes are part of the body’s lymphatic system, a major component of the immune system that produces white blood cells and transports them throughout the body.

Cancer cells can form secondary tumors in the lymph nodes. They can also travel through the bloodstream or lymphatic system to other locations in the body, where they can continue to reproduce, forming additional secondary tumors. Although breast cancer cells may spread to any part of the body, the most common locations for metastases are the lungs, bones, liver and brain.

Because oncologists name cancer based on the primary site of origin, breast cancer that spreads to other areas of the body is still called breast cancer—just as cancer that begins in the lungs and spreads to another location in the body is still lung cancer, and prostate cancer that spreads to another location in the body is still prostate cancer.

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Diagnosing metastatic breast cancer

While metastatic breast cancer refers to any breast cancer that has spread to other locations in the body, the disease is somewhat different for every patient. A diagnosis must take into account a number of factors and variables about that specific type of cancer. These include information about the parts of the body the cancer has spread to, the size of the tumors and the biology of the cancer cells, including whether they use certain hormones or proteins to fuel growth.

These factors will determine a precise diagnosis of the MBC, including the cancer stage and the grade of cancer cells. Grading is a system used to describe how abnormal cancer cells look in comparison to normal cells.

These factors will also be used when deciding on a treatment plan. While there is no cure for MBC, there are numerous treatment options available, which are helping more patients live longer, better lives while managing the disease.