Metastatic breast cancer is a point where a cancerous (malignant) tumor in the breast has spread past its origin into other tissues in the body.

Metastasis is the term used for the spreading of a cancer. Cancer cells break away from the original tumor and enter the lymphatic system (lymph nodes) or blood stream and spread to other parts of the body, forming metastatic tumors.

X-rays and other types of tests are used to check for the extent of metastatic breast cancer. The most common sites for metastatic breast cancer are:

  • bones
  • lungs
  • liver
  • brain

No matter where the cancer spreads, the cancer's origin determines its type. For example, if metastatic breast cancer spreads to the lungs, it is still considered breast cancer, not lung cancer. Early diagnosis and treatment prevents cancer from spreading further.

In the typical stages of cancer, metastasis would begin at Stage II, when the cancer would just have begun to spread to the lymph nodes. By Stage IV, the cancer would be affecting organs far away from the breast.

In diagnosis, metastatic breast cancer is designated by the following:

  • MO: No cancer metastasis detected.
  • cMO(i+): Special tests discover small amounts of cancer cells in the blood or bone marrow or the cancer has spread to the lymph nodes.
  • M1: Metastatic breast cancer has spread to distant organs.

Visit the Breast Cancer Learning Center to learn more.