Cancer begins when healthy cells in the breast change and grow uncontrollably, forming a mass or sheet of cells called a tumor. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body. A benign tumor means the tumor can grow but will not spread.

Breast cancer spreads when the cancer grows into other parts of the body or when breast cancer cells move to other parts of the body through the blood vessels and/or lymph vessels. This is called metastasis. Breast cancer most commonly spreads to the regional lymph nodes. The regional lymph nodes are located under the arm, in the neck, under the chest bone, or just above the collarbone. When the cancer spreads further through the body, it most commonly spreads to the bones, lungs, and liver. Less often, breast cancer may spread to the brain. If cancer comes back after initial treatment, it can recur locally, meaning in the breast and/or regional lymph nodes. It can also recur elsewhere in the body, called distant metastases.