Breast cancer is cancer that forms in the cells of the breast. It occurs when mutated breast cells grow and divide rapidly. These cells eventually form a tumor that is cancerous, or malignant. In some cases, malignant tumors can invade nearby healthy tissue. Cancer can also spread to lymph nodes under the armpit, above the collarbone, and in the chest. In addition, invasive cancer can spread to other parts of the body.

A healthy breast consists of glandular tissue called lobes, sectioned off into lobules, which produce milk carried to the nipple by small ducts, which are like pipelines. Fatty and connective tissue as well as blood and lymph vessels surround all the glandular tissue. Most breast cancer starts in the lining of the ducts and the lobules.

While breast cancer is the most common cancer among women in the United States, in rare cases, it can also occur in men.