Metastatic breast cancer occurs when cancer cells break away from the original tumor and spread to other parts of the body through the lymph nodes or bloodstream. The most common sites of metastasis from breast cancer are the bones, lungs, brain, and liver. A metastatic tumor is made up of breast cancer cells, not cells from the organ they spread to. For example, a bone metastasis would be made up of breast cancer cells, not bone cells. Breast cancer most often spreads from the lining of the milk ducts in the breast.