Sometimes breast cancer cells spread to one or both lungs through the blood or lymph system. It’s not uncommon for the lymph nodes of the chest (mediastinum) to be affected. The cells that have spread to the lungs are breast cancer cells. It is not the same as having cancer that starts in the lungs (a lung cancer). When breast cancer spreads to the lungs it can be treated but it can't be cured. The lungs sit on either side of the chest and are protected by the rib cage.