Metastatic breast cancer is a stage of breast cancer where the disease has spread to distant metastases. It is a complication of primary breast cancer, usually occurring several years after resection of the primary breast cancer. Metastatic breast cancer cells frequently differ from the preceding primary breast cancer in properties such as receptor status, have often developed resistance to several lines of previous treatment and acquired special properties that permit them metastasize to distant sites, making them especially dangerous. The prognosis is often poor, distant metastases are the cause of about 90% of deaths due to breast cancer.
Breast cancer primarily metastasizes to the bone, lungs, regional lymph nodes, liver and brain, with the most common site being the bone. Lymph node metastsasis into the sentinel node and few surrounding nodes is regarded as a treatable local event and not metastatic breast cancer, both when occurring at primary presentation or later.
Typical environmental barriers in a metastatic event include physical (a basement membrane), chemical (reactive oxygen species or ROS, hypoxia and low pH) and biological (immune surveillance, inhibitory cytokines and regulatory extra-cellular matrix (ECM) peptides) components. Organ-specific anatomic considerations also influence metastasis; these include blood-flow patterns from the primary tumor and the homing ability of cancer cells to certain tissues. The targeting by cancer cells of specific organs is probably regulated by chemo-attractant factors and adhesion molecules produced by the target organ, along with cell-surface receptors expressed by the tumor cells.