Metastases form when circulating tumor cells (CTCs) from a primary tumor enter distal tissues and form secondary, aggressive tumors17. CTCs seed new and maintain existing tumors2, 18, 19.  CTCs can be identified by the presence of epithelial cell adhesion molecule (EpCAM) 3, 4, 5.  The metastatic process begins when cancer cells enter the circulatory system through newly formed, imperfect and leaky blood vessels of the tumor (Figure 2).  CTCs travel through the circulatory system and, if they escape the immune system, can invade tissues to form micro metastases that can develop into secondary, metastatic tumors.  A large number of studies has demonstrated (20-,21,22,23,24,25,26,27,28,29) that

  • Poor outcome after surgery and chemotherapy is observed when CTCs remain in the body
  • The number of CTCs in the blood is highly correlated with disease progression
  • Blood vessels and the lymphatic system act as conduits to transport malignant circulating tumor cells19