Figure 1. Schematic representation of tumor progression.

Primary carcinomas are induced through multiple mutations and the outgrowth of malignant cells in situ. Upon epithelial-to-mesenchymal transition (EMT) cells acquire migratory and invasive traits, detach from primary tumors and locally invade surrounding tissue. In a next step, locally invading cells gain access to blood or lymph vessels through intravasation and become circulating tumor cells (CTCs). After extravasation, CTCs settle in novel organs/sites and are termed disseminated tumor cells (DTCs), which can give rise to micro- and macrometastases in various organs.