ETH Zurich Jan 24 2025 Certain tumor types do not remain at their point of origin but spread throughout the body and form metastases. This is because the primary tumor continuously releases cancer cells into the blood. These circulating tumor cells (CTCs) can join together into small clusters of up to a dozen cells and settle in other organs. There, the clusters grow into larger tumors, known as metastases. Metastatic tumors are still a major medical problem: every year, around seven million