Cancer can spread through the body when cells break off from a tumor and try to metastasize to other organs. These circulating tumor cells are now considered an important biomarker in determining the progression of cancer, and even the success of therapy, says KFC at the Physics arXiv Blog. Unfortunately, however, they're really hard to find. "In a single milliliter of blood there are usually several billion red blood cells, several million white blood cells but fewer than ten circulating tumor cells," KFC says. "And there is the only one way to find them.