Cancer progression is usually thought of as a process involving the growth of a primary tumor followed by metastasis, in which cancer cells leave the primary tumor and spread to distant organs. A newly released study by scientists at Memorial Sloan-Kettering Cancer Center shows that circulating tumor cells cancer cells that break away from a primary tumor and disseminate to other areas of the body can also return to and grow in their tumor of origin, a newly discovered process called "self-seeding."