A small tumor no larger than 1 millimeter in diameter can sustain itself in such a manner indefinitely. But it cannot grow any larger unless the tumor itself begins to generate a network of blood vessels to help supply additional nutrients. A clinically detectable tumor is about 1 gram in weight and made up of at least 1 billion cells.
Metastatic tumors form when large progressive tumors shed tumor cells.
These tumor cells must be able to grow and function apart from the primary tumor. Metastatic tumor cells move throughout the body, usually through the circulatory system or the lymphatic system . Metastatic tumor cells often lodge in a blood capillary , where they may or may not grow. The tumor cells that actually grow are somehow more suited to the new location. Metastatic tumor cells tend to mutate more quickly than normal cells, giving them a greater ability to adapt to their environment as well as a greater ability to resist therapy.