It has been known for many years that cancer cells originating in a primary tumor can spread to another organ and form tiny, microscopic tumor masses (metastases) that can remain dormant for years. A likely explanation for this tumor dormancy is that no angiogenesis occurred, so the small tumor lacked the new blood vessels needed for continued growth.

One possible reason for tumor dormancy may be that some primary tumors secrete the inhibitor angiostatin into the bloodstream, which then circulates throughout the body and inhibits blood vessel growth at other sites. This could prevent microscopic metastases from growing into visible tumors.