Dormancy, Quiescence, and Cellular Senescence

Abstract

Dormant tumors are defined as microscopic (diameter of ∼1 mm) human cancers, either primary, recurrent or metastatic, and can remain in an asymptomatic, non-detectable, and occult form for a long period of time. In other words, this phenomenon of long-term persistence of cancer cells that do not grow is called tumor dormancy (hibernation). Patients can carry cancer cells for a very long time, in some cases indefinitely without relapsing. Based on tumor type and stage, the dormancy period may range from years to even decades between the initial therapy and the occurrence of relapsed tumors or recurrent metastatic disease. As seen in autopsies of adults who have died from non-cancer causes, tumors can be dormant as long as a lifetime without ever becoming clinically evident. Dormant tumors represent the earliest stages in tumor development and are highly prevalent in humans. This phenomenon occurs early in tumor development, indicating that tumor growth is not continuous, and may pass through a long period of subclinical equilibrium. Dormant tumors have been identified in many organs, including thyroid, breast, and prostate, and are being described as cancer without disease. Therefore, clinical relevancy of tumor dormancy is important.