Tumor dormancy of primary and secondary cancers.

Tumor dormancy is a phenomenon whereby cancer cells persist below the threshold of diagnostic detection for months to decades. This condition may arise due to either cell cycle arrest or a dynamic equilibrium state in which cell proliferation is in balance with cells undergoing apoptosis. Tumor dormancy is usually a reference to occult cancer cells that persist for an extended period of time after treatment, but primary cancers can also exhibit extended growth plateaus below the limits of detection. For example, autopsies of individuals who died of trauma reveal that most individuals harbor microscopic primary cancers. Mechanisms that operate independently or successively may restrict tumor expansion throughout tumor progression from incipiency to late-stage cancer. Proposed mechanisms include cell cycle withdrawal, immune surveillance, and blocked angiogenesis. The precise mechanisms underlying dormancy remain to be established, and relevant models will have an important impact on diagnostic and therapeutic strategies for treating cancer. This review summarizes the phenomenon of tumor dormancy, experimental models, and potential mechanisms.