Unusually long latency periods between the treatment of primary tumors and metastatic recurrences are commonly thought to prove the existence and relevance of clinical tumor dormancy. However, careful consideration of disease courses and cancer growth rates leads to the conclusion that clinical dormancy may be everything from non-existent to much more frequent than originally thought. On the other hand, cellular dormancy defined as a non-productive state of disseminated tumor cells is very frequent, while homeostatic mechanisms such as angiogenic and immunological control contribute to the chronicity of cancer. This review attempts to provide a conceptual framework for the study of dormancy, which may guide clinical and experimental research.
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