Cancer progression and the invisible phase of metastatic colonization

Metastatic dissemination occurs very early in the malignant progression of a cancer but the clinical manifestation of metastases often takes years. In recent decades, 5-year survival of patients with many solid cancers has increased due to earlier detection, local disease control and adjuvant therapies. As a consequence, we are confronted with an increase in late relapses as more antiproliferative cancer therapies prolong disease courses, raising questions about how cancer cells survive, evolve or stop growing and finally expand during periods of clinical latency. I argue here that the understanding of early metastasis formation, particularly of the currently invisible phase of metastatic colonization, will be essential for the next stage in adjuvant therapy development that reliably prevents metachronous metastasis. Metastatic dissemination can occur early during cancer progression, yet clinically overt metastases are often not detected for many years after surgical removal of the primary tumour. In this Perspective, Klein argues that understanding the ‘invisible’ phase of metastatic colonization is necessary to explain this phenomenon and develop better therapies to prevent metastasis.