Pathogenesis and biology.

Metastasis to the brain is an increasing complication of solid cancers. Fortunately, our understanding of its pathogenesis has greatly increased in the last decade, with crucial insights into the molecular and cellular determinants of successful brain colonization; some aspects remain less well understood. The latter include the exact features of brain metastasis-initiating cancer cells, and a potential premetastatic niche. It is clear that a brain-arrested cancer cell has to master a sequence of steps to eventually grow to a clinically relevant brain metastasis. Various brain-specific cell types and molecular niches promote or hinder brain colonization in a dynamic and reciprocal manner. After mandatory extravasation and colonization of a brain-specific perivascular niche, the cancer cell can stay dormant, or further grow by dynamic interactions with cerebral blood vessels. In addition, the activation of certain molecular pathways on site of the cancer cell which are related to growth, motility, survival, and adaptation to the brain environment appears also important, given their characteristic modification in brain metastases of patients. A deeper understanding of the most vulnerable steps of the brain metastatic cascade may foster the development of novel preventive approaches, and that of core biologic mechanisms for macrometastatic growth and persistence will help to develop better therapeutics.