Pre-metastatic niche should be known to better understand that the "soil" is already established even before cancer cells arrive at the metastatic foci.  Inflammatory cytokines such as TNF-alpha and IL-6 promote the formation of the pre-metastatic niche via the production of S100A8/9 and subsequently the recruitment of the tumor-entrained neutrophils and macrophages.  Recent research also emphasizes the importance of exosomes in thd formation of pre-metastatic niche.  

http://www.ncbi.nlm.nih.gov/pubmed/22635005