For a cell to move out of a tissue and enter the blood stream, in the very least case, tissue permeability is a necessary condition. That means the surround cells “drop hands” in a sense (breach of cell-cell interactions) and move a little as to create a space big enough for the cells that wants to get past, to squeeze through. This has been extensively studied in the processes of inflammation.
“Well hey”, you might say, “there is often inflammation associated with cancer so maybe it takes advantage of conditions already present”.
Not all cancers have associated inflammation but also metastasize. But here we see deception. When it comes to cancer cells they no longer talk about permeability but say that the cancer cells breach either the basement membrane or the cell-cell interactions. However if you look closer at what they are saying it is none other than tissue permability, just said in a different way. And it requires the FULL cooperation of immune system cells.
On entering the bloodstream the cancer cells are accompanied by other cells, from the tumour site, which are normal immune system cells… Ah I forgot to mention a tumor is not just “a mass of cells”. Far from it.