What is Invasion and metastasis?

To metastasise, a carcinoma cell must leave the epithelium — lose its cell junctions and polarity and acquire motility, an epithelial–mesenchymal transition driven by transcription factors normally used in embryonic development — degrade the basement membrane and matrix with secreted proteases, cross the wall of a blood or lymph vessel, survive in the circulation (fewer than one in ten thousand do, killed by shear, by the absence of attachment, and by natural killer cells), lodge in a capillary bed of another organ, cross out, and grow there. Where it grows is not random: breast cancer goes to bone, lung, liver and brain, colon cancer to the liver, prostate cancer to bone — Paget’s seed and soil of 1889, explained partly by blood flow and partly by the fit between the cell and the tissue’s signals. A disseminated cell may lie dormant for years before it grows, which is why cancers recur a decade after apparent cure. Metastasis causes nine deaths in ten from solid tumours, and it is the step least understood.