Research into mechanisms of hematogenous metastasis has largely become genetic in focus, attempting to understand the molecular basis of `seed-soil relationships. Preceeding this biological mechanism is the physical process of dissemination of circul
ating tumour cells (CTCs). We utilize a `filter-flow paradigm to show that assumptions about CTC dynamics strongly affect metastatic efficiency: without data on CTC dynamics, any attempt to predict metastatic spread in individual patients is impossible.
