Most cancer deaths are not caused by growth of the primary tumour, but rather result from its invasive spread to secondary sites and the resulting formation of metastasis (see figure).
Malignant tumour cells can
(1) detach from the primary tumour and
(2) migrate into neighbouring tissues as well as
(3) blood or lymph vessels.
(4) They are swept to distant sites,
(5) where they attach to the vessel wall,
(6) exit the vessel,
(7) migrate into the surrounding tissue and
(8) establish new tumours.
(9) New blood vessels are formed (angiogenesis) to support the growth of the tumour (metastases).
In many cases, these metastases ultimately lead to the death of the patient.