Tumours can also move away from their original tissue by invading blood vessels or lymphatic ducts and being carried to distant sites. This process is called metastasis.
Tumour cells that are carried through lymphatics will usually metastasise to local lymph nodes; this is the reason that surgeons may remove lymph nodes as well as the original tumour, to treat a cancer.
Tumours that metastasise via the bloodstream must first invade a blood vessel at the initial tumour site, and then exit the blood vessels in a different organ to establish a new tumour site. Such an event is relatively rare for any individual tumour cell; nevertheless, metastasis accounts for 90 per cent of cancer-related deaths, so identification of metastatic tumours is important both for prognosis and treatment.
Pathologists recognise metastatic tumours, because the affected organ contains clumps of cells that are completely uncharacteristic. In some cases, the primary tumour type can be recognised because it has retained some distinctive characteristics of the original cell type. However, the original identity of tumour cells is not always self-evident and this is particularly true of metastatic tumours.
Therefore, it may be possible to observe a metastatic tumour in a tissue, but be impossible to identify the primary cell type and the original site of the tumour, at least by H&E staining. In this case additional staining, particularly immunohistochemistry, is valuable to identify the original cell type, because it can provide an important guide for patient scanning, further surgery, radiotherapy and drug treatment.