Why is the liver such a common site for cancer metastasis?

Recall that most cancers spread haematogenously (i.e. via the blood stream); even the epithelial cancers that predominantly spread via the lymphatics can do so on occasion. Once you understand this, then it seems logical that the organs that receive the majority of the heart's blood tend to attract the majority of metastases (all things being equal).

The liver gets around 25% of the total cardiac output, via not one, but two sources:

• Arterial supply from the hepatic artery
• Venous supply from the portal vein

The latter drains blood from the gut, pancreas and spleen, and the former sends a whole chunk of the the blood from anywhere heading towards the liver.

The same factor is believed to be equally important in understanding why the lungs and brain are frequently afflicted by metastasis. Obviously, what makes a site ripe for metastasis isn't only the percentage of the cardiac output that passes through it, but it is certainly an important factor.