Why gut cancers metastasise to the liver
The portal vein (the vein carrying gut blood to the liver) often delivers cells shed from gut cancers to the liver first, where they can grow as metastases (secondary tumours).
Remember
Most blood from the abdominal digestive tract reaches the liver first through the portal vein, so gut cancers commonly seed the liver, and a deposit from a bowel cancer remains bowel cancer, not a new liver cancer.
Most blood from the digestive tract does not go straight back to the heart. It travels first in the portal vein (the large vein that collects blood from the stomach, intestines, pancreas and spleen and carries it to the liver), so the liver meets it before the rest of the body does.
Cancer cells that break away from a tumour in the stomach, bowel or pancreas can enter the portal system and be carried to the liver. There they can lodge in the sinusoids (the blood-filled channels between liver cells) and grow into metastases, which are secondary tumours: cancer that has spread from its original site to another organ. Gut blood passes through the liver before reaching the general circulation, so this is a first-pass route.
That is why, in the UK, liver metastases are far more common than primary liver cancer, which is usually hepatocellular carcinoma (cancer of the liver cells) and is seen particularly with chronic liver disease and cirrhosis (scarring of the liver). A metastasis from a bowel cancer is still bowel cancer, not a new liver cancer.
Do not confuse the vessels. The portal vein brings blood into the liver, whereas the hepatic veins carry it out into the inferior vena cava. About 75% of the liver's blood comes from the portal vein and about 25% from the hepatic artery.
One exception is that some drainage, through the middle and inferior rectal veins, enters the systemic circulation and bypasses the liver, so rectal cancer can also spread to the lungs.
Key facts
- Route
- Most blood from the abdominal digestive tract reaches the liver through the portal circulation.
- Frequency
- Liver metastases are far more common than primary liver cancer in the UK.
- Blood supply
- The liver receives about 75% of its blood from the portal vein and about 25% from the hepatic artery.
- Vessels
- The portal vein brings blood into the liver, whereas the hepatic veins carry blood out of it.
- Exception
- Drainage through the middle and inferior rectal veins goes into the systemic circulation and bypasses the liver.
- Identity
- A deposit from a bowel cancer is still bowel cancer, not a new liver cancer.
Exam traps
- The hepatic veins do not bring gut blood into the liver, because they carry blood out of the liver into the inferior vena cava; the portal vein is the vessel that brings it in.
- The hepatic artery does not carry tumour cells shed from the bowel to the liver, because those cells enter the veins that drain the gut and reach the liver through the portal vein; the hepatic artery supplies only about a quarter of the liver's blood.
- Fast liver cell turnover does not explain how gut cancer cells reach the liver, because hepatocytes (liver cells) are normally quiescent, meaning resting, and the route depends on blood flow.
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