A risk factor is anything that affects your chance of getting a disease such as cancer. Different cancers have different risk factors. For example, exposing skin to strong sunlight is a risk factor for skin cancer. Smoking is a risk factor for cancers of the lung, larynx (voice box), colon, bladder, kidney, and many other organs.

But risk factors don't tell us everything. Having a risk factor, or even several risk factors, does not necessarily mean that a person will get the disease. And many people who get the disease may not have had any known risk factors.

Researchers have found several risk factors that make a person more likely to develop bile duct cancer.

Certain diseases of the liver or bile ducts

People who have chronic (long-standing) inflammation of the bile duct have an increased risk of developing bile duct cancer. Several conditions of the liver or bile duct may cause this.

• Primary sclerosing cholangitis is a condition in which inflammation of the bile duct (cholangitis) leads to the formation of scar tissue (sclerosis) and an increased risk of bile duct cancer. The cause of the inflammation is not usually known.

• Ulcerative colitis is a condition that results in inflammation of the large intestine. Some people with ulcerative colitis also develop inflammation in the bile duct, which puts them at increased risk for bile duct cancer.

• Bile duct stones, which are similar to, but much smaller than gallstones, can also cause inflammation that increases the risk of bile duct cancer.

• Choledochal cysts are bile-filled sacs that are connected to the bile duct. (Choledochal means having to do with the common bile duct.) The cells lining the sac often have areas of pre-cancerous changes, which increase a person's risk for developing bile duct cancer.

• Other abnormalities of the bile ducts: Some people have abnormalities where the bile duct and pancreatic duct normally meet that allow digestive juices from the pancreas to reflux (flow back "upstream") into the bile ducts. This backward flow also prevents the bile from being emptied through the bile ducts as quickly as normal. These people are at higher risk of bile duct cancer.

• Cirrhosis is damage to the liver due to irritants such as alcohol and diseases such as hepatitis that causes scar tissue to form. Some studies have found it raises the risk of bile duct cancer.

Other rare diseases of the liver and bile duct that may increase the risk of developing bile duct cancer include polycystic liver disease and Caroli syndrome (a dilation of the intrahepatic bile ducts that is present at birth).

Liver fluke infections: In some Asian countries, infection by liver flukes, which are food -- or water-borne parasite worms that invade the bile duct, is a major cause of bile duct cancer. There are several types of liver flukes. The ones most closely related to bile duct cancer risk are called Clonorchis sinensis and Opisthorchis viverrini.

Aging: While it can occur at younger ages, older people are more likely to get bile duct cancer. More than 2 out of 3 patients with bile duct cancer are older than age 65.

Obesity: Being overweight or obese can increase the risk of developing cancers of the gallbladder and bile ducts. This may be because obesity increases the risk of gallstones and bile duct stones. However, there may be other ways that being overweight can lead to bile duct cancers, such as changes in certain hormones.

Exposure to Thorotrast: A radioactive substance called Thorotrast (thorium dioxide), which was used as a contrast agent for x-rays until the 1950s, can lead to bile duct cancer, as well as to some types of liver cancer. For this reason, Thorotrast is no longer used.

Family history: Most bile duct cancers are not found in people with a family history of the disease. A history of bile duct cancer in the family seems to increase a person's chances of developing this cancer, but the risk is still low because this is a rare disease.

Other possible risk factors: Studies have found several other possible risk factors for bile duct cancers. More research is needed to confirm these possible links:

• smoking
• diabetes
• pancreatitis (inflammation of the pancreas)
• infection with hepatitis B virus or hepatitis C virus
• exposure to asbestos
• exposure to radon or other radioactive chemicals
• exposure to dioxin, nitrosamines, or polychlorinated biphenyls (PCBs)