The cause of biliary system cancer is unknown, although a possibility is gallstones. When gallstones are present, bile is released more slowly. If the bile contains carcinogens, the gallbladder tissue is exposed to these carcinogens for a longer period of time. Approximately 1% of all cholecystectomy specimens are found to be cancerous. Because of the risk of cancer, even for asymptomatic cholelithiasis, a cholecystectomy is recommended. Primary carcinoma of the gallbladder is rare and is usually associated with cholecystitis. Most biliary cancer is from metastasis, commonly from the head of the pancreas.