Cholecystectomy protects against extrahepatic bile duct cancer: Is this a result of the removal of gallstones? Ekbom A, Hsieh C-C, Yuen J, Trichopoulous D, McLaughlin JK, Lan S-J, Adami H-O. Risk of extrahepatic bile duct cancer after cholecystectomy. Lancet 1993;342:1262–1265