The epidemiology of cancer of the small bowel.
Despite its anatomical location between two regions of high cancer risk, the small bowel rarely develops a malignant tumor. However, in recent years, small bowel cancer incidence rates have begun to rise. The purpose of this review is to explore the descriptive and analytic epidemiology of small bowel cancer for those factors that protect this organ and those factors associated with loss of this protection. Within the small intestine, the sites at the highest risk are the duodenum, for adenocarcinomas, and the ileum, for carcinoids and lymphomas. In industrialized countries, small bowel cancers are predominantly adenocarcinomas; in developing countries, lymphomas are much more common. The incidence of small bowel cancer rises with age and has generally been higher among males than among females. The risk factors for small bowel cancer include dietary factors similar to those implicated in large bowel cancer, cigarette smoking, alcohol intake, and other medical conditions, including Crohn's disease, familial adenomatous polyposis, cholecystectomy, peptic ulcer disease, and cystic fibrosis. The protective factors may include rapid cell turnover, a general absence of bacteria, an alkaline environment, and low levels of activating enzymes of precarcinogens. Adenocarcinomas of the small and large bowel are similar in risk factors and geographic distribution but not in recent time trends; colorectal cancer incidence rates in the United States have been falling since the mid-1980s. Small bowel lymphoma may be associated with infectious agents, such as HIV. Given the differences in anatomic and geographic location among histological subtypes, much may be learned from well-designed, histology-specific epidemiological and genetic studies of cancer of the small bowel.