THE EPIDEMIOLOGY OF CANCER OF THE SMALL-BOWEL

Despite its anatomical location between two regions of high cancer ris k, the small bowel rarely develops a malignant tumor, However, in rece nt years, small bowel cancer incidence rates have begun to rise, The p urpose of this review is to explore the descriptive and analytic epide miology of small bowel cancer for those factors that protect this orga n and those factors associated with loss of this protection, Within th e small intestine, the sites at the highest risk are the duodenum, for adenocarcinomas, and the ileum, for carcinoids and lymphomas, In indu strialized countries, small bowel cancers are predominantly adenocarci nomas; in developing countries, lymphomas are much more common, The in cidence of small bowel cancer rises with age and has generally been hi gher among males than among females, The risk factors for small bowel cancer include dietary factors similar to those implicated in large bo wel cancer, cigarette smoking, alcohol intake, and other medical condi tions, including Crohn's disease, familial adenomatous polyposis, chol ecystectomy, peptic ulcer disease, and cystic fibrosis, The protective factors may include rapid cell turnover, a general absence of bacteri a, an alkaline environment, and low levels of activating enzymes of pr ecarcinogens, Adenocarcinomas of the small and large bowel are similar in risk factors and geographic distribution but not in recent time tr ends; colorectal cancer incidence rates in the United States have been falling since the mid-1980s, Small bowel lymphoma may be associated w ith infectious agents, such as HIV. Given the differences in anatomic and geographic location among histological subtypes, much may be learn ed from well-designed, histology-specific epidemiological and genetic studies of cancer of the small bowel.