Malignant neoplasms of the small bowel tend to have a characteristic anatomic distribution. Which of the following statements are true?

a. Adenocarcinomas of the small intestine show a distinct polarity with decreasing frequency from duodenum to ileum

b. Adenocarcinoma of the small intestine associated with Crohn’s disease occurs primarily in the ileum

d. The vast majority of carcinoid tumors of the small intestine occur in the ileum

Anatomically, adenocarcinomas of the small intestine show a distinct polarity with a decreasing frequency from duodenum to ileum. Given the difference in length between the duodenum, jejunum and ileum, the duodenal epithelium shows a substantially greater propensity towards malignant transformation. Even within the duodenum, two-thirds of the carcinomas occur in the periampullary region suggesting that the periampullary mucosa or luminal content (ingested potential carcinogens) interacts with pancreaticobiliary secretions to induce local neoplastic changes. Crohn’s disease primarily is a disease involving the terminal ileum and therefore it is not surprising that most adenocarcinomas developing in association with Crohn’s disease occur also at this site. Small intestinal lymphomas arise from lymphoid tissue within the wall of the bowel. Therefore lymphomas predominate in the ileum, where the greatest concentration of gut lymphoid tissue occurs. Next to the appendix, which harbors 85% of all carcinoid tumors, the small intestine is by far the next most common site of origin. Approximately ninety percent of small intestinal carcinoids are located in the ileum with 40% found within two feet of the ileocecal junction. Multiple primary tumors may be present in 30% of patients.