I thank Cory Sandone for her artwork on small bowel neoplasms.
Neoplasms of the small intestine are rare, accounting for 3% to 6% of gastrointestinal neoplasms overall and 1% to 3% of primary gastrointestinal cancers. The incidence of small bowel cancer is higher in North American and Western European men compared with Asian men and women, and a higher incidence also has been noted in African American men and women. The prevalence of small bowel cancer, particularly carcinoid tumors, is increasing. The mean age at presentation is 65 years, with the incidence increasing after age 40 years.
More than 40 different histologic types of neoplasm may arise in the small intestine, and nearly 50% are malignant. Kopácová et al found that 42% of small intestinal neoplasms discovered in 170 patients by double balloon endoscopy were malignant; adenocarcinoma was the most common. Other malignancies include carcinoid, lymphoma, and gastrointestinal stromal tumor (GIST). Benign polyps include adenomas and hamartomas. Several hereditary syndromes have small intestinal polyps in their phenotypes (see Fig. 76-1 ).
The rarity of small bowel neoplasms is likely a result of several factors. The transit time through the small intestine is rapid, giving a brief mucosal contact time for potential carcinogens. Bacterial counts in the small intestine are lower than in the colon, so there is less processing of intestinal contents into carcinogens. A higher concentration of gastrointestinal lymphoid tissue and higher levels of immunoglobulin also are present. Finally, small intestinal contents are alkaline and liquid, which results in less chemical and mechanical stress on the mucosa and thus less inflammation.
Risk factors for small bowel cancer include Crohn disease and celiac disease. In both, the risk increases with the duration of the disease. In Crohn disease, the incidence of adenocarcinoma is higher among males with fistulizing disease of the distal jejunum or ileum. Celiac disease is associated with both lymphoma and adenocarcinoma. Lifestyle factors that have been reported in some studies to increase risk include red meat consumption, alcohol, smoking, and obesity. Gallstones appear to be associated with a higher risk for carcinoid tumors.
The presentation of small intestine neoplasms varies. Symptoms include gastrointestinal bleeding and abdominal pain, whereas more advanced disease may present with bowel obstruction and weight loss. Some tumors are seen incidentally during an imaging study, and patients with lymphomas may present with fever, weight loss, and drenching night sweats. A small number of patients with liver metastases from carcinoid tumors present with the carcinoid syndrome, suggested by flushing, diarrhea, and bronchospasm.
Until recently, small bowel tumors have been difficult to image or see. The best available diagnostic tool used to be the poorly tolerated and relatively inaccurate small bowel enteroclysis. Current methods for the diagnosis of small bowel tumors are much more effective and include computerized tomography enterography (CTE), magnetic resonance (MR) enterography, capsule endoscopy, and double-balloon enteroscopy (DBE).
CTE can only detect lesions that are 0.85 cm or larger. MR enterography offers greater resolution without the exposure to ionizing radiation and provides information about extraintestinal involvement. It has a sensitivity of 0.86 and specificity of 0.98 for small bowel neoplasms, and more recent studies have identified imaging characteristics on MR that may help differentiate benign from malignant neoplasms. These characteristics included solitary lesions, nonpedunculated lesions, longer lesions, mesenteric fat infiltration, and enlarged mesenteric lymph nodes.
Both video capsule endoscopy and DBE allow small bowel tumors to be seen. However, one major limitation of video capsule endoscopy is the fact that it does not cover the complete small bowel mucosa in nearly 35% of patients. Furthermore, the average capsule retention rate is 10% to 25% and is higher in patients with a small bowel tumor. DBE is more sensitive than CTE and permits suspicious lesions to be biopsied. However, it can be time consuming and is invasive, with a 1% to 4% complication rate. Understanding the prevalence and distribution of the different types of small bowel tumors helps in calling the odds. In 1987, adenocarcinoma was the most common small bowel tumor (45%), followed by carcinoid (29%), lymphoma (16%), and sarcoma (10%). However, in 2009, carcinoid was the most common small-bowel tumor. In the three decades between 1985 and 2005, the proportion of patients with carcinoid tumors increased from 28% to 44%, whereas the incidence of adenocarcinoma decreased from 42% to 33%. The proportion of patients with lymphoma and GIST has remained stable. A small proportion of small intestinal malignancies are metastases from lung, breast, prostate, and melanoma.
The location of tumors of the small intestine is relatively consistent and is demonstrated in Figure 76-2 . Adenocarcinoma is most frequently found in the duodenum. When an adenocarcinoma is associated with an inflammatory disease (Crohn disease, celiac disease, and Meckel diverticulum), the lesion is most commonly found in the ileum. Primary intestinal lymphoma parallels the distribution of lymphoid tissue and tends to be found in the lymphoid-rich terminal ileum. Carcinoid tumors are most commonly found in the ileum within 60 cm of the ileocecal valve. Endoscopically, a small protrusion within the submucosa is seen. A desmoplastic reaction in the submucosa is common and can be seen on CT imaging (see Fig. 76-3 ). Small bowel carcinoids can be associated with multiple endocrine neoplasia type 1. Secondary malignancies of the large bowel, stomach, breast, lung, pancreas, and gallbladder are the most common tumors associated with carcinoid. GISTs can be found anywhere in the small intestine, but the ileum is the most common location. Endoscopically, they are round, submucosal lesions, sometimes with a central ulcer.