Malignant Tumors of the Small Intestine

Malignant tumors of the small intestine are rare. Their frequency in a large autopsy study was lower than 0.1%. Although the small bowel is the largest gastrointestinal organ, less than 5% of malignant tumors arise in the small intestine. The reason for this remains unclear. Adenocarcinoma (nonampullary) is the most common malignancy and accounts for 30% to 50% of malignant tumors. Most develop in the duodenum or the jejunum. Predisposing factors appear to be alcohol intake, Crohn disease, celiac disease, and neurofibromatosis. A predisposing factor appears to be a preexisting adenoma, and more than 40% of patients with familial adenomatous polyposis (FAP) have polyps in the proximal small bowel, and more than 5% develop adenocarcinoma. Fig. 73.1 shows the morphologic types and local consequences of malignant tumors of the small intestine.

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Malignant Tumors of the Small Intestine.

Carcinoma of the ampulla of Vater, together with the other adenocarcinomas, makes up most of the malignancies in the small bowel. Again, these tumors are rare, but they are the most common sites of extracolonic malignancy in FAP. Lymphomas are the third and leiomyosarcomas the fourth most common small intestinal malignancies. Lymphomas make up 15% to 20% of all malignant small bowel tumors, of which non-Hodgkin lymphomas are most common.

Leiomyosarcomas are now classified as gastrointestinal stromal tumors (GISTs). It is often difficult for pathologists to define whether these are benign or malignant. Differentiation is usually based on a mitotic index. If there is less than 1 mitosis per 30 high-power fields, metastases are less than 1%. If there are more than 10 mitoses per high-power field, metastases are 100% and 5-year survival rates decrease to 5%, with no 10-year survival reported. A mitotic index between 0 and 10 has grades of 5-year survival and metastases. The GISTs are of great interest (see Chapter 37 ), and more are being reported. It is not yet clear whether the increase results from better diagnostic studies or whether there is a true increased incidence.