Tumors of the Small Intestine

Small intestinal (SI) tumors (tumors of the duodenum, jejunum, and ileum) are uncommon in comparison with those occurring elsewhere in the GI tract. Fifty percent of SI tumors are metastases from adenocarcinomas or sarcomas; however, incidence rates of primary SI tumors have increased throughout the last 3 decades. SI tumors are histologically diverse in nature owing to the fact that they are derived from both epithelial and mesenchymal components of the SI tract. Changes in their epidemiology resulted in a shift of adenocarcinomas to neuroendocrine tumors as the most common type of SI malignancy. Although the majority of SI tumors develop in the absence of identifiable risk factors, several conditions have been identified as significant risk factors such as inflammatory bowel disease, genetic defects in mismatch repair genes and others. Our understanding of genetic and molecular aberrations of SI tumors has increased and resulted in the development of novel molecularly targeted therapies for different types of primary SI malignancies. Unfortunately, the prognosis for most forms of SI malignancies still carries poor prognoses owing to their late clinical presentation. This chapter will summarize the different aspects of the specific subtypes of SI tumors and discuss the latest developments.