A rare cancer that forms in tissues of the small intestine (the part of the digestive tract between the stomach and the large intestine). The most common type is adenocarcinoma (cancer that begins in cells that make and release mucus and other fluids). Other types of small intestine cancer include sarcoma (cancer that begins in connective or supportive tissue), carcinoid tumor (a slow-growing type of cancer), gastrointestinal stromal tumor (a type of soft tissue sarcoma), and lymphoma (cancer that begins in immune system cells).