The small intestine is the longest organ in the human body, about 5-6 meters long, and it accounts for about 75% of the entire gastrointestinal tract, but its tumor incidence is only 3%-6% of gastrointestinal tract tumors, and its malignancy incidence is only 1% of gastrointestinal tract malignancies. The main function of the small intestine is to digest and absorb nutrients. The three major nutrients needed by the human body: glucose, amino acids and fatty acids are all absorbed by the small intestine. The small intestine contains bile, pancreatic fluid and small intestinal fluid needed for the digestion of food, and these digestive fluids are very digestive and contain a variety of digestive enzymes, and the pH becomes significantly alkaline. In clinical practice, my hospital is a large national oncology specialty hospital, especially gastrointestinal tumor is a specialty department, and every year we encountered single-digit small bowel cancers, and more than 90% of them are proximal to the small bowel, duodenal cancers, and we almost never encountered cancers of the jejunum and ileum. The incidence of small bowel cancer is much lower than that of colorectal cancer and esophageal and gastric cancers, and the definitive reasons are inconclusive. The basic consensus reasons are: 1. Peristalsis in the small intestine is much faster than in the rest of the digestive tract; peristalsis in the esophagus and stomach is slow and rhythmic. Peristalsis in the large intestine is also slow, and many people have chronic constipation. In contrast, peristalsis in the small intestine is fast, almost 24 hours a day, and not necessarily regular. The high-speed transit of the small intestine may reduce the contact time between the small intestine wall and the cancer-inducing factors in the intestinal contents. 2. The small intestine is low in bacteria, almost all of which are probiotics that aid in digestion. 3, The small intestine contains a large number of lymphocytes, even large clusters of lymphocytes, under the mucosa. Lymphocytes are the body’s defenders and can kill bacteria, toxins, and even cancer cells. Once there is a foreign body, it will be eliminated by lymphocytes at an early stage. Small bowel cancer (adenocarcinoma of the small bowel) has a very low incidence, but it does occur. It is basically the proximal part of the small intestine, near the descending part of the duodenum. Diagnosis of small bowel cancer is also basically detected by gastroscopy, because gastroscopy can see inside the duodenum. In addition, abdominal CT can also observe duodenal occupation and thickening of the intestinal wall. However, ordinary colonoscopy is unable to detect small bowel lesions. The treatment of adenocarcinoma of the small intestine is basically similar to that of gastric cancer and colorectal cancer. Early and middle stages are mainly treated with radical surgery, and late stages are mainly treated with chemotherapy. There is no standard program or clinical guideline for chemotherapy for small bowel cancer, and it is carried out according to the chemotherapy program for colon cancer, but the prognosis is not as good as that of colon cancer. In addition to adenocarcinoma of small intestine, mesenchymal tumors of small intestine are more common, which can occur in any segment of small intestine, in the intestinal wall or in the mesentery of small intestine. Small bowel lymphoma is also a relatively common small bowel tumor, and the main incidence site is at the end of the small bowel, that is, the end of the ileum.