The time of cancer formation is uncertain. Different types of cancers or different groups of people may take different amounts of time to form cancer, and it may take several years, or even more than 10 or 20 years. It may start as a relatively small lesion, which may still be benign, and gradually develop into precancerous lesions or carcinoma in situ, and then further develop into cancer, and this process will take at least a few years. If the intervention treatment is done in advance at the time of inflammation or carcinoma in situ, it is possible to prevent cancer from occurring, so it is important to have regular medical checkups. Gastric cancer may start with atrophic gastritis, gradually develop to intestinal epithelial hyperplasia and then to atypical hyperplasia, and if it is severe atypical hyperplasia, it belongs to pre-cancer, and then develops to carcinoma in situ and gastric cancer. Therefore, it is necessary to take active treatments from the beginning of gastritis, including the treatment of anti-Helicobacter pylori bacteria, paying attention to dietary regulation, and reviewing gastroscopy once a year, and so on.