CT of head and neck can mainly detect primary malignant tumors of head and secondary malignant tumors of head; CT of chest can detect lung cancer and secondary malignant tumors of lung; CT of abdomen mainly detects malignant tumors of gastrointestinal tract, including pancreatic cancer, liver cancer, gallbladder cancer, gynecological tumors and bladder tumors. With the exception of gastric cancer and colon cancer, because stomach and colon are cavity organs, CT is not the most important method to detect them. Gastroscopy and colonoscopy should be improved, and pathology should be taken if necessary to clarify malignant tumors in stomach and colon. If malignant tumor of a certain part is suspected, the examination of the corresponding part should be performed, and if necessary, whole body examination by PET-CT can be performed, which can detect both primary and secondary malignant tumors.