Primary cancer is the gradual transformation of normal cells of normal tissues and organs into cancer cells under the long-term effect of various internal and external carcinogenic factors, and then into cancer cell masses, which is called “primary cancer” or “primary malignant tumor”. Primary cancer accounts for the major part of clinical malignant tumors, and it can occur in almost all parts of human body and all organs and tissues except finger (toe) nails and hair. Recurrent cancer refers to the growth of new cancer tumor on the organ where the primary cancer is located after the primary cancer has subsided through treatment, and the new cancer tumor grown is called recurrent cancer. The reasons for cancer recurrence are many, among which the main factor is incomplete treatment of primary cancer. For example, surgery is not clean, radiotherapy or chemotherapy is not complete. At that time, the cancer tumor disappears on the surface, but there are still some remaining cancer cells, which can cause cancer recurrence under the effect of certain internal and external triggers. Metastatic cancer is cancer cells invading into blood vessels, lymphatic vessels or body cavities from the primary site, running with blood or body fluids, and forming the same type of cancer as the primary cancer in distant sites or organs. Metastatic cancer must meet two conditions: first, the site of occurrence must be the distant part of the primary cancer; second, the nature of the cancer must be the same as the primary cancer. Metastasis prompts the spread of malignant tumors, causing greater and more extensive harm to the organism, and also makes cancer treatment very difficult. The extensive metastasis of cancer is often the main reason why advanced cancer cannot be cured surgically. Since cancer is prone to metastasis, sometimes metastatic cancer is detected first and the primary cancer is identified later, such as swollen lymph nodes in the neck, which is sometimes the first clinical symptom found in patients with nasopharyngeal cancer and only discovered to be nasopharyngeal cancer after further examination.