There are no cancer cells in the blood of normal people because the human body has a strong immune system with NK cells and T lymphocytes. If there are cancer cells in the blood, these immune cells will detect and destroy them. If there are cancer cells in the blood, they are commonly found in two cases: the first one is patients with acute leukemia, where leukemia cells can be detected in the blood because they proliferate in the bone marrow and are released into the peripheral blood. After acute leukemia is diagnosed, the most effective method is to give chemotherapy to kill leukemia cells through chemotherapeutic drugs, and after remission, hematopoietic stem cell therapy can be given to potentially cure leukemia. The second type is cancer in other parts of the body that has metastasized in the blood, which can also cause the appearance of cancer cells in the blood, such as patients with stomach cancer can metastasize to the lungs, brain, liver, etc. In this case, the most effective method is also to give chemotherapy. If the patient cannot tolerate chemotherapy, only symptomatic supportive treatment can be given.