If unfortunately you have cancer, of course surgery is the first choice. After surgery, although the cancer is not visible, the disease is not cured. The recurrence of metastasis after surgery is the most worrying thing. Almost all deaths caused by cancer are caused by metastasis. So how to prevent recurrence and metastasis after surgery? First let’s understand why metastasis occurs. Scholars have proposed theories such as secondary mutations, microsatellite foci, clinical subfoci, and postoperative tumor cell remnants. Although there are many debates on the theories, there is a consensus that tumor cells are still present in the body after surgery. This is the seed of recurrence. These tumor cells start to be undetectable by ultrasound, CT and other examinations. When tumor cells divide from one to two, from two to four, four to eight …… and soon form tumors that can be detected by CT, they have reached advanced stage and lost the chance of treatment. Therefore, early removal of these postoperative residual tumor cells is the key to stop recurrence, and post-surgery is the best time to stop these tumor seeds from germinating. To remove these tumor cells, the first step is to detect whether they are present or not. This is because they are not present in all patients. Cancer cells metastasize along the blood stream. Cancer cells first have to enter the bloodstream and then circulate to reach various parts of the body to form metastases. This part of the cancer cells that enter the blood circulation, we call them circulating tumor cells. The number of this part is very small, often less than 2 cells/ml. In 1 ml of blood, there are billions of normal cells, so a tumor cell will be hidden in billions of normal blood cells, so locating a tumor cell among billions of cells has always been a great challenge. However, science is advancing, and in 2009, the US FDA approved a technology to detect circulating tumor cells for clinical use. This revolutionary test is rapidly gaining widespread clinical use in Europe and the United States. Circulating tumor cells can be captured by immunomagnetic selection, seen and photographed directly under a microscope. After surgery, this test becomes the only means to directly detect tumor cells when neither radiographs nor CT examinations can detect cancer. In this way, the presence of tumor cells after surgery can be directly monitored. Numerous studies have shown that the number of circulating tumor cells is an independent predictor of the length of survival of cancer patients. After treatment, if the number of circulating tumor cells continues to increase, the patient’s survival will also be shortened. Conversely, if the number of circulating tumor cells is reduced after treatment, survival is prolonged. So can chemotherapy destroy circulating tumor cells? Let’s first look at the principle of chemotherapy drugs. Chemotherapeutic drugs mainly kill all the rapidly growing cells in the body. Tumor cells will be eliminated because of their vigorous growth, and of course the normal growing cells in the body will also be eliminated. But many circulating tumor cells are in a dormant state. Some studies have found that circulating tumor cells can exist for more than 7-15 years after surgery, and these also confirm that circulating tumor cells are in a dormant state. According to the principle of action of chemotherapeutic drugs, it is impossible to destroy circulating tumor cells in dormant state. This is the reason why the recurrence and metastasis rate is still high after postoperative adjuvant chemotherapy. The latest research shows that the key to prevent postoperative metastasis and postoperative treatment does not lie in how many chemotherapy treatments or how long radiotherapy is required, but in whether the residual tumor cells in the body are removed. Imagine, if chemotherapy cannot remove and destroy the residual tumor cells, what is the point of doing more chemotherapy? In 2006, I was sent by the National Scholarship Council to focus on the research on the mechanism of circulating tumor cell metastasis in the UK. After five years of intensive research in the UK, I first proposed the idea of removing circulating tumor cells as a treatment to prevent recurrence and metastasis after surgery, and successfully used photodynamic immunotherapy to remove the residual cancer cells. Under normal circumstances, the immune system exists in the human body, and cancer cells are quickly captured and killed by the immune system. However, cancer cells have the function of immune evasion, making themselves dress up as good cells, which are difficult to be recognized by the immune system, thus evading the capture of the immune system. Our photodynamic immunotherapy is to use photodynamics to kill tumor cells, so that these tumor cells lose their immune evasion deformation ability, which will then be recognized and remembered by the immune cells, and the immune cells will capture and kill other cancer cells according to the memory. The mechanism of photodynamic killing of tumor cells is to take advantage of the ability of tumor cells to absorb a large amount of photosensitizer, which excites the production of monomorphic oxygen by the action of laser light, thus selectively destroying cancer cells. The efficacy can also be accurately assessed by detecting the number of cells circulating in the peripheral blood, which means that the efficacy is visible and can be said to be immediate. Treatment after surgery to reduce the risk of recurrence and metastasis It is a better treatment option for patients who still have cancer cells remaining after radiotherapy.