The residual cancer cells at the time of surgery is the fundamental reason why “radical resection” in the sense of treatment method cannot achieve the efficacy of “tumor eradication”. The “seeds” of post-operative recurrent metastatic lesions are all derived from the cancer cells left during surgery. If not, then it is a new second tumor. Why are there residual cancer cells? There are three reasons: the first one is the residual flesh and microscopic residue in the surgical area. The so-called naked eye residue means that you can tell there is residual cancer by your eyes, but there is no way to remove it. Because the cancer is too strong and invades the surrounding tissues, important organs or large blood vessels, and cannot be removed together. Let’s say you have gone to a lot of trouble to dig out the stump of a tree, but you will see many more small roots extending in all directions. Same thing. Microscopic residual means that there is no residual cancer as judged by the naked eye, but under the microscope, cancer cells are found to remain at the cut edge. In order to reduce the occurrence of microscopic residual, the scope of surgery is very specifically defined. For example, for resection of gastrointestinal tumors, the resection line of the stomach or intestinal canal is usually 5 cm from the edge of the tumor. Once the residual tumor is removed by visual or microscopic means, the surgery will not be a “radical resection”, but a relatively palliative surgery, and the efficacy will be significantly affected. Once the cancer cells have expanded beyond the stage of carcinoma in situ, they may enter the blood vessels directly or through the lymphatic system, and then circulate in the blood vessels with the blood, i.e. circulating tumor cells, which is the second type of residual cancer cells. The third kind is that these circulating state tumor cells will run down the blood circulation to the liver, lung, bone and other organs to lurk and wait for the opportunity to open up revolutionary bases. It is because of the residual cancer cells in three forms that cancer patients cannot be cured after surgery and thus remove the “cancer” label. Therefore, theoretically speaking, if a cancer patient lives long enough, once the body’s immune system fails to supervise the cancer cells, the residual cancer cells in the body may continue to expand and thus recur and metastasize.