Why does cancer come back after removing the area where it occurred? Why does cancer recur after the site where it occurred is removed?” The key to understanding this question is that a cancer lesion is not a simple mass with a single behavior and nature; in the vast majority of cases, we should see it as a complex collection of numerous cells with common characteristics (infinite proliferation, transformation and metastasis). — Although it is currently proposed that attention should be paid to the heterogeneity among cancer cells in tumors, this is not very relevant to the present topic and will be omitted. Our body is composed of 40 trillion – 60 trillion cells that are constantly undergoing complex biochemical reactions as well as differentiation and proliferation. Although there are sophisticated regulatory mechanisms that have evolved over billions of years, based on their large base, there are always some cells that mutate during proliferation, and most of these mutated cells are detected and removed by the body’s own monitoring mechanisms, and those that remain are the culprits of cancer. By the time they have grown enough to cause symptoms or to be detected by instruments, the number of these little devils can often be counted in the millions. The figure below shows colonies of human and rat breast cancer cells cultured in vitro at different magnifications of the microscope. It should be noted that even the countless number of cells as densely packed as in a and e is just a single layer of cells on a fraction of a square millimeter. This is the first reason: the tumor lesion is composed of an extremely large number of cancer cells. Just as in the case of the huge number of the population of China, there will always be a handful of people who do not like to dream of China and want to defect to the U.S. Empire to break the fortress of capitalism from within in a legal or illegal way. Among these countless cancer cells, there will also be some that are restless and want to develop elsewhere. After the tumor cells in situ (figure a below) break through the anatomical barrier of their site (figure b below), our body’s two major vascular systems, lymph and blood, provide convenient access to the tumor cells (figures c and d below). In addition, tumors born in cavernous organs can, like plants sowing seeds, metastasize by dropping directly to metastasis by a small number of cells that fall elsewhere. This is the second reason: tumor cells can develop into metastatic cancer through lymphatic metastasis, hematogenous metastasis and implantation metastasis. In summary, even if cancer lesions are found and surgically removed, a small amount of cancer cells have often metastasized to other parts of the body by this time. Because the number is very small, we often cannot know the exact metastasis site and number, and these cells themselves are the winners of the struggle with the body’s defense mechanism and have the strong ability to reproduce and escape from the blow, so even if the remaining number is very small, they are likely to resurface. Depending on the source, various tumors have different sites of metastasis, for example, the common sites of metastasis for lung cancer are brain, bone and liver. Currently, adjuvant radiotherapy and/or chemotherapy are often required after tumor surgery to maximize the removal of these metastatic cancer cells to reduce the possibility of recurrence. Unfortunately, with current medical technology it is not yet possible to precisely and completely eliminate metastatic cancer cells, and the probability of recurrence asked by the subject in the question description is also determined by the specific condition and treatment. Because of the above points, very early diagnosis and precise targeted therapy are the two hot spots in tumor research. Although the survival time and quality of life of many kinds of tumors have been greatly improved, we still have a long way to go in the face of cancer cells, a powerful opponent that can overcome the body’s immune system.