What are circulating tumor cells and what is their clinical role? Circulating tumor cells (CTCs) are tumor cells that are released into the peripheral blood during the development of malignant tumors (primary or metastatic foci) and survive after evading immune killing, and are closely related to tumor staging, metastasis, prognosis and drug resistance. As a new tumor marker, CTC is gradually and widely used in tumor staging, prognosis assessment, postoperative detection of recurrence and efficacy evaluation. It is a non-invasive liquid biopsy method to analyze circulating tumor cells in peripheral blood with the help of advanced detection tools and kits, which helps to diagnose patients with metastasis, monitor tumor recurrence and metastasis in postoperative patients, evaluate the sensitivity of anti-tumor drugs and patient prognosis, and select individualized treatment strategies. Does the detection of circulating tumor cells mean that the metastasis is recurrent or will definitely recur? No. CTCs will only form metastases under certain conditions, usually when the number of CTCs reaches or increases significantly and the patient is immunocompromised, the risk of tumor recurrence increases significantly. This is similar to the relationship of “seed-soil”, if there is a seed, if the soil environment is not suitable, it will not take root and sprout. In general, if the patient’s immune function is normal, even if a small amount of cancer cells spread into the blood, they may be killed by the immune system and will not form metastases. If there are circulating tumor cells, can chemotherapy kill them? The main role of postoperative adjuvant chemotherapy is to kill the cancer cells that may remain, including circulating tumor cells, through systemic application of chemotherapy drugs. In general, adjuvant chemotherapy drugs can kill circulating tumor cells, but there is no evidence that they can necessarily kill them completely. It exists whether the tumor cells are sensitive to the drugs, whether they develop new immune escape mechanisms or new alterations in drug resistance genes. Therefore, if circulating tumor cells are found during postoperative review, they can be reviewed during the course of treatment to dynamically detect changes in circulating tumor cells. Circulating tumor cells are a powerful complement to the TNM staging system and can be referred to when developing treatment plans in the clinic. At present, the main basis of postoperative adjuvant chemotherapy is still the classical TNM staging system, which determines whether postoperative adjuvant chemotherapy is needed according to the postoperative pathological staging. This staging system is currently the most widely used internationally and has the most practical guidance value. Nevertheless, the system is not perfect, and there are still many areas that need to be improved or added. Circulating tumor cell detection is an effective supplement to the TNM staging system. In particular, as the research on tumor mechanism is deepening, diagnosis and treatment have been carried out at the molecular and genetic levels, and the trend of individualized treatment of tumors is becoming more and more obvious, and the crude treatment mode of measuring all patients with one ruler will gradually transition to the individualized and refined treatment mode. Circulating tumor cell detection, which is a kind of liquid biopsy, will be increasingly used in clinical applications, but there are still many problems to be solved. For example, the consistency and reliability of the detection tool, the conclusion of large sample studies on the number of circulating tumor cells and prognosis of survival, and the basis of drug treatment for patients with positive circulating tumor cells alone all need to be supported by further research findings. But in any case, thankfully, the development of science and technology will continue to promote the advancement of antitumor tumors, screening for effective therapeutic agents, screening for effective treatment populations, improving treatment efficacy, and reducing treatment side effects. Circulating tumor cell count can assist in evaluating tumor treatment effect In the traditional evaluation of tumor treatment efficacy, imaging (CT or MRI) is usually used to observe and measure tumor size change or enhancement change to evaluate tumor treatment effect, which has a certain lag and usually takes 1-3 months of treatment before evaluation. However, CTC can detect tumor progression earlier than imaging. The change of peripheral blood CTC count before, during and after mid-treatment can help to evaluate whether the tumor progresses and the effect of drug treatment. Studies have shown that patients with a high number of CTCs in peripheral blood before chemotherapy wall a significantly shorter progression-free survival and overall survival in patients with a low number of CTCs. The role and detection of circulating tumor cells still have certain problems to be solved, and the specific clinical application and whether it is used as the basis of anti-tumor therapy need to be decided by the treating physician according to the specific condition of the patient, and must not be blindly copied and reproduced.