In cancer diagnosis and treatment, physicians typically rely on invasive biopsies and non-invasive imaging to track tumor size, expansion, and response to treatment. This approach can be not only traumatic for patients, but also expensive. However, the advent of CirculatingTumorCells (CTCs) assays has broken this stalemate. CTCs are tumor cells that detach from tumor lesions and disseminate into the bloodstream, and are an important cause of postoperative recurrence and distant metastasis in patients with malignancies, as well as a significant contributor to death in tumor patients. Compared with other histological specimens such as bone marrow, peripheral blood specimens are easy to obtain and less invasive, and are a more ideal source of specimens for routine clinical testing. In recent years, CTCs have gradually emerged as one of the most promising non-invasive diagnostic and real-time efficacy monitoring tools for tumor diagnosis and surveillance. A large number of experiments have confirmed that CTCs testing can help in the early diagnosis of tumors, determine patient prognosis, evaluate the efficacy of anti-tumor drugs and formulate individualized treatment plans. Technical route of tumor diagnosis by CTCs assay Compared with traditional diagnostic methods, CTCs assay can detect changes in disease more sensitively and without side effects to patients. However, trying to detect these rare CTCs is very challenging. This is because there may be only 1-10 such cells in 1 ml of a patient’s blood sample, and it is produced continuously, with a dynamic distribution in the blood, which can be lagging and highly heterogeneous. Given the absolute strength shown by CTCs detection and the existence of some obstacles, many scientists from all over the world persist in the struggle in this field. Recently, Prof. Qihui Shi, a Young Thousand Talents Program expert from the School of Biomedical Engineering, Shanghai Jiao Tong University, gave a speech on the topic of “Detection of Circulating Tumor/epithelial Cells in Human Peripheral Blood” at the 2015 Shanghai Forum on Development and Cooperation in the Greater Health Industry. Clinical application of CTCs detection Blood has always been a good window to detect human health. The traditional method for detecting diseases is to measure molecular class markers, including proteins, metabolites, nucleic acids, microRNAs, exosomes, etc. However, for cancer detection, in the last decade, there has been an increasing interest in the detection of cellular markers (i.e. rare cells detached from organ tissues into the blood), including CTCs. Today, CTCs detection has become a novel cancer detection tool, but a search of the earliest literature reveals that the concept of CTCs was introduced as early as 1869 by the Australian scholar Ashworth. However, after nearly 150 years of development, the field has not made much progress, mainly because CTCs are very rare. Therefore, it is a very difficult task to identify and isolate CTCs from complex blood. Among other things, the isolation process needs to reach a very high capture rate and purity, and to ensure the activity of the cells. In addition, since the number of CTCs is very small, it may involve the integration of multiple assays into a single cell. Currently, most technical as well as clinical studies are limited to the counting of CTCs, and less research has been done on the molecular characterization of CTCs. However, the results of counting may fluctuate as the rate of cancer cell shedding changes, and the functional heterogeneity of different CTCs is large, so the information obtained by CTCs counting is actually very limited. The only approved product: the CellSearch system Currently, the only FDA-approved CTC clinical test that effectively defines CTCs of epithelial origin is JanssenDiagnostics’ CellSearch, a semi-automated system based on EpCAM (epithelial cell antigen ) magnetic bead capture, staining, and counting of DAPI+/CK+/CD45- cells under a fluorescent microscope. The U.S. FDA approved the CellSearch system in 2004, 2007 and 2008 for prognostic assessment, progression-free survival and overall survival prediction in metastatic breast, colorectal and prostate cancers, respectively. Our CFDA approved the CellSearch system for prognostic assessment of metastatic breast cancer in 2012. Among them, in a paper entitled CirculatingTumorCells,DiseaseProgression,andSurvivalinMetastaticBreastCancer, published in NEJM in 2004, researchers used the CellSearch system for the prognosis of metastatic breast cancer The results showed that 7.5 ml of blood with less than 5 CTCs had a better prognosis. CTC device sharing, the strongest analysis of detection principles According to Prof. Shih, there are 40-50 companies engaged in the development of CTC devices internationally, and the following table lists some of the companies’ related information. According to the detection technology, CTC devices are mainly divided into in vivo detection and in vitro detection. The former is a test system that is left in the blood vessels of the body; currently only GILUPI, a German company, is developing it, and it is an EpCAM antibody-based positive selection. Other than that, all other CTC devices are in vitro tests, i.e., they are performed by drawing 7.5 ml of blood. In vitro assays are divided into two categories, including direct assays without capture (enrichment) and post-capture (enrichment) assays, with the latter being the dominant method. There are also two types of post-capture (enrichment) assays, namely positive and negative selection. Positive selection is mainly based on CTC surface markers or physical properties of cells (size, density); the former is based on immunomagnetic sphere technology and microfluidic chip technology, and the latter is based on the principle of filtration and centrifugation. Negative selection, on the other hand, indirectly captures CTCs by leukocyte surface markers. Comparison of the characteristics of different capture methods The most reliable method for the identification of benign/malignant circulating tumor cells is sequencing, but it is more expensive. Alternatively, pathological identification is an alternative method, but there is currently not much reliable clinical evidence on whether this method can reliably identify free cells in the blood. In fact, there are many other types of cells in the blood besides erythrocytes, leukocytes, and CTCs, including mature megakaryocytes, immature myeloid cells, and lymphocytes, endothelial cells, squamous epithelial cells, and mesothelial cells. The specificity of any sorting method is limited, and these mixed-in cells will bring some influence to the subsequent detection. Therefore, scientists and related companies at home and abroad have been developing new technologies to improve this. It is believed that in the near future, CTC will be more widely used in clinical cancer diagnosis and treatment.