Abstract:In the treatment and prognostic studies of malignant tumors, metastasis is a rather tricky problem and an important indicator that affects the prognosis of patients with tumors and the judgment of their therapeutic efficacy. In recent years, the study of malignant tumor metastasis has developed to the cellular and molecular level, and the concept of tumor micrometastasis has been gradually established. The mechanism of micrometastasis is mainly related to lymphangiogenesis, cytokines and microenvironmental changes, and the study of tumor micrometastasis has become a hot spot in tumor research. The detection of tumor micrometastasis has also been paid more and more attention by the tumor professional circles. This paper analyzes the mechanism of malignant tumor micrometastasis, the detection status and the current problems. [Keywords] malignant tumor; micrometastasis; mechanism; detection Clinically, there are always some patients with malignant tumors who, despite the fact that there is no residual residue after the first surgical treatment and the routine postoperative radiotherapy, may still find signs of metastasis when they review the original lesion and the related organs in a few months or a few years. If we can accurately determine whether there is tumor metastasis and the extent of metastasis in a timely manner, we can greatly improve the efficiency of tumor treatment, prolong the survival period of patients and improve their quality of life. Generally speaking, it is difficult to detect micro-metastatic tumor cells in clinical and routine pathological examinations, and early detection and diagnosis can only be made through special examinations such as immunohistochemistry or PCR. In recent years, the detection of micrometastasis of malignant tumors has been gradually developed and paid attention to, now we would like to make a brief review about the mechanism of micrometastasis of malignant tumors, its detection and the current research progress. The concept of malignant tumor micrometastasis The traditional concept of tumor metastasis refers to the emergence of new identical or similar foci outside the tumor foci, which are often found months and years after the primary foci are discovered. On the other hand, tumor micrometastasis refers to the tumor residues detected in various tissues, body fluids and cellular grafts at microscopic and submicroscopic levels, and it is the metastasis of non-hematological malignant tumors that cannot be detected by conventional clinicopathological methods and are hidden in tissues other than the primary foci. The mechanism of malignant tumor micrometastasis At present, there are two main theories about tumor metastasis, one is the “seed and soil hypothesis”: it is believed that the formation of tumor metastasis depends on whether the environment of the tissues in the site of metastasis is suitable for the residence and growth of primary tumor cells. The second one is “tumor heterogeneity theory”: it is believed that due to the instability of genetic traits of tumor cells, tumor cells of monoclonal origin will be heterogeneous in the process of proliferation, which leads to the differentiation of the metastatic potential of tumor cells. The mechanism of micrometastasis is mainly based on the theories of lymphangiogenesis, cytokines and microenvironmental changes, etc. Since the 90’s, with the increasing of the number of metastatic tumors in the past decade, the tumor metastasis has become a major problem in the world. Since the 1990s, with the in-depth study of tumor microvessels, it has been found that the metastasis of malignant tumors is closely related to the microangiogenesis of tumors. The research in this field has been paid more and more attention by related experts. Detection methods Currently, there are two types of detection methods: morphological and non-morphological methods. Morphological methods include continuous section HE staining, immunocytochemistry and immunofluorescence; non-morphological methods include flow cytometry, RT-PCR and immuno-magnetic separation, etc. Conventional continuous section HE staining method Continuous section method is generally used for micrometastasis detection of lymph nodes, and its detection rate has a great relationship with the spacing of the sections, and the meticulous analysis of continuous sections can be obtained from the conventional single-slice examination of negative lymph nodes. The detailed analysis of consecutive sections can detect about 10% of micrometastases in lymph nodes that are negative in conventional single-slice examination. However, consecutive section method needs to cut dozens to hundreds of slices for one specimen, which is a big workload and difficult to be promoted in clinic, and it is difficult to detect small amount of cancer cells that have not formed metastases, so it has been seldom used in recent years. 3.2 Immunohistochemistry has been applied to the clinic earlier because of its easier operation, relatively lower cost, higher sensitivity and specificity. However, its shortcomings are time-consuming, expensive, and prone to false-positive and false-negative problems. 3.3 Flow cytometry The basic principle of flow cytometry is to use fluorescently labeled monoclonal antibody to bind to tumor cells, and then detect the tumor cells by flow cytometry, which is faster and more convenient than immunocytochemistry in processing and scanning samples. The process is fully automated and eliminates subjective interference. 3.4 Polymerase Chain Reaction and Reverse Transcription Polymerase Chain Reaction (PCR and RT-PCR) The principle of this technique is to confirm the presence of tumor cells by amplifying tumor cell signature genes or target RNAs in the patient’s blood, lymph nodes, or bone marrow. PCR is used to amplify the abnormal DNA present in the tumor cells, and the greatest advantage of this technique is its high sensitivity. With the discovery of new tumor-specific markers and the optimization of PCR reaction conditions, RT-PCR may become a routine method for detecting tumor micrometastases. 3.5 Immunomagnetic separation This is a newly established method to improve the detection of circulating single tumor cells. FITC2-conjugated anti-whole-cell keratin monoclonal antibody-positive cells are coupled with superparamagnetic anti-FITC microbeads and then subjected to a high-gradient magnetic field with an immunomagnetic activated cell screener. This method is 10-100 times more sensitive than using RT-PCR or immunohistochemistry alone, thus reducing false positives and false negatives. 3. 6 Immunofluorescence technique It has the following advantages: (1) Low false-positive rate. (2) The absolute number of tumor cells can be determined and quantified simultaneously. (3) Microcomputer storage is available, and there is a possibility of repositioning for correlation analysis. (4) Not only in vitro cell detection, but also in vivo micrometastasis detection can be performed. Some Problems of Tumor Micrometastasis Research Tumor micrometastasis is a hotspot in tumor research, which is still in the stage of data accumulation and mechanism exploration. The current problem is that some gradually eliminated micrometastasis detection methods, such as conventional cytopathology and some immunohistochemical methods, are still being applied, such as the detection of lymph node metastasis, breast cancer and colorectal cancer by serial sectioning. The phenomenon of misuse of markers is also common, such as several markers that have long been recognized for certain tumors or their outdated detection methods are still proving, analyzing and comparing their value, but lack of innovative research perspectives and levels [5]. There are many methods for detecting micrometastases of malignant tumors, and various tumor markers are various, and their clinical significance and prognostic value need to be further confirmed and clarified. In addition, there are many standards for the detection of micrometastases, therefore, it is more and more important to standardize the detection standards.