Specific study of biological tumor cytology: a narrative review

Tumor metastasis is a very complex invasion. The concept of circulating tumor cells (CTCs), which describes the tumor cells diffusing into the blood, is being increasingly recognized for its role in cancer metastasis as well as it is considered the key step of tumor blood metastasis. Epithelial-mesenchymal transition (EMT) is the reverse transition from mesenchymal phenotypes to an epithelial one. Tumor cells undergoing this process have stronger metastatic potential. Though the biological characteristics of CTCs after EMT can help explain the unknown phenomena in tumor metastasis, the biomarkers of CTCs after EMT are not accurate. Also the correlation between CTCs after EMT and the prognosis of patients and the dynamic transformation of cell groups during treatment are not clear. Tumor metastasis initiating cells (MIC), which eventually lead to metastasis in CTCs, are types of cancer stem cells, or at least tumor cells with many stem cell characteristics. Further investigation of the maintenance of tumor stem cell characteristics of CTCs is required. The circulating tumor microemboli (CTM) can prevent CTCs from losing their nests, and the presence of host cells facilitates special metastasis and helps tumor cells escape immune surveillance. In this paper, we reviewed research on the biological cytological characteristics of CTCs in recent years, and provided relevant supporting evidence for monitoring tumor recurrence, assessing patient prognosis, evaluating the sensitivity of anti-tumor biological drugs, and selecting an individualized treatment plan.