Introduction
Tumors are often difficult to be cured and have a high mortality rate, making them a cause for concern. Currently, surgery and chemotherapy are the main treatment methods for early to mid-stage tumors, which can achieve certain results, including remission. However, tumor recurrence is often an unavoidable fact [
1], so cancer patients often undergo regular check-ups [
2,
3]. However, most check-up methods rely on imaging, which can only observe tumors after they have reached a certain size. This often results in missing the best treatment window. Early observation or monitoring of tumor development is undoubtedly the key to early detection and treatment of tumors [
4].
CTCs are tumor cells that detach from primary or metastatic lesions and enter the bloodstream through the vascular and lymphatic systems [
5]. CTCs can be thought of as the seed cells of malignant tumor metastasis. Several CTCs (at least three cells) can aggregate to form circulating tumor microemboli (CTM), which have a survival advantage over single cells because they can undergo epithelial-mesenchymal transition (EMT) to lose adhesion and become more easily metastasized while maintaining internal connections to resist apoptosis [
6,
7].
CTCs detection is a novel liquid biopsy technology that facilitates precision diagnosis and treatment of tumors [
8]. Compared with invasive tissue biopsy, CTC detection has the advantages of easy sample acquisition and the ability to provide dynamic monitoring information. Compared with another liquid biopsy marker, circulating tumor DNA (ctDNA), CTC is a complete tumor cell that carries multiple omics information (genome, transcriptome, proteome, metabolome, etc.) of tumor cells [
8]. Moreover, the use of live CTCs can achieve in vitro analysis of tumor cell morphology and function. CTCs counting, molecular typing, and downstream analysis have broad prospects for application in tumor efficacy evaluation, prognosis assessment, and auxiliary treatment decision-making [
9].
Currently, multiple CTCs detection methods have been developed based on unique molecular substances within tumor cells as detection targets to achieve identification and classification of tumor cells [
10,
11]. This method has high sensitivity and specificity and can identify individual tumor cells. However, due to the complexity of tumor, the detection efficiency of CTCs in different types of tumors and their practical guidance for the occurrence and development of tumor are issues that need to be studied further. This paper aims to explore the differences in CTC detection in different types of tumors by detecting the number and types of CTCs, and investigate the guidance value of CTC detection for the occurrence and development of different types of tumors.
Discussion
Early detection and treatment of cancer greatly increase the chances of cure. Therefore, researchers have been trying to find biological tumor markers that can assist in early cancer diagnosis from various types of specimens [
14‐
16]. However, due to the biological complexity and heterogeneity of tumorigenesis and tumors, no satisfactory tumor-specific biomarkers for early screening have been identified. Although serum tumor markers such as CEA [
17], CA-199 [
18], PSA [
19], AFP [
20], CA-125 [
21], etc., which are commonly used in clinical tumor screening tests, have been widely used, none of them can accurately and specifically predict the presence or absence of tumors. EMT is a phenomenon in which epithelial cells transform into mesenchymal cells under certain physiological and pathological conditions [
22,
23]. EMT mainly occurs during the process of tumor cells entering the peripheral blood circulation, characterized by the loss of epithelial cell phenotype and the acquisition of mesenchymal cell phenotype [
24]. Studies suggested that EMT markers could be used for the detection or capture of CTCs. The significance and value of CTCs detection for the early diagnosis, treatment monitoring, and prognosis of tumor diseases are being gradually confirmed. Based on our results, it can be indicated a detection rate of 100% for CTCs in liver cancers, cervical cancer, nasopharyngeal carcinoma, breast cancer, and prostate cancer, 96.7% in lung cancer, 94.2% in colon cancer, and 90% in gastric cancer (Table
2). The overall detection rate of CTCs in the peripheral blood of cancer patients was 97.6% (data not shown).
The most important aspect of CTCs detection is the enrichment of CTCs, which determines the sensitivity of CTCs detection. Currently, there are several commercial systems available for CTCs enrichment, including the Canpatrol™ system, ISET system, ScreenCell system, and CellSearch system. Three systems utilize microfiltration membranes with specific pore sizes to enrich CTCs, while the CellSearch system uses a positive enrichment strategy based on the epithelial cell adhesion molecule (EpCAM) positive CTCs. In this system, EpCAM antibodies are coated on magnetic beads and specifically bind to the surface of CTCs, which are then selected under the action of an external magnetic field. The CellSearch system, based on EpCAM, is the only commercially available product approved by both the FDA (approved in 2004) and CFDA (approved in 2012) for CTCs detection in malignant tumors. Although CTCs enrichment systems based on physical properties, such as Canpatrol™, may have the disadvantages of potential contamination by other blood cells and missing certain tumor cells, their economic cost and ease of operation make them widely used methods for CTCs detection. Regarding the CellSearch system, studies have found that not all CTCs express EpCAM during the EMT process, which limits the clinical application of EpCAM-based technologies to some extent. It is important to note that each method has its own unique advantages and disadvantages, and therefore the choice of method primarily depends on our research goals and requirements. Although there are various CTCs enrichment technologies based on physical or biological characteristics, neither can effectively explain the issues of missing CTCs or leukocyte contamination, which indicates the need for further research in this area.
The detection rate of CTCs can be influenced by various factors, including tumor type, disease stage, technical sensitivity, and sample processing. In our study, we utilized the commercial product Canpatrol™ system. According to previous reports, this system has shown a detection rate for CTCs ranging from 65 to 90.18% [
13,
25,
26] or even higher. In this study, to maintain the accuracy of CTCs enrichment, stringent protocols are employed during sample processing. Stringent measures are implemented to prevent membrane filtration blockage. In cases of blockage occurs, it is necessary to redraw blood samples to ensure the efficient capture of CTCs and achieve a high detection rate in cancer patients. Of course, what cannot be ignored is false positive results potentially caused by leukocytes cannot be entirely ruled out. The high detection rate of CTCs in cancer patients may provide effective assistance for early cancer diagnosis.
We conducted an analysis to investigate the association between the total number and three subtypes of CTCs in peripheral blood of cancer patients and clinical characteristics including age, lymph node metastasis, and hematogenous metastasis. Our results showed a significant increase in the number of mesenchymal CTCs in patients with lymph node metastasis compared to those without lymph node metastasis. Similarly, the total number of CTCs and the number of mixed phenotypic CTCs and mesenchymal CTCs were significantly increased in patients with hematogenous metastasis compared to those without hematogenous metastasis (Table
1). In metastatic breast cancer patients, the presence of ≥ 5 CTCs in 7.5 mL peripheral blood indicated poor prognosis [
27], while in patients without colon cancer, the presence of ≥ 1 CTCs in 7.5 mL peripheral blood indicated a greater risk of recurrence and metastasis and worse prognosis. [
28]. Our results also revealed that an increase in the number of CTCs or mesenchymal CTCs indicates the occurrence of tumor metastasis. Shiyang Wu, et al. study showed that mesenchymal CTCs were more common to be found in metastatic stages of cancer [
13]. Our findings are consistent with previous reports indicating that mesenchymal CTCs are associated with metastasis and disease progression. [
29,
30].
Studies have shown that the epithelial-to-mesenchymal transition EMT of tumor cells can promote the generation of CTCs and facilitate their survival in blood vessels, which is an important prerequisite for tumors to have high metastatic ability [
31‐
34]. During EMT, epithelial tumor cells become more invasive and migratory, making it easier for CTCs to enter distant organs through blood vessels or lymphatic vessels. We analyzed the correlation between clinical staging (TNM) and CTCs detection. The data basically showed that the average proportion of epithelial CTCs decreased in later stages of liver cancer, gastric cancer, and lung cancer, compared to earlier stages. In contrast, the average proportion of mesenchymal CTCs increased in later stages of cancer, compared to earlier stages. The average proportion of mixed phenotypic CTCs in later stages seem to be consistent with earlier stages (Table
3; Fig.
3). Which means the counts change of epithelial CTCs and mesenchymal CTCs are more helpful for tumor monitoring. Moreover, we consecutively monitored CTC counts and subtypes more than three times in eight tumor patients with poor prognosis and observed a significant increase in CTC counts in the second or third detection. Especially, the number of mixed phenotypic CTCs increased significantly in the second or third detection (Fig.
4). These findings suggest that changes in CTC subtypes are related to the prognosis of cancer.
CTM are tumor cell clusters and are associated with high metastatic potential [
7].One case of nasopharyngeal carcinoma patient was found to have CTM consisting of 2 mixed phenotypic CTCs and 1 mesenchymal CTC. Among the 26 detected mesenchymal CTCs, a phenomenon of two CTCs adhering together was observed (Fig.
2).
In conclusion, our study suggests that CTC detection can provide valuable information for tumor diagnosis and prognosis. The results indicate that different tumor types have different detection rates and subtypes of CTCs. Moreover, our findings suggest that changes in CTC counts and subtypes may be useful for monitoring disease progression and treatment response.
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