Circulating Tumor Cells (CTC) are cells that detach from a primary tumor and circulate throughout the bloodstream. These cells are part of the metastasizing process that leads to the progression of the disease. CTCs in the blood promise to be a very valuable test for early indication of presence of a cancer and to monitor residual disease. It is widely believed that the detection of CTCs in blood is an important factor in monitoring an individual’s baseline condition and ongoing response to therapy. The theory being that CTCs are suspected to act as the seeds for further growth of additional tumors in tissues distant from the original source.

The value of capturing and counting CTCs is evolving as more research data is gathered about the utility of these markers in monitoring disease progression and potentially guiding personalized cancer therapy. The detection and enumeration of circulating tumor cells (CTCs) has shown significant clinical utility with respect to prognosis in breast, colorectal and prostate cancers. Emerging studies show that CTCs can provide pharmacodynamic information to aid therapy decision making. CTCs as a virtual and real-time biopsy have clear potential to facilitate exploration of tumor biology, and in particular, the process of metastasis.

The vast majority of studies find an association between the presence of CTCs and the clinical outcome parameters such as the Disease Free Survival (DFS, i.e. the time span between the end of a therapy and the relapse), the Progression Free Survival (PFS, i.e.in the metastatic the time span between the end of a therapy and the progression) or the Overall Survival (OS, i.e. the time span between the end of the therapy and the final patient’s death).

CTC detection and enumeration in peripheral blood have been examined in prospective multi-centric studies for metastatic breast, colorectal, and prostate cancers and found to be associated with decreased progression-free and overall survival. An increasing number of clinical studies are validating these observations and extending them to other cancers and even to earlier disease stages.

Role of CTCs in clinic is further detailed as follows:

Isolation and detection of CTCs from blood constitute a virtual and real time biopsy as compared to difficult to reach tumor tissues especially during metastatic cancer.

CTCs represent changing biological behavior of secondary tumors and hence CTCs can be used as a source of biological material to monitor drug response during the course of drug treatment.

CTCs are associated with decreased progression free survival and decreased overall survival in patients treated for metastatic breast, colorectal or prostate cancer.

Presence of CTCs could be a prerequisite for tumor metastasis, detection of CTCs may indicate potential risk of metastatic relapse or metastatic progression in a patient.

CTC detection and characterization may help in stratification of patient for therapy as well as real-time monitoring of patient during treatment.

Detailed characterization of CTCs may lead to identification of novel therapeutic targets and understanding of development of drug resistance during therapy.

Molecular characterization of metastatic tumor from bone which is a common site of spread is generally not feasible due to the difficult access and CTCs can be used as a source of tissue material for analysis.