Circulating Tumor Cells: Detection and Applications

What are Circulating Tumor Cells?

Circulating tumor cells (CTCs) are cells that detach from a primary tumor and travel to other areas in the body through the bloodstream.

CTCs can act as seeds for additional tumors (metastases) in a process termed metastasis. Only 0.1% of CTCs form metastases. For CTCs to form new metastases:

1. Cells detach from the primary tumor.
2. The detached cells (the CTCs) enter the bloodstream.
3. Some CTCs survive by avoiding immune detection and enter the microvessels of other tissues or organs.
4. The tumor cells achieve neoangiogenesis (when new blood vessels grow, supporting tumor growth and survival) in the new tissue or organ. At this point, these cells are known as disseminated tumor cells.

The spreading of cancer throughout the body via metastasis is the primary cause of cancer-related deaths. Nine out of ten cancer deaths are caused by tumor metastases.

CTCs in the bloodstream are found as either single CTCs or CTC clusters. There are many types of CTCs, each with unique characteristics. Doctors use CTC data in disease prognosis and treatment.

Why are CTCs so valuable?

CTCs provide doctors with valuable information that can be used for:

• Early cancer detection
• Disease prognosis
• Developing appropriate cancer treatments
• Monitoring cancer progress during and after treatment
• Looking for signs of cancer relapse after cancer treatment

CTCs are obtained through liquid biopsies, which are cheaper, less invasive and safer than surgical biopsies. Because of this, liquid biopsies are a valuable tool for monitoring a cancer patient before, during and after treatment.

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Applications in cancer diagnosis, prognosis and treatment monitoring

Today, most CTC technology is still in the research phase. In clinical practice, CTCs are used in the prognosis of certain forms of cancer, including:

• Breast
• Lung
• Prostate
• Colon
• Pancreas
• Melanoma
• Glioblastoma

In the future, research may lead to tools that doctors can use to diagnose, treat, and monitor cancer.

Doctors and patients have a better chance of beating cancer the earlier that it is caught. Using liquid biopsies from blood collected from patients, researchers hope to be able to detect CTCs before a detectable metastatic growth occurs. One goal is to create a blood test that can be used to screen for multiple types of cancer.

Figure 1: The stages of metastasis. 1. Cells detach from the primary tumor. 2. The detached cells (the CTCs) enter the bloodstream. 3. Some CTCs survive by avoiding immune detection and enter the microvessels of other tissues or organs. 4. The tumor cells achieve neoangiogenesis (when new blood vessels grow, supporting tumor growth and survival) in the new tissue or organ. At this point, these cells are known as disseminated tumor cells. Credit: Technology Networks

Once detected, the CTCs’ genetic and molecular information could be analyzed and used to create a more effective treatment plan. Thanks to the non-invasive characteristics of liquid biopsies, they can be taken more frequently. This frequency could allow doctors to track a patient’s response to treatment more often than with conventional surgical biopsies.

Challenges of detecting CTCs

CTCs exist in low quantities in the bloodstream. In 1 mL of blood there are around 1-10 CTCs. As such, they are difficult to identify, isolate and study. The Cellsearch system remains the only FDA-approved CTC detection method. This technique involves positive selection, where staining and separation techniques are used to isolate cells with CTC characteristics. One issue with these technologies is that CTCs have the ability to alter their expression of certain antigens dynamically. As such, negative selection techniques, such as size-based exclusion, are also under development, which involve depleted non-CTC cells to make detection simpler. 

Another challenge with using CTCs in early cancer detection is determining if CTCs that are found in the blood are indicative of a dangerous metastasis. Not all cancerous growths are aggressive, and therefore not every CTC would indicate a cancer that needs to be treated. More strategies are needed to determine which CTCs necessitate action and which do not.

New single-cell technologies are proving fruitful in these efforts. They have helped reveal the genomic and transcriptomic heterogeneity between different CTCs.

Novel methods of detection, separation, isolation

Today, CTCs are used in limited clinical settings. Scientists are actively researching expanded uses. As such, there are novel methods in development to detect, separate and isolate CTCs. 

Some of the current CTC detection systems include:

• RT-PCR (reverse transcriptase polymerase chain reaction)
• Immunofluorescence and immunocytochemistry
• FISH (fluorescence in situ hybridization)
• SE-iFISH (immunostaining-FISH combined with subtraction enrichment)
• FACS (fluorescence assisted cell sorting)

Current systems for separating and isolating CTCs include:

• Ficoll
• Size-based enrichment
• RosetteSep
• Magnetophoretic mobility-based separation
• Microfluidic devices
• FAST (fiber-optic array scanning technology)
• AdnaTest
• CellSearch
• Ariol system

CellSearch is the only commercially available and FDA-approved technique for analyzing blood samples to enumerate CTCs. It is used in prognosis for patients with breast cancer, prostate cancer, and colorectal cancer.