Circulating tumor cells (CTCs) are cells that have detached from a primary tumor and entered the bloodstream. These cells can travel through the circulatory system and potentially form new tumors in distant organs, a process known as metastasis. Detecting and counting these cells in a patient’s blood sample is the basis of a “liquid biopsy.” Unlike a traditional tissue biopsy that requires a surgical procedure, a liquid biopsy only needs a standard blood draw, allowing doctors to gather information about a cancer safely and repeatedly over time.
The Purpose of CTC Detection
The detection of CTCs serves several purposes in cancer care and research:
- Prognostic Information: The number of CTCs in a blood sample helps doctors understand the potential aggressiveness of a cancer. Studies across various cancer types have shown a correlation between higher CTC counts and poorer patient outcomes. For example, in patients with metastatic breast cancer, finding five or more CTCs in a 7.5 mL blood sample is associated with shorter survival.
- Treatment Monitoring: Tracking CTC levels over time offers a way to monitor how well a treatment is working. A blood sample can be taken before therapy to establish a baseline, and subsequent tests can show if the number of CTCs is decreasing, which suggests the therapy is effective.
- Guiding Therapy Decisions: The genetic makeup of captured CTCs can be analyzed to identify specific mutations driving the cancer’s growth. This molecular information can help doctors select targeted therapies designed to attack cancer cells with those particular vulnerabilities, leading to more personalized treatment.
- Understanding Metastasis: Researchers examine these cells to learn how they survive in the bloodstream, evade the immune system, and establish new tumors. This knowledge is used to develop new strategies aimed at preventing or treating metastatic cancer.
Methods for Isolating Circulating Tumor Cells
The main challenge in CTC detection is their extreme rarity, as there may be only one tumor cell among billions of normal blood cells. Isolation techniques generally fall into two categories based on the properties they exploit to separate CTCs from other blood cells.
One strategy relies on the physical characteristics of tumor cells, which often differ from normal blood cells. Techniques based on microfiltration, for instance, work like a microscopic sieve. Because cancer cells are typically larger and less deformable than red and white blood cells, they can be separated. When a blood sample is passed through a filter with precisely sized pores, the smaller, more flexible blood cells pass through while the larger CTCs are trapped for collection.
Another approach is immunoaffinity-based separation, which uses antibodies that bind to specific proteins (antigens) on the surface of cancer cells. In a process called ‘positive selection,’ antibodies target an epithelial protein called EpCAM. Since many cancers originate in epithelial tissues, their CTCs often have this protein while normal blood cells do not. The antibodies are coated onto magnetic beads, which latch onto the EpCAM-positive CTCs and allow them to be pulled from the sample with a magnet.
A contrasting immunoaffinity method is ‘negative selection.’ Instead of targeting CTCs directly, this technique uses antibodies that bind to proteins common on the surface of normal blood cells, such as the CD45 antigen on white blood cells. These antibodies are used to remove the vast population of normal blood cells. The cells left behind are highly enriched for CTCs and can be collected for study.
Analyzing Captured Tumor Cells
Once isolated, the first step is enumeration, which is counting the captured CTCs. This count provides the initial data point for the prognostic assessments discussed earlier. The U.S. Food and Drug Administration has cleared the CELLSEARCH® system for counting CTCs in patients with metastatic breast, prostate, and colorectal cancers.
Beyond counting, captured cells undergo molecular and genetic analysis to extract more detailed information. Scientists can perform genomic sequencing on the DNA of individual CTCs to identify specific mutations or genetic alterations. For example, identifying a mutation in a specific gene might indicate that a drug targeting the protein made by that gene would be an effective treatment.
A more advanced analysis involves growing captured CTCs in a laboratory, a process known as creating cell cultures or patient-derived xenografts. This creates a living ‘avatar’ of the patient’s cancer. These lab-grown cells can then be used to test the effectiveness of various chemotherapy drugs or other treatments, helping to predict which therapies are most likely to be successful.
Challenges in Clinical Application
Despite its promise, several practical hurdles prevent CTC detection from becoming a routine test for all types of cancer.
A significant issue is the biological diversity of tumor cells. CTCs are not identical, and some may lack the surface proteins, like EpCAM, that many detection systems use for capture. This means certain CTCs can evade detection, leading to an undercount and potentially misleading results.
Another challenge is the lack of standardization across different detection platforms. Numerous laboratories have developed their own technologies for isolating and analyzing CTCs, each with different methods and levels of sensitivity. This variation makes it difficult to compare results from one test to another or to combine data from different clinical studies. A standardized approach is needed to ensure that a CTC count from one lab means the same thing as a count from another.
The viability and purity of isolated cells also present a challenge. For molecular analysis or cell culture, CTCs must be captured alive and intact, but the isolation process can be harsh. Ensuring the final sample is pure and free from contamination by other blood cells is also necessary for obtaining reliable genetic data.