The human body constantly produces and deals with abnormal cells. It has sophisticated mechanisms to monitor and address these cells, maintaining health.
What Are Circulating Tumor Cells?
Circulating Tumor Cells (CTCs) are cells that have detached from a primary tumor, or sometimes precancerous lesions, and entered the bloodstream or lymphatic system. These cells are distinct from normal cells due to characteristics acquired from their tumor origin. CTCs can be found in the peripheral blood of individuals with various types of cancer, including breast, prostate, lung, and colon cancer. Their presence indicates cells with tumor-like properties are in circulation, but does not automatically mean a cancer diagnosis. CTCs play a role in metastasis, the process where cancer spreads from its original site to other parts of the body. They can exist as single cells or in clusters. While their numbers in the blood can vary significantly, even reaching hundreds or thousands per milliliter in some cancer patients, these cells are generally rare compared to normal blood cells.
The Journey from Cell to Disease
For a single circulating tumor cell to establish a new tumor, it must overcome numerous biological challenges. Most CTCs do not lead to new tumors because they face a hostile environment in the bloodstream. They must survive the physical stress of circulation and evade detection by the body’s immune system.
Even if a CTC survives in the bloodstream, it then needs to exit the circulation and find a suitable site in a distant organ. This new environment, often called the metastatic niche, must support the cell’s survival, proliferation, and the formation of a new blood supply for the developing tumor. Only a very small fraction, estimated to be around 0.01% of detached tumor cells, successfully form new metastases. The presence of CTC clusters may increase the likelihood of successful metastasis compared to individual CTCs.
The Body’s Defense Against Abnormal Cells
The human body has a defense system that monitors and eliminates abnormal or damaged cells, including those with cancerous potential. This protective mechanism is primarily carried out by the immune system, which identifies and destroys these cells before they can proliferate or form tumors. Specific immune cells, such as natural killer (NK) cells and T-cells, are important in this defense. NK cells can identify and destroy abnormal cells by targeting their surface, while T-cells recognize and kill cancer cells. This protective system is effective in most healthy individuals, preventing the progression of many abnormal cells into clinical cancer. The lymphatic system, a part of the immune system, also helps filter out damaged and abnormal cells.
Detecting and Monitoring These Cells
Advanced technologies are used to detect circulating tumor cells. These methods are sensitive, capable of detecting even a few CTCs in a blood sample. For example, the CellSearch system is an FDA-approved method used for CTC detection in certain cancers like breast, colorectal, and prostate cancer. Other techniques involve isolating cells based on physical properties like size, or using specific antibodies to identify tumor markers.
These detection methods are primarily used in research settings and for monitoring cancer progression in diagnosed patients. However, CTC detection in healthy individuals is an area of ongoing research, and these methods are not used as general screening tools for the public. The detection of CTCs in healthy individuals is not a definitive diagnostic marker for future cancer development.