CTC Biomarker

What are CTCs? How are they used as a biomarker?

Circulating tumor cells (CTCs) are cells that break away from a tumor and travel through the bloodstream to other parts of the body. They can travel in the blood to distant organs​ or tissues​, where they may form a metastatic tumor.

The detection of CTCs in the blood is currently used as a predictive and prognostic biomarker in metastatic CRC. Some ways that your medical care team may use CTC testing include:

• To predict prognosis – the number of CTCs present in the blood can be used to predict a favorable or unfavorable prognosis.
• To determine changes in prognosis – a change in the number of CTCs present in the blood over time can be used to determine a change in original prognosis.
• To monitor for disease progression – an increase in CTC levels may suggest that the cancer is progressing.
• ​​To guide a treatment plan – a change in CTC counts over time may be used to suggest whether a treatment is working. ​ ​​
• ​​​To detect other biomarkers, such as PD-​L1​ or HER2 – ​CTCs ​may reflect different biomarkers than the primary tumor, which may open additional therapeutic options​​​​
• ​​​To monitor for recurrence – During remission, the presence of, or an increase in CTCs over time can indicate a possible recurrence of cancer​​​​

Research exploring other uses for CTC testing is ongoing.

How is CTC testing performed? What do CTC test results mean?

CTCs are isolated from blood samples, or liquid biopsies. Liquid biopsy samples are sent to a lab where the number of CTCs present in the sample is counted. This process is called CTC enumeration.

CTC testing results are generally reported as the number of CTCs present per 7.5 mL of blood. The number of CTCs in your sample will be compared to a predetermined cutoff number. Currently, the cutoff number for mCRC is 3 CTCs per 7.5 mL of blood​ when using the CellSearch CTC ​​test​.

Whether your CTC count is above or below the predetermined cutoff number can be used to predict prognosis.

• Patients with CTC counts below the cutoff number (0-2) are predicted to have a more favorable prognosis.
• Patients with CTC counts above the cutoff number (3+) are predicted to have a less favorable prognosis.

CTC testing may be done at multiple timepoints throughout treatment or during monitoring. This is sometimes referred to as serial CTC testing. Changes in CTC count can also be used to determine changes in prognosis.

• CTC counts that remain or drop below the cutoff (0-2) are predicted to have a more favorable prognosis. These results may indicate that the current treatment is working or that the cancer is not progressing.
• CTC counts that remain or rise above the cutoff (3+) are predicted to have a less favorable prognosis. These results may indicate that the current treatment is not working or that the cancer may be progressing.

What’s the difference between CTC and ctDNA tests?

CTCs and ctDNA are both indicators obtained via blood draw that can be used to guide treatment decisions and monitor disease progression. However, there are differences between the two types of testing which include:

• Stage of disease – ctDNA testing may be used in patients with stage II, III, or IV CRC, while CTC testing ​can be performed with patients at any stage, the cut-off value ​is currently limited to patients with metastatic (stage IV) CRC.
• Biomarker testing – ctDNA is​ genetic material​ used for next-generation sequencing (NGS) to determine the status of ​a wide range of ​other biomarkers. ​CTC testing is whole cells that can have select biomarkers detected on or in the cell.​

Is CTC testing the same thing as ​specific ​biomarker testing?

CTC testing is not the same as biomarker testing. Additional testing​, such as PD-L1 or HER2 testing on the surface of CTCs, or NGS of DNA/RNA​ ​from CTCs ​is required to determine the status of other biomarkers.

Is CTC testing for all patients?

Patients with​ ​​any stage of​ CRC may benefit from CTC testing. The best way to determine whether CTC testing is right for you is to ask your medical care provider.

There is ongoing research exploring the potential uses for CTC testing at earlier stages of disease.

Who orders tests on CTC? Who pays for CTC testing? What should I do if I want my doctor to run CTC tests on me?

Currently, the CELLSEARCH​® CTC test is the only FDA-approved blood test for detecting CTC​ ​in CRC patients. CELLSEARCH​® CTC testing is covered by most insurance plans.

Only health care professionals can order a CTC test. You should talk to your health care provider if you would like CTC testing.