When Cancer Leaves a Trace in the Blood: How ctDNA Could Change Cancer Monitoring

What if a simple blood sample could reveal clues about what a tumor is doing?

That idea is driving growing interest in ctDNA cancer testing—an approach that analyzes tiny fragments of DNA released by cancer cells into the bloodstream.

Known as circulating tumor DNA (ctDNA), these fragments can carry genetic information from a tumor and may provide insights that complement imaging, tissue biopsies, and other clinical tests.

A Liquid Biopsy With a Different Perspective

Traditional tumor biopsies require doctors to obtain tissue from the cancer. A liquid biopsy takes a different route: instead of collecting a piece of the tumor, clinicians analyze substances in a blood sample.

ctDNA can contain tumor-specific genetic alterations. Depending on the cancer and clinical situation, testing may help identify molecular changes, support treatment planning, or monitor how cancer is responding.

That makes the concept particularly interesting for precision oncology.

Could Blood Reveal Treatment Response Earlier?

Cancer treatment is often monitored using imaging, laboratory tests, symptoms, and other clinical assessments.

Researchers are investigating whether changes in ctDNA could provide an additional signal of how cancer is responding.

For example, active NCI trials are studying whether ctDNA can help determine treatment response, identify residual disease, or guide treatment decisions in cancers including colorectal, lung, breast, and germ-cell cancers.

The attraction is simple:

A blood sample can potentially be collected repeatedly.

That creates an opportunity to study cancer dynamically rather than relying only on occasional snapshots.

The Hidden Cancer Problem

One of the most intriguing applications is detecting minimal residual disease (MRD)—small amounts of cancer that may remain after treatment but are too limited to be easily identified through conventional approaches.

Researchers are studying whether ctDNA can help identify patients who may have residual cancer after treatment and who might benefit from closer monitoring or additional therapy.

But an important distinction matters:

ctDNA testing is not a universal replacement for scans, biopsies, or clinical evaluation.

Its usefulness depends on the cancer type, assay, disease setting, and clinical evidence available.

From Detection to Treatment Decisions

The field is moving beyond simply asking whether ctDNA can detect cancer.

Researchers are asking whether its results can actually change clinical decisions.

In 2026, NCI’s ComboMATCH program incorporated ctDNA sequencing as a non-invasive way to screen some patients for eligibility in a precision-medicine clinical trial based on specific tumor alterations.

Other ongoing trials are investigating whether ctDNA results can help determine when treatment should be intensified, continued, changed, or potentially reduced.

The Future of Liquid Biopsy

The long-term vision is powerful: combine ctDNA, tissue biomarkers, imaging, molecular profiling, and clinical information to build a more dynamic picture of cancer.

Instead of asking only:

“Is the cancer still there?”

the future may increasingly ask:

“What is the cancer doing right now—and what should happen next?”

The Takeaway

ctDNA cancer testing represents one of the most fascinating developments in liquid biopsy research. It could add another layer of information to cancer monitoring and personalized treatment, although researchers are still defining where and how it should be used routinely.

The next breakthrough in cancer care may not always require another scan. Sometimes, the clue could be hiding in the blood.

This article is for general educational purposes only and should not replace personalized medical advice, diagnosis, or treatment from a qualified healthcare professional.