The Waiting Game Between Scans
After surgery, radiation, and chemotherapy, most glioblastoma patients have follow-up MRI scans every two to three months. That schedule makes clinical sense. MRI remains the gold standard for tracking tumor changes in the brain. But for patients and families, the weeks between scans can feel unsettling. Questions fill the gap: Is the tumor growing right now? Did last month's headache mean something? Is treatment still working?
A technology called circulating tumor DNA (ctDNA), a type of liquid biopsy, may one day help answer those questions between MRI appointments. Research in this area has grown rapidly, though the science is still maturing and no liquid biopsy test is currently FDA-approved specifically for glioblastoma monitoring. It's worth understanding what liquid biopsy can and cannot do right now if you're tracking your own care.
What Is a Liquid Biopsy?
A liquid biopsy is a test done on a fluid sample, usually blood or cerebrospinal fluid (CSF), that looks for cancer-related material the tumor has shed into circulation. Tumors constantly release small fragments of DNA, proteins, and other molecules into surrounding fluids. In people with glioblastoma, those fragments may include:
- Circulating tumor DNA (ctDNA) — tiny fragments of tumor-specific genetic material that carry the mutations of the original tumor
- Extracellular vesicles (EVs) and exosomes — nano-sized particles released by tumor cells that carry genetic and protein cargo
- Circulating tumor cells (CTCs) — intact cancer cells that have entered the bloodstream
- Tumor-educated platelets (TEPs) — platelets that have absorbed RNA signals from the tumor
- Cell-free RNA and methylation patterns — additional molecular signals that may reflect tumor activity and gene regulation
Unlike a surgical biopsy, a liquid biopsy requires only a blood draw or a lumbar puncture. It's far less invasive. Because it can be repeated over time, it offers the chance for real-time monitoring that a single tumor tissue sample cannot provide.
Research on glioblastoma molecular profiling through liquid biopsy (PMC, 2025) shows that liquid biopsy components can reflect the full range of genetic variation in a glioblastoma tumor, capturing signals from different tumor regions that a single surgical sample might miss.
The Big Obstacle: The Blood-Brain Barrier
Glioblastoma is a brain tumor. That location creates a specific challenge that makes liquid biopsy harder here than in many other cancers.
The blood-brain barrier (BBB) is a tight network of cells surrounding blood vessels in the brain. It blocks many substances, including tumor-derived DNA fragments, from entering the bloodstream. As a result, ctDNA concentrations in blood of glioblastoma patients tend to be very low compared to patients with lung or colon cancer, for example.
A systematic review in PMC (2025) found ctDNA detection rates in cerebrospinal fluid of about 82%, compared to roughly 16% in plasma. The gap is substantial. CSF bathes the brain directly and contains far more tumor-derived material than blood does.
Blood-based liquid biopsy isn't useless for glioblastoma. Current technology just has to work harder to detect low signals. CSF-based testing tends to offer better sensitivity when detection is the main goal.
Blood Versus Cerebrospinal Fluid: The Trade-Off
Both blood and CSF have advantages and disadvantages.
Blood (plasma): A standard blood draw is safe, quick, and can be repeated frequently without significant burden or risk. The downside is low signal. ctDNA from a brain tumor crosses the BBB in very small amounts. Newer technologies like droplet digital PCR (ddPCR) and next-generation sequencing (NGS) have improved detection of these small signals, but sensitivity is still a challenge. The half-life of cell-free DNA is also under 1.5 hours, which means samples must be processed quickly to preserve results.
Cerebrospinal fluid (CSF): CSF provides much richer tumor-derived material because it directly contacts brain tissue. Detection rates are meaningfully higher. The downside is that collecting CSF requires a lumbar puncture (spinal tap). This is more invasive than a blood draw, carries procedural risks, and can't be done frequently. Repeated lumbar punctures also mean more procedural risk over time that doctors need to weigh carefully.
Researchers are exploring a technique called sonobiopsy. It uses low-intensity focused ultrasound to temporarily open the blood-brain barrier at the tumor site before a blood draw, releasing more tumor material into the bloodstream. Early data suggest this approach may boost ctDNA detection rates in blood, though it's still experimental and not yet in routine use.
A 2025 review on liquid biopsies for gliomas and brain metastases suggests that combining blood and CSF results with imaging data may give a better picture of tumor activity than either test alone.
What Can Liquid Biopsy Actually Detect?
When a liquid biopsy detects tumor-derived material, it can show several important things.
Tumor-specific mutations: ctDNA carries the same genetic mutations found in the original tumor, such as changes in genes like EGFR, PTEN, TP53, TERT promoter, and MGMT methylation status. Detecting these mutations in a blood or CSF sample can confirm the signal is truly tumor-derived. It can also reveal whether the tumor has acquired new mutations during treatment, a process called clonal evolution, which may affect sensitivity to specific therapies. To learn how individual mutations might affect your treatment, see our articles on TP53 Mutation in Glioblastoma and What Molecular Tests Your Newly Diagnosed Glioblastoma Actually Needs.
Methylation profiling: Tumor cells often have distinctive DNA methylation patterns, which are chemical tags on DNA that affect how genes are turned on or off. Methylation profiling of cell-free DNA in blood or CSF is being studied and may allow tumor classification and recurrence detection even when ctDNA levels are too low to find mutations directly.
Extracellular vesicles and exosomes: Glioblastoma cells release many exosomes, which are tiny particles loaded with proteins, RNA, and DNA. These particles may cross the blood-brain barrier more easily than ctDNA fragments. This makes them useful targets for blood-based testing. Exosome cargo appears to reflect tumor biology, treatment response, and drug resistance.
One of the Most Promising Applications: Pseudoprogression Versus True Recurrence
One of the hardest problems in glioblastoma management is interpreting MRI changes after chemoradiation. When a scan shows a larger-looking lesion, it may represent true tumor growth. Or it may represent pseudoprogression, an inflammatory reaction to treatment that looks like growth on imaging but isn't actual tumor expansion. This distinction is very important. If the enlargement is pseudoprogression, continuing current therapy may be the right call. If it is true recurrence, the care team may need to pivot to salvage treatment.
Liquid biopsy may provide another clue. Rising ctDNA levels in blood or CSF may point to true tumor growth rather than treatment-related swelling. This could help guide decisions when the imaging is unclear. Researchers are actively studying this use of liquid biopsy in neuro-oncology. For more about this imaging challenge, see our article on Radiation Necrosis After Glioblastoma Treatment: How to Tell It Apart From Tumor Recurrence.
A 2024 analysis on circulating liquid biopsy biomarkers in glioblastoma identifies pseudoprogression differentiation as one area where serial liquid biopsy shows promise in the near term.
Can Liquid Biopsy Catch Recurrence Before an MRI Does?
This is one of the most important questions now being studied in neuro-oncology. Rising ctDNA levels in blood or CSF may appear before imaging shows changes. Researchers call this the molecular lead time. In theory, this window could let doctors think about changing treatment before the tumor looks clearly different on a scan.
A clinical trial (NCT05539339) is looking at personalized ctDNA-guided treatment decisions for glioblastoma patients at molecular relapse. These are patients whose liquid biopsy suggests recurrence before imaging shows it. This is one of the most advanced uses of this technology being studied right now.
But this idea is still being tested. Early detection via liquid biopsy hasn't yet been proven in randomized clinical trials to improve outcomes for glioblastoma patients. The current goal is to validate that molecular signals reliably predict clinical recurrence. Then researchers want to test whether acting on those signals earlier actually changes patient outcomes.
Personalized Assays and Emerging Technology
The low amount of ctDNA in glioblastoma blood samples has driven researchers toward increasingly sensitive laboratory methods. Several approaches are being developed:
- Droplet digital PCR (ddPCR): Partitions a blood sample into thousands of tiny droplets, each analyzed independently, allowing detection of very rare mutant DNA molecules in a large background of normal DNA
- Next-generation sequencing (NGS): Reads millions of DNA sequences simultaneously, enabling detection of multiple mutations and broader genomic profiling from a single sample
- Personalized ctDNA assays: Designed from a patient's own tumor genome, these tests look specifically for that tumor's unique mutations in blood samples over time, improving sensitivity compared to generic off-the-shelf panels
- Methylation-based classifiers: Use the tumor's methylation signature rather than DNA sequence mutations—useful when ctDNA quantity is very low and mutation-based detection fails
A 2025 review on liquid biopsy-derived tumor biomarkers for clinical applications in glioblastoma shows that personalized assays designed from a patient's tumor profile can improve detection sensitivity compared to generic panels. This is how precision oncology works: design the test around your specific tumor, not population averages.
Where Things Stand Today
Despite the promise, liquid biopsy for glioblastoma monitoring is not yet a standard part of clinical care. No blood-based or CSF-based ctDNA test is FDA-approved for glioblastoma surveillance. Most testing in this area is happening within clinical trials or at specialized academic medical centers with dedicated neuro-oncology research programs.
Several factors are driving rapid progress:
- Sequencing technology that can now detect lower concentrations of tumor DNA than was possible just five years ago
- Growing recognition that MRI alone has limitations—especially around pseudoprogression and heterogeneous tumor response across different tumor regions
- Multiple prospective clinical trials now running that include serial liquid biopsy as a primary or secondary endpoint
- Interest in combining liquid biopsy with advanced imaging, AI-based scan analysis, and other biomarkers into integrated real-time monitoring frameworks
If you are enrolled in or thinking about joining a clinical trial, ask whether liquid biopsy collection is part of the protocol. Clinical trials are currently one of the main ways this technology is being studied before it can become standard care.
When to Talk to Your Doctor
Ask your neuro-oncologist or care team about liquid biopsy if:
- You want to know what monitoring options beyond routine MRI may be available at your center or through a clinical trial
- Your most recent MRI showed ambiguous changes that may represent pseudoprogression or true recurrence and you want to explore additional testing
- You are considering enrolling in a clinical trial and want to know whether liquid biopsy is included in the protocol
- You are at a point of recurrence and want to understand whether molecular re-profiling—including liquid biopsy—might inform salvage treatment options
Most liquid biopsy tests for glioblastoma are currently research tools, not standard clinical tests. Your oncologist can tell you what is available at your specific institution and whether any open trials include this technology.
This article is for general information and is not a substitute for medical advice. Always consult your oncologist or care team about your specific situation.