What Is Liquid Biopsy?
A liquid biopsy is a non-invasive test to diagnose or analyze cancer tumors. In a traditional biopsy, a solid tissue sample is used for diagnosis and analysis. A liquid biopsy, in contrast, uses only blood or other fluid samples.
Liquid biopsy methodologies use next-generation sequencing to identify mutations. While liquid biopsy can analyze many types of cell-free DNA (cfDNA), cancer research is concerned with circulating tumor DNA (ctDNA).
In This Article
- What Can a Liquid Biopsy Extract?
- How Does Liquid Biopsy Work?
- Liquid Biopsy v. Tissue Biopsy
- Benefits of Liquid Biopsy Over Tissue Biopsy
- Uses of Liquid Biopsy in Oncology
- Current Liquid Biopsy Challenges
What Can a Liquid Biopsy Extract?
A liquid biopsy can contain:
- Circulating tumor cells (CTCs)
- Fragments of tumor DNA (ctDNA)
- Tumor proteins, which can indicate the cancer’s capability of spreading.
- Tumor exosomes, which contain messengers that help cancer develop and metastasize.
How Does Liquid Biopsy Work?
- A tumor cell releases DNA fragments into the patient’s bloodstream, either during apoptosis, necrosis, or secretion from live cells. This is circulating tumor DNA.
- A clinician orders a sample of the patient’s blood. Blood is the most common fluid used for liquid biopsy. However, ctDNA can also be detected in gastric juices, saliva, blood serum, and urine.
- Plasma or serum is separated from the rest of the blood with a centrifuge. cfDNA can then be extracted from the plasma or serum. ctDNA is a fraction of this cfDNA sample.
- Next-generation sequencing is used to amplify and analyze the DNA. NGS converts DNA into a library containing unique identifying sequences. The DNA then undergoes amplification and the sequence is read many times. This sequence is then analyzed for mutations and anomalies.
Droplet-based digital PCR can also be used for detection of mutations and tumor status in plasma samples. With this methodology, a plasma sample is diluted and separated into a large number of droplets. When NGS is used, it looks at a huge number of genes; PCR can be useful when researchers know they are looking for more specific indicators.
Liquid Biopsy v. Tissue Biopsy
In a traditional biopsy, clinicians must take a sample of the patient’s tumor tissue. This is done in one of several ways.
- If a tumor is on or near the skin’s surface, tools are used to scrape or excise a sample.
- Specialized needles can be used to biopsy suspicious tissue in lymph nodes, breast tissue, or bone marrow.
- If the tissue is part of the digestive system, an endoscopy can be used to find the tissue and insert tools in through the mouth, rectum, urinary tract, or incision in the skin.
- If it cannot be reached using one of the methods above, the clinician may recommend a surgical biopsy.
Once the tissue sample is extracted, the clinician will send the sample to a laboratory. Lab technicians will examine the tissue under a microscope to determine a few things:
- Whether or not the tissue is cancerous.
- What type of cancer it is, and where in the body it originated.
- How aggressive the cancer is.
During a tissue biopsy, genetic sequencing is sometimes conducted. Understanding a tumor’s specific genetic mutations is an important way to make targeted treatment decisions.
With these results, the clinician can make decisions about patient treatment. Once the patient has been treated, additional scans and blood work can determine if the treatment was effective. They can also monitor patients for cancer recurrence.
Benefits of Liquid Biopsy Over Tissue Biopsy
Heterogeneous tumor insights
Many tumors are heterogeneous, meaning they have a diverse set of cells. This can limit the ability of a single, tissue-based biopsy to access the tumor’s full molecular profile.
In contrast, liquid biopsies may detect tumor DNA shed from numerous sites within the tumor. This paints a more comprehensive genomic picture. Clinicians can be more informed when using molecularly targeted therapies.
Liquid biopsies are noninvasive. They’re done using blood, plasma, saliva, gastric juices, serum, or urine. For patients who are hesitant to undergo a procedure (or patients who've already been through a lot), this can be an attractive option.
A blood sample can detect ctDNA from tumors anywhere in the body. Traditional biopsies, however, must have access to the actual tumor tissue. A nonlocalized, minimally invasive approach provides more comfort for patients while advancing oncological care.
It also makes it possible to understand tumors in hard-to-reach parts of the body. This is especially valuable where a traditional biopsy would be difficult or impossible to conduct.
Liquid biopsies are more cost effective than traditional biopsies, even the simplest of which can cost thousands of dollars. Insurance doesn’t always cover traditional biopsies either, which makes it more financially strenuous on patients. Liquid biopsies are a fraction of the cost, and some insurance companies will cover the cost partially or in full.
Uses of Liquid Biopsy in Oncology
As cancer rates continue to increase globally, liquid biopsies present a better solution for diagnoses of various forms of the disease. Cell-free DNA is the most common substance used in liquid biopsies and one of the most researched.
ctDNA can be used to distinguish cancer patients from healthy ones. ctDNA levels are higher in those with cancer and increase as the stages of cancer progress. With tumor-specific analysis, cancer genotyping is concise and accurate. This genotyping also provides a more comprehensive diagnosis, especially in blood ctDNA that corresponds with tumor tissues across cancer variants.
Liquid biopsy can detect microsatellite instability (MSI). This is a change that occurs in cancer cells and some other cells, where the number of repeated DNA bases in a microsatellite (a short, repeated sequence of DNA) is different from what it was when the microsatellite was inherited. High MSI (MSI-H) can be an indicator in specific cancers. It can be a result of deficiencies specific to DNA repair protein complexes.
For example, MSI is an indicator for Lynch syndrome, and a risk factor in cancer development. Not only can MSI be used to predict cancer risk, its detection can indicate specific cancers because of irregularities in DNA repair protein complexes.
Liquid biopsies help solve one of the toughest challenges in oncological care: early detection and diagnosis. Diagnoses can be delayed for many reasons. However, delays most often happen because the patient or their provider does not recognize symptoms in time. With cancer claiming nearly 10 million lives in 2018, delayed diagnosis is a medical crisis.
Early cancer detection can save countless lives. Many forms of cancer can be treated effectively when diagnosed in an early stage. Liquid biopsies, therefore, are an important tool for early detection. They provide a quick and efficient way to screen for cancer using multigene assays by use of cfDNA extraction.
Ovarian cancer, for example, is notoriously difficult to detect, with only 20% of all cases caught in early stages. A panel developed in 2017 was able to distinguish between healthy women and women with ovarian cancer with a specificity of 90.7%. Researchers estimate this screening could detect certain ovarian cancers up to two years earlier than the current average diagnosis.
ctDNA analysis can determine the tumor mutational burden (TMB), or the number of non-inherited mutations per million bases. This indicator can help predict the effectiveness of certain treatments, especially immune checkpoint inhibitor therapy (such as the PD-L1 pathway).
Liquid biopsies may also be a more efficient and innovative method than tissue biopsies to select patients for clinical trials. The broader analysis of mutations can help guide appropriate therapy. Liquid biopsy can also be used for patient segmentation or inclusion and for patient monitoring.
In many cases, clinicians conduct a liquid biopsy and a tissue biopsy. This helps to compare results and establish patient monitoring for tumor changes.
Traditional biopsies are invasive and cannot always be conducted on a routine basis. Liquid biopsies offer something that traditional biopsies do not: regimen. Tracking genetic anomalies and genomic evolution makes cancer recurrence prediction possible and accessible. If a patient’s tumor is growing, or has come back after treatment, a liquid biopsy can detect those changes early.
Liquid biopsies can detect minimum residual disease (MRD), or a small number of cancer cells remaining in the body after treatment. MRD can speak to the effectiveness of a treatment and the potential for recurrence of a cancer.
In an article published in BMC Medicine, researchers considered 230 patients with early stage colorectal cancer. These patients had their tumors removed, then had their blood drawn at their first follow up appointment.
100% of patients with ctDNA in their blood at their first follow up appointment relapsed within three years. In comparison, only 10% of patients without ctDNA in their blood at their first follow up relapsed.
Lung, breast, ovarian, and skin cancers have had similar results in research studies. This information can help doctors determine how aggressive their treatment plan should be.
Current Liquid Biopsy Challenges
The current standard in oncology is still a solid tumor biopsy. In part, this is because liquid biopsy still presents challenges to clinicians and researchers, including:
- Low yield. Some clinicians are understandably hesitant to perform blood draws on a sick patient. Samples sometimes contain only small amounts of DNA and RNA, and yield varies widely. There is no good prediction for how valuable a sample will be until after it is sequenced.
- Low abundance of ctDNA. The amount of ctDNA in a sample depends on the type and stage of tumor, how and if the tumor’s cells are shedding DNA, and where the patient is in their treatment. Detecting ctDNA is not a foolproof way of determining whether or not a patient has a cancerous tumor.
- Rare messages. There are over 100 types of cancer, and even more genetic indicators. If a cancer is rare, or if the genetic indicator is uncommon, researchers might not have the context they need to interpret ctDNA results.
- Classification of tumor origin. ctDNA can't always determine where the tumor is, or what type of cancer the patient has. Further diagnostic testing is often necessary.
Research is underway to address these challenges, and ctDNA is understood more and more every day. In some contexts, liquid biopsy is already being adopted. For example, the tides are shifting in clinical trials. Blood draws are often a standard part of tracking patient progress and comparing treatment efficacy between patients.
As scientific advancements continue, liquid biopsy will be a more attractive, cost-effective, and sensitive option for cancer treatment.