A liquid biopsy is a test done on a sample of blood to diagnose cancer and is a simple, non-invasive alternative to surgical biopsies. In this test, cells or fragments of DNA from a tumor which are circulating in the blood are detected.
Biopsy is a procedure where depending on the lesion size, the whole tissue or part of the tissue is removed surgically and sent for histopathological examination. The term histopathological examination involves preparing the tissue by processing and cutting it to make slides and examining the tissue to conclude the nature of lesion.
Biopsy and histopathological examination are two different procedures. First one is usually performed by surgeon and second involves pathologist and laboratory staff.
However, over the period the term biposy has covered both the aspect. Thus in routine clinician talk, the report is referred to as biopsy report and in operation theater the procedure biopsy means that the removed tissue would be sent to department of pathology for making that biopsy report.
With that background knowledge, let us discuss further.
In conventional biopsy,When a patient has a lump (superficial lump visible with naked eye or an internal mass detected by imaging or based on clinical symptoms), the diagnosis is confirmed by a tissue biopsy. In this procedure, either a small piece of the lump (incisional biopsy) or the entire lump (excisional biopsy) is removed. This removed tissue is processed and then examined by a histopathologist under the microscope. Based on the morphological appearance of the cells under the microscope, it can be determined whether cancer is present or not. It can also give information regarding the type of cancer and the prognosis. Additional testing including immunohistochemistry, molecular and genetic testing may also be carried out on this removed tissue piece to gather further information about the tumor.
For a long time, this “conventional” or “standard” biopsy has been the mainstay as well as gold standard for confirming or ruling out a cancer diagnosis.
Tissue biopsies require the use of a large needle, an endoscope, or open (minor or major) surgery to retrieve the tissue from the body. Thus it is an invasive, risky and painful procedure. It also requires admitting the patient to the hospital thereby, increasing the cost.
In some cases, tissue biopsy may not be feasible due to inaccessibility of the tumors as in case of brain tumors. Old age or other health conditions may prevent some people from undergoing this procedure. Since, it is impractical to perform repeated biopsies on a patient, so these tests cannot be used as a method to track tumors and response to treatment.
Recent research has been focusing on a different approach that could act as an adjunct or in some cases, serve as an alternative to tissue biopsies. This technique, often called a liquid biopsy, is based on detecting and analyzing fragments of tumor tissue—either whole cells or molecules of cells that are circulating in body fluids such as blood, urine, cerebrospinal fluid or saliva.
Like conventional biopsy, liquid biopsy has a role in diagnosis and monitoring of diseases like cancer. Its major advantage over conventional biopsy is that it is largely non-invasive; requiring only a routine blood sample or other body fluids which can be easily obtained.
It can be used as a screening test to diagnose cancer in early stages, even before the appearance of symptoms. Since it can be done more frequently, so it can be used to track tumors and mutations over a period of time. It may also be used to determine the effectiveness of a treatment regimen by taking multiple liquid biopsy samples in the span of a few weeks. It may also be used to detect any relapse after successful completion of treatment.
Inspite of its obvious advantages over conventional biopsy, this relatively new technique is still in experimental stages and its clinical implementation is not yet widespread.
Currently liquid biopsy has just been approved by the USFDA for clinical use is as a prognostic survival tool with no potential to guide treatment decisions.
Although liquid biopsies have their major utility in cancer diagnosis, they can also be used in certain other conditions. Circulating endothelial cells (CECs) have a role in heart attack diagnosis. To diagnose prenatal conditions, cell-free fetal DNA (cffDNA) can be isolated from maternal blood or amniotic fluid and analyzed.
Liquid biopsies detect either circulating tumor cells (CTC) or cell-free tumor fragments like DNA (ctDNA), RNA or proteins. Advances in sequencing and mapping the human genome have enabled the scientists to detect genetic mutations in cancer fragments in blood.
Uses of Liquid biopsies
Screening Test to Detect Early Cancer
Liquid biopsies can be used to detect cancer at an early stage which can have a huge impact on survival and prognosis. In several studies, ctDNA was detected in blood from patients several months before they were diagnosed with cancer by conventional methods.
The drawback was that these tests sometimes yielded false-positive results that is, tumor DNA was detected in blood while no cancer actually developed in future. Also these tests might detect early-stage tumors that are unlikely to progress further or harm the patient in his lifetime. Over- treating these slow-growing tumors may cause more harm to the patient.
Further studies are needed to establish that detection of ctDNA in a patient’s blood can be used as a confirmatory marker for early-stage cancer and whether treatment given to such patients will actually prove useful to the patient.
To Provide Targeted Cancer Treatment
Liquid biopsies can identify unique molecular characteristics, such as specific mutations in an individual cancer. Optimal treatment could be then provided to the patient to treat that particular mutation.
Cobas® EGFR Mutation Test was approved by FDA in 2016 for the detection of EGFR gene mutations in ctDNA of patients with lung cancer. This liquid biospy test helped to identify patients who could be benefited by treatment with erlotinib and osimeritinib . Both these are targeted therapies that attack cancer cells with EGFR mutations. Since there was a concern of a false-negative test result, it was recommended by FDA that a tissue biopsy be performed, if the liquid biopsy was negative that is, it did not detect an EGFR mutation.
To Monitor Response to Treatment
Being non-invasive in nature, liquid biopsy they can be used to monitor response of a patient to a particular drug regimen both during treatment and after it is completed.
This can enable clinicians to make appropriate changes in treatment, if the test results indicate that the therapy is not effective.
Present monitoring is done by imaging like CT/MRI but these are not able to detect small changes in tumor size and are also costly. Also levels of tumor cell fragments in blood can be used to detect recurrence of the disease after completion of treatment much earlier than other conventional methods.
In a research carried out in lymphoma patients, it was observed that changes in ctDNA levels correlated with positive response to chemotherapy.
Similarly, changes in ctDNA level were found to correlate with patients’ responses to immunotherapy treatment.
This could be really helpful because only a small proportion of patients respond to immunotherapy treatment.
To Provide Molecular Information About Cancer
Liquid biopsy tests can provide molecular information about the cancer, which can change during and after treatment. This could help in tracking the development of drug resistance and making more personalized treatment decisions.
For example, most patients with lung cancer develop drug resistance to tyrosine kinase inhibitors within 1 or 2 years of starting treatment. Majority of these patients who developed the drug resistance showed a common genetic mutation which had resulted in drug resistance.
Limitations of ctDNA-based Liquid Biopsies
Majority of the cancers do not have biomarkers discovered yet which can be used to detect or monitor a cancer.
DNA mutations can be different for different patients even with the same cancer type. Thus, although a particular genetic alteration may be commonly seen in one type of cancer, many patients with that cancer type may not have it. Also tumors are heterogeneous which means that even in a single patient, mutations can vary between different cancer cells. It remains a huge challenge to identify different biomarkers for every cancer type.
Tumor fragments released in the body fluids may not be true representative of the actual tumor. So information revealed by these cells or fragments, may not be ideal for taking treatment decisions.
It is also unknown whether the mutations found are “driver” mutations, that is mutations which lead to development of cancer or “passenger” mutations, that is, changes that occur due to the development cancer and not causing it.
The most important issue is whether liquid biopsy tests ultimately have an impact on prognosis, survival and quality of life of the patient.
Future and Further Research
Liquid biopsy is still in experimental stages and a lot of research has been undergoing in this technique. Within a span of few years, it has emerged as a completely new and different field of study.
It is believed that in future, these tests could be used as a routine prescreening test just like mammography and colonoscopy in healthy individuals. Also, instead of extensive imaging and invasive tissue biopsies, liquid biopsies could be used to guide cancer treatment decisions. This would revolutionize and entirely change the diagnostic tools and management of cancer in near future.
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