I came across the term liquid biopsy in 2010 and posted my first note about the term followed by two more about it (see: Does the New Term "Liquid Biopsy" Make Any Sense?; Rapid Adoption of the Term "Liquid Biopsy" on the Web; Continuing Discussion about the "Liquid Biopsy") Here's a quote from the first of them:
...[The use of the term liquid biopsy]... may offer some benefits. It differentiates between serum biomarkers that are elaborated by circulating tumor cells (CTCs) as opposed to, say, those that are released into the blood stream from a diseased organ or solid tumor. However, it's also possible that all, or perhaps most, serum biomarkers are the result of CTCs.
As a pathologist, I was influenced the idea that the word biopsy indicate that cells are being obtained and analyzed. In the above passage, I suggested that the term liquid biopsy should be limited to biomarkers identified in serum that were released by circulating tumor cells. On the basis of a recent article, however, it seems that this distinction is being lost and liquid biopsy is being applied to any cancer serum biomarker (see: The Liquid Biopsy: A Noninvasive Tumor Tracker). It skirts the definitional cell/serum issue by referring to a liquid biopsy as "a blood test that detects evidence of cancer in the circulation." A biopsy is thus being considered as a type of test. Below is an excerpt from this article but read the whole thing to understand the broader picture.
...{R]esearch published in ...Science Translational Medicine shows how liquid biopsies can provide a noninvasive, ongoing picture of a patient's cancer, offering valuable insight into how best to fight it. Work from [two] different groups shows how liquid biopsies are being used in the lab to identify tumors at a very early stage, monitor them for metastasis, and even pick up signs of early treatment resistance. In the future, instead of extensive imaging and invasive tissue biopsies, liquid biopsies could be used to guide cancer treatment decisions and perhaps even screen for tumors that are not yet visible on imaging....[Researchers have demonstrated] that a liquid biopsy measuring the serum level of circulating tumor (ct)DNA could one day be a very useful tool in cancer decision-making, giving clues about what type of cancer a patient has and whether it has spread. "Mutant DNA fragments are found at relatively high concentrations in the circulation of most patients with metastatic cancer and at lower but detectable concentrations in a substantial fraction of patients with localized cancers," [according to the researchers] .The team found this to be particularly true in cases of breast, colon, pancreas, and gastroesophageal tumors, where "detectable levels of ctDNA were present in 49% to 78% of patients with localized tumors and 86% to 100% of patients with metastatic tumors."...."In addition to offering clues about stage and spread, liquid biopsies can be used to monitor the effects of cancer treatment, give an early warning about possible recurrence, and offer clues to the reasons for treatment resistance"
I fear that this definitional battle has now been lost and that the term liquid biopsy will henceforth be used for any molecular or cellular evidence of a malignant tumor in serum. This evidence can be in the form of circulating tumor cells (CTCs), DNA released from such circulating cancer cells, or DNA that has been released by the in situ cells of a primary or metastatic cancer. In fact, in many cases, it may not be possible to differentiate between cancer DNA from circulating cells versus in situ cells.
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