A liquid biopsy is the collection and evaluation of tumor material that is noninvasively derived from body fluids, such as blood, saliva, urine, stool, cerebrospinal fluid, pleural fluid, and ascites.1 The tumor material that is assessed can be intact tumor cells, subcellular particles, extracellular vesicles, proteins, or nucleic acids. In the context of cancer, liquid biopsy is increasingly being used to refer to the analysis of tumor-specific cell-free DNA (cfDNA), or circulating tumor DNA (ctDNA). The advantage of the liquid biopsy is convenience and lack of invasive testing, whereas a tissue biopsy enables histologic assessment of the tumor, including expression of non–DNA-based alterations. In addition, analysis of ctDNA is useful when inadequate tumor material is collected during a tissue biopsy, or for patients who are unable to undergo biopsy or for those who have tumors that are inaccessible.

Essentially, ctDNA is cfDNA released by tumor cells. Other, normal cells also release cfDNA into circulation. The proportion of cfDNA that is made up of ctDNA varies widely between patients with the same type of cancer and also between cancer types, although it is thought that this proportion is generally associated with tumor burden within individual patients.

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ctDNA Collection and Analysis

Because ctDNA is present at very low concentrations in the blood and has a very short half-life, collection and analysis can be challenging. Collection of ctDNA requires either the use of specialized collection tubes or nearly immediate processing to ensure stability.

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Analysis of ctDNA requires ultrasensitive methods, particularly to detect any genetic aberrations such as point mutations, copy-number variations, or other changes. Techniques that are used include targeted approaches such as ultrasensitive polymerase chain reaction (PCR) modalities, or more broad profiling approaches, such as next-generation sequencing (NGS).

Clinical Applications

There are many potential clinical applications for ctDNA — diagnosis and molecular profiling, monitoring treatment response, identification of treatment resistance, residual disease detection, and cancer screening.1,2 It is important to understand, however, that in most cases, the use of ctDNA for these applications has not yet undergone rigorous testing in prospective, randomized-controlled trials.1 A joint review by the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) concluded in mid-2018 that there is currently no evidence of clinical validity or utility for ctDNA assays for advanced cancers, except for the assays that have received US Food and Drug Administration (FDA) approval.2 The expert panel reached similar conclusions for the use of ctDNA for cancer screening, use in early-stage cancer, treatment monitoring, and for residual disease detection after surgery or adjuvant therapy.