Circulating tumour DNA - looking beyond the blood

Over the past decade, various liquid biopsy techniques have emerged as viable alternatives to the analysis of traditional tissue biopsy samples. Such surrogate 'biopsies' offer numerous advantages, including the relative ease of obtaining serial samples and overcoming the issues of interpreting one or more small tissue samples that might not reflect the entire tumour burden. To date, the majority of research in the area of liquid biopsies has focused on blood-based biomarkers, predominantly using plasma-derived circulating tumour DNA (ctDNA). However, ctDNA can also be obtained from various non-blood sources and these might offer unique advantages over plasma ctDNA. In this Review, we discuss advances in the analysis of ctDNA from non-blood sources, focusing on urine, cerebrospinal fluid, and pleural or peritoneal fluid, but also consider other sources of ctDNA. We discuss how these alternative sources can have a distinct yet complementary role to that of blood ctDNA analysis and consider various technical aspects of non-blood ctDNA assay development. We also reflect on the settings in which non-blood ctDNA can offer distinct advantages over plasma ctDNA and explore some of the challenges associated with translating these alternative assays from academia into clinical use. Advances in circulating tumour DNA (ctDNA) detection and analysis are beginning to be implemented in clinical practice. Nonetheless, much of this development has thus far focused on plasma ctDNA. Theoretically, all bodily fluids, including urine, cerebrospinal fluid, saliva, pleural fluid and others, can also contain measurable ctDNA and can provide several advantages over the reliance on plasma ctDNA. In this Review, Tivey et al. describe the potential roles of ctDNA obtained from non-plasma sources in optimizing the outcomes of patients with cancer.