Final Report Summary - CANCEREXOMESINPLASMA (Non-invasive genomic analysis of cancer using circulating tumour DNA) Liquid biopsy can be used to non-invasively sample a cancer patient’s tumour, potentially offering an alternative to traditional invasive sampling techniques. Recent years have seen an explosion in interest in circulating tumour DNA (ctDNA), fragments of DNA released into the blood circulation by tumour cells, and it is this marker that has been the focus of our efforts during this project. We have built on our previous research showing that ctDNA can be used effectively as a tool to aid in the clinical care of cancer patients.Throughout this project we have made several important contributions to the field:• We assessed and established potential applications of ctDNA across multiple cancer types • We established characteristics of cell-free DNA and ctDNA that provide clues about the biology of ctDNA. These provide the foundation for improved analytical methods and future clinical applications: • We led (and continue to lead) clinical studies to collect plasma and other bodily fluid samples, and collaborated to support multiple projects• We explored mechanisms of treatment resistance by unbiased whole exome sequencing of patient fluid samples.• We developed methods for the ultra-sensitive detection of ctDNA when tumour burden is low, such as in early stage disease or after treatment. Using our optimised methods, we can detect ctDNA to individual minute amounts.• We demonstrated that it is feasible to detect, quantify and characterise ctDNA in small volumes of blood (such as in a dried blood spot) or in ‘sub-optimally’ processed blood samples (such as in archived blood samples)These advances we have led and promoted have played a key part in the liquid biopsy revolution. Whilst initial ctDNA assays have been approved and are entering clinical used, there remain considerable opportunities to improve the utility of this biomarker. For example, the technological improvements we developed and demonstrated will support the expansion of applications into additional areas, including earlier-stage disease where ctDNA levels are lower and harder to detect, and research questions which require samples which are difficult to obtain or are highly limiting.